1 Introduction

Dynamic frameworks like Rails, Django and TurboGears helped pave the way to a more modern way of thinking about web applications. Grails builds on these concepts and dramatically reduces the complexity of building web applications on the Java platform. What makes it different, however, is that it does so by building on already established Java technologies like Spring and Hibernate.

Grails is a full stack framework and attempts to solve as many pieces of the web development puzzle through the core technology and its associated plugins. Included out the box are things like:

• An easy to use Object Relational Mapping (ORM) layer built on Hibernate
• An expressive view technology called Groovy Server Pages (GSP)
• A controller layer built on Spring MVC
• A command line scripting environment built on the Groovy-powered Gant
• An embedded Tomcat container which is configured for on the fly reloading
• Dependency injection with the inbuilt Spring container
• Support for internationalization (i18n) built on Spring's core MessageSource concept
• A transactional service layer built on Spring's transaction abstraction

All of these are made easy to use through the power of the Groovy language and the extensive use of Domain Specific Languages (DSLs)

This documentation will take you through getting started with Grails and building web applications with the Grails framework.

1.1 What's new in Grails 2.2?

Namespace Support

Grails 2.2 includes improved support for managing naming conflicts between artifacts provided by an application and its plugins.

Bean names for Service artifacts provided by a plugin are now prefixed with the plugin name. For example, if a Service named com.publishing.AuthorService is provided by a plugin named PublishingUtilities and another Service named com.bookutils.AuthorService is provided by a plugin named BookUtilities, the bean names for those services will be publishingUtilitiesAuthorService and bookUtilitiesAuthorService respectively. If a plugin provides a Service that does not have a name which conflicts with any other Service, then a bean alias will automatically be created that does not contain the prefix and the alias will refer to the bean referenced by the prefixed name. Service artifacts provided directly by the application will have no prefix added to the relevant bean name. See the dependency injection and services docs.

Domain classes provided by a plugin will have their default database table name prefixed with the plugin name if the grails.gorm.table.prefix.enabled config property is set to true. For example, if the PublishingUtilities plugin provides a domain class named Book, the default table name for that domain class will be PUBLISHING_UTILITIES_BOOK if the grails.gorm.table.prefix.enabled config property is set to true.

URL Mappings may now include a plugin attribute to indicate that the controller referenced in the mapping is provided by a particular plugin.

static mappings = {
    // requests to /bookAuthors will be handled by the
    // AuthorController provided by the BookUtilities plugin
    "/bookAuthors" {
        controller = 'author'
        plugin = 'bookUtilities'
    }
    // requests to /publishingAuthors will be handled by the
    // AuthorController provided by the Publishing plugin
    "/publishingAuthors" {
        controller = 'author'
        plugin = 'publishing'
    }
}

See the namespaced controllers docs for more information.

Controller methods and GSP Tags which accept a controller name as a paramater now support an optional parameter indicating that the controller is provided by a specific plugin.

<g:link controller="user" plugin="springSecurity">Manage Users</g:link>

class DemoController {
    def index() {
        redirect controller: 'user', action: 'list', plugin: 'springSecurity'
    }
}

Forked Tomcat Execution

Grails 2.2 supports forked JVM execution of the Tomcat container in development mode. This has several benefits including:

• Reduced memory consumption, since the Grails build system can exit
• Isolation of the build classpath from the runtime classpath
• The ability to deploy other Grails/Spring applications in parallel without conflicting dependencies

See the documentation on using forked mode for more information.

SQL Projections In Criteria Queries

Grails 2.2 adds new functionality to criteria queries to provide access to Hibernate's SQL projection API.

// Use SQL projections to retrieve the perimeter and area of all of the Box instances…
def c = Box.createCriteria()
def results = c.list {
    projections {
      sqlProjection '(2 * (width + height)) as perimiter, (width * height) as area', ['perimeter', 'area'], [INTEGER, INTEGER]
    }
}

See the Criteria section for more information.

Groovy 2

Grails 2.2 ships with Groovy 2.0, which has a bunch of new features itself.

1.2 What's new in Grails 2.1?

Maven Improvements / Multi Module Build Support

Grails' Maven support has been improved in a number of significant ways. Firstly it is now possible to specify plugins within your pom.xml file:

<dependency>
    <groupId>org.grails.plugins</groupId>
    <artifactId>hibernate</artifactId>
    <version>2.1.0</version>
    <type>zip</type>
    <scope>compile</scope>
</dependency>

The Maven plugin now resolves plugins as well as jar dependencies (previously jar dependencies were resolved by Maven and plugins by Ivy). Ivy is completely disabled leaving all dependency resolution up to Maven ensuring that evictions work as expected.

There is also a new Grails create-multi-project-build script which features initial support for Maven (Gradle coming in a future release). This script can be run from a parent directory containing Grails applications and plugins and it will generate a Maven multi-module build.

Enabling Maven in a project has been made easier with the inclusion of the create-pom command:

grails create-app myapp
cd myapp
grails create-pom com.mycompany
mvn package

To create a multi-module Maven build follow these steps:

grails create-app myapp
grails create-plugin plugin-a
grails create-plugin plugin-b
grails create-multi-project-build com.mycompany:parent:1.0-SNAPSHOT
mvn install

Grails Wrapper

The Grails Wrapper allows a Grails application to build without having to install Grails and configure a GRAILS_HOME environment variable. The wrapper includes a small shell script and a couple of small bootstrap jar files that typically would be checked in to source code control along with the rest of the project. The first time the wrapper is executed it will download and configure a Grails installation. This wrapper makes it more simple to setup a development environment, configure CI and manage upgrades to future versions of Grails. When the application is upgraded to the next version of Grails, the wrapper is updated and checked in to the source code control system and the next time developers update their workspace and run the wrapper, they will automatically be using the correct version of Grails.

See the Wrapper Documentation for more details.

Debug Option

The grails command now supports a -debug option which will startup the remote debug agent. This behavior used to be provided by the grails-debug command. grails-debug is still available but is deprecated and may be removed from a future release.

grails -debug run-app

Grails Command Aliases

The alias command may be used to define aliases for grails commands.

The following command creates an alias named rit (short for "run integration tests"):

grails alias rit test-app integration:

See the alias docs for more info.

Cache Plugin

Grails 2.1 installs the cache plugin by default. This plugin provides powerful and easy to use cache functionality to applications and plugins. The main plugin provides basic map backed caching support. For more robust caching options one of the implementation plugins should be installed and configured. See the cache-redis docs and the cache-ehcache docs for details.

See the main plugin documentation for details on how to configure and use the plugin.

New GORM Methods

In Grails 2.1.1 domain classes now have static methods named first and last to retrieve the first and last instances from the datastore. See the first and last documentation for details.

1.3 What's new in Grails 2.0?

1.3.1 Development Environment Features

Interactive Mode and Console Enhancements

Grails 2.0 features brand new console output that is more concise and user friendly to consume. An example of the new output when running tests can be seen below:

In general Grails makes its best effort to display update information on a single line and only present the information that is crucial. This means that while in previous versions of Grails the war command produced many lines of output, in Grails 2.0 only 1 line of output is produced:

In addition simply typing 'grails' at the command line activates the new interactive mode which features TAB completion, command history and keeps the JVM running to ensure commands execute much quicker than otherwise

For more information on the new features of the console refer to the section of the user guide that covers the console and interactive mode.

Reloading Agent

Grails 2.0 reloading mechanism no longer uses class loaders, but instead uses a JVM agent to reload changes to class files. This results in greatly improved reliability when reloading changes and also ensures that the class files stored in disk remain consistent with the class files loaded in memory, which reduces the need to run the clean command.

New Test Report and Documentation Templates

There are new templates for displaying test results that are clearer and more user friendly than the previous reports:

In addition, the Grails documentation engine has received a facelift with a new template for presenting Grails application and plugin documentation:

See the section on the documentation engine for more usage info.

Use a TOC for Project Docs

The old documentation engine relied on you putting section numbers into the gdoc filenames. Although convenient, this effectively made it difficult to restructure your user guide by inserting new chapters and sections. In addition, any such restructuring or renaming of section titles resulted in breaking changes to the URLs.

You can now use logical names for your gdoc files and define the structure and section titles in a YAML table-of-contents file, as described in the section on the documentation engine. The logical names appear in the URLs, so as long as you don't change those, your URLs will always remain the same no matter how much restructuring or changing of titles you do.

Grails 2.0 even provides a migrate-docs command to aid you in migrating existing gdoc user guides.

Enhanced Error Reporting and Diagnosis

Error reporting and problem diagnosis has been greatly improved with a new errors view that analyses stack traces and recursively displays problem areas in your code:

In addition stack trace filtering has been further enhanced to display only relevant trace information:

Line | Method
->>   9 | getValue     in Book.groovy
- - - - - - - - - - - - - - - - - - - - - - - - -
|     7 | getBookValue in BookService.groovy
|   886 | runTask . .  in ThreadPoolExecutor.java
|   908 | run          in     ''
^   662 | run . . . .  in Thread.java

H2 Database and Console

Grails 2.0 now uses the H2 database instead of HSQLDB, and enables the H2 database console in development mode (at the URI /dbconsole) so that the in-memory database can be easily queried from the browser:

Plugin Usage Tracking

To enhance community awareness of the most popular plugins an opt-in plugin usage tracking system has been included where users can participate in providing feedback to the plugin community on which plugins are most popular.

This will help drive the roadmap and increase support of key plugins while reducing the need to support older or less popular plugins thus helping plugin development teams focus their efforts.

Dependency Resolution Improvements

There are numerous improvements to dependency resolution handling via Ivy including:

• Grails now makes a best effort to cache the previous resolve and avoid resolving again unless you change BuildConfig.groovy.
• Plugins dependencies now appear in the dependency report generated by grails dependency-report
• Plugins published with the release plugin now publish their transitive plugin dependencies in the generated POM which are later resolved.
• It is now possible to customize the ivy cache directory via BuildConfig.groovy

grails.project.dependency.resolution = {
    cacheDir "target/ivy-cache"
}

• You can change the ivy cache directory for all projects via settings.groovy

grails.dependency.cache.dir = "${userHome}/.ivy2/cache"

• It is now possible to completely disable resolution from inherited repositories (repositories defined by other plugins):

grails.project.dependency.resolution = {
    repositories {
        inherits false // Whether to inherit repository definitions from plugins
        …
    }
    …
}

• It is now possible to easily disable checksum validation errors:

grails.project.dependency.resolution = {
    checksums false // whether to verify checksums or not
}

1.3.2 Core Features

Binary Plugins

Grails plugins can now be packaged as JAR files and published to standard maven repositories. This even works for GSP and static resources (with resources plugin 1.0.1). See the section on Binary plugins for more information.

Groovy 1.8

Grails 2.0 comes with Groovy 1.8 which includes many new features and enhancements

Spring 3.1 Profile Support

Grails' existing environment support has been bridged into the Spring 3.1 profile support. For example when running with a custom Grails environment called "production", a Spring profile of "production" is activated so that you can use Spring's bean configuration APIs to configure beans for a specific profile.

1.3.3 Web Features

Controller Actions as Methods

It is now possible to define controller actions as methods instead of using closures as in previous versions of Grails. In fact this is now the preferred way of expressing an action. For example:

// action as a method
def index() {
}
// action as a closure
def index = {
}

Binding Primitive Method Action Arguments

It is now possible to bind form parameters to action arguments where the name of the form element matches the argument name. For example given the following form:

<g:form name="myForm" action="save">
    <input name="name" />
    <input name="age" />
</g:form>

You can define an action that declares arguments for each input and automatically converts the parameters to the appropriate type:

def save(String name, int age) {
    // remaining
}

Static Resource Abstraction

A new static resource abstraction is included that allows declarative handling of JavaScript, CSS and image resources including automatic ordering, compression, caching and gzip handling.

Servlet 3.0 Async Features

Grails now supports Servlet 3.0 including the Asynchronous programming model defined by the specification:

def index() {
    def ctx = startAsync()
    ctx.start {
        new Book(title:"The Stand").save()
        render template:"books", model:[books:Book.list()]
        ctx.complete()
    }
}

Link Generation API

A general purpose LinkGenerator class is now available that is usable anywhere within a Grails application and not just within the context of a controller. For example if you need to generate links in a service or an asynchronous background job outside the scope of a request:

LinkGenerator grailsLinkGenerator
def generateLink() {
    grailsLinkGenerator.link(controller:"book", action:"list")
}

Page Rendering API

Like the LinkGenerator the new PageRenderer can be used to render GSP pages outside the scope of a web request, such as in a scheduled job or web service. The PageRenderer class features a very similar API to the render method found within controllers:

grails.gsp.PageRenderer groovyPageRenderer
void welcomeUser(User user) {
    def contents = groovyPageRenderer.render(view:"/emails/welcomeLetter", model:[user: user])
    sendEmail {
        to user.email
        body contents
    }
}

The PageRenderer service also allows you to pre-process GSPs into HTML templates:

new File("/path/to/welcome.html").withWriter { w ->
    groovyPageRenderer.renderTo(view:"/page/content", w)
}

Filter Exclusions

Filters may now express controller, action and uri exclusions to offer more options for expressing to which requests a particular filter should be applied.

filter1(actionExclude: 'log*') {
    before = {
        // …
    }
}
filter2(controllerExclude: 'auth') {
    before = {
        // …
    }
}
filter3(uriExclude: '/secure*') {
    before = {
        // …
    }
}

Performance Improvements

Performance of GSP page rendering has once again been improved by optimizing the GSP compiler to inline method calls where possible.

HTML5 Scaffolding

There is a new HTML5-based scaffolding UI:

jQuery by Default

The jQuery plugin is now the default JavaScript library installed into a Grails application. For backwards compatibility a Prototype plugin is available. Refer to the documentation on the Prototype plugin for installation instructions.

Easy Date Parsing

A new date method has been added to the params object to allow easy, null-safe parsing of dates:

def val = params.date('myDate', 'dd-MM-yyyy')
// or a list for formats
def val = params.date('myDate', ['yyyy-MM-dd', 'yyyyMMdd', 'yyMMdd'])
// or the format read from messages.properties via the key 'date.myDate.format'
def val = params.date('myDate')

Customizable URL Formats

The default URL Mapping mechanism supports camel case names in the URLs. The default URL for accessing an action named addNumbers in a controller named MathHelperController would be something like /mathHelper/addNumbers. Grails allows for the customization of this pattern and provides an implementation which replaces the camel case convention with a hyphenated convention that would support URLs like /math-helper/add-numbers. To enable hyphenated URLs assign a value of "hyphenated" to the grails.web.url.converter property in grails-app/conf/Config.groovy.

// grails-app/conf/Config.groovy
grails.web.url.converter = 'hyphenated'

Arbitrary strategies may be plugged in by providing a class which implements the UrlConverter interface and adding an instance of that class to the Spring application context with the bean name of grails.web.UrlConverter.BEAN_NAME. If Grails finds a bean in the context with that name, it will be used as the default converter and there is no need to assign a value to the grails.web.url.converter config property.

// src/groovy/com/myapplication/MyUrlConverterImpl.groovy
package com.myapplication
class MyUrlConverterImpl implements grails.web.UrlConverter {
    String toUrlElement(String propertyOrClassName) {
        // return some representation of a property or class name that should be used in URLs…
    }
}

// grails-app/conf/spring/resources.groovy
beans = {
    "${grails.web.UrlConverter.BEAN_NAME}"(com.myapplication.MyUrlConverterImpl)
}

Web Flow input and output

1.3.4 Persistence Features

The GORM API

The GORM API has been formalized into a set of classes (GormStaticApi, GormInstanceApi and GormValidationApi) that get statically wired into every domain class at the byte code level. The result is better code completion for IDEs, better integration with Java and the potential for more GORM implementations for other types of data stores.

Detached Criteria and Where Queries

Grails 2.0 features support for DetachedCriteria which are criteria queries that are not associated with any session or connection and thus can be more easily reused and composed:

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}
def results = criteria.list(max:4, sort:"firstName")

To support the addition of DetachedCriteria queries and encourage their use a new where method and DSL has been introduced to greatly reduce the complexity of criteria queries:

def query = Person.where {
    (lastName != "Simpson" && firstName != "Fred") || (firstName == "Bart" && age > 9)
}
def results = query.list(sort:"firstName")

See the documentation on DetachedCriteria and Where Queries for more information.

New findOrCreate and findOrSave Methods

Domain classes have support for the findOrCreateWhere, findOrSaveWhere, findOrCreateBy and findOrSaveBy query methods which behave just like findWhere and findBy methods except that they should never return null. If a matching instance cannot be found in the database then a new instance is created, populated with values represented in the query parameters and returned. In the case of findOrSaveWhere and findOrSaveBy, the instance is saved before being returned.

def book = Book.findOrCreateWhere(author: 'Douglas Adams', title: "The Hitchiker's Guide To The Galaxy")
def book = Book.findOrSaveWhere(author: 'Daniel Suarez', title: 'Daemon')
def book = Book.findOrCreateByAuthorAndTitle('Daniel Suarez', 'Daemon')
def book = Book.findOrSaveByAuthorAndTitle('Daniel Suarez', 'Daemon')

Abstract Inheritance

GORM now supports abstract inheritance trees which means you can define queries and associations linking to abstract classes:

abstract class Media {
    String title
    …
}
class Book extends Media {
}
class Album extends Media {
}
class Account {
    static hasMany = [purchasedMedia:Media]
}
..
def allMedia = Media.list()

Multiple Data Sources Support

It is now possible to define multiple datasources in DataSource.groovy and declare one or more datasources a particular domain uses by default:

class ZipCode {
   String code
   static mapping = {
      datasource 'ZIP_CODES'
   }
}

If multiple datasources are specified for a domain then you can use the name of a particular datasource as a namespace in front of any regular GORM method:

def zipCode = ZipCode.auditing.get(42)

For more information see the section on Multiple Data Sources in the user guide.

Database Migrations

A new database migration plugin has been designed and built for Grails 2.0 allowing you to apply migrations to your database, rollback changes and diff your domain model with the current state of the database.

Database Reverse Engineering

A new database reverse engineering plugin has been designed and built for Grails 2.0 that allows you to generate a domain model from an existing database schema.

Hibernate 3.6

Grails 2.0 is now built on Hibernate 3.6

Bag Collections

You can now use Hibernate Bags for mapped collections to avoid the memory and performance issues of loading large collections to enforce Set uniqueness or List order.

For more information see the section on Sets, Lists and Maps in the user guide.

1.3.5 Testing Features

New Unit Testing Console Output

Test output from the test-app command has been improved:

New Unit Testing API

There is a new unit testing API based on mixins that supports JUnit 3, 4 and Spock style tests (with Spock 0.6 and above). Example:

import grails.test.mixin.TestFor
@TestFor(SimpleController)
class SimpleControllerTests  {
    void testIndex() {
        controller.home()
        assert view == "/simple/homePage"
        assert model.title == "Hello World"
    }
}

The documentation on testing has also been re-written around this new framework.

Unit Testing GORM

A new in-memory GORM implementation is present that supports many more features of the GORM API making unit testing of criteria queries, named queries and other previously unsupported methods possible.

Faster Unit Testing with Interactive Mode

The new interactive mode (activated by typing 'grails') greatly improves the execution time of running unit and integration tests.

Unit Test Scaffolding

A unit test is now generated for scaffolded controllers

2 Getting Started

2.1 Installation Requirements

• Windows
• Linux
• Mac OS X

These will show you how to install Grails too, not just the JDK.

A JDK is required in your Grails development environment. A JRE is not sufficient.

On some platforms (for example OS X) the Java installation is automatically detected. However in many cases you will want to manually configure the location of Java. For example:

export JAVA_HOME=/Library/Java/Home
export PATH="$PATH:$JAVA_HOME/bin"

if you're using bash or another variant of the Bourne Shell.

2.2 Downloading and Installing

• Download a binary distribution of Grails and extract the resulting zip file to a location of your choice
• Set the GRAILS_HOME environment variable to the location where you extracted the zip
• On Unix/Linux based systems this is typically a matter of adding something like the following export GRAILS_HOME=/path/to/grails to your profile
• On Windows this is typically a matter of setting an environment variable under My Computer/Advanced/Environment Variables
• Then add the bin directory to your PATH variable:
• On Unix/Linux based systems this can be done by adding export PATH="$PATH:$GRAILS_HOME/bin" to your profile
• On Windows this is done by modifying the Path environment variable under My Computer/Advanced/Environment Variables

If Grails is working correctly you should now be able to type grails -version in the terminal window and see output similar to this:

Grails version: 2.0.0

2.3 Creating an Application

grails [command name]

Run create-app to create an application:

grails create-app helloworld

This will create a new directory inside the current one that contains the project. Navigate to this directory in your console:

cd helloworld

2.4 A Hello World Example

$ cd helloworld
$ grails

You should see a prompt that looks like this:

What we want is a simple page that just prints the message "Hello World!" to the browser. In Grails, whenever you want a new page you just create a new controller action for it. Since we don't yet have a controller, let's create one now with the create-controller command:

grails> create-controller hello

Don't forget that in the interactive console, we have auto-completion on command names. So you can type "cre" and then press <tab> to get a list of all create-* commands. Type a few more letters of the command name and then <tab> again to finish.

The above command will create a new controller in the grails-app/controllers/helloworld directory called HelloController.groovy. Why the extra helloworld directory? Because in Java land, it's strongly recommended that all classes are placed into packages, so Grails defaults to the application name if you don't provide one. The reference page for create-controller provides more detail on this.

We now have a controller so let's add an action to generate the "Hello World!" page. The code looks like this:

package helloworld
class HelloController {
    def index() {
        render "Hello World!"
    }
}

The action is simply a method. In this particular case, it calls a special method provided by Grails to render the page.

Job done. To see your application in action, you just need to start up a server with another command called run-app:

grails> run-app

This will start an embedded server on port 8080 that hosts your application. You should now be able to access your application at the URL http://localhost:8080/helloworld/ - try it!

If you see the error "Server failed to start for port 8080: Address already in use", then it means another server is running on that port. You can easily work around this by running your server on a different port using -Dserver.port=9090 run-app. '9090' is just an example: you can pretty much choose anything within the range 1024 to 49151.

The result will look something like this:

This is the Grails intro page which is rendered by the grails-app/view/index.gsp file. It detects the presence of your controllers and provides links to them. You can click on the "HelloController" link to see our custom page containing the text "Hello World!". Voila! You have your first working Grails application.

One final thing: a controller can contain many actions, each of which corresponds to a different page (ignoring AJAX at this point). Each page is accessible via a unique URL that is composed from the controller name and the action name: /<appname>/<controller>/<action>. This means you can access the Hello World page via /helloworld/hello/index, where 'hello' is the controller name (remove the 'Controller' suffix from the class name and lower-case the first letter) and 'index' is the action name. But you can also access the page via the same URL without the action name: this is because 'index' is the default action . See the end of the controllers and actions section of the user guide to find out more on default actions.

2.5 Using Interactive Mode

For more information on the capabilities of interactive mode refer to the section on Interactive Mode in the user guide.

2.6 Getting Set Up in an IDE

IntelliJ IDEA

IntelliJ IDEA and the JetGroovy plugin offer good support for Groovy and Grails developers. Refer to the section on Groovy and Grails support on the JetBrains website for a feature overview.

IntelliJ IDEA comes in two flavours; the open source "Community Edition" and the commercial "Ultimate Edition". Both offers support for Groovy, but only Ultimate Edition offers Grails support.

With Ultimate Edition, there is no need to use the grails integrate-with --intellij command, as Ultimate Edition understands Grails projects natively. Just open the project with File -> New Project -> Create project from existing sources.

You can still use Community Edition for Grails development, but you will miss out on all the Grails specific features like automatic classpath management, GSP editor and quick access to Grails commands. To integrate Grails with Community Edition run the following command to generate appropriate project files:

grails integrate-with --intellij

Eclipse

We recommend that users of Eclipse looking to develop Grails application take a look at Groovy/Grails Tool Suite, which offers built in support for Grails including automatic classpath management, a GSP editor and quick access to Grails commands. See the STS Integration page for an overview.

NetBeans

NetBeans provides a Groovy/Grails plugin that automatically recognizes Grails projects and provides the ability to run Grails applications in the IDE, code completion and integration with the Glassfish server. For an overview of features see the NetBeans Integration guide on the Grails website which was written by the NetBeans team.

TextMate

Since Grails' focus is on simplicity it is often possible to utilize more simple editors and TextMate on the Mac has an excellent Groovy/Grails bundle available from the Texmate bundles SVN.

To integrate Grails with TextMate run the following command to generate appropriate project files:

grails integrate-with --textmate

Alternatively TextMate can easily open any project with its command line integration by issuing the following command from the root of your project:

mate .

2.7 Convention over Configuration

Here is a breakdown and links to the relevant sections:

• grails-app - top level directory for Groovy sources
• conf - Configuration sources.
• controllers - Web controllers - The C in MVC.
• domain - The application domain.
• i18n - Support for internationalization (i18n).
• services - The service layer.
• taglib - Tag libraries.
• utils - Grails specific utilities.
• views - Groovy Server Pages - The V in MVC.
• scripts - Gant scripts.
• src - Supporting sources
• groovy - Other Groovy sources
• java - Other Java sources
• test - Unit and integration tests.

2.8 Running an Application

grails run-app

You can specify a different port by using the server.port argument:

grails -Dserver.port=8090 run-app

Note that it is better to start up the application in interactive mode since a container restart is much quicker:

$ grails
grails> run-app
| Server running. Browse to http://localhost:8080/helloworld
| Application loaded in interactive mode. Type 'stop-app' to shutdown.
| Downloading: plugins-list.xml
grails> stop-app
| Stopping Grails server
grails> run-app
| Server running. Browse to http://localhost:8080/helloworld
| Application loaded in interactive mode. Type 'stop-app' to shutdown.
| Downloading: plugins-list.xml

More information on the run-app command can be found in the reference guide.

2.9 Testing an Application

To execute tests you run the test-app command as follows:

grails test-app

2.10 Deploying an Application

grails war

This will produce a WAR file under the target directory which can then be deployed as per your container's instructions.

Unlike most scripts which default to the development environment unless overridden, the war command runs in the production environment by default. You can override this like any script by specifying the environment name, for example:

grails dev war

NEVER deploy Grails using the run-app command as this command sets Grails up for auto-reloading at runtime which has a severe performance and scalability implications

When deploying Grails you should always run your containers JVM with the -server option and with sufficient memory allocation. A good set of VM flags would be:

-server -Xmx512M -XX:MaxPermSize=256m

2.11 Supported Java EE Containers

• Tomcat 7
• Tomcat 6
• SpringSource tc Server
• Eclipse Virgo
• GlassFish 3
• GlassFish 2
• Resin 4
• Resin 3
• JBoss 6
• JBoss 5
• Jetty 8
• Jetty 7
• Jetty 6
• Oracle Weblogic 10.3
• Oracle Weblogic 10
• Oracle Weblogic 9
• IBM WebSphere 8.5
• IBM WebSphere 8.0
• IBM WebSphere 7.0
• IBM WebSphere 6.1

It's required to set "-Xverify:none" in "Application servers > server > Process Definition > Java Virtual Machine > Generic JVM arguments" for WebSphere.

Some containers have bugs however, which in most cases can be worked around. A list of known deployment issues can be found on the Grails wiki.

2.12 Generating an Application

grails generate-all Book

2.13 Creating Artefacts

These are just for your convenience and you can just as easily use an IDE or your favourite text editor.

grails create-domain-class book

This will result in the creation of a domain class at grails-app/domain/Book.groovy such as:

class Book {
}

There are many such create-* commands that can be explored in the command line reference guide.

To decrease the amount of time it takes to run Grails scripts, use the interactive mode.

3 Upgrading from previous versions of Grails

• Dependency resolution has been changed to only use data from the POMs to resolve, this can impact plugins and you may need to republish a plugin with corrected dependency data
• environment bean added by Spring 3.1, which will be auto-wired into properties of the same name.
• Logging by convention packages have changed, so you may not see the logging output you expect. Update your logging configuration as described below.
• HSQLDB has been replaced with H2 as default in-memory database. If you use the former, either change your data source to H2 or add HSQLDB as a runtime dependency.
• The release-plugin command has been removed. You must now install the Release plugin and use its publish-plugin command instead.
• The redirect() method no longer commits the response, so isCommitted() will return false. If you use that method, then call request.isRedirected() instead.
• The redirect() method now uses the grails.serverURL config setting to generate the redirect URL. You may need to remove the setting, particularly from the development and test environments.
• withFormat() no longer takes account of the request content type. If you want to do something based on the request content type, use request.withFormat().
• Adaptive AJAX tags using Prototype will break. In this situation you must install the new Prototype plugin.
• If you install Resources (or it is installed automatically), tags like <g:javascript> won't write anything to the page until you add the <r:layoutResources/> tags to your layout.
• Resources adds a '/static' URL, so you may have to update your access control rules accordingly.
• Some plugins may fail to install because one or more of their dependencies can not be found. If this happens, the plugin probably has a custom repository URL that you need to add to your project's BuildConfig.groovy.
• The behaviour of abstract domain classes has changed, so if you use them you will either have to move the abstract classes to 'src/groovy' or migrate your database schema and data.
• Criteria queries default to INNER_JOIN for associations rather than OUTER_JOIN. This may affect some of your result data.
• Constraints declared for non-existent properties will now throw an exception.
• beforeValidate() may be called two or more times during a request, for example once on save() and once just before the view is rendered.
• Public methods in controllers will now be treated as actions. If you don't want this, make them protected or private.
• The new unit testing framework won't work with the old GrailsUnitTestCase class hierarchy. Your old tests will continue to work, but if you wish to use the new annotations, do not extend any of the *UnitTestCase classes.
• Output from Ant tasks is now hidden by default. If your scripts are using ant.echo(), ant.input(), etc. you might want to use alternative mechanisms for output.
• Domain properties of type java.net.URL may no longer work with your existing data. The serialisation mechanism for them appears to have changed. Consider migrating your data and domain models to String.
• The Ivy cache location has changed. If you want to use the old location, configure the appropriate global setting (see below) but be aware that you may run into problems running Grails 1.3.x and 2.x projects side by side.
• With new versions of various dependencies, some APIs (such as the Servlet API) may have changed. If you have code that implements any of those APIs, you will need to update it. Problems will typically manifest as compilation errors.
• The following deprecated classes have been removed: grails.web.JsonBuilder and grails.web.OpenRicoBuilder.

Upgrading to 2.2 from 2.1 or 2.0

Groovy 2.0

Grails 2.2 ships with Groovy 2.0 which has some language level changes that may require changes to your code or plugins that you use.

Dependency resolution

Grails 2.2 no longer uses the BuildConfig of the plugin for dependency resolution and only uses data provided by POMs, this may impact some plugins that had previously incorrectly specified dependency information.

If you don't want to immediately deal with the changes necessary to upgrade, then you can open BuildConfig and set the legacyResolve settings to true:

grails.project.dependency.resolution = {    
    …
    legacyResolve false 
    …
}

This is not recommended however, as it will re-enable the previous behavior of using both POM data and BuildConfig to resolve dependencies. The most commmon problem you will face is with plugins that express their dependencies in a scope that is not valid inside a POM (example: "build" scope).

Plugins like this will need to be re-publish with a corrected scope of "compile". If you then specify the plugin as "build" scope in your application, transitive compile and runtime scoped dependencies will be converted to "build" scope as well.

Upgrading from Grails 1.3.x

Changes to web.xml template

If you have customized the web.xml provided by grails install-templates then you will need to update this customized template with the latest version provided by Grails. Failing to do so will lead to a ClassNotFoundException for the org.codehaus.groovy.grails.web.util.Log4jConfigListener class.

Groovy 1.8 Changes

Groovy 1.8 is a little stricter in terms of compilation so you may be required to fix compilation errors in your application that didn't occur under Grails 1.3.x.

Groovy 1.8 also requires that you update many of the libraries that you may be using in your application. Libraries known to require an upgrade include:

• Spock
• Geb
• GMock (upgrade unavailable as of this writing)

New 'environment' bean

Spring 3.1 adds a new bean with the name 'environment'. It's of type Environment (in package org.springframework.core.env) and it will automatically be autowired into properties with the same name. This seems to cause particular problems with domain classes that have an environment property. In this case, adding the method:

void setEnvironment(org.springframework.core.env.Environment env) {}

HSQLDB Has Been Replaced With H2

HSQLDB is still bundled with Grails but is not configured as a default runtime dependency. Upgrade options include replacing HSQLDB references in DataSource.groovy with H2 references or adding HSQLDB as a runtime dependency for the application.

If you want to run an application with different versions of Grails, it's simplest to add HSQLDB as a runtime dependency, which you can do in BuildConfig.groovy:

grails.project.dependency.resolution = {
    inherits("global") {
    }
    repositories {
        grailsPlugins()
        grailsHome()
        grailsCentral()
    }
    dependencies {
        // Add HSQLDB as a runtime dependency
        runtime 'hsqldb:hsqldb:1.8.0.10'
    }
}

A default DataSource.groovy which is compatible with H2 looks like this:

dataSource {
    driverClassName = "org.h2.Driver"
    username = "sa"
    password = ""
}
// environment specific settings
environments {
    development {
        dataSource {
            dbCreate = "create-drop" // one of 'create', 'create-drop','update'
            url = "jdbc:h2:mem:devDb"
        }
    }
    test {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:mem:testDb"
        }
    }
    production {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:prodDb"
        }
    }
}

Another significant difference between H2 and HSQLDB is in the handling of byte[] domain class properties. HSQLDB's default BLOB size is large and so you typically don't need to specify a maximum size. But H2 defaults to a maximum size of 255 bytes! If you store images in the database, the saves are likely to fail because of this. The easy fix is to add a maxSize constraint to the byte[] property:

class MyDomain {
    byte[] data
    static constraints = {
        data maxSize: 1024 * 1024 * 2 // 2MB
    }
}

This constraint influences schema generation, so in the above example H2 will have the data column set to BINARY(2097152) by Hibernate.

Abstract Inheritance Changes

In previous versions of Grails abstract classes in grails-app/domain were not treated as persistent. This is no longer the case and has a significant impact on upgrading your application. For example consider the following domain model in a Grails 1.3.x application:

abstract class Sellable {
}
class Book extends Sellable {
}

In Grails 1.3.x you would get a BOOK table and the properties from the Sellable class would be stored within the BOOK table. However, in Grails 2.x you will get a SELLABLE table and the default table-per-hierarchy inheritance rules apply with all properties of the Book stored in the SELLABLE table.

You have two options when upgrading in this scenario:

1. Move the abstract Sellable class into the src/groovy package. If the Sellable class is in the src/groovy directory it will no longer be regarded as persistent.
2. Use the database migration plugin to apply the appropriate changes to the database (typically renaming the table to the root abstract class of the inheritance tree).

Criteria Queries Default to INNER JOIN

The previous default of LEFT JOIN for criteria queries across associations is now INNER JOIN.

Invalid Constraints Now Thrown an Exception

Previously if you defined a constraint on a property that doesn't exist no error would be thrown:

class Person {
    String name
    static constraints = {
        bad nullable:false // invalid property, no error thrown
    }
}

Now the above code will result in an exception

Logging By Convention Changes

The packages that you should use for Grails artifacts have mostly changed. In particular:

• service -> services
• controller -> controllers
• tagLib -> taglib (case change)
• bootstrap -> conf
• dataSource -> conf

You can find out more about logging by convention in the main part of the user guide, under "Configuring loggers". This change is a side-effect of injecting the log property into artefacts at compile time.

jQuery Replaces Prototype

The Protoype Javascript library has been removed from Grails core and now new Grails applications have the jQuery plugin configured by default. This will only impact you if you are using Prototype with the adaptive AJAX tags in your application, e.g. <g:remoteLink/> etc, because those tags will break as soon as you upgrade.

To resolve this issue, simply install the Prototype plugin in your application. You can also remove the prototype files from your web-app/js/prototype directory if you want.

The Resources Plugin

The Resources plugin is a great new feature of Grails that allows you to manage static web resources better than before, but you do need to be aware that it adds an extra URL at /static. If you have access control in your application, this may mean that the static resources require an authenticated user to load them! Make sure your access rules take account of the /static URL.

Controller Public Methods

As of Grails 2.0, public methods of controllers are now treated as actions in addition to actions defined as traditional Closures. If you were relying on the use of methods for privacy controls or as helper methods then this could result in unexpected behavior. To resolve this issue you should mark all methods of your application that are not to be exposed as actions as private methods.

Command Object Constraints

As of Grails 2.0, constrained properties in command object classes are no longer nullable by default. Nullable command object properties must be explicitly configured as such in the same way that nullable persistent properties in domain classes are configured.

The redirect Method

The redirect method no longer commits the response. The result of this is code that relies of this behavior will break in 2.0. For example:

redirect action: "next"
if (response.committed) {
    // do something
}

In this case in Grails 1.3.x and below the response.committed property would return true and the if block will execute. In Grails 2.0 this is no longer the case and you should instead use the new isRedirected() method of the request object:

redirect action: "next"
if (request.redirected) {
    // do something
}

Another side-effect of the changes to the redirect method is that it now always uses the grails.serverURL configuration option if it's set. Previous versions of Grails included default values for all the environments, but when upgrading to Grails 2.0 those values more often than not break redirection. So, we recommend you remove the development and test settings for grails.serverURL or replace them with something appropriate for your application.

Content Negotiation

As of Grails 2.0 the withFormat method of controllers no longer takes into account the request content type (dictated by the CONTENT_TYPE header), but instead deals exclusively with the response content type (dictated by the ACCEPT header or file extension). This means that if your application has code that relies on reading XML from the request using withFormat this will no longer work:

def processBook() {
    withFormat {
        xml {
            // read request XML
        }
        html {
            // read request parameters
        }
    }
}

Instead you use the withFormat method provided on the request object:

def processBook() {
    request.withFormat {
        xml {
            // read request XML
        }
        html {
            // read request parameters
        }
    }
}

Unit Test Framework

Grails 2 introduces a new unit testing framework that is simpler and behaves more consistently than the old one. The old framework based on the GrailsUnitTestCase class hierarchy is still available for backwards compatibility, but it does not work with the new annotations.

Migrating unit tests to the new approach is non-trivial, but recommended. Here are a set of mappings from the old style to the new:

1. Remove extends *UnitTestCase and add a @TestFor annotation to the class if you're testing a core artifact (controller, tag lib, domain class, etc.) or @TestMixin(GrailsUnitTestMixin) for non-core artifacts and non-artifact classes.
2. Add @Mock annotation for domain classes that must be mocked and use new MyDomain().save() in place of mockDomain().
3. Replace references to mockRequest, mockResponse and mockParams with request, response and params.
4. Remove references to renderArgs and use the view and model properties for view rendering, or response.text for all others.
5. Replace references to redirectArgs with response.redirectedUrl. The latter takes into account the URL mappings as is a string URL rather than a map of redirect() arguments.
6. The mockCommandObject() method is no longer needed as Grails automatically detects whether an action requires a command object or not.

There are other differences, but these are the main ones. We recommend that you read the chapter on testing thoroughly to understand everything that has changed.

Note that the Grails annotations don't need to be imported in your test cases to run them from the command line, but your IDE may need them. So, here are the relevant classes with packages:

• grails.test.mixin.TestFor
• grails.test.mixin.TestMixin
• grails.test.mixin.Mock
• grails.test.mixin.support.GrailsUnitTestMixin
• grails.test.mixin.domain.DomainClassUnitTestMixin
• grails.test.mixin.services.ServiceUnitTestMixin
• grails.test.mixin.web.ControllerUnitTestMixin
• grails.test.mixin.web.FiltersUnitTestMixin
• grails.test.mixin.web.GroovyPageUnitTestMixin
• grails.test.mixin.web.UrlMappingsUnitTestMixin
• grails.test.mixin.webflow/WebFlowUnitTestMixin

Note that you're only ever likely to use the first two explicitly. The rest are there for reference.

Command Line Output

Ant output is now hidden by default to keep the noise in the terminal to a minimum. That means if you use ant.echo in your scripts to communicate messages to the user, we recommend switching to an alternative mechanism.

For status related messages, you can use the event system:

event "StatusUpdate", ["Some message"]
event "StatusFinal",  ["Some message"]
event "StatusError",  ["Some message"]

For more control you can use the grailsConsole script variable, which gives you access to an instance of GrailsConsole. In particular, you can log information messages with log() or info(), errors and warnings with error() and warning(), and request user input with userInput().

Custom Plugin Repositories

Many plugins have dependencies, both other plugins and straight JAR libraries. These are often located in Maven Central, the Grails core repository or the Grails Central Plugin Repository in which case applications are largely unaffected if they upgrade to Grails 2. But sometimes such dependencies are located elsewhere and Grails must be told where they can be found.

Due to changes in the way Grails handles the resolution of dependencies, Grails 2.0 requires you to add any such custom repository locations to your project if an affected plugin is to install properly.

Ivy Cache Location Has Changed

The default Ivy cache location for Grails has changed. If the thought of yet another cache of JARs on your disk horrifies you, then you can change this in your settings.groovy:

grails.dependency.cache.dir = "${userHome}/.ivy2/cache"

If you do this, be aware that you may run into problems running Grails 2 and earlier versions of Grails side-by-side. These problems can be avoided by excluding "xml-apis" and "commons-digester" from the inherited global dependencies in Grails 1.3 and earlier projects.

URL Domain Properties

If your domain model has any properties of type java.net.URL, they may cease to work once you upgrade to Grails 2. It seems that the default mapping of URL to database column has changed with the new version of Hibernate. This is a tricky problem to solve, but in the long run it's best if you migrate your URL properties to strings. One technique is to use the database migration plugin to add a new text column and then execute some code in BootStrap (using Grails 1.3.x or earlier) to fetch each row of the table as a domain instance, convert the URL properties to string URLs, and then write those values to the new column.

Updated Underlying APIs

Grails 2.0 contains updated dependencies including Servlet 3.0, Tomcat 7, Spring 3.1, Hibernate 3.6 and Groovy 1.8. This means that certain plugins and applications that depend on earlier versions of these APIs may no longer work. For example the Servlet 3.0 HttpServletRequest interface includes new methods, so if a plugin implements this interface for Servlet 2.5 but not for Servlet 3.0 then said plugin will break. The same can be said of any Spring interface.

Removal of release-plugin Command

The built in release-plugin command for releases plugins to the central Grails plugin repository has been removed. The new release plugin should be used instead which provides an equivalent publish-plugin command.

Removal of Deprecated Classes

The following deprecated classes have been removed: grails.web.JsonBuilder, grails.web.OpenRicoBuilder

Upgrading from Grails 1.2.x

Plugin Repositories

As of Grails 1.3, Grails no longer natively supports resolving plugins against secured SVN repositories. The plugin resolution mechanism in Grails 1.2 and below has been replaced by one built on Ivy, the upside of which is that you can now resolve Grails plugins against Maven repositories as well as regular Grails repositories.

Ivy supports a much richer set of repository resolvers for resolving plugins, including support for Webdav, HTTP, SSH and FTP. See the section on resolvers in the Ivy docs for all the available options and the section of plugin repositories in the user guide which explains how to configure additional resolvers.

If you still need support for resolving plugins against secured SVN repositories then the IvySvn project provides a set of resolvers for SVN repositories.

Upgrading from Grails 1.1.x

Plugin paths

In Grails 1.1.x typically a pluginContextPath variable was used to establish paths to plugin resources. For example:

<g:resource dir="${pluginContextPath}/images" file="foo.jpg" />

In Grails 1.2 views have been made plugin aware and this is no longer necessary:

<g:resource dir="images" file="foo.jpg" />

Additionally the above example will no longer link to an application image from a plugin view. To do so change the above to:

<g:resource contextPath="" dir="images" file="foo.jpg" />

The same rules apply to the javascript and render tags.

Tag and Body return values

Tags no longer return java.lang.String instances but instead return a Grails StreamCharBuffer instance. The StreamCharBuffer class implements all the same methods as String but doesn't extend String, so code like this will break:

def foo = body()
if (foo instanceof String) {
    // do something
}

In these cases you should check for the java.lang.CharSequence interface, which both String and StreamCharBuffer implement:

def foo = body()
if (foo instanceof CharSequence) {
    // do something
}

New JSONBuilder

There is a new version of JSONBuilder which is semantically different from the one used in earlier versions of Grails. However, if your application depends on the older semantics you can still use the deprecated implementation by setting the following property to true in Config.groovy:

grails.json.legacy.builder=true

Validation on Flush

Grails now executes validation routines when the underlying Hibernate session is flushed to ensure that no invalid objects are persisted. If one of your constraints (such as a custom validator) executes a query then this can cause an additional flush, resulting in a StackOverflowError. For example:

static constraints = {
    author validator: { a ->
        assert a != Book.findByTitle("My Book").author
    }
}

The above code can lead to a StackOverflowError in Grails 1.2. The solution is to run the query in a new Hibernate session (which is recommended in general as doing Hibernate work during flushing can cause other issues):

static constraints = {
    author validator: { a ->
        Book.withNewSession {
            assert a != Book.findByTitle("My Book").author
        }
    }
}

Upgrading from Grails 1.0.x

Groovy 1.6

Grails 1.1 and above ship with Groovy 1.6 and no longer supports code compiled against Groovy 1.5. If you have a library that was compiled with Groovy 1.5 you must recompile it against Groovy 1.6 or higher before using it with Grails 1.1.

Java 5.0

Grails 1.1 now no longer supports JDK 1.4, if you wish to continue using Grails then it is recommended you stick to the Grails 1.0.x stream until you are able to upgrade your JDK.

Configuration Changes

1) The setting grails.testing.reports.destDir has been renamed to grails.project.test.reports.dir for consistency.

2) The following settings have been moved from grails-app/conf/Config.groovy to grails-app/conf/BuildConfig.groovy:

• grails.config.base.webXml
• grails.project.war.file (renamed from grails.war.destFile)
• grails.war.dependencies
• grails.war.copyToWebApp
• grails.war.resources

3) The grails.war.java5.dependencies option is no longer supported, since Java 5.0 is now the baseline (see above).

4) The use of jsessionid (now considered harmful) is disabled by default. If your application requires jsessionid you can re-enable its usage by adding the following to grails-app/conf/Config.groovy:

grails.views.enable.jsessionid=true

5) The syntax used to configure Log4j has changed. See the user guide section on Logging for more information.

Plugin Changes

As of version 1.1, Grails no longer stores plugins inside your PROJECT_HOME/plugins directory by default. This may result in compilation errors in your application unless you either re-install all your plugins or set the following property in grails-app/conf/BuildConfig.groovy:

grails.project.plugins.dir="./plugins"

Script Changes

1) If you were previously using Grails 1.0.3 or below the following syntax is no longer support for importing scripts from GRAILS_HOME:

Ant.property(environment:"env")
grailsHome = Ant.antProject.properties."env.GRAILS_HOME"
includeTargets << new File("${grailsHome}/scripts/Bootstrap.groovy")

Instead you should use the new grailsScript method to import a named script:

includeTargets << grailsScript("_GrailsBootstrap")

2) Due to an upgrade of Gant all references to the variable Ant should be changed to ant.

3) The root directory of the project is no longer on the classpath, so loading a resource like this will no longer work:

def stream = getClass().classLoader.getResourceAsStream(
                   "grails-app/conf/my-config.xml")

Instead you should use the Java File APIs with the basedir property:

new File("${basedir}/grails-app/conf/my-config.xml").withInputStream { stream ->
    // read the file
}

Command Line Changes

The run-app-https and run-war-https commands no longer exist and have been replaced by an argument to run-app:

grails run-app -https

Data Mapping Changes

1) Enum types are now mapped using their String value rather than the ordinal value. You can revert to the old behavior by changing your mapping as follows:

static mapping = {
    someEnum enumType:"ordinal"
}

2) Bidirectional one-to-one associations are now mapped with a single column on the owning side and a foreign key reference. You shouldn't need to change anything; however you should drop column on the inverse side as it contains duplicate data.

REST Support

Incoming XML requests are now no longer automatically parsed. To enable parsing of REST requests you can do so using the parseRequest argument inside a URL mapping:

"/book"(controller:"book",parseRequest:true)

Alternatively, you can use the new resource argument, which enables parsing by default:

"/book"(resource:"book")

4 Configuration

4.1 Basic Configuration

• grails-app/conf/BuildConfig.groovy
• grails-app/conf/Config.groovy

Both of them use Groovy's ConfigSlurper syntax. The first, BuildConfig.groovy, is for settings that are used when running Grails commands, such as compile, doc, etc. The second file, Config.groovy, is for settings that are used when your application is running. This means that Config.groovy is packaged with your application, but BuildConfig.groovy is not. Don't worry if you're not clear on the distinction: the guide will tell you which file to put a particular setting in.

The most basic syntax is similar to that of Java properties files with dot notation on the left-hand side:

foo.bar.hello = "world"

Note that the value is a Groovy string literal! Those quotes around 'world' are important. In fact, this highlights one of the advantages of the ConfigSlurper syntax over properties files: the property values can be any valid Groovy type, such as strings, integers, or arbitrary objects!

Things become more interesting when you have multiple settings with the same base. For example, you could have the two settings

foo.bar.hello = "world"
foo.bar.good = "bye"

both of which have the same base: foo.bar. The above syntax works but it's quite repetitive and verbose. You can remove some of that verbosity by nesting properties at the dots:

foo {
    bar {
        hello = "world"
        good = "bye"
    }
}

or by only partially nesting them:

foo {
    bar.hello = "world"
    bar.good = "bye"
}

However, you can't nest after using the dot notation. In other words, this won't work:

// Won't work!
foo.bar {
    hello = "world"
    good = "bye"
}

Within both BuildConfig.groovy and Config.groovy you can access several implicit variables from configuration values:

For example:

my.tmp.dir = "${userHome}/.grails/tmp"

In addition, BuildConfig.groovy has

and Config.groovy has

Those are the basics of adding settings to the configuration file, but how do you access those settings from your own application? That depends on which config you want to read.

The settings in BuildConfig.groovy are only available from command scripts and can be accessed via the grailsSettings.config property like so:

target(default: "Example command") {
    def maxIterations = grailsSettings.config.myapp.iterations.max
    …
}

If you want to read runtime configuration settings, i.e. those defined in Config.groovy, use the grailsApplication object, which is available as a variable in controllers and tag libraries:

class MyController {
    def hello() {
        def recipient = grailsApplication.config.foo.bar.hello
        render "Hello ${recipient}"
    }
}

and can be easily injected into services and other Grails artifacts:

class MyService {
    def grailsApplication
    String greeting() {
        def recipient = grailsApplication.config.foo.bar.hello
        return "Hello ${recipient}"
    }
}

As you can see, when accessing configuration settings you use the same dot notation as when you define them.

4.1.1 Built in options

Build settings

Let's start with some important build settings. Although Grails requires JDK 6 when developing your applications, it is possible to deploy those applications to JDK 5 containers. Simply set the following in BuildConfig.groovy:

grails.project.source.level = "1.5"
grails.project.target.level = "1.5"

Note that source and target levels are different to the standard public version of JDKs, so JDK 5 -> 1.5, JDK 6 -> 1.6, and JDK 7 -> 1.7.

In addition, Grails supports Servlet versions 2.5 and above but defaults to 2.5. If you wish to use newer features of the Servlet API (such as 3.0 async support) you should configure the grails.servlet.version setting appropriately:

grails.servlet.version = "3.0"

Runtime settings

On the runtime front, i.e. Config.groovy, there are quite a few more core settings:

• grails.config.locations - The location of properties files or addition Grails Config files that should be merged with main configuration. See the section on externalised config.
• grails.enable.native2ascii - Set this to false if you do not require native2ascii conversion of Grails i18n properties files (default: true).
• grails.views.default.codec - Sets the default encoding regime for GSPs - can be one of 'none', 'html', or 'base64' (default: 'none'). To reduce risk of XSS attacks, set this to 'html'.
• grails.views.gsp.encoding - The file encoding used for GSP source files (default: 'utf-8').
• grails.mime.file.extensions - Whether to use the file extension to dictate the mime type in Content Negotiation (default: true).
• grails.mime.types - A map of supported mime types used for Content Negotiation.
• grails.serverURL - A string specifying the server URL portion of absolute links, including server name e.g. grails.serverURL="http://my.yourportal.com". See createLink. Also used by redirects.
• grails.views.gsp.sitemesh.preprocess - Determines whether SiteMesh preprocessing happens. Disabling this slows down page rendering, but if you need SiteMesh to parse the generated HTML from a GSP view then disabling it is the right option. Don't worry if you don't understand this advanced property: leave it set to true.
• grails.reload.excludes and grails.reload.includes - Configuring these directives determines the reload behavior for project specific source files. Each directive takes a list of strings that are the class names for project source files that should be excluded from reloading behavior or included accordingly when running the application in development with the run-app command. If the grails.reload.includes directive is configured, then only the classes in that list will be reloaded.

War generation

• grails.project.war.file - Sets the name and location of the WAR file generated by the war command
• grails.war.dependencies - A closure containing Ant builder syntax or a list of JAR filenames. Lets you customise what libaries are included in the WAR file.
• grails.war.copyToWebApp - A closure containing Ant builder syntax that is legal inside an Ant copy, for example "fileset()". Lets you control what gets included in the WAR file from the "web-app" directory.
• grails.war.resources - A closure containing Ant builder syntax. Allows the application to do any other other work before building the final WAR file

For more information on using these options, see the section on deployment

4.1.2 Logging

The Basics

Grails uses its common configuration mechanism to provide the settings for the underlying Log4j log system, so all you have to do is add a log4j setting to the file grails-app/conf/Config.groovy.

So what does this log4j setting look like? Here's a basic example:

log4j = {
    error  'org.codehaus.groovy.grails.web.servlet',  //  controllers
           'org.codehaus.groovy.grails.web.pages' //  GSP
    warn   'org.apache.catalina'
}

This says that for loggers whose name starts with 'org.codehaus.groovy.grails.web.servlet' or 'org.codehaus.groovy.grails.web.pages', only messages logged at 'error' level and above will be shown. Loggers with names starting with 'org.apache.catalina' logger only show messages at the 'warn' level and above. What does that mean? First of all, you have to understand how levels work.

Logging levels

The are several standard logging levels, which are listed here in order of descending priority:

1. off
2. fatal
3. error
4. warn
5. info
6. debug
7. trace
8. all

When you log a message, you implicitly give that message a level. For example, the method log.error(msg) will log a message at the 'error' level. Likewise, log.debug(msg) will log it at 'debug'. Each of the above levels apart from 'off' and 'all' have a corresponding log method of the same name.

The logging system uses that message level combined with the configuration for the logger (see next section) to determine whether the message gets written out. For example, if you have an 'org.example.domain' logger configured like so:

warn 'org.example.domain'

then messages with a level of 'warn', 'error', or 'fatal' will be written out. Messages at other levels will be ignored.

Before we go on to loggers, a quick note about those 'off' and 'all' levels. These are special in that they can only be used in the configuration; you can't log messages at these levels. So if you configure a logger with a level of 'off', then no messages will be written out. A level of 'all' means that you will see all messages. Simple.

Loggers

Loggers are fundamental to the logging system, but they are a source of some confusion. For a start, what are they? Are they shared? How do you configure them?

A logger is the object you log messages to, so in the call log.debug(msg), log is a logger instance (of type Log). These loggers are cached and uniquely identified by name, so if two separate classes use loggers with the same name, those loggers are actually the same instance.

There are two main ways to get hold of a logger:

1. use the log instance injected into artifacts such as domain classes, controllers and services;
2. use the Commons Logging API directly.

If you use the dynamic log property, then the name of the logger is 'grails.app.<type>.<className>', where type is the type of the artifact, for example 'controllers' or 'services', and className is the fully qualified name of the artifact. For example, if you have this service:

package org.example
class MyService {
    …
}

then the name of the logger will be 'grails.app.services.org.example.MyService'.

For other classes, the typical approach is to store a logger based on the class name in a constant static field:

package org.other
import org.apache.commons.logging.LogFactory
class MyClass {
    private static final log = LogFactory.getLog(this)
    …
}

This will create a logger with the name 'org.other.MyClass' - note the lack of a 'grails.app.' prefix since the class isn't an artifact. You can also pass a name to the getLog() method, such as "myLogger", but this is less common because the logging system treats names with dots ('.') in a special way.

Configuring loggers

You have already seen how to configure loggers in Grails:

log4j = {
    error  'org.codehaus.groovy.grails.web.servlet'
}

This example configures loggers with names starting with 'org.codehaus.groovy.grails.web.servlet' to ignore any messages sent to them at a level of 'warn' or lower. But is there a logger with this name in the application? No. So why have a configuration for it? Because the above rule applies to any logger whose name begins with 'org.codehaus.groovy.grails.servlet.' as well. For example, the rule applies to both the org.codehaus.groovy.grails.web.servlet.GrailsDispatcherServlet class and the org.codehaus.groovy.grails.web.servlet.mvc.GrailsWebRequest one.

In other words, loggers are hierarchical. This makes configuring them by package much simpler than it would otherwise be.

The most common things that you will want to capture log output from are your controllers, services, and other artifacts. Use the convention mentioned earlier to do that: grails.app.<artifactType>.<className> . In particular the class name must be fully qualifed, i.e. with the package if there is one:

log4j = {
    // Set level for all application artifacts
    info "grails.app"
    // Set for a specific controller in the default package
    debug "grails.app.controllers.YourController"
    // Set for a specific domain class
    debug "grails.app.domain.org.example.Book"
    // Set for all taglibs
    info "grails.app.taglib"
}

The standard artifact names used in the logging configuration are:

• conf - For anything under grails-app/conf such as BootStrap.groovy (but excluding filters)
• filters - For filters
• taglib - For tag libraries
• services - For service classes
• controllers - For controllers
• domain - For domain entities

Grails itself generates plenty of logging information and it can sometimes be helpful to see that. Here are some useful loggers from Grails internals that you can use, especially when tracking down problems with your application:

• org.codehaus.groovy.grails.commons - Core artifact information such as class loading etc.
• org.codehaus.groovy.grails.web - Grails web request processing
• org.codehaus.groovy.grails.web.mapping - URL mapping debugging
• org.codehaus.groovy.grails.plugins - Log plugin activity
• grails.spring - See what Spring beans Grails and plugins are defining
• org.springframework - See what Spring is doing
• org.hibernate - See what Hibernate is doing

So far, we've only looked at explicit configuration of loggers. But what about all those loggers that don't have an explicit configuration? Are they simply ignored? The answer lies with the root logger.

The Root Logger

All logger objects inherit their configuration from the root logger, so if no explicit configuration is provided for a given logger, then any messages that go to that logger are subject to the rules defined for the root logger. In other words, the root logger provides the default configuration for the logging system.

Grails automatically configures the root logger to only handle messages at 'error' level and above, and all the messages are directed to the console (stdout for those with a C background). You can customise this behaviour by specifying a 'root' section in your logging configuration like so:

log4j = {
    root {
        info()
    }
    …
}

The above example configures the root logger to log messages at 'info' level and above to the default console appender. You can also configure the root logger to log to one or more named appenders (which we'll talk more about shortly):

log4j = {
    appenders {
        file name:'file', file:'/var/logs/mylog.log'
    }
    root {
        debug 'stdout', 'file'
    }
}

In the above example, the root logger will log to two appenders - the default 'stdout' (console) appender and a custom 'file' appender.

For power users there is an alternative syntax for configuring the root logger: the root org.apache.log4j.Logger instance is passed as an argument to the log4j closure. This lets you work with the logger directly:

log4j = { root ->
    root.level = org.apache.log4j.Level.DEBUG
    …
}

For more information on what you can do with this Logger instance, refer to the Log4j API documentation.

Those are the basics of logging pretty well covered and they are sufficient if you're happy to only send log messages to the console. But what if you want to send them to a file? How do you make sure that messages from a particular logger go to a file but not the console? These questions and more will be answered as we look into appenders.

Appenders

Loggers are a useful mechanism for filtering messages, but they don't physically write the messages anywhere. That's the job of the appender, of which there are various types. For example, there is the default one that writes messages to the console, another that writes them to a file, and several others. You can even create your own appender implementations!

This diagram shows how they fit into the logging pipeline:

As you can see, a single logger may have several appenders attached to it. In a standard Grails configuration, the console appender named 'stdout' is attached to all loggers through the default root logger configuration. But that's the only one. Adding more appenders can be done within an 'appenders' block:

log4j = {
    appenders {
        rollingFile name: "myAppender",
                    maxFileSize: 1024,
                    file: "/tmp/logs/myApp.log"
    }
}

The following appenders are available by default:

Each named argument passed to an appender maps to a property of the underlying Appender implementation. So the previous example sets the name, maxFileSize and file properties of the RollingFileAppender instance.

You can have as many appenders as you like - just make sure that they all have unique names. You can even have multiple instances of the same appender type, for example several file appenders that log to different files.

If you prefer to create the appender programmatically or if you want to use an appender implementation that's not available in the above syntax, simply declare an appender entry with an instance of the appender you want:

import org.apache.log4j.*
log4j = {
    appenders {
        appender new RollingFileAppender(
                name: "myAppender",
                maxFileSize: 1024,
                file: "/tmp/logs/myApp.log")
    }
}

This approach can be used to configure JMSAppender, SocketAppender, SMTPAppender, and more.

Once you have declared your extra appenders, you can attach them to specific loggers by passing the name as a key to one of the log level methods from the previous section:

error myAppender: "grails.app.controllers.BookController"

This will ensure that the 'grails.app.controllers.BookController' logger sends log messages to 'myAppender' as well as any appenders configured for the root logger. To add more than one appender to the logger, then add them to the same level declaration:

error myAppender:      "grails.app.controllers.BookController",
      myFileAppender:  ["grails.app.controllers.BookController",
                        "grails.app.services.BookService"],
      rollingFile:     "grails.app.controllers.BookController"

The above example also shows how you can configure more than one logger at a time for a given appender (myFileAppender) by using a list.

Be aware that you can only configure a single level for a logger, so if you tried this code:

error myAppender:      "grails.app.controllers.BookController"
debug myFileAppender:  "grails.app.controllers.BookController"
fatal rollingFile:     "grails.app.controllers.BookController"

you'd find that only 'fatal' level messages get logged for 'grails.app.controllers.BookController'. That's because the last level declared for a given logger wins. What you probably want to do is limit what level of messages an appender writes.

An appender that is attached to a logger configured with the 'all' level will generate a lot of logging information. That may be fine in a file, but it makes working at the console difficult. So we configure the console appender to only write out messages at 'info' level or above:

log4j = {
    appenders {
        console name: "stdout", threshold: org.apache.log4j.Level.INFO
    }
}

The key here is the threshold argument which determines the cut-off for log messages. This argument is available for all appenders, but do note that you currently have to specify a Level instance - a string such as "info" will not work.

Custom Layouts

By default the Log4j DSL assumes that you want to use a PatternLayout. However, there are other layouts available including:

• xml - Create an XML log file
• html - Creates an HTML log file
• simple - A simple textual log
• pattern - A Pattern layout

You can specify custom patterns to an appender using the layout setting:

log4j = {
    appenders {
        console name: "customAppender",
                layout: pattern(conversionPattern: "%c{2} %m%n")
    }
}

This also works for the built-in appender "stdout", which logs to the console:

log4j = {
    appenders {
        console name: "stdout",
                layout: pattern(conversionPattern: "%c{2} %m%n")
    }
}

Environment-specific configuration

Since the logging configuration is inside Config.groovy, you can put it inside an environment-specific block. However, there is a problem with this approach: you have to provide the full logging configuration each time you define the log4j setting. In other words, you cannot selectively override parts of the configuration - it's all or nothing.

To get around this, the logging DSL provides its own environment blocks that you can put anywhere in the configuration:

log4j = {
    appenders {
        console name: "stdout",
                layout: pattern(conversionPattern: "%c{2} %m%n")
        environments {
            production {
                rollingFile name: "myAppender", maxFileSize: 1024,
                            file: "/tmp/logs/myApp.log"
            }
        }
    }
    root {
        //…
    }
    // other shared config
    info "grails.app.controller"
    environments {
        production {
            // Override previous setting for 'grails.app.controller'
            error "grails.app.controllers"
        }
    }
}

The one place you can't put an environment block is inside the root definition, but you can put the root definition inside an environment block.

Full stacktraces

When exceptions occur, there can be an awful lot of noise in the stacktrace from Java and Groovy internals. Grails filters these typically irrelevant details and restricts traces to non-core Grails/Groovy class packages.

When this happens, the full trace is always logged to the StackTrace logger, which by default writes its output to a file called stacktrace.log. As with other loggers though, you can change its behaviour in the configuration. For example if you prefer full stack traces to go to the console, add this entry:

error stdout: "StackTrace"

This won't stop Grails from attempting to create the stacktrace.log file - it just redirects where stack traces are written to. An alternative approach is to change the location of the 'stacktrace' appender's file:

log4j = {
    appenders {
        rollingFile name: "stacktrace", maxFileSize: 1024,
                    file: "/var/tmp/logs/myApp-stacktrace.log"
    }
}

or, if you don't want to the 'stacktrace' appender at all, configure it as a 'null' appender:

log4j = {
    appenders {
        'null' name: "stacktrace"
    }
}

You can of course combine this with attaching the 'stdout' appender to the 'StackTrace' logger if you want all the output in the console.

Finally, you can completely disable stacktrace filtering by setting the grails.full.stacktrace VM property to true:

grails -Dgrails.full.stacktrace=true run-app

Masking Request Parameters From Stacktrace Logs

When Grails logs a stacktrace, the log message may include the names and values of all of the request parameters for the current request. To mask out the values of secure request parameters, specify the parameter names in the grails.exceptionresolver.params.exclude config property:

grails.exceptionresolver.params.exclude = ['password', 'creditCard']

Request parameter logging may be turned off altogether by setting the grails.exceptionresolver.logRequestParameters config property to false. The default value is true when the application is running in DEVELOPMENT mode and false for all other modes.

grails.exceptionresolver.logRequestParameters=false

Logger inheritance

Earlier, we mentioned that all loggers inherit from the root logger and that loggers are hierarchical based on '.'-separated terms. What this means is that unless you override a parent setting, a logger retains the level and the appenders configured for that parent. So with this configuration:

log4j = {
    appenders {
        file name:'file', file:'/var/logs/mylog.log'
    }
    root {
        debug 'stdout', 'file'
    }
}

all loggers in the application will have a level of 'debug' and will log to both the 'stdout' and 'file' appenders. What if you only want to log to 'stdout' for a particular logger? Change the 'additivity' for a logger in that case.

Additivity simply determines whether a logger inherits the configuration from its parent. If additivity is false, then its not inherited. The default for all loggers is true, i.e. they inherit the configuration. So how do you change this setting? Here's an example:

log4j = {
    appenders {
        …
    }
    root {
        …
    }
    info additivity: false
         stdout: ["grails.app.controllers.BookController",
                  "grails.app.services.BookService"]
}

So when you specify a log level, add an 'additivity' named argument. Note that you when you specify the additivity, you must configure the loggers for a named appender. The following syntax will not work:

info additivity: false, ["grails.app.controllers.BookController",
                         "grails.app.services.BookService"]

Customizing stack trace printing and filtering

Stacktraces in general and those generated when using Groovy in particular are quite verbose and contain many stack frames that aren't interesting when diagnosing problems. So Grails uses a implementation of the org.codehaus.groovy.grails.exceptions.StackTraceFilterer interface to filter out irrelevant stack frames. To customize the approach used for filtering, implement that interface in a class in src/groovy or src/java and register it in Config.groovy:

grails.logging.stackTraceFiltererClass =
         'com.yourcompany.yourapp.MyStackTraceFilterer'

In addition, Grails customizes the display of the filtered stacktrace to make the information more readable. To customize this, implement the org.codehaus.groovy.grails.exceptions.StackTracePrinter interface in a class in src/groovy or src/java and register it in Config.groovy:

grails.logging.stackTracePrinterClass =
         'com.yourcompany.yourapp.MyStackTracePrinter'

Finally, to render error information in the error GSP, an HTML-generating printer implementation is needed. The default implementation is org.codehaus.groovy.grails.web.errors.ErrorsViewStackTracePrinter and it's registered as a Spring bean. To use your own implementation, either implement the org.codehaus.groovy.grails.exceptions.StackTraceFilterer directly or subclass ErrorsViewStackTracePrinter and register it in grails-app/conf/spring/resources.groovy as:

import com.yourcompany.yourapp.MyErrorsViewStackTracePrinter
beans = {
    errorsViewStackTracePrinter(MyErrorsViewStackTracePrinter,
                                ref('grailsResourceLocator'))
}

Alternative logging libraries

By default, Grails uses Log4J to do its logging. For most people this is absolutely fine, and many users don't even care what logging library is used. But if you're not one of those and want to use an alternative, such as the JDK logging package or logback, you can do so by simply excluding a couple of dependencies from the global set and adding your own:

grails.project.dependency.resolution = {
    inherits("global") {
        excludes "grails-plugin-logging", "log4j"
    }
    …
    dependencies {
        runtime "ch.qos.logback:logback-core:0.9.29"
        …
    }
    …
}

If you do this, you will get unfiltered, standard Java stacktraces in your log files and you won't be able to use the logging configuration DSL that's just been described. Instead, you will have to use the standard configuration mechanism for the library you choose.

4.1.3 GORM

• grails.gorm.failOnError - If set to true, causes the save() method on domain classes to throw a grails.validation.ValidationException if validation fails during a save. This option may also be assigned a list of Strings representing package names. If the value is a list of Strings then the failOnError behavior will only be applied to domain classes in those packages (including sub-packages). See the save method docs for more information.

For example, to enable failOnError for all domain classes:

grails.gorm.failOnError=true

and to enable failOnError for domain classes by package:

grails.gorm.failOnError = ['com.companyname.somepackage',
                           'com.companyname.someotherpackage']

• grails.gorm.autoFlush = If set to true, causes the merge, save and delete methods to flush the session, replacing the need to explicitly flush using save(flush: true).

4.2 Environments

Per Environment Configuration

Grails supports the concept of per environment configuration. The Config.groovy, DataSource.groovy, and BootStrap.groovy files in the grails-app/conf directory can use per-environment configuration using the syntax provided by ConfigSlurper. As an example consider the following default DataSource definition provided by Grails:

dataSource {
    pooled = false
    driverClassName = "org.h2.Driver"
    username = "sa"
    password = ""
}
environments {
    development {
        dataSource {
            dbCreate = "create-drop"
            url = "jdbc:h2:mem:devDb"
        }
    }
    test {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:mem:testDb"
        }
    }
    production {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:prodDb"
        }
    }
}

Notice how the common configuration is provided at the top level and then an environments block specifies per environment settings for the dbCreate and url properties of the DataSource.

Packaging and Running for Different Environments

Grails' command line has built in capabilities to execute any command within the context of a specific environment. The format is:

grails [environment] [command name]

In addition, there are 3 preset environments known to Grails: dev, prod, and test for development, production and test. For example to create a WAR for the test environment you wound run:

grails test war

To target other environments you can pass a grails.env variable to any command:

grails -Dgrails.env=UAT run-app

Programmatic Environment Detection

Within your code, such as in a Gant script or a bootstrap class you can detect the environment using the Environment class:

import grails.util.Environment
...
switch (Environment.current) {
    case Environment.DEVELOPMENT:
        configureForDevelopment()
        break
    case Environment.PRODUCTION:
        configureForProduction()
        break
}

Per Environment Bootstrapping

It's often desirable to run code when your application starts up on a per-environment basis. To do so you can use the grails-app/conf/BootStrap.groovy file's support for per-environment execution:

def init = { ServletContext ctx ->
    environments {
        production {
            ctx.setAttribute("env", "prod")
        }
        development {
            ctx.setAttribute("env", "dev")
        }
    }
    ctx.setAttribute("foo", "bar")
}

Generic Per Environment Execution

The previous BootStrap example uses the grails.util.Environment class internally to execute. You can also use this class yourself to execute your own environment specific logic:

Environment.executeForCurrentEnvironment {
    production {
        // do something in production
    }
    development {
        // do something only in development
    }
}

4.3 The DataSource

If you use a database other than H2 you need a JDBC driver. For example for MySQL you would need Connector/J

Drivers typically come in the form of a JAR archive. It's best to use Ivy to resolve the jar if it's available in a Maven repository, for example you could add a dependency for the MySQL driver like this:

grails.project.dependency.resolution = {
    inherits("global")
    log "warn"
    repositories {
        grailsPlugins()
        grailsHome()
        grailsCentral()
        mavenCentral()
    }
    dependencies {
        runtime 'mysql:mysql-connector-java:5.1.16'
    }
}

Note that the built-in mavenCentral() repository is included here since that's a reliable location for this library.

If you can't use Ivy then just put the JAR in your project's lib directory.

Once you have the JAR resolved you need to get familiar Grails' DataSource descriptor file located at grails-app/conf/DataSource.groovy. This file contains the dataSource definition which includes the following settings:

• driverClassName - The class name of the JDBC driver
• username - The username used to establish a JDBC connection
• password - The password used to establish a JDBC connection
• url - The JDBC URL of the database
• dbCreate - Whether to auto-generate the database from the domain model - one of 'create-drop', 'create', 'update' or 'validate'
• pooled - Whether to use a pool of connections (defaults to true)
• logSql - Enable SQL logging to stdout
• formatSql - Format logged SQL
• dialect - A String or Class that represents the Hibernate dialect used to communicate with the database. See the org.hibernate.dialect package for available dialects.
• readOnly - If true makes the DataSource read-only, which results in the connection pool calling setReadOnly(true) on each Connection
• properties - Extra properties to set on the DataSource bean. See the Commons DBCP BasicDataSource documentation.

A typical configuration for MySQL may be something like:

dataSource {
    pooled = true
    dbCreate = "update"
    url = "jdbc:mysql://localhost/yourDB"
    driverClassName = "com.mysql.jdbc.Driver"
    dialect = org.hibernate.dialect.MySQL5InnoDBDialect
    username = "yourUser"
    password = "yourPassword"
}

When configuring the DataSource do not include the type or the def keyword before any of the configuration settings as Groovy will treat these as local variable definitions and they will not be processed. For example the following is invalid:

dataSource {
    boolean pooled = true // type declaration results in ignored local variable
    …
}

Example of advanced configuration using extra properties:

dataSource {
    pooled = true
    dbCreate = "update"
    url = "jdbc:mysql://localhost/yourDB"
    driverClassName = "com.mysql.jdbc.Driver"
    dialect = org.hibernate.dialect.MySQL5InnoDBDialect
    username = "yourUser"
    password = "yourPassword"
    properties {
        maxActive = 50
        maxIdle = 25
        minIdle = 5
        initialSize = 5
        minEvictableIdleTimeMillis = 60000
        timeBetweenEvictionRunsMillis = 60000
        maxWait = 10000
        validationQuery = "/* ping */"
    }
}

More on dbCreate

Hibernate can automatically create the database tables required for your domain model. You have some control over when and how it does this through the dbCreate property, which can take these values:

• create - Drops the existing schemaCreates the schema on startup, dropping existing tables, indexes, etc. first.
• create-drop - Same as create, but also drops the tables when the application shuts down cleanly.
• update - Creates missing tables and indexes, and updates the current schema without dropping any tables or data. Note that this can't properly handle many schema changes like column renames (you're left with the old column containing the existing data).
• validate - Makes no changes to your database. Compares the configuration with the existing database schema and reports warnings.
• any other value - does nothing

You can also remove the dbCreate setting completely, which is recommended once your schema is relatively stable and definitely when your application and database are deployed in production. Database changes are then managed through proper migrations, either with SQL scripts or a migration tool like Liquibase (the Database Migration plugin uses Liquibase and is tightly integrated with Grails and GORM).

4.3.1 DataSources and Environments

Grails' DataSource definition is "environment aware", however, so you can do:

dataSource {
    pooled = true
    driverClassName = "com.mysql.jdbc.Driver"
    dialect = org.hibernate.dialect.MySQL5InnoDBDialect
    // other common settings here
}
environments {
    production {
        dataSource {
            url = "jdbc:mysql://liveip.com/liveDb"
            // other environment-specific settings here
        }
    }
}

4.3.2 JNDI DataSources

Referring to a JNDI DataSource

Most Java EE containers supply DataSource instances via Java Naming and Directory Interface (JNDI). Grails supports the definition of JNDI data sources as follows:

dataSource {
    jndiName = "java:comp/env/myDataSource"
}

The format on the JNDI name may vary from container to container, but the way you define the DataSource in Grails remains the same.

Configuring a Development time JNDI resource

The way in which you configure JNDI data sources at development time is plugin dependent. Using the Tomcat plugin you can define JNDI resources using the grails.naming.entries setting in grails-app/conf/Config.groovy:

grails.naming.entries = [
    "bean/MyBeanFactory": [
        auth: "Container",
        type: "com.mycompany.MyBean",
        factory: "org.apache.naming.factory.BeanFactory",
        bar: "23"
    ],
    "jdbc/EmployeeDB": [
        type: "javax.sql.DataSource", //required
        auth: "Container", // optional
        description: "Data source for Foo", //optional
        driverClassName: "org.h2.Driver",
        url: "jdbc:h2:mem:database",
        username: "dbusername",
        password: "dbpassword",
        maxActive: "8",
        maxIdle: "4"
    ],
    "mail/session": [
        type: "javax.mail.Session,
        auth: "Container",
        "mail.smtp.host": "localhost"
    ]
]

4.3.3 Automatic Database Migration

• create
• create-drop
• update
• validate
• no value

In development mode dbCreate is by default set to "create-drop", but at some point in development (and certainly once you go to production) you'll need to stop dropping and re-creating the database every time you start up your server.

It's tempting to switch to update so you retain existing data and only update the schema when your code changes, but Hibernate's update support is very conservative. It won't make any changes that could result in data loss, and doesn't detect renamed columns or tables, so you'll be left with the old one and will also have the new one.

Grails supports migrations via the Database Migration plugin which can be installed by declaring the plugin in grails-app/conf/BuildConfig.groovy:

grails.project.dependency.resolution = {
    …
    plugins {
        runtime ':database-migration:1.3.1'
    }
}

The plugin uses Liquibase and and provides access to all of its functionality, and also has support for GORM (for example generating a change set by comparing your domain classes to a database).

4.3.4 Transaction-aware DataSource Proxy

If this were not the case, then retrieving a connection from the dataSource would be a new connection, and you wouldn't be able to see changes that haven't been committed yet (assuming you have a sensible transaction isolation setting, e.g. READ_COMMITTED or better).

The "real" unproxied dataSource is still available to you if you need access to it; its bean name is dataSourceUnproxied.

You can access this bean like any other Spring bean, i.e. using dependency injection:

class MyService {
   def dataSourceUnproxied
   …
}

or by pulling it from the ApplicationContext:

def dataSourceUnproxied = ctx.dataSourceUnproxied

4.3.5 Database Console

You can access the console by navigating to http://localhost:8080/appname/dbconsole in a browser. The URI can be configured using the grails.dbconsole.urlRoot attribute in Config.groovy and defaults to '/dbconsole'.

The console is enabled by default in development mode and can be disabled or enabled in other environments by using the grails.dbconsole.enabled attribute in Config.groovy. For example you could enable the console in production using

environments {
    production {
        grails.serverURL = "http://www.changeme.com"
        grails.dbconsole.enabled = true
        grails.dbconsole.urlRoot = '/admin/dbconsole'
    }
    development {
        grails.serverURL = "http://localhost:8080/${appName}"
    }
    test {
        grails.serverURL = "http://localhost:8080/${appName}"
    }
}

If you enable the console in production be sure to guard access to it using a trusted security framework.

Configuration

By default the console is configured for an H2 database which will work with the default settings if you haven't configured an external database - you just need to change the JDBC URL to jdbc:h2:mem:devDB. If you've configured an external database (e.g. MySQL, Oracle, etc.) then you can use the Saved Settings dropdown to choose a settings template and fill in the url and username/password information from your DataSource.groovy.

4.3.6 Multiple Datasources

Configuring Additional DataSources

The default DataSource configuration in grails-app/conf/DataSource.groovy looks something like this:

dataSource {
    pooled = true
    driverClassName = "org.h2.Driver"
    username = "sa"
    password = ""
}
hibernate {
    cache.use_second_level_cache = true
    cache.use_query_cache = true
    cache.provider_class = 'net.sf.ehcache.hibernate.EhCacheProvider'
}
environments {
    development {
        dataSource {
            dbCreate = "create-drop"
            url = "jdbc:h2:mem:devDb"
        }
    }
    test {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:mem:testDb"
        }
    }
    production {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:prodDb"
        }
    }
}

This configures a single DataSource with the Spring bean named dataSource. To configure extra DataSources, add another dataSource block (at the top level, in an environment block, or both, just like the standard DataSource definition) with a custom name, separated by an underscore. For example, this configuration adds a second DataSource, using MySQL in the development environment and Oracle in production:

environments {
    development {
        dataSource {
            dbCreate = "create-drop"
            url = "jdbc:h2:mem:devDb"
        }
        dataSource_lookup {
            dialect = org.hibernate.dialect.MySQLInnoDBDialect
            driverClassName = 'com.mysql.jdbc.Driver'
            username = 'lookup'
            password = 'secret'
            url = 'jdbc:mysql://localhost/lookup'
            dbCreate = 'update'
        }
    }
    test {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:mem:testDb"
        }
    }
    production {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:prodDb"
        }
        dataSource_lookup {
            dialect = org.hibernate.dialect.Oracle10gDialect
            driverClassName = 'oracle.jdbc.driver.OracleDriver'
            username = 'lookup'
            password = 'secret'
            url = 'jdbc:oracle:thin:@localhost:1521:lookup'
            dbCreate = 'update'
        }
    }
}

You can use the same or different databases as long as they're supported by Hibernate.

Configuring Domain Classes

If a domain class has no DataSource configuration, it defaults to the standard 'dataSource'. Set the datasource property in the mapping block to configure a non-default DataSource. For example, if you want to use the ZipCode domain to use the 'lookup' DataSource, configure it like this;

class ZipCode {
   String code
   static mapping = {
      datasource 'lookup'
   }
}

A domain class can also use two or more DataSources. Use the datasources property with a list of names to configure more than one, for example:

class ZipCode {
   String code
   static mapping = {
      datasources(['lookup', 'auditing'])
   }
}

If a domain class uses the default DataSource and one or more others, use the special name 'DEFAULT' to indicate the default DataSource:

class ZipCode {
   String code
   static mapping = {
      datasources(['lookup', 'DEFAULT'])
   }
}

If a domain class uses all configured DataSources use the special value 'ALL':

class ZipCode {
   String code
   static mapping = {
      datasource 'ALL'
   }
}

Namespaces and GORM Methods

If a domain class uses more than one DataSource then you can use the namespace implied by each DataSource name to make GORM calls for a particular DataSource. For example, consider this class which uses two DataSources:

class ZipCode {
   String code
   static mapping = {
      datasources(['lookup', 'auditing'])
   }
}

The first DataSource specified is the default when not using an explicit namespace, so in this case we default to 'lookup'. But you can call GORM methods on the 'auditing' DataSource with the DataSource name, for example:

def zipCode = ZipCode.auditing.get(42)
…
zipCode.auditing.save()

As you can see, you add the DataSource to the method call in both the static case and the instance case.

Hibernate Mapped Domain Classes

You can also partition annotated Java classes into separate datasources. Classes using the default datasource are registered in grails-app/conf/hibernate/hibernate.cfg.xml. To specify that an annotated class uses a non-default datasource, create a hibernate.cfg.xml file for that datasource with the file name prefixed with the datasource name.

For example if the Book class is in the default datasource, you would register that in grails-app/conf/hibernate/hibernate.cfg.xml:

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
          '-//Hibernate/Hibernate Configuration DTD 3.0//EN'
          'http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd'>
<hibernate-configuration>
   <session-factory>
      <mapping class='org.example.Book'/>
   </session-factory>
</hibernate-configuration>

and if the Library class is in the "ds2" datasource, you would register that in grails-app/conf/hibernate/ds2_hibernate.cfg.xml:

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
          '-//Hibernate/Hibernate Configuration DTD 3.0//EN'
          'http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd'>
<hibernate-configuration>
   <session-factory>
      <mapping class='org.example.Library'/>
   </session-factory>
</hibernate-configuration>

The process is the same for classes mapped with hbm.xml files - just list them in the appropriate hibernate.cfg.xml file.

Services

Like Domain classes, by default Services use the default DataSource and PlatformTransactionManager. To configure a Service to use a different DataSource, use the static datasource property, for example:

class DataService {
   static datasource = 'lookup'
   void someMethod(...) {
      …
   }
}

A transactional service can only use a single DataSource, so be sure to only make changes for domain classes whose DataSource is the same as the Service.

Note that the datasource specified in a service has no bearing on which datasources are used for domain classes; that's determined by their declared datasources in the domain classes themselves. It's used to declare which transaction manager to use.

What you'll see is that if you have a Foo domain class in dataSource1 and a Bar domain class in dataSource2, and WahooService uses dataSource1, a service method that saves a new Foo and a new Bar will only be transactional for Foo since they share the datasource. The transaction won't affect the Bar instance. If you want both to be transactional you'd need to use two services and XA datasources for two-phase commit, e.g. with the Atomikos plugin.

XA and Two-phase Commit

Grails has no native support for XA DataSources or two-phase commit, but the Atomikos plugin makes it easy. See the plugin documentation for the simple changes needed in your DataSource definitions to reconfigure them as XA DataSources.

4.4 Externalized Configuration

grails.config.locations = [
    "classpath:${appName}-config.properties",
    "classpath:${appName}-config.groovy",
    "file:${userHome}/.grails/${appName}-config.properties",
    "file:${userHome}/.grails/${appName}-config.groovy" ]

In the above example we're loading configuration files (both Java Properties files and ConfigSlurper configurations) from different places on the classpath and files located in USER_HOME.

It is also possible to load config by specifying a class that is a config script.

grails.config.locations = [com.my.app.MyConfig]

This can be useful in situations where the config is either coming from a plugin or some other part of your application. A typical use for this is re-using configuration provided by plugins across multiple applications.

Ultimately all configuration files get merged into the config property of the GrailsApplication object and are hence obtainable from there.

Values that have the same name as previously defined values will overwrite the existing values, and the pointed to configuration sources are loaded in the order in which they are defined.

Config Defaults

The configuration values contained in the locations described by the grails.config.locations property will override any values defined in your application Config.groovy file which may not be what you want. You may want to have a set of default values be be loaded that can be overridden in either your application's Config.groovy file or in a named config location. For this you can use the grails.config.defaults.locations property.

This property supports the same values as the grails.config.locations property (i.e. paths to config scripts, property files or classes), but the config described by grails.config.defaults.locations will be loaded before all other values and can therefore be overridden. Some plugins use this mechanism to supply one or more sets of default configuration that you can choose to include in your application config.

Grails also supports the concept of property place holders and property override configurers as defined in Spring For more information on these see the section on Grails and Spring

4.5 Versioning

Versioning Basics

Grails has built in support for application versioning. The version of the application is set to 0.1 when you first create an application with the create-app command. The version is stored in the application meta data file application.properties in the root of the project.

To change the version of your application you can edit the file manually, or run the set-version command:

grails set-version 0.2

The version is used in various commands including the war command which will append the application version to the end of the created WAR file.

Detecting Versions at Runtime

You can detect the application version using Grails' support for application metadata using the GrailsApplication class. For example within controllers there is an implicit grailsApplication variable that can be used:

def version = grailsApplication.metadata['app.version']

You can retrieve the the version of Grails that is running with:

def grailsVersion = grailsApplication.metadata['app.grails.version']

or the GrailsUtil class:

import grails.util.GrailsUtil
…
def grailsVersion = GrailsUtil.grailsVersion

4.6 Project Documentation

The documentation engine uses a variation on the Textile syntax to automatically create project documentation with smart linking, formatting etc.

Creating project documentation

To use the engine you need to follow a few conventions. First, you need to create a src/docs/guide directory where your documentation source files will go. Then, you need to create the source docs themselves. Each chapter should have its own gdoc file as should all numbered sub-sections. You will end up with something like:

+ src/docs/guide/introduction.gdoc
+ src/docs/guide/introduction/changes.gdoc
+ src/docs/guide/gettingStarted.gdoc
+ src/docs/guide/configuration.gdoc
+ src/docs/guide/configuration/build.gdoc
+ src/docs/guide/configuration/build/controllers.gdoc

Note that you can have all your gdoc files in the top-level directory if you want, but you can also put sub-sections in sub-directories named after the parent section - as the above example shows.

Once you have your source files, you still need to tell the documentation engine what the structure of your user guide is going to be. To do that, you add a src/docs/guide/toc.yml file that contains the structure and titles for each section. This file is in YAML format and basically represents the structure of the user guide in tree form. For example, the above files could be represented as:

introduction:
  title: Introduction
  changes: Change Log
gettingStarted: Getting Started
configuration:
  title: Configuration
  build:
    title: Build Config
    controllers: Specifying Controllers

The format is pretty straightforward. Any section that has sub-sections is represented with the corresponding filename (minus the .gdoc extension) followed by a colon. The next line should contain title: plus the title of the section as seen by the end user. Every sub-section then has its own line after the title. Leaf nodes, i.e. those without any sub-sections, declare their title on the same line as the section name but after the colon.

That's it. You can easily add, remove, and move sections within the toc.yml to restructure the generated user guide. You should also make sure that all section names, i.e. the gdoc filenames, should be unique since they are used for creating internal links and for the HTML filenames. Don't worry though, the documentation engine will warn you of duplicate section names.

Creating reference items

Reference items appear in the Quick Reference section of the documentation. Each reference item belongs to a category and a category is a directory located in the src/docs/ref directory. For example, suppose you have defined a new controller method called renderPDF. That belongs to the Controllers category so you would create a gdoc text file at the following location:

+ src/docs/ref/Controllers/renderPDF.gdoc

Configuring Output Properties

There are various properties you can set within your grails-app/conf/Config.groovy file that customize the output of the documentation such as:

• grails.doc.title - The title of the documentation
• grails.doc.subtitle - The subtitle of the documentation
• grails.doc.authors - The authors of the documentation
• grails.doc.license - The license of the software
• grails.doc.copyright - The copyright message to display
• grails.doc.footer - The footer to use

Other properties such as the version are pulled from your project itself. If a title is not specified, the application name is used.

You can also customise the look of the documentation and provide images by setting a few other options:

• grails.doc.css - The location of a directory containing custom CSS files (type java.io.File)
• grails.doc.js - The location of a directory containing custom JavaScript files (type java.io.File)
• grails.doc.style - The location of a directory containing custom HTML templates for the guide (type java.io.File)
• grails.doc.images - The location of a directory containing image files for use in the style templates and within the documentation pages themselves (type java.io.File)

One of the simplest ways to customise the look of the generated guide is to provide a value for grails.doc.css and then put a custom.css file in the corresponding directory. Grails will automatically include this CSS file in the guide. You can also place a custom-pdf.css file in that directory. This allows you to override the styles for the PDF version of the guide.

Generating Documentation

Once you have created some documentation (refer to the syntax guide in the next chapter) you can generate an HTML version of the documentation using the command:

grails doc

This command will output an docs/manual/index.html which can be opened in a browser to view your documentation.

Documentation Syntax

As mentioned the syntax is largely similar to Textile or Confluence style wiki markup. The following sections walk you through the syntax basics.

Basic Formatting

Monospace: monospace

@monospace@

Italic: italic

_italic_

Bold: bold

*bold*

Image:

!http://grails.org/images/new/grailslogo_topNav.png!

You can also link to internal images like so:

!someFolder/my_diagram.png!

This will link to an image stored locally within your project. There is currently no default location for doc images, but you can specify one with the grails.doc.images setting in Config.groovy like so:

grails.doc.images = new File("src/docs/images")

In this example, you would put the my_diagram.png file in the directory 'src/docs/images/someFolder'.

Linking

There are several ways to create links with the documentation generator. A basic external link can either be defined using confluence or textile style markup:

[SpringSource|http://www.springsource.com/]

or

"SpringSource":http://www.springsource.com/

For links to other sections inside the user guide you can use the guide: prefix with the name of the section you want to link to:

[Intro|guide:introduction]

The section name comes from the corresponding gdoc filename. The documentation engine will warn you if any links to sections in your guide break.

To link to reference items you can use a special syntax:

[controllers|renderPDF]

In this case the category of the reference item is on the left hand side of the | and the name of the reference item on the right.

Finally, to link to external APIs you can use the api: prefix. For example:

[String|api:java.lang.String]

The documentation engine will automatically create the appropriate javadoc link in this case. To add additional APIs to the engine you can configure them in grails-app/conf/Config.groovy. For example:

grails.doc.api.org.hibernate=
            "http://docs.jboss.org/hibernate/stable/core/javadocs"

The above example configures classes within the org.hibernate package to link to the Hibernate website's API docs.

Lists and Headings

Headings can be created by specifying the letter 'h' followed by a number and then a dot:

h3.<space>Heading3
h4.<space>Heading4

Unordered lists are defined with the use of the * character:

* item 1
** subitem 1
** subitem 2
* item 2

Numbered lists can be defined with the # character:

# item 1

Tables can be created using the table macro:

{table}
 *Name* | *Number*
 Albert | 46
 Wilma | 1348
 James | 12
{table}

Code and Notes

You can define code blocks with the code macro:

class Book {
    String title
}

{code}
class Book {
    String title
}
{code}

The example above provides syntax highlighting for Java and Groovy code, but you can also highlight XML markup:

<hello>world</hello>

{code:xml}
<hello>world</hello>
{code}

There are also a couple of macros for displaying notes and warnings:

Note:

This is a note!

{note}
This is a note!
{note}

Warning:

This is a warning!

{warning}
This is a warning!
{warning}

4.7 Dependency Resolution

You specify a grails.project.dependency.resolution property inside the grails-app/conf/BuildConfig.groovy file that configures how dependencies are resolved:

grails.project.dependency.resolution = {
   // config here
}

The default configuration looks like the following:

grails.project.class.dir = "target/classes"
grails.project.test.class.dir = "target/test-classes"
grails.project.test.reports.dir = "target/test-reports"
//grails.project.war.file = "target/${appName}-${appVersion}.war"
grails.project.dependency.resolution = {
    // inherit Grails' default dependencies
    inherits("global") {
        // uncomment to disable ehcache
        // excludes 'ehcache'
    }
    log "warn"
    repositories {
        grailsPlugins()
        grailsHome()
        grailsCentral()
        // uncomment these to enable remote dependency resolution
        // from public Maven repositories
        //mavenCentral()
        //mavenLocal()
        //mavenRepo "http://snapshots.repository.codehaus.org"
        //mavenRepo "http://repository.codehaus.org"
        //mavenRepo "http://download.java.net/maven/2/"
        //mavenRepo "http://repository.jboss.com/maven2/"
    }
    dependencies {
        // specify dependencies here under either 'build', 'compile',
        // 'runtime', 'test' or 'provided' scopes eg.
        // runtime 'mysql:mysql-connector-java:5.1.16'
    }
    plugins {
        compile ":hibernate:$grailsVersion"
        compile ":jquery:1.6.1.1"
        compile ":resources:1.0"
        build ":tomcat:$grailsVersion"
    }
}

The details of the above will be explained in the next few sections.

4.7.1 Configurations and Dependencies

• build: Dependencies for the build system only
• compile: Dependencies for the compile step
• runtime: Dependencies needed at runtime but not for compilation (see above)
• test: Dependencies needed for testing but not at runtime (see above)
• provided: Dependencies needed at development time, but not during WAR deployment

Within the dependencies block you can specify a dependency that falls into one of these configurations by calling the equivalent method. For example if your application requires the MySQL driver to function at runtime you can specify that like this:

runtime 'com.mysql:mysql-connector-java:5.1.16'

This uses the string syntax: group:name:version. You can also use a Map-based syntax:

runtime group: 'com.mysql',
        name: 'mysql-connector-java',
        version: '5.1.16'

In Maven terminology, group corresponds to an artifact's groupId and name corresponds to its artifactId.

Multiple dependencies can be specified by passing multiple arguments:

runtime 'com.mysql:mysql-connector-java:5.1.16',
        'net.sf.ehcache:ehcache:1.6.1'
// Or
runtime(
    [group:'com.mysql', name:'mysql-connector-java', version:'5.1.16'],
    [group:'net.sf.ehcache', name:'ehcache', version:'1.6.1']
)

Disabling transitive dependency resolution

By default, Grails will not only get the JARs and plugins that you declare, but it will also get their transitive dependencies. This is usually what you want, but there are occasions where you want a dependency without all its baggage. In such cases, you can disable transitive dependency resolution on a case-by-case basis:

runtime('com.mysql:mysql-connector-java:5.1.16',
        'net.sf.ehcache:ehcache:1.6.1') {
    transitive = false
}
// Or
runtime group:'com.mysql',
        name:'mysql-connector-java',
        version:'5.1.16',
        transitive:false

Excluding specific transitive dependencies

A far more common scenario is where you want the transitive dependencies, but some of them cause issues with your own dependencies or are unnecessary. For example, many Apache projects have 'commons-logging' as a transitive dependency, but it shouldn't be included in a Grails project (we use SLF4J). That's where the excludes option comes in:

runtime('com.mysql:mysql-connector-java:5.1.16',
        'net.sf.ehcache:ehcache:1.6.1') {
    excludes "xml-apis", "commons-logging"
}
// Or
runtime(group:'com.mysql', name:'mysql-connector-java', version:'5.1.16') {
    excludes([ group: 'xml-apis', name: 'xml-apis'],
             [ group: 'org.apache.httpcomponents' ],
             [ name: 'commons-logging' ])

As you can see, you can either exclude dependencies by their artifact ID (also known as a module name) or any combination of group and artifact IDs (if you use the Map notation). You may also come across exclude as well, but that can only accept a single string or Map:

runtime('com.mysql:mysql-connector-java:5.1.16',
        'net.sf.ehcache:ehcache:1.6.1') {
    exclude "xml-apis"
}

Using Ivy module configurations

If you use Ivy module configurations and wish to depend on a specific configuration of a module, you can use the dependencyConfiguration method to specify the configuration to use.

provided("my.org:web-service:1.0") {
    dependencyConfiguration "api"
}

If the dependency configuration is not explicitly set, the configuration named "default" will be used (which is also the correct value for dependencies coming from Maven style repositories).

Where are the JARs?

With all these declarative dependencies, you may wonder where all the JARs end up. They have to go somewhere after all. By default Grails puts them into a directory, called the dependency cache, that resides on your local file system at user.home/.grails/ivy-cache. You can change this either via the settings.groovy file:

grails.dependency.cache.dir = "${userHome}/.my-dependency-cache"

or in the dependency DSL:

grails.project.dependency.resolution = {
    …
    cacheDir "target/ivy-cache"
    …
}

The settings.groovy option applies to all projects, so it's the preferred approach.

4.7.2 Dependency Repositories

Remote Repositories

Initially your BuildConfig.groovy does not use any remote public Maven repositories. There is a default grailsHome() repository that will locate the JAR files Grails needs from your Grails installation. To use a public repository, specify it in the repositories block:

repositories {
    mavenCentral()
}

In this case the default public Maven repository is specified. To use the SpringSource Enterprise Bundle Repository you can use the ebr() method:

repositories {
    ebr()
}

You can also specify a specific Maven repository to use by URL:

repositories {
    mavenRepo "http://repository.codehaus.org"
}

and even give it a name:

repositories {
    mavenRepo name: "Codehaus", root: "http://repository.codehaus.org"
}

so that you can easily identify it in logs.

Controlling Repositories Inherited from Plugins

A plugin you have installed may define a reference to a remote repository just as an application can. By default your application will inherit this repository definition when you install the plugin.

If you do not wish to inherit repository definitions from plugins then you can disable repository inheritance:

repositories {
    inherit false
}

In this case your application will not inherit any repository definitions from plugins and it is down to you to provide appropriate (possibly internal) repository definitions.

Offline Mode

There are times when it is not desirable to connect to any remote repositories (whilst working on the train for example!). In this case you can use the offline flag to execute Grails commands and Grails will not connect to any remote repositories:

grails --offline run-app

Note that this command will fail if you do not have the necessary dependencies in your local Ivy cache

You can also globally configure offline mode by setting grails.offline.mode to true in ~/.grails/settings.groovy or in your project's BuildConfig.groovy file:

grails.offline.mode=true

Local Resolvers

If you do not wish to use a public Maven repository you can specify a flat file repository:

repositories {
    flatDir name:'myRepo', dirs:'/path/to/repo'
}

To specify your local Maven cache (~/.m2/repository) as a repository:

repositories {
    mavenLocal()
}

Custom Resolvers

If all else fails since Grails builds on Apache Ivy you can specify an Ivy resolver:

/*
 * Configure our resolver.
 */
def libResolver = new org.apache.ivy.plugins.resolver.URLResolver()
['libraries', 'builds'].each {
    libResolver.addArtifactPattern(
            "http://my.repository/${it}/" +
            "[organisation]/[module]/[revision]/[type]s/[artifact].[ext]")
    libResolver.addIvyPattern(
            "http://my.repository/${it}/" +
            "[organisation]/[module]/[revision]/[type]s/[artifact].[ext]")
}
libResolver.name = "my-repository"
libResolver.settings = ivySettings
resolver libResolver

It's also possible to pull dependencies from a repository using SSH. Ivy comes with a dedicated resolver that you can configure and include in your project like so:

import org.apache.ivy.plugins.resolver.SshResolver
…
repositories {
    ...
    def sshResolver = new SshResolver(
            name: "myRepo",
            user: "username",
            host: "dev.x.com",
            keyFile: new File("/home/username/.ssh/id_rsa"),
            m2compatible: true)
    sshResolver.addArtifactPattern(
            "/home/grails/repo/[organisation]/[artifact]/" +
            "[revision]/[artifact]-[revision].[ext]")
    sshResolver.latestStrategy =
            new org.apache.ivy.plugins.latest.LatestTimeStrategy()
    sshResolver.changingPattern = ".*SNAPSHOT"
    sshResolver.setCheckmodified(true)
    resolver sshResolver
}

Download the JSch JAR and add it to Grails' classpath to use the SSH resolver. You can do this by passing the path in the Grails command line:

grails -classpath /path/to/jsch compile|run-app|etc.

You can also add its path to the CLASSPATH environment variable but be aware this it affects many Java applications. An alternative on Unix is to create an alias for grails -classpath ... so that you don't have to type the extra arguments each time.

Authentication

If your repository requires authentication you can configure this using a credentials block:

credentials {
    realm = ".."
    host = "localhost"
    username = "myuser"
    password = "mypass"
}

This can be placed in your USER_HOME/.grails/settings.groovy file using the grails.project.ivy.authentication setting:

grails.project.ivy.authentication = {
    credentials {
        realm = ".."
        host = "localhost"
        username = "myuser"
        password = "mypass"
    }
}

4.7.3 Debugging Resolution

// log level of Ivy resolver, either 'error', 'warn',
// 'info', 'debug' or 'verbose'
log "warn"

A common issue is that the checksums for a dependency don't match the associated JAR file, and so Ivy rejects the dependency. This helps ensure that the dependencies are valid. But for a variety of reasons some dependencies simply don't have valid checksums in the repositories, even if they are valid JARs. To get round this, you can disable Ivy's dependency checks like so:

grails.project.dependency.resolution = {
    …
    log "warn"
    checksums false
    …
}

This is a global setting, so only use it if you have to.

4.7.4 Inherited Dependencies

inherits "global"

Inside the BuildConfig.groovy file. To exclude specific inherited dependencies you use the excludes method:

inherits("global") {
    excludes "oscache", "ehcache"
}

4.7.5 Providing Default Dependencies

The dependency resolution DSL provides a mechanism to express that all of the default dependencies will be provided by the container. This is done by invoking the defaultDependenciesProvided method and passing true as an argument:

grails.project.dependency.resolution = {
    defaultDependenciesProvided true // all of the default dependencies will
                                     // be "provided" by the container
    inherits "global" // inherit Grails' default dependencies
    repositories {
        grailsHome()
        …
    }
    dependencies {
        …
    }
}

defaultDependenciesProvided must come before inherits, otherwise the Grails dependencies will be included in the war.

4.7.6 Snapshots and Other Changing Dependencies

Whenever you have a changing dependency, Grails will always check the remote repository for a new version. More specifically, when a changing dependency is encountered during dependency resolution its last modified timestamp in the local cache is compared against the last modified timestamp in the dependency repositories. If the version on the remote server is deemed to be newer than the version in the local cache, the new version will be downloaded and used.

{info} Be sure to read the next section on “Dependency Resolution Caching” in addition to this one as it affects changing dependencies. {info}

All dependencies (jars and plugins) with a version number ending in -SNAPSHOT are implicitly considered to be changing by Grails. You can also explicitly specify that a dependency is changing by setting the changing flag in the dependency DSL:

runtime ('org.my:lib:1.2.3') {
    changing = true
}

There is a caveat to the support for changing dependencies that you should be aware of. Grails will stop looking for newer versions of a dependency once it finds a remote repository that has the dependency.

Consider the following setup:

grails.project.dependency.resolution = {
    repositories {
        mavenLocal()
        mavenRepo "http://my.org/repo"
    }
    dependencies {
        compile "myorg:mylib:1.0-SNAPSHOT"
    }

In this example we are using the local maven repository and a remote network maven repository. Assuming that the local Grails dependency and the local Maven cache do not contain the dependency but the remote repository does, when we perform dependency resolution the following actions will occur:

• maven local repository is searched, dependency not found
• maven network repository is searched, dependency is downloaded to the cache and used

Note that the repositories are checked in the order they are defined in the BuildConfig.groovy file.

If we perform dependency resolution again without the dependency changing on the remote server, the following will happen:

• maven local repository is searched, dependency not found
• maven network repository is searched, dependency is found to be the same “age” as the version in the cache so will not be updated (i.e. downloaded)

Later on, a new version of mylib 1.0-SNAPSHOT is published changing the version on the server. The next time we perform dependency resolution, the following will happen:

• maven local repository is searched, dependency not found
• maven network repository is searched, dependency is found to newer than version in the cache so will be updated (i.e. downloaded to the cache)

So far everything is working well.

Now we want to test some local changes to the mylib library. To do this we build it locally and install it to the local Maven cache (how doesn't particularly matter). The next time we perform a dependency resolution, the following will occur:

• maven local repository is searched, dependency is found to newer than version in the cache so will be updated (i.e. downloaded to the cache)
• maven network repository is NOT searched as we've already found the dependency

This is what we wanted to occur.

Later on, a new version of mylib 1.0-SNAPSHOT is published changing the version on the server. The next time we perform dependency resolution, the following will happen:

• maven local repository is searched, dependency is found to be the same “age” as the version in the cache so will not be updated (i.e. downloaded)
• maven network repository is NOT searched as we've already found the dependency

This is likely to not be the desired outcome. We are now out of sync with the latest published snapshot and will continue to keep using the version from the local maven repository.

The rule to remember is this: when resolving a dependency, Grails will stop searching as soon as it finds a repository that has the dependency at the specified version number. It will not continue searching all repositories trying to find a more recently modified instance.

To remedy this situation (i.e. build against the newer version of mylib 1.0-SNAPSHOT in the remote repository), you can either:

• Delete the version from the local maven repository, or
• Reorder the repositories in the BuildConfig.groovy file

Where possible, prefer deleting the version from the local maven repository. In general, when you have finished building against a locally built SNAPSHOT always try to clear it from the local maven repository.

This changing dependency behaviour is an unmodifiable characteristic of the underlying dependency management system that Grails uses, Apache Ivy. It is currently not possible to have Ivy search all repositories to look for newer versions (in terms of modification date) of the same dependency (i.e. the same combination of group, name and version).

4.7.7 Dependency Reports

To obtain a report of an application's dependencies you can run the dependency-report command:

grails dependency-report

By default this will generate reports in the target/dependency-report directory. You can specify which configuration (scope) you want a report for by passing an argument containing the configuration name:

grails dependency-report runtime

4.7.8 Plugin JAR Dependencies

Specifying Plugin JAR dependencies

The way in which you specify dependencies for a plugin is identical to how you specify dependencies in an application. When a plugin is installed into an application the application automatically inherits the dependencies of the plugin.

To define a dependency that is resolved for use with the plugin but not exported to the application then you can set the export property of the dependency:

test('org.spockframework:spock-core:0.5-groovy-1.8') {
    export = false
}

In this case the Spock dependency will be available only to the plugin and not resolved as an application dependency. Alternatively, if you're using the Map syntax:

test group: 'org.spockframework', name: 'spock-core',
     version: '0.5-groovy-1.8', export: false

You can use exported = false instead of export = false, but we recommend the latter because it's consistent with the Map argument.

Overriding Plugin JAR Dependencies in Your Application

If a plugin is using a JAR which conflicts with another plugin, or an application dependency then you can override how a plugin resolves its dependencies inside an application using exclusions. For example:

plugins {
    compile(":hibernate:$grailsVersion") {
        excludes "javassist"
    }
}
dependencies {
    runtime "javassist:javassist:3.4.GA"
}

In this case the application explicitly declares a dependency on the "hibernate" plugin and specifies an exclusion using the excludes method, effectively excluding the javassist library as a dependency.

4.7.9 Maven Integration

However, if you would like to continue using Grails regular commands like run-app, test-app and so on then you can tell Grails' command line to load dependencies from the Maven pom.xml file instead.

To do so simply add the following line to your BuildConfig.groovy:

grails.project.dependency.resolution = {
    pom true
    ..
}

The line pom true tells Grails to parse Maven's pom.xml and load dependencies from there.

4.7.10 Deploying to a Maven Repository

The plugin provides the ability to publish Grails projects and plugins to local and remote Maven repositories. There are two key additional targets added by the plugin:

• maven-install - Installs a Grails project or plugin into your local Maven cache
• maven-deploy - Deploys a Grails project or plugin to a remote Maven repository

By default this plugin will automatically generate a valid pom.xml for you unless a pom.xml is already present in the root of the project, in which case this pom.xml file will be used.

maven-install

The maven-install command will install the Grails project or plugin artifact into your local Maven cache:

grails maven-install

In the case of plugins, the plugin zip file will be installed, whilst for application the application WAR file will be installed.

maven-deploy

The maven-deploy command will deploy a Grails project or plugin into a remote Maven repository:

grails maven-deploy

It is assumed that you have specified the necessary <distributionManagement> configuration within a pom.xml or that you specify the id of the remote repository to deploy to:

grails maven-deploy --repository=myRepo

The repository argument specifies the 'id' for the repository. Configure the details of the repository specified by this 'id' within your grails-app/conf/BuildConfig.groovy file or in your $USER_HOME/.grails/settings.groovy file:

grails.project.dependency.distribution = {
    localRepository = "/path/to/my/local"
    remoteRepository(id: "myRepo", url: "http://myserver/path/to/repo")
}

The syntax for configuring remote repositories matches the syntax from the remoteRepository element in the Ant Maven tasks. For example the following XML:

<remoteRepository id="myRepo" url="scp://localhost/www/repository">
    <authentication username="..." privateKey="${user.home}/.ssh/id_dsa"/>
</remoteRepository>

Can be expressed as:

remoteRepository(id: "myRepo", url: "scp://localhost/www/repository") {
    authentication username: "...", privateKey: "${userHome}/.ssh/id_dsa"
}

By default the plugin will try to detect the protocol to use from the URL of the repository (ie "http" from "http://.." etc.), however to specify a different protocol you can do:

grails maven-deploy --repository=myRepo --protocol=webdav

The available protocols are:

• http
• scp
• scpexe
• ftp
• webdav

Groups, Artifacts and Versions

Maven defines the notion of a 'groupId', 'artifactId' and a 'version'. This plugin pulls this information from the Grails project conventions or plugin descriptor.

Projects

For applications this plugin will use the Grails application name and version provided by Grails when generating the pom.xml file. To change the version you can run the set-version command:

grails set-version 0.2

The Maven groupId will be the same as the project name, unless you specify a different one in Config.groovy:

grails.project.groupId="com.mycompany"

Plugins

With a Grails plugin the groupId and version are taken from the following properties in the GrailsPlugin.groovy descriptor:

String groupId = 'myOrg'
String version = '0.1'

The 'artifactId' is taken from the plugin name. For example if you have a plugin called FeedsGrailsPlugin the artifactId will be "feeds". If your plugin does not specify a groupId then this defaults to "org.grails.plugins".

4.7.11 Plugin Dependencies

grails.project.dependency.resolution = {
    …
    repositories {
        …
    }
    plugins {
        runtime ':hibernate:1.2.1'
    }
    dependencies {
        …
    }
    …
}

If you don't specify a group id the default plugin group id of org.grails.plugins is used. You can specify to use the latest version of a particular plugin by using "latest.integration" as the version number:

plugins {
    runtime ':hibernate:latest.integration'
}

Integration vs. Release

The "latest.integration" version label will also include resolving snapshot versions. To not include snapshot versions then use the "latest.release" label:

plugins {
    runtime ':hibernate:latest.release'
}

The "latest.release" label only works with Maven compatible repositories. If you have a regular SVN-based Grails repository then you should use "latest.integration".

And of course if you use a Maven repository with an alternative group id you can specify a group id:

plugins {
    runtime 'mycompany:hibernate:latest.integration'
}

Plugin Exclusions

You can control how plugins transitively resolves both plugin and JAR dependencies using exclusions. For example:

plugins {
    runtime(':weceem:0.8') {
        excludes "searchable"
    }
}

Here we have defined a dependency on the "weceem" plugin which transitively depends on the "searchable" plugin. By using the excludes method you can tell Grails not to transitively install the searchable plugin. You can combine this technique to specify an alternative version of a plugin:

plugins {
    runtime(':weceem:0.8') {
        excludes "searchable" // excludes most recent version
    }
    runtime ':searchable:0.5.4' // specifies a fixed searchable version
}

You can also completely disable transitive plugin installs, in which case no transitive dependencies will be resolved:

plugins {
    runtime(':weceem:0.8') {
        transitive = false
    }
    runtime ':searchable:0.5.4' // specifies a fixed searchable version
}

4.7.12 Caching of Dependency Resolution Results

Grails only performs dependency resolution under the following circumstances:

• The project is clean (i.e. fresh checkout or after grails clean)
• The BuildConfig.groovy file has changed since the last command was run
• The --refresh-dependencies command line switch is provided to the command (any command)
• The refresh-dependencies command is the command being executed

Generally, this strategy works well and you can ignore dependency resolution caching. Every time you change your dependencies (i.e. modify BuildConfig.groovy) Grails will do the right thing and resolve your new dependencies.

However, when you have changing or dynamic dependencies you will have to consider dependency resolution caching.

{info} A changing dependency is one whose version number does not change, but its contents do (like a SNAPSHOT). A dynamic dependency is one that is defined as one of many possible options (like a dependency with a version range, or symbolic version number like latest.integration). {info}

Both changing and dynamic dependencies are influenced by the environment. With caching active, any changes to the environment are effectively ignored. For example, your project may not automatically fetch the very latest version of a dependency when using latest.integration. Or if you declare a SNAPSHOT dependency, you may not automatically get the latest that's available on the server.

To ensure you have the correct version of a changing or dynamic dependency in your project, you can:

• clean the project
• run the refresh-dependencies command
• run any command with the --refresh-dependencies switch; or
• make a change to BuildConfig.groovy

If you have your CI builds configured to not perform clean builds, it may be worth adding the --refresh-dependencies switch to the command you use to build your projects.

5 The Command Line

However, Grails takes it further through the use of convention and the grails command. When you type:

grails [command name]

Grails searches in the following directories for Gant scripts to execute:

• USER_HOME/.grails/scripts
• PROJECT_HOME/scripts
• PROJECT_HOME/plugins/*/scripts
• GRAILS_HOME/scripts

Grails will also convert command names that are in lower case form such as run-app into camel case. So typing

grails run-app

Results in a search for the following files:

• USER_HOME/.grails/scripts/RunApp.groovy
• PROJECT_HOME/scripts/RunApp.groovy
• PLUGINS_HOME/*/scripts/RunApp.groovy
• GLOBAL_PLUGINS_HOME/*/scripts/RunApp.groovy
• GRAILS_HOME/scripts/RunApp.groovy

If multiple matches are found Grails will give you a choice of which one to execute.

When Grails executes a Gant script, it invokes the "default" target defined in that script. If there is no default, Grails will quit with an error.

To get a list of all commands and some help about the available commands type:

grails help

which outputs usage instructions and the list of commands Grails is aware of:

Usage (optionals marked with *):
grails [environment]* [target] [arguments]*
Examples:
grails dev run-app
grails create-app books
Available Targets (type grails help 'target-name' for more info):
grails bootstrap
grails bug-report
grails clean
grails compile
...

Refer to the Command Line reference in the Quick Reference menu of the reference guide for more information about individual commands

It's often useful to provide custom arguments to the JVM when running Grails commands, in particular with run-app where you may for example want to set a higher maximum heap size. The Grails command will use any JVM options provided in the general JAVA_OPTS environment variable, but you can also specify a Grails-specific environment variable too:

export GRAILS_OPTS="-Xmx1G -Xms256m -XX:MaxPermSize=256m"
grails run-app

non-interactive mode

When you run a script manually and it prompts you for information, you can answer the questions and continue running the script. But when you run a script as part of an automated process, for example a continuous integration build server, there's no way to "answer" the questions. So you can pass the --non-interactive switch to the script command to tell Grails to accept the default answer for any questions, for example whether to install a missing plugin.

For example:

grails war --non-interactive

5.1 Interactive Mode

If you need to open a file whilst within interactive mode you can use the open command which will TAB complete file paths:

Even better, the open command understands the logical aliases 'test-report' and 'dep-report', which will open the most recent test and dependency reports respectively. In other words, to open the test report in a browser simply execute open test-report. You can even open multiple files at once: open test-report test/unit/MyTests.groovy will open the HTML test report in your browser and the MyTests.groovy source file in your text editor.

TAB completion also works for class names after the create-* commands:

If you need to run an external process whilst interactive mode is running you can do so by starting the command with a !:

Note that with ! (bang) commands, you get file path auto completion - ideal for external commands that operate on the file system such as 'ls', 'cat', 'git', etc.

The stop-app command will stop an application that has been run with the run-app command.

To exit interactive mode enter the exit command. Note that if the Grails application has been run with run-app normally it will terminate when the interactive mode console exits because the JVM will be terminated. An exception to this would be if the application were running in forked mode which means the application is running in a different JVM. In that case the application will be left running afer the interactive mode console terminates. If you want to exit interactive mode and stop an application that is running in forked mode, use the quit command. The quit command will stop the running application and then close interactive mode.

5.2 Forked Tomcat Execution

Forked Tomcat Execution

Grails 2.2 and above support forked JVM execution of the Tomcat container in development mode. This has several benefits including:

• Reduced memory consumption, since the Grails build system can exit
• Isolation of the build classpath from the runtime classpath
• The ability to deploy other Grails/Spring applications in parallels without conflicting dependencies

To enable forked execution you can set the grails.project.fork.run property to true:

grails.project.fork.run=true

Then just us the regular run-app command as per normal. Note that in forked mode the grails process will exit and leave the container running in the background. To stop the server there is a new stop-app command:

grails stop-app

To customize the JVM arguments passed to the forked JVM you can specify a map instead:

grails.project.fork.run= [maxMemory:1024, minMemory:64, debug:false, maxPerm:256, jvmArgs: '..arbitrary JVM arguments..']

Auto-deploying additional WAR files in Forked Mode

Since forked execution isolates classpaths more effectively than embedded execution you can deploy additional WAR files (such as other Grails or Spring applications) to the container.

The easiest way to do so is to drop the WAR files into the src/autodeploy directory (if it doesn't exist you can create it).

You can customize the location of the autodeploy directory by specifying an alternative location in BuildConfig.groovy:

grails.project.autodeploy.dir="/path/to/my/war/files"

Customizing the Forked Tomcat instance

If you want to programmatically customize the forked Tomcat instance you can do so by implementing a class named org.grails.plugins.tomcat.ForkedTomcatCustomizer which provides a method with the following signature:

void customize(Tomcat tomcat) {
 // your code here
}

5.3 Creating Gant Scripts

grails create-script compile-sources

Will create a script called scripts/CompileSources.groovy. A Gant script itself is similar to a regular Groovy script except that it supports the concept of "targets" and dependencies between them:

target(default:"The default target is the one that gets executed by Grails") {
    depends(clean, compile)
}
target(clean:"Clean out things") {
    ant.delete(dir:"output")
}
target(compile:"Compile some sources") {
    ant.mkdir(dir:"mkdir")
    ant.javac(srcdir:"src/java", destdir:"output")
}

As demonstrated in the script above, there is an implicit ant variable (an instance of groovy.util.AntBuilder) that allows access to the Apache Ant API.

In previous versions of Grails (1.0.3 and below), the variable was Ant, i.e. with a capital first letter.

You can also "depend" on other targets using the depends method demonstrated in the default target above.

The default target

In the example above, we specified a target with the explicit name "default". This is one way of defining the default target for a script. An alternative approach is to use the setDefaultTarget() method:

target("clean-compile": "Performs a clean compilation on the app source") {
    depends(clean, compile)
}
target(clean:"Clean out things") {
    ant.delete(dir:"output")
}
target(compile:"Compile some sources") {
    ant.mkdir(dir:"mkdir")
    ant.javac(srcdir:"src/java", destdir:"output")
}
setDefaultTarget("clean-compile")

This lets you call the default target directly from other scripts if you wish. Also, although we have put the call to setDefaultTarget() at the end of the script in this example, it can go anywhere as long as it comes after the target it refers to ("clean-compile" in this case).

Which approach is better? To be honest, you can use whichever you prefer - there don't seem to be any major advantages in either case. One thing we would say is that if you want to allow other scripts to call your "default" target, you should move it into a shared script that doesn't have a default target at all. We'll talk some more about this in the next section.

5.4 Re-using Grails scripts

The bootstrap script for example lets you bootstrap a Spring ApplicationContext instance to get access to the data source and so on (the integration tests use this):

includeTargets << grailsScript("_GrailsBootstrap")
target ('default': "Database stuff") {
    depends(configureProxy, packageApp, classpath, loadApp, configureApp)
    Connection c
    try {
        c = appCtx.getBean('dataSource').getConnection()
        // do something with connection
    }
    finally {
        c?.close()
    }
}

Pulling in targets from other scripts

Gant lets you pull in all targets (except "default") from another Gant script. You can then depend upon or invoke those targets as if they had been defined in the current script. The mechanism for doing this is the includeTargets property. Simply "append" a file or class to it using the left-shift operator:

includeTargets << new File("/path/to/my/script.groovy")
includeTargets << gant.tools.Ivy

Core Grails targets

As you saw in the example at the beginning of this section, you use neither the File- nor the class-based syntax for includeTargets when including core Grails targets. Instead, you should use the special grailsScript() method that is provided by the Grails command launcher (note that this is not available in normal Gant scripts, just Grails ones).

The syntax for the grailsScript() method is pretty straightforward: simply pass it the name of the Grails script to include, without any path information. Here is a list of Grails scripts that you could reuse:

There are many more scripts provided by Grails, so it is worth digging into the scripts themselves to find out what kind of targets are available. Anything that starts with an "_" is designed for reuse.

Script architecture

You maybe wondering what those underscores are doing in the names of the Grails scripts. That is Grails' way of determining that a script is internal , or in other words that it has not corresponding "command". So you can't run "grails _grails-settings" for example. That is also why they don't have a default target.

Internal scripts are all about code sharing and reuse. In fact, we recommend you take a similar approach in your own scripts: put all your targets into an internal script that can be easily shared, and provide simple command scripts that parse any command line arguments and delegate to the targets in the internal script. For example if you have a script that runs some functional tests, you can split it like this:

./scripts/FunctionalTests.groovy:
includeTargets << new File("${basedir}/scripts/_FunctionalTests.groovy")
target(default: "Runs the functional tests for this project.") {
    depends(runFunctionalTests)
}
./scripts/_FunctionalTests.groovy:
includeTargets << grailsScript("_GrailsTest")
target(runFunctionalTests: "Run functional tests.") {
    depends(...)
    …
}

Here are a few general guidelines on writing scripts:

• Split scripts into a "command" script and an internal one.
• Put the bulk of the implementation in the internal script.
• Put argument parsing into the "command" script.
• To pass arguments to a target, create some script variables and initialise them before calling the target.
• Avoid name clashes by using closures assigned to script variables instead of targets. You can then pass arguments direct to the closures.

5.5 Hooking into Events

The mechanism is deliberately simple and loosely specified. The list of possible events is not fixed in any way, so it is possible to hook into events triggered by plugin scripts, for which there is no equivalent event in the core target scripts.

Defining event handlers

Event handlers are defined in scripts called _Events.groovy. Grails searches for these scripts in the following locations:

• USER_HOME/.grails/scripts - user-specific event handlers
• PROJECT_HOME/scripts - applicaton-specific event handlers
• PLUGINS_HOME/*/scripts - plugin-specific event handlers
• GLOBAL_PLUGINS_HOME/*/scripts - event handlers provided by global plugins

Whenever an event is fired, all the registered handlers for that event are executed. Note that the registration of handlers is performed automatically by Grails, so you just need to declare them in the relevant _Events.groovy file.

Event handlers are blocks defined in _Events.groovy, with a name beginning with "event". The following example can be put in your /scripts directory to demonstrate the feature:

eventCreatedArtefact = { type, name ->
   println "Created $type $name"
}
eventStatusUpdate = { msg ->
   println msg
}
eventStatusFinal = { msg ->
   println msg
}

You can see here the three handlers eventCreatedArtefact, eventStatusUpdate, eventStatusFinal. Grails provides some standard events, which are documented in the command line reference guide. For example the compile command fires the following events:

• CompileStart - Called when compilation starts, passing the kind of compile - source or tests
• CompileEnd - Called when compilation is finished, passing the kind of compile - source or tests

Triggering events

To trigger an event simply include the Init.groovy script and call the event() closure:

includeTargets << grailsScript("_GrailsEvents")
event("StatusFinal", ["Super duper plugin action complete!"])

Common Events

Below is a table of some of the common events that can be leveraged:

5.6 Customising the build

The defaults

The core of the Grails build configuration is the grails.util.BuildSettings class, which contains quite a bit of useful information. It controls where classes are compiled to, what dependencies the application has, and other such settings.

Here is a selection of the configuration options and their default values:

The BuildSettings class has some other properties too, but they should be treated as read-only:

Of course, these properties aren't much good if you can't get hold of them. Fortunately that's easy to do: an instance of BuildSettings is available to your scripts as the grailsSettings script variable. You can also access it from your code by using the grails.util.BuildSettingsHolder class, but this isn't recommended.

Overriding the defaults

All of the properties in the first table can be overridden by a system property or a configuration option - simply use the "config option" name. For example, to change the project working directory, you could either run this command:

grails -Dgrails.project.work.dir=work compile

grails.project.work.dir = "work"

What happens if you use both a system property and a configuration option? Then the system property wins because it takes precedence over the BuildConfig.groovy file, which in turn takes precedence over the default values.

The BuildConfig.groovy file is a sibling of grails-app/conf/Config.groovy - the former contains options that only affect the build, whereas the latter contains those that affect the application at runtime. It's not limited to the options in the first table either: you will find build configuration options dotted around the documentation, such as ones for specifying the port that the embedded servlet container runs on or for determining what files get packaged in the WAR file.

Available build settings

Reloading Agent Cache Directory

Grails uses an agent based reloading system in the development environment that allows source code changes to be picked up while the application is running. This reloading agent caches information needed to carry out the reloading efficiently. By default this information is stored under <USER_HOME_DIR>/.grails/.slcache/. The GRAILS_AGENT_CACHE_DIR environment variable may be assigned a value to cause this cache information to be stored somewhere else. Note that this is an operating system environment variable, not a JVM system property or a property which may be defined in BuildConfig.groovy. This setting must be defined as an environment variable because the agent cache directory must be configured very early in the JVM startup process, before any Grails code is executed.

5.7 Ant and Maven

Ant Integration

When you create a Grails application with the create-app command, Grails doesn't automatically create an Ant build.xml file but you can generate one with the integrate-with command:

grails integrate-with --ant

This creates a build.xml file containing the following targets:

• clean - Cleans the Grails application
• compile - Compiles your application's source code
• test - Runs the unit tests
• run - Equivalent to "grails run-app"
• war - Creates a WAR file
• deploy - Empty by default, but can be used to implement automatic deployment

Each of these can be run by Ant, for example:

ant war

The build file is configured to use Apache Ivy for dependency management, which means that it will automatically download all the requisite Grails JAR files and other dependencies on demand. You don't even have to install Grails locally to use it! That makes it particularly useful for continuous integration systems such as CruiseControl or Jenkins.

It uses the Grails Ant task to hook into the existing Grails build system. The task lets you run any Grails script that's available, not just the ones used by the generated build file. To use the task, you must first declare it:

<taskdef name="grailsTask"
         classname="grails.ant.GrailsTask"
         classpathref="grails.classpath"/>

This raises the question: what should be in "grails.classpath"? The task itself is in the "grails-bootstrap" JAR artifact, so that needs to be on the classpath at least. You should also include the "groovy-all" JAR. With the task defined, you just need to use it! The following table shows you what attributes are available:

The task also supports the following nested elements, all of which are standard Ant path structures:

• classpath - The build classpath (used to load Gant and the Grails scripts).
• compileClasspath - Classpath used to compile the application's classes.
• runtimeClasspath - Classpath used to run the application and package the WAR. Typically includes everything in @compileClasspath.
• testClasspath - Classpath used to compile and run the tests. Typically includes everything in runtimeClasspath.

How you populate these paths is up to you. If you use the home attribute and put your own dependencies in the lib directory, then you don't even need to use any of them. For an example of their use, take a look at the generated Ant build file for new apps.

Maven Integration

Grails provides integration with Maven 2 with a Maven plugin.

Preparation

In order to use the Maven plugin, all you need is Maven 2 installed and set up. This is because you no longer need to install Grails separately to use it with Maven!

The Maven 2 integration for Grails has been designed and tested for Maven 2.0.9 and above. It will not work with earlier versions.

The default mvn setup DOES NOT supply sufficient memory to run the Grails environment. We recommend that you add the following environment variable setting to prevent poor performance:

export MAVEN_OPTS="-Xmx512m -XX:MaxPermSize=256"

Creating a Grails Maven Project

Using the create-pom command you can generate a valid Maven pom.xml file for any existing Grails project. The below presents an example:

$ grails create-app myapp
$ cd myapp
$ grails create-pom com.mycompany

The create-pom command expects a group id as an argument. The name and the version are taken from the application.properties of the application. The Maven plugin will keep the version in the pom.xml in sync with the version in application.properties.

The following standard Maven commands are then possible:

• compile - Compiles a Grails project
• package - Builds a WAR file from the Grails project.
• install - Builds a WAR file (or plugin zip/jar if a plugin) and installs it into your local Maven cache
• test - Runs the tests of a Grails project
• clean - Cleans the Grails project

Other standard Maven commands will likely work too.

You can also use some of the Grails commands that have been wrapped as Maven goals:

• grails:create-controller - Calls the create-controller command
• grails:create-domain-class - Calls the create-domain-class command
• grails:create-integration-test - Calls the create-integration-test command
• grails:create-pom - Creates a new Maven POM for an existing Grails project
• grails:create-script - Calls the create-script command
• grails:create-service - Calls the create-service command
• grails:create-taglib - Calls the create-tag-lib command
• grails:create-unit-test - Calls the create-unit-test command
• grails:exec - Executes an arbitrary Grails command line script
• grails:generate-all - Calls the generate-all command
• grails:generate-controller - Calls the generate-controller command
• grails:generate-views - Calls the generate-views command
• grails:install-templates - Calls the install-templates command
• grails:list-plugins - Calls the list-plugins command
• grails:package - Calls the package command
• grails:run-app - Calls the run-app command

For a complete, up to date list, run mvn grails:help

Creating a Grails Maven Project using the Archetype

You can create a Maven Grails project without having Grails installed, simply run the following command:

mvn archetype:generate -DarchetypeGroupId=org.grails \
    -DarchetypeArtifactId=grails-maven-archetype \
    -DarchetypeVersion=2.1.0.RC1 \
    -DgroupId=example -DartifactId=my-app

Choose whichever grails version, group ID and artifact ID you want for your application, but everything else must be as written. This will create a new Maven project with a POM and a couple of other files. What you won't see is anything that looks like a Grails application. So, the next step is to create the project structure that you're used to. But first, to set target JDK to Java 6, do that now. Open my-app/pom.xml and change

<plugin>
  <artifactId>maven-compiler-plugin</artifactId>
  <configuration>
    <source>1.5</source>
    <target>1.5</target>
  </configuration>
</plugin>

<plugin>
  <artifactId>maven-compiler-plugin</artifactId>
  <configuration>
    <source>1.6</source>
    <target>1.6</target>
  </configuration>
</plugin>

Then you're ready to create the project structure:

cd my-app
mvn initialize

Defining Plugin Dependencies

All Grails plugins are published to a standard Maven repository located at . When using the Maven plugin for Grails you must ensure that this repository is declared in your list of remote repositories:

<repository>
    <id>grails-plugins</id>
    <name>grails-plugins</name>
    <url>http://repo.grails.org/grails/plugins</url>
</repository>

With this done you can declare plugin dependencies within your pom.xml file:

<dependency>
    <groupId>org.grails.plugins</groupId>
    <artifactId>database-migration</artifactId>
    <version>1.1</version>
    <scope>runtime</scope>
    <type>zip</type>
</dependency>

Note that the type element must be set to zip.

Forked Grails Execution

By default the Maven plugin will run Grails commands in-process, meaning that the Grails process occupies the same JVM as the Maven process. This can put strain on the Maven process for particularly large applications.

In this case it is recommended to use forked execution. Forked execution can be configured in the configuration element of the plugin:

<plugin>
    <groupId>org.grails</groupId>
    <artifactId>grails-maven-plugin</artifactId>
    <version>${grails.version}</version>
    <configuration>
        <!-- Whether for Fork a JVM to run Grails commands -->
        <fork>true</fork>
    </configuration>
    <extensions>true</extensions>
</plugin>

With this configuration in place a separate JVM will be forked when running Grails commands. If you wish to debug the JVM that is forked you can add the forkDebug element:

<!-- Whether for Fork a JVM to run Grails commands -->
    <fork>true</fork>
    <forkDebug>true</forkDebug>

If you need to customize the memory of the forked process the following elements are available:

• forkMaxMemory - The maximum amount of heap (default 1024)
• forkMinMemory - The minimum amount of heap (default 512)
• forkPermGen - The amount of permgen (default 256)

Multi Module Maven Builds

The Maven plugin can be used to power multi-module Grails builds. The easiest way to set this is up is with the create-multi-project-build command:

$ grails create-app myapp
$ grails create-plugin plugin1
$ grails create-plugin plugin2
$ grails create-multi-project-build org.mycompany:parent:1.0

Running mvn install will build all projects together. To enable the 'grails' command to read the POMs you can modify BuildConfig.groovy to use the POM and resolve dependencies from your Maven local cache:

grails.project.dependency.resolution = {
    …
    pom true
    repositories {
        …
        mavenLocal()    
    }
}

By reading the pom.xml file you can do an initial mvn install from the parent project to build all plugins and install them into your local maven cache and then cd into your project and use the regular grails run-app command to run your application. All previously built plugins will be resolved from the local Maven cache.

Adding Grails commands to phases

The standard POM created for you by Grails already attaches the appropriate core Grails commands to their corresponding build phases, so "compile" goes in the "compile" phase and "war" goes in the "package" phase. That doesn't help though when you want to attach a plugin's command to a particular phase. The classic example is functional tests. How do you make sure that your functional tests (using which ever plugin you have decided on) are run during the "integration-test" phase?

Fear not: all things are possible. In this case, you can associate the command to a phase using an extra "execution" block:

<plugin>
    <groupId>org.grails</groupId>
    <artifactId>grails-maven-plugin</artifactId>
    <version>2.1.0.RC2</version>
    <extensions>true</extensions>
    <executions>
        <execution>
            <goals>
            …
            </goals>
        </execution>
        <!-- Add the "functional-tests" command to the "integration-test" phase -->
        <execution>
            <id>functional-tests</id>
            <phase>integration-test</phase>
            <goals>
                <goal>exec</goal>
            </goals>
            <configuration>
                <command>functional-tests</command>
            </configuration>
        </execution>
    </executions>
</plugin>

This also demonstrates the grails:exec goal, which can be used to run any Grails command. Simply pass the name of the command as the command system property, and optionally specify the arguments with the args property:

mvn grails:exec -Dcommand=create-webtest -Dargs=Book

Debugging a Grails Maven Project

Maven can be launched in debug mode using the "mvnDebug" command. To launch your Grails application in debug, simply run:

mvnDebug grails:run-app

The process will be suspended on startup and listening for a debugger on port 8000.

If you need more control of the debugger, this can be specified using the MAVEN_OPTS environment variable, and launch Maven with the default "mvn" command:

MAVEN_OPTS="-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005"
mvn grails:run-app

Raising issues

If you come across any problems with the Maven integration, please raise a JIRA issue.

5.8 Grails Wrapper

Generating The Wrapper

The wrapper command can be used to generate the wrapper shell scripts and supporting jar files. Execute the wrapper command at the top of an existing Grails project.

grails wrapper

In order to do this of course Grails must be installed and configured. This is only a requirement for bootstrapping the wrapper. Once the wrapper is generated there is no need to have a Grails installation configured in order to use the wrapper.

See the wrapper command documentation for details about command line arguments.

By default the wrapper command will generate a grailsw shell script and grailsw.bat batch file at the top of the project. In addition to those, a wrapper/ directory (the name of the directory is configurable via command line options) is generated which contains some support files which are necessary to run the wrapper. All of these files should be checked into the source code control system along with the rest of the project. This allows developers to check the project out of source code control and immediately start using the wrapper to execute Grails commands without having to install and configure Grails.

Using The Wrapper

The wrapper script accepts all of the same arguments as the normal grails command.

./grailsw create-domain-class com.demo.Person
./grailsw run-app
./grailsw test-app unit:
etc...

6 Object Relational Mapping (GORM)

GORM is Grails' object relational mapping (ORM) implementation. Under the hood it uses Hibernate 3 (a very popular and flexible open source ORM solution) and thanks to the dynamic nature of Groovy with its static and dynamic typing, along with the convention of Grails, there is far less configuration involved in creating Grails domain classes.

You can also write Grails domain classes in Java. See the section on Hibernate Integration for how to write domain classes in Java but still use dynamic persistent methods. Below is a preview of GORM in action:

def book = Book.findByTitle("Groovy in Action")
book
  .addToAuthors(name:"Dierk Koenig")
  .addToAuthors(name:"Guillaume LaForge")
  .save()

6.1 Quick Start Guide

grails create-domain-class helloworld.Person

If no package is specified with the create-domain-class script, Grails automatically uses the application name as the package name.

This will create a class at the location grails-app/domain/helloworld/Person.groovy such as the one below:

package helloworld
class Person {
}

If you have the dbCreate property set to "update", "create" or "create-drop" on your DataSource, Grails will automatically generate/modify the database tables for you.

You can customize the class by adding properties:

class Person {
    String name
    Integer age
    Date lastVisit
}

Once you have a domain class try and manipulate it with the shell or console by typing:

grails console

This loads an interactive GUI where you can run Groovy commands with access to the Spring ApplicationContext, GORM, etc.

6.1.1 Basic CRUD

Create

To create a domain class use Map constructor to set its properties and call save:

def p = new Person(name: "Fred", age: 40, lastVisit: new Date())
p.save()

The save method will persist your class to the database using the underlying Hibernate ORM layer.

Read

Grails transparently adds an implicit id property to your domain class which you can use for retrieval:

def p = Person.get(1)
assert 1 == p.id

This uses the get method that expects a database identifier to read the Person object back from the database. You can also load an object in a read-only state by using the read method:

def p = Person.read(1)

In this case the underlying Hibernate engine will not do any dirty checking and the object will not be persisted. Note that if you explicitly call the save method then the object is placed back into a read-write state.

In addition, you can also load a proxy for an instance by using the load method:

def p = Person.load(1)

This incurs no database access until a method other than getId() is called. Hibernate then initializes the proxied instance, or throws an exception if no record is found for the specified id.

Update

To update an instance, change some properties and then call save again:

def p = Person.get(1)
p.name = "Bob"
p.save()

Delete

To delete an instance use the delete method:

def p = Person.get(1)
p.delete()

6.2 Domain Modelling in GORM

These are modeled in GORM as Groovy classes, so a Book class may have a title, a release date, an ISBN number and so on. The next few sections show how to model the domain in GORM.

To create a domain class you run the create-domain-class command as follows:

grails create-domain-class org.bookstore.Book

The result will be a class at grails-app/domain/org/bookstore/Book.groovy:

package org.bookstore
class Book {
}

This class will map automatically to a table in the database called book (the same name as the class). This behaviour is customizable through the ORM Domain Specific Language

Now that you have a domain class you can define its properties as Java types. For example:

package org.bookstore
class Book {
    String title
    Date releaseDate
    String ISBN
}

Each property is mapped to a column in the database, where the convention for column names is all lower case separated by underscores. For example releaseDate maps onto a column release_date. The SQL types are auto-detected from the Java types, but can be customized with Constraints or the ORM DSL.

6.2.1 Association in GORM

6.2.1.1 Many-to-one and one-to-one

Example A

class Face {
    Nose nose
}

class Nose {
}

In this case we have a unidirectional many-to-one relationship from Face to Nose. To make this relationship bidirectional define the other side as follows (and see the section on controlling the ends of the association just below):

Example B

class Face {
    Nose nose
}

class Nose {
    static belongsTo = [face:Face]
}

In this case we use the belongsTo setting to say that Nose "belongs to" Face. The result of this is that we can create a Face, attach a Nose instance to it and when we save or delete the Face instance, GORM will save or delete the Nose. In other words, saves and deletes will cascade from Face to the associated Nose:

new Face(nose:new Nose()).save()

The example above will save both face and nose. Note that the inverse is not true and will result in an error due to a transient Face:

new Nose(face:new Face()).save() // will cause an error

Now if we delete the Face instance, the Nose will go too:

def f = Face.get(1)
f.delete() // both Face and Nose deleted

To make the relationship a true one-to-one, use the hasOne property on the owning side, e.g. Face:

Example C

class Face {
    static hasOne = [nose:Nose]
}

class Nose {
    Face face
}

Note that using this property puts the foreign key on the inverse table to the previous example, so in this case the foreign key column is stored in the nose table inside a column called face_id. Also, hasOne only works with bidirectional relationships.

Finally, it's a good idea to add a unique constraint on one side of the one-to-one relationship:

class Face {
    static hasOne = [nose:Nose]
    static constraints = {
        nose unique: true
    }
}

class Nose {
    Face face
}

Controlling the ends of the association

Occasionally you may find yourself with domain classes that have multiple properties of the same type. They may even be self-referential, i.e. the association property has the same type as the domain class it's in. Such situations can cause problems because Grails may guess incorrectly the type of the association. Consider this simple class:

class Person {
    String name
    Person parent
    static belongsTo = [ supervisor: Person ]
    static constraints = { supervisor nullable: true }
}

As far as Grails is concerned, the parent and supervisor properties are two directions of the same association. So when you set the parent property on a Person instance, Grails will automatically set the supervisor property on the other Person instance. This may be what you want, but if you look at the class, what we in fact have are two unidirectional relationships.

To guide Grails to the correct mapping, you can tell it that a particular association is unidirectional through the mappedBy property:

class Person {
    String name
    Person parent
    static belongsTo = [ supervisor: Person ]
    static mappedBy = [ supervisor: "none", parent: "none" ]
    static constraints = { supervisor nullable: true }
}

You can also replace "none" with any property name of the target class. And of course this works for normal domain classes too, not just self-referential ones. Nor is the mappedBy property limited to many-to-one and one-to-one associations: it also works for one-to-many and many-to-many associations as you'll see in the next section.

If you have a property called "none" on your domain class, this approach won't work currently! The "none" property will be treated as the reverse direction of the association (or the "back reference"). Fortunately, "none" is not a common domain class property name.

6.2.1.2 One-to-many

class Author {
    static hasMany = [books: Book]
    String name
}

class Book {
    String title
}

In this case we have a unidirectional one-to-many. Grails will, by default, map this kind of relationship with a join table.

The ORM DSL allows mapping unidirectional relationships using a foreign key association instead

Grails will automatically inject a property of type java.util.Set into the domain class based on the hasMany setting. This can be used to iterate over the collection:

def a = Author.get(1)
for (book in a.books) {
    println book.title
}

The default fetch strategy used by Grails is "lazy", which means that the collection will be lazily initialized on first access. This can lead to the n+1 problem if you are not careful.

If you need "eager" fetching you can use the ORM DSL or specify eager fetching as part of a query

The default cascading behaviour is to cascade saves and updates, but not deletes unless a belongsTo is also specified:

class Author {
    static hasMany = [books: Book]
    String name
}

class Book {
    static belongsTo = [author: Author]
    String title
}

If you have two properties of the same type on the many side of a one-to-many you have to use mappedBy to specify which the collection is mapped:

class Airport {
    static hasMany = [flights: Flight]
    static mappedBy = [flights: "departureAirport"]
}

class Flight {
    Airport departureAirport
    Airport destinationAirport
}

This is also true if you have multiple collections that map to different properties on the many side:

class Airport {
    static hasMany = [outboundFlights: Flight, inboundFlights: Flight]
    static mappedBy = [outboundFlights: "departureAirport",
                       inboundFlights: "destinationAirport"]
}

class Flight {
    Airport departureAirport
    Airport destinationAirport
}

6.2.1.3 Many-to-many

class Book {
    static belongsTo = Author
    static hasMany = [authors:Author]
    String title
}

class Author {
    static hasMany = [books:Book]
    String name
}

Grails maps a many-to-many using a join table at the database level. The owning side of the relationship, in this case Author, takes responsibility for persisting the relationship and is the only side that can cascade saves across.

For example this will work and cascade saves:

new Author(name:"Stephen King")
        .addToBooks(new Book(title:"The Stand"))
        .addToBooks(new Book(title:"The Shining"))
        .save()

However this will only save the Book and not the authors!

new Book(name:"Groovy in Action")
        .addToAuthors(new Author(name:"Dierk Koenig"))
        .addToAuthors(new Author(name:"Guillaume Laforge"))
        .save()

This is the expected behaviour as, just like Hibernate, only one side of a many-to-many can take responsibility for managing the relationship.

Grails' Scaffolding feature does not currently support many-to-many relationship and hence you must write the code to manage the relationship yourself

6.2.1.4 Basic Collection Types

class Person {
    static hasMany = [nicknames: String]
}

GORM will map an association like the above using a join table. You can alter various aspects of how the join table is mapped using the joinTable argument:

class Person {
    static hasMany = [nicknames: String]
    static mapping = {
       hasMany joinTable: [name: 'bunch_o_nicknames',
                           key: 'person_id',
                           column: 'nickname',
                           type: "text"]
    }
}

The example above will map to a table that looks like the following:

bunch_o_nicknames Table

---------------------------------------------
| person_id         |     nickname          |
---------------------------------------------
|   1               |      Fred             |
---------------------------------------------

6.2.2 Composition in GORM

class Person {
    Address homeAddress
    Address workAddress
    static embedded = ['homeAddress', 'workAddress']
}
class Address {
    String number
    String code
}

The resulting mapping would looking like this:

If you define the Address class in a separate Groovy file in the grails-app/domain directory you will also get an address table. If you don't want this to happen use Groovy's ability to define multiple classes per file and include the Address class below the Person class in the grails-app/domain/Person.groovy file

6.2.3 Inheritance in GORM

class Content {
     String author
}

class BlogEntry extends Content {
    URL url
}

class Book extends Content {
    String ISBN
}

class PodCast extends Content {
    byte[] audioStream
}

In the above example we have a parent Content class and then various child classes with more specific behaviour.

Considerations

At the database level Grails by default uses table-per-hierarchy mapping with a discriminator column called class so the parent class (Content) and its subclasses (BlogEntry, Book etc.), share the same table.

Table-per-hierarchy mapping has a down side in that you cannot have non-nullable properties with inheritance mapping. An alternative is to use table-per-subclass which can be enabled with the ORM DSL

However, excessive use of inheritance and table-per-subclass can result in poor query performance due to the use of outer join queries. In general our advice is if you're going to use inheritance, don't abuse it and don't make your inheritance hierarchy too deep.

Polymorphic Queries

The upshot of inheritance is that you get the ability to polymorphically query. For example using the list method on the Content super class will return all subclasses of Content:

def content = Content.list() // list all blog entries, books and podcasts
content = Content.findAllByAuthor('Joe Bloggs') // find all by author
def podCasts = PodCast.list() // list only podcasts

6.2.4 Sets, Lists and Maps

Sets of Objects

By default when you define a relationship with GORM it is a java.util.Set which is an unordered collection that cannot contain duplicates. In other words when you have:

class Author {
    static hasMany = [books: Book]
}

The books property that GORM injects is a java.util.Set. Sets guarantee uniquenes but not order, which may not be what you want. To have custom ordering you configure the Set as a SortedSet:

class Author {
    SortedSet books
    static hasMany = [books: Book]
}

In this case a java.util.SortedSet implementation is used which means you must implement java.lang.Comparable in your Book class:

class Book implements Comparable {
    String title
    Date releaseDate = new Date()
    int compareTo(obj) {
        releaseDate.compareTo(obj.releaseDate)
    }
}

The result of the above class is that the Book instances in the books collection of the Author class will be ordered by their release date.

Lists of Objects

To keep objects in the order which they were added and to be able to reference them by index like an array you can define your collection type as a List:

class Author {
    List books
    static hasMany = [books: Book]
}

In this case when you add new elements to the books collection the order is retained in a sequential list indexed from 0 so you can do:

author.books[0] // get the first book

The way this works at the database level is Hibernate creates a books_idx column where it saves the index of the elements in the collection to retain this order at the database level.

When using a List, elements must be added to the collection before being saved, otherwise Hibernate will throw an exception (org.hibernate.HibernateException: null index column for collection):

// This won't work!
def book = new Book(title: 'The Shining')
book.save()
author.addToBooks(book)
// Do it this way instead.
def book = new Book(title: 'Misery')
author.addToBooks(book)
author.save()

Bags of Objects

If ordering and uniqueness aren't a concern (or if you manage these explicitly) then you can use the Hibernate Bag type to represent mapped collections.

The only change required for this is to define the collection type as a Collection:

class Author {
   Collection books
   static hasMany = [books: Book]
}

Since uniqueness and order aren't managed by Hibernate, adding to or removing from collections mapped as a Bag don't trigger a load of all existing instances from the database, so this approach will perform better and require less memory than using a Set or a List.

Maps of Objects

If you want a simple map of string/value pairs GORM can map this with the following:

class Author {
    Map books // map of ISBN:book names
}
def a = new Author()
a.books = ["1590597583":"Grails Book"]
a.save()

If you want a Map of objects then you can do this:

class Book {
    Map authors
    static hasMany = [authors: Author]
}
def a = new Author(name:"Stephen King")
def book = new Book()
book.authors = [stephen:a]
book.save()

The static hasMany property defines the type of the elements within the Map. The keys for the map must be strings.

A Note on Collection Types and Performance

The Java Set type doesn't allow duplicates. To ensure uniqueness when adding an entry to a Set association Hibernate has to load the entire associations from the database. If you have a large numbers of entries in the association this can be costly in terms of performance.

The same behavior is required for List types, since Hibernate needs to load the entire association to maintain order. Therefore it is recommended that if you anticipate a large numbers of records in the association that you make the association bidirectional so that the link can be created on the inverse side. For example consider the following code:

def book = new Book(title:"New Grails Book")
def author = Author.get(1)
book.author = author
book.save()

In this example the association link is being created by the child (Book) and hence it is not necessary to manipulate the collection directly resulting in fewer queries and more efficient code. Given an Author with a large number of associated Book instances if you were to write code like the following you would see an impact on performance:

def book = new Book(title:"New Grails Book")
def author = Author.get(1)
author.addToBooks(book)
author.save()

You could also model the collection as a Hibernate Bag as described above.

6.3 Persistence Basics

Grails automatically binds a Hibernate session to the currently executing request. This lets you use the save and delete methods as well as other GORM methods transparently.

Transactional Write-Behind

A useful feature of Hibernate over direct JDBC calls and even other frameworks is that when you call save or delete it does not necessarily perform any SQL operations at that point. Hibernate batches up SQL statements and executes them as late as possible, often at the end of the request when flushing and closing the session. This is typically done for you automatically by Grails, which manages your Hibernate session.

Hibernate caches database updates where possible, only actually pushing the changes when it knows that a flush is required, or when a flush is triggered programmatically. One common case where Hibernate will flush cached updates is when performing queries since the cached information might be included in the query results. But as long as you're doing non-conflicting saves, updates, and deletes, they'll be batched until the session is flushed. This can be a significant performance boost for applications that do a lot of database writes.

Note that flushing is not the same as committing a transaction. If your actions are performed in the context of a transaction, flushing will execute SQL updates but the database will save the changes in its transaction queue and only finalize the updates when the transaction commits.

6.3.1 Saving and Updating

def p = Person.get(1)
p.save()

This save will be not be pushed to the database immediately - it will be pushed when the next flush occurs. But there are occasions when you want to control when those statements are executed or, in Hibernate terminology, when the session is "flushed". To do so you can use the flush argument to the save method:

def p = Person.get(1)
p.save(flush: true)

Note that in this case all pending SQL statements including previous saves, deletes, etc. will be synchronized with the database. This also lets you catch any exceptions, which is typically useful in highly concurrent scenarios involving optimistic locking:

def p = Person.get(1)
try {
    p.save(flush: true)
}
catch (org.springframework.dao.DataIntegrityViolationException e) {
    // deal with exception
}

Another thing to bear in mind is that Grails validates a domain instance every time you save it. If that validation fails the domain instance will not be persisted to the database. By default, save() will simply return null in this case, but if you would prefer it to throw an exception you can use the failOnError argument:

def p = Person.get(1)
try {
    p.save(failOnError: true)
}
catch (ValidationException e) {
    // deal with exception
}

You can even change the default behaviour with a setting in Config.groovy, as described in the section on configuration. Just remember that when you are saving domain instances that have been bound with data provided by the user, the likelihood of validation exceptions is quite high and you won't want those exceptions propagating to the end user.

You can find out more about the subtleties of saving data in this article - a must read!

6.3.2 Deleting Objects

def p = Person.get(1)
p.delete()

As with saves, Hibernate will use transactional write-behind to perform the delete; to perform the delete in-place you can use the flush argument:

def p = Person.get(1)
p.delete(flush: true)

Using the flush argument lets you catch any errors that occur during a delete. A common error that may occur is if you violate a database constraint, although this is normally down to a programming or schema error. The following example shows how to catch a DataIntegrityViolationException that is thrown when you violate the database constraints:

def p = Person.get(1)
try {
    p.delete(flush: true)
}
catch (org.springframework.dao.DataIntegrityViolationException e) {
    flash.message = "Could not delete person ${p.name}"
    redirect(action: "show", id: p.id)
}

Note that Grails does not supply a deleteAll method as deleting data is discouraged and can often be avoided through boolean flags/logic.

If you really need to batch delete data you can use the executeUpdate method to do batch DML statements:

Customer.executeUpdate("delete Customer c where c.name = :oldName",
                       [oldName: "Fred"])

6.3.3 Understanding Cascading Updates and Deletes

Whether it is a one-to-one, one-to-many or many-to-many, defining belongsTo will result in updates cascading from the owning class to its dependant (the other side of the relationship), and for many-/one-to-one and one-to-many relationships deletes will also cascade.

If you do not define belongsTo then no cascades will happen and you will have to manually save each object (except in the case of the one-to-many, in which case saves will cascade automatically if a new instance is in a hasMany collection).

Here is an example:

class Airport {
    String name
    static hasMany = [flights: Flight]
}

class Flight {
    String number
    static belongsTo = [airport: Airport]
}

If I now create an Airport and add some Flights to it I can save the Airport and have the updates cascaded down to each flight, hence saving the whole object graph:

new Airport(name: "Gatwick")
        .addToFlights(new Flight(number: "BA3430"))
        .addToFlights(new Flight(number: "EZ0938"))
        .save()

Conversely if I later delete the Airport all Flights associated with it will also be deleted:

def airport = Airport.findByName("Gatwick")
airport.delete()

However, if I were to remove belongsTo then the above cascading deletion code would not work. To understand this better take a look at the summaries below that describe the default behaviour of GORM with regards to specific associations. Also read part 2 of the GORM Gotchas series of articles to get a deeper understanding of relationships and cascading.

Bidirectional one-to-many with belongsTo

class A { static hasMany = [bees: B] }

class B { static belongsTo = [a: A] }

In the case of a bidirectional one-to-many where the many side defines a belongsTo then the cascade strategy is set to "ALL" for the one side and "NONE" for the many side.

Unidirectional one-to-many

class A { static hasMany = [bees: B] }

class B {  }

In the case of a unidirectional one-to-many where the many side defines no belongsTo then the cascade strategy is set to "SAVE-UPDATE".

Bidirectional one-to-many, no belongsTo

class A { static hasMany = [bees: B] }

class B { A a }

In the case of a bidirectional one-to-many where the many side does not define a belongsTo then the cascade strategy is set to "SAVE-UPDATE" for the one side and "NONE" for the many side.

Unidirectional one-to-one with belongsTo

class A {  }

class B { static belongsTo = [a: A] }

In the case of a unidirectional one-to-one association that defines a belongsTo then the cascade strategy is set to "ALL" for the owning side of the relationship (A->B) and "NONE" from the side that defines the belongsTo (B->A)

Note that if you need further control over cascading behaviour, you can use the ORM DSL.

6.3.4 Eager and Lazy Fetching

class Airport {
    String name
    static hasMany = [flights: Flight]
}

class Flight {
    String number
    Location destination
    static belongsTo = [airport: Airport]
}

class Location {
    String city
    String country
}

Given the above domain classes and the following code:

def airport = Airport.findByName("Gatwick")
for (flight in airport.flights) {
    println flight.destination.city
}

GORM will execute a single SQL query to fetch the Airport instance, another to get its flights, and then 1 extra query for each iteration over the flights association to get the current flight's destination. In other words you get N+1 queries (if you exclude the original one to get the airport).

Configuring Eager Fetching

An alternative approach that avoids the N+1 queries is to use eager fetching, which can be specified as follows:

class Airport {
    String name
    static hasMany = [flights: Flight]
    static mapping = {
        flights lazy: false
    }
}

In this case the flights association will be loaded at the same time as its Airport instance, although a second query will be executed to fetch the collection. You can also use fetch: 'join' instead of lazy: false , in which case GORM will only execute a single query to get the airports and their flights. This works well for single-ended associations, but you need to be careful with one-to-manys. Queries will work as you'd expect right up to the moment you add a limit to the number of results you want. At that point, you will likely end up with fewer results than you were expecting. The reason for this is quite technical but ultimately the problem arises from GORM using a left outer join.

So, the recommendation is currently to use fetch: 'join' for single-ended associations and lazy: false for one-to-manys.

Be careful how and where you use eager loading because you could load your entire database into memory with too many eager associations. You can find more information on the mapping options in the section on the ORM DSL.

Using Batch Fetching

Although eager fetching is appropriate for some cases, it is not always desirable. If you made everything eager you could quite possibly load your entire database into memory resulting in performance and memory problems. An alternative to eager fetching is to use batch fetching. You can configure Hibernate to lazily fetch results in "batches". For example:

class Airport {
    String name
    static hasMany = [flights: Flight]
    static mapping = {
        flights batchSize: 10
    }
}

In this case, due to the batchSize argument, when you iterate over the flights association, Hibernate will fetch results in batches of 10. For example if you had an Airport that had 30 flights, if you didn't configure batch fetching you would get 1 query to fetch the Airport and then 30 queries to fetch each flight. With batch fetching you get 1 query to fetch the Airport and 3 queries to fetch each Flight in batches of 10. In other words, batch fetching is an optimization of the lazy fetching strategy. Batch fetching can also be configured at the class level as follows:

class Flight {
    …
    static mapping = {
        batchSize 10
    }
}

Check out part 3 of the GORM Gotchas series for more in-depth coverage of this tricky topic.

6.3.5 Pessimistic and Optimistic Locking

Optimistic Locking

By default GORM classes are configured for optimistic locking. Optimistic locking is a feature of Hibernate which involves storing a version value in a special version column in the database that is incremented after each update.

The version column gets read into a version property that contains the current versioned state of persistent instance which you can access:

def airport = Airport.get(10)
println airport.version

When you perform updates Hibernate will automatically check the version property against the version column in the database and if they differ will throw a StaleObjectException. This will roll back the transaction if one is active.

This is useful as it allows a certain level of atomicity without resorting to pessimistic locking that has an inherit performance penalty. The downside is that you have to deal with this exception if you have highly concurrent writes. This requires flushing the session:

def airport = Airport.get(10)
try {
    airport.name = "Heathrow"
    airport.save(flush: true)
}
catch (org.springframework.dao.OptimisticLockingFailureException e) {
    // deal with exception
}

The way you deal with the exception depends on the application. You could attempt a programmatic merge of the data or go back to the user and ask them to resolve the conflict.

Alternatively, if it becomes a problem you can resort to pessimistic locking.

The version will only be updated after flushing the session.

Pessimistic Locking

Pessimistic locking is equivalent to doing a SQL "SELECT * FOR UPDATE" statement and locking a row in the database. This has the implication that other read operations will be blocking until the lock is released.

In Grails pessimistic locking is performed on an existing instance with the lock method:

def airport = Airport.get(10)
airport.lock() // lock for update
airport.name = "Heathrow"
airport.save()

Grails will automatically deal with releasing the lock for you once the transaction has been committed. However, in the above case what we are doing is "upgrading" from a regular SELECT to a SELECT..FOR UPDATE and another thread could still have updated the record in between the call to get() and the call to lock().

To get around this problem you can use the static lock method that takes an id just like get:

def airport = Airport.lock(10) // lock for update
airport.name = "Heathrow"
airport.save()

In this case only SELECT..FOR UPDATE is issued.

As well as the lock method you can also obtain a pessimistic locking using queries. For example using a dynamic finder:

def airport = Airport.findByName("Heathrow", [lock: true])

Or using criteria:

def airport = Airport.createCriteria().get {
    eq('name', 'Heathrow')
    lock true
}

6.3.6 Modification Checking

isDirty

You can use the isDirty method to check if any field has been modified:

def airport = Airport.get(10)
assert !airport.isDirty()
airport.properties = params
if (airport.isDirty()) {
   // do something based on changed state
}

isDirty() does not currently check collection associations, but it does check all other persistent properties and associations.

You can also check if individual fields have been modified:

def airport = Airport.get(10)
assert !airport.isDirty()
airport.properties = params
if (airport.isDirty('name')) {
   // do something based on changed name
}

getDirtyPropertyNames

You can use the getDirtyPropertyNames method to retrieve the names of modified fields; this may be empty but will not be null:

def airport = Airport.get(10)
assert !airport.isDirty()
airport.properties = params
def modifiedFieldNames = airport.getDirtyPropertyNames()
for (fieldName in modifiedFieldNames) {
   // do something based on changed value
}

getPersistentValue

You can use the getPersistentValue method to retrieve the value of a modified field:

def airport = Airport.get(10)
assert !airport.isDirty()
airport.properties = params
def modifiedFieldNames = airport.getDirtyPropertyNames()
for (fieldName in modifiedFieldNames) {
    def currentValue = airport."$fieldName"
    def originalValue = airport.getPersistentValue(fieldName)
    if (currentValue != originalValue) {
        // do something based on changed value
    }
}

6.4 Querying with GORM

• Dynamic Finders
• Where Queries
• Criteria Queries
• Hibernate Query Language (HQL)

In addition, Groovy's ability to manipulate collections with GPath and methods like sort, findAll and so on combined with GORM results in a powerful combination.

However, let's start with the basics.

Listing instances

Use the list method to obtain all instances of a given class:

def books = Book.list()

The list method supports arguments to perform pagination:

def books = Book.list(offset:10, max:20)

as well as sorting:

def books = Book.list(sort:"title", order:"asc")

Here, the sort argument is the name of the domain class property that you wish to sort on, and the order argument is either asc for ascending or desc for descending.

Retrieval by Database Identifier

The second basic form of retrieval is by database identifier using the get method:

def book = Book.get(23)

You can also obtain a list of instances for a set of identifiers using getAll:

def books = Book.getAll(23, 93, 81)

6.4.1 Dynamic Finders

Instead, a method is auto-magically generated using code synthesis at runtime, based on the properties of a given class. Take for example the Book class:

class Book {
    String title
    Date releaseDate
    Author author
}

class Author {
    String name
}

The Book class has properties such as title, releaseDate and author. These can be used by the findBy and findAllBy methods in the form of "method expressions":

def book = Book.findByTitle("The Stand")
book = Book.findByTitleLike("Harry Pot%")
book = Book.findByReleaseDateBetween(firstDate, secondDate)
book = Book.findByReleaseDateGreaterThan(someDate)
book = Book.findByTitleLikeOrReleaseDateLessThan("%Something%", someDate)

Method Expressions

A method expression in GORM is made up of the prefix such as findBy followed by an expression that combines one or more properties. The basic form is:

Book.findBy([Property][Comparator][Boolean Operator])?[Property][Comparator]

The tokens marked with a '?' are optional. Each comparator changes the nature of the query. For example:

def book = Book.findByTitle("The Stand")
book =  Book.findByTitleLike("Harry Pot%")

In the above example the first query is equivalent to equality whilst the latter, due to the Like comparator, is equivalent to a SQL like expression.

The possible comparators include:

• InList - In the list of given values
• LessThan - less than a given value
• LessThanEquals - less than or equal a give value
• GreaterThan - greater than a given value
• GreaterThanEquals - greater than or equal a given value
• Like - Equivalent to a SQL like expression
• Ilike - Similar to a Like, except case insensitive
• NotEqual - Negates equality
• Between - Between two values (requires two arguments)
• IsNotNull - Not a null value (doesn't take an argument)
• IsNull - Is a null value (doesn't take an argument)

Notice that the last three require different numbers of method arguments compared to the rest, as demonstrated in the following example:

def now = new Date()
def lastWeek = now - 7
def book = Book.findByReleaseDateBetween(lastWeek, now)
books = Book.findAllByReleaseDateIsNull()
books = Book.findAllByReleaseDateIsNotNull()

Boolean logic (AND/OR)

Method expressions can also use a boolean operator to combine two or more criteria:

def books = Book.findAllByTitleLikeAndReleaseDateGreaterThan(
                      "%Java%", new Date() - 30)

In this case we're using And in the middle of the query to make sure both conditions are satisfied, but you could equally use Or:

def books = Book.findAllByTitleLikeOrReleaseDateGreaterThan(
                      "%Java%", new Date() - 30)

You can combine as many criteria as you like, but they must all be combined with And or all Or. If you need to combine And and Or or if the number of criteria creates a very long method name, just convert the query to a Criteria or HQL query.

Querying Associations

Associations can also be used within queries:

def author = Author.findByName("Stephen King")
def books = author ? Book.findAllByAuthor(author) : []

In this case if the Author instance is not null we use it in a query to obtain all the Book instances for the given Author.

Pagination and Sorting

The same pagination and sorting parameters available on the list method can also be used with dynamic finders by supplying a map as the final parameter:

def books = Book.findAllByTitleLike("Harry Pot%",
               [max: 3, offset: 2, sort: "title", order: "desc"])

6.4.2 Where Queries

Basic Querying

The where method accepts a closure that looks very similar to Groovy's regular collection methods. The closure should define the logical criteria in regular Groovy syntax, for example:

def query = Person.where {
   firstName == "Bart"
}
Person bart = query.find()

The returned object is a DetachedCriteria instance, which means it is not associated with any particular database connection or session. This means you can use the where method to define common queries at the class level:

class Person {
    static simpsons = where {
         lastName == "Simpson"
    }
    …
}
…
Person.simpsons.each {
    println it.firstname
}

Query execution is lazy and only happens upon usage of the DetachedCriteria instance. If you want to execute a where-style query immediately there are variations of the findAll and find methods to accomplish this:

def results = Person.findAll {
     lastName == "Simpson"
}
def results = Person.findAll(sort:"firstName") {
     lastName == "Simpson"
}
Person p = Person.find { firstName == "Bart" }

Each Groovy operator maps onto a regular criteria method. The following table provides a map of Groovy operators to methods:

It is possible use regular Groovy comparison operators and logic to formulate complex queries:

def query = Person.where {
    (lastName != "Simpson" && firstName != "Fred") || (firstName == "Bart" && age > 9)
}
def results = query.list(sort:"firstName")

The Groovy regex matching operators map onto like and ilike queries unless the expression on the right hand side is a Pattern object, in which case they map onto an rlike query:

def query = Person.where {
     firstName ==~ ~/B.+/
}

Note that rlike queries are only supported if the underlying database supports regular expressions

A between criteria query can be done by combining the in keyword with a range:

def query = Person.where {
     age in 18..65
}

Finally, you can do isNull and isNotNull style queries by using null with regular comparison operators:

def query = Person.where {
     middleName == null
}

Query Composition

Since the return value of the where method is a DetachedCriteria instance you can compose new queries from the original query:

def query = Person.where {
     lastName == "Simpson"
}
def bartQuery = query.where {
     firstName == "Bart"
}
Person p = bartQuery.find()

Note that you cannot pass a closure defined as a variable into the where method unless it has been explicitly cast to a DetachedCriteria instance. In other words the following will produce an error:

def callable = {
    lastName == "Simpson"
}
def query = Person.where(callable)

The above must be written as follows:

import grails.gorm.DetachedCriteria
def callable = {
    lastName == "Simpson"
} as DetachedCriteria<Person>
def query = Person.where(callable)

As you can see the closure definition is cast (using the Groovy as keyword) to a DetachedCriteria instance targeted at the Person class.

Conjunction, Disjunction and Negation

As mentioned previously you can combine regular Groovy logical operators (|| and &&) to form conjunctions and disjunctions:

def query = Person.where {
    (lastName != "Simpson" && firstName != "Fred") || (firstName == "Bart" && age > 9)
}

You can also negate a logical comparison using !:

def query = Person.where {
    firstName == "Fred" && !(lastName == 'Simpson')
}

Property Comparison Queries

If you use a property name on both the left hand and right side of a comparison expression then the appropriate property comparison criteria is automatically used:

def query = Person.where {
   firstName == lastName
}

The following table described how each comparison operator maps onto each criteria property comparison method:

Querying Associations

Associations can be queried by using the dot operator to specify the property name of the association to be queried:

def query = Pet.where {
    owner.firstName == "Joe" || owner.firstName == "Fred"
}

You can group multiple criterion inside a closure method call where the name of the method matches the association name:

def query = Person.where {
    pets { name == "Jack" || name == "Joe" }
}

This technique can be combined with other top-level criteria:

def query = Person.where {
     pets { name == "Jack" } || firstName == "Ed"
}

For collection associations it is possible to apply queries to the size of the collection:

def query = Person.where {
       pets.size() == 2
}

The following table shows which operator maps onto which criteria method for each size() comparison:

Subqueries

It is possible to execute subqueries within where queries. For example to find all the people older than the average age the following query can be used:

final query = Person.where {
  age > avg(age)
}

The following table lists the possible subqueries:

You can apply additional criteria to any subquery by using the of method and passing in a closure containing the criteria:

def query = Person.where {
  age > avg(age).of { lastName == "Simpson" } && firstName == "Homer"
}

Since the property subquery returns multiple results, the criterion used compares all results. For example the following query will find all people younger than people with the surname "Simpson":

Person.where {
    age < property(age).of { lastName == "Simpson" }
}

Other Functions

There are several functions available to you within the context of a query. These are summarized in the table below:

Currently functions can only be applied to properties or associations of domain classes. You cannot, for example, use a function on a result of a subquery.

For example the following query can be used to find all pet's born in 2011:

def query = Pet.where {
    year(birthDate) == 2011
}

You can also apply functions to associations:

def query = Person.where {
    year(pets.birthDate) == 2009
}

Batch Updates and Deletes

Since each where method call returns a DetachedCriteria instance, you can use where queries to execute batch operations such as batch updates and deletes. For example, the following query will update all people with the surname "Simpson" to have the surname "Bloggs":

def query = Person.where {
    lastName == 'Simpson'
}
int total = query.updateAll(lastName:"Bloggs")

Note that one limitation with regards to batch operations is that join queries (queries that query associations) are not allowed.

To batch delete records you can use the deleteAll method:

def query = Person.where {
    lastName == 'Simpson'
}
int total = query.deleteAll()

6.4.3 Criteria

Criteria can be used either with the createCriteria or withCriteria methods. The builder uses Hibernate's Criteria API. The nodes on this builder map the static methods found in the Restrictions class of the Hibernate Criteria API. For example:

def c = Account.createCriteria()
def results = c {
    between("balance", 500, 1000)
    eq("branch", "London")
    or {
        like("holderFirstName", "Fred%")
        like("holderFirstName", "Barney%")
    }
    maxResults(10)
    order("holderLastName", "desc")
}

This criteria will select up to 10 Account objects in a List matching the following criteria:

• balance is between 500 and 1000
• branch is 'London'
• holderFirstName starts with 'Fred' or 'Barney'

The results will be sorted in descending order by holderLastName.

If no records are found with the above criteria, an empty List is returned.

Conjunctions and Disjunctions

As demonstrated in the previous example you can group criteria in a logical OR using an or { } block:

or {
    between("balance", 500, 1000)
    eq("branch", "London")
}

This also works with logical AND:

and {
    between("balance", 500, 1000)
    eq("branch", "London")
}

And you can also negate using logical NOT:

not {
    between("balance", 500, 1000)
    eq("branch", "London")
}

All top level conditions are implied to be AND'd together.

Querying Associations

Associations can be queried by having a node that matches the property name. For example say the Account class had many Transaction objects:

class Account {
    …
    static hasMany = [transactions: Transaction]
    …
}

We can query this association by using the property name transactions as a builder node:

def c = Account.createCriteria()
def now = new Date()
def results = c.list {
    transactions {
        between('date', now - 10, now)
    }
}

The above code will find all the Account instances that have performed transactions within the last 10 days. You can also nest such association queries within logical blocks:

def c = Account.createCriteria()
def now = new Date()
def results = c.list {
    or {
        between('created', now - 10, now)
        transactions {
            between('date', now - 10, now)
        }
    }
}

Here we find all accounts that have either performed transactions in the last 10 days OR have been recently created in the last 10 days.

Querying with Projections

Projections may be used to customise the results. Define a "projections" node within the criteria builder tree to use projections. There are equivalent methods within the projections node to the methods found in the Hibernate Projections class:

def c = Account.createCriteria()
def numberOfBranches = c.get {
    projections {
        countDistinct('branch')
    }
}

When multiple fields are specified in the projection, a List of values will be returned. A single value will be returned otherwise.

SQL Projections

The criteria DSL provides access to Hibernate's SQL projection API.

// Box is a domain class…
class Box {
    int width
    int height
}

// Use SQL projections to retrieve the perimeter and area of all of the Box instances…
def c = Box.createCriteria()
def results = c.list {
    projections {
      sqlProjection '(2 * (width + height)) as perimeter, (width * height) as area', ['perimeter', 'area'], [INTEGER, INTEGER]
    }
}

The first argument to the sqlProjection method is the SQL which defines the projections. The second argument is a list of Strings which represent column aliases corresponding to the projected values expressed in the SQL. The third argument is a list of org.hibernate.type.Type instances which correspond to the projected values expressed in the SQL. The API supports all org.hibernate.type.Type objects but constants like INTEGER, LONG, FLOAT etc. are provided by the DSL which correspond to all of the types defined in org.hibernate.type.StandardBasicTypes.

Consider that the following table represents the data in the BOX table.

The query above would return results like this:

[[18, 14], [20, 16], [22, 18], [26, 36]]

Each of the inner lists contains the 2 projected values for each Box, perimeter and area.

Note that if there are other references in scope wherever your criteria query is expressed that have names that conflict with any of the type constants described above, the code in your criteria will refer to those references, not the type constants provided by the DSL. In the unlikely event of that happening you can disambiguate the conflict by referring to the fully qualified Hibernate type. For example StandardBasicTypes.INTEGER instead of INTEGER.

If only 1 value is being projected, the alias and the type do not need to be included in a list.

def results = c.list {
    projections {
      sqlProjection 'sum(width * height) as totalArea', 'totalArea', INTEGER
    }
}

That query would return a single result with the value of 84 as the total area of all of the Box instances.

The DSL supports grouped projections with the sqlGroupProjection method.

def results = c.list {
    projections {
        sqlGroupProjection 'width, sum(height) as combinedHeightsForThisWidth', 'width', ['width', 'combinedHeightsForThisWidth'], [INTEGER, INTEGER]
    }
}

The first argument to the sqlProjection method is the SQL which defines the projections. The second argument represents the group by clause that should be part of the query. That string may be single column name or a comma separated list of column names. The third argument is a list of Strings which represent column aliases corresponding to the projected values expressed in the SQL. The fourth argument is a list of org.hibernate.type.Type instances which correspond to the projected values expressed in the SQL.

The query above is projecting the combined heights of boxes grouped by width and would return results that look like this:

[[2, 24], [4, 9]]

Each of the inner lists contains 2 values. The first value is a box width and the second value is the sum of the heights of all of the boxes which have that width.

Using SQL Restrictions

You can access Hibernate's SQL Restrictions capabilities.

def c = Person.createCriteria()
def peopleWithShortFirstNames = c.list {
    sqlRestriction "char_length(first_name) <= 4"
}

SQL Restrictions may be parameterized to deal with SQL injection vulnerabilities related to dynamic restrictions.

def c = Person.createCriteria()
def peopleWithShortFirstNames = c.list {
    sqlRestriction "char_length(first_name) < ? AND char_length(first_name) > ?", [maxValue, minValue]
}

Note that the parameter there is SQL. The first_name attribute referenced in the example refers to the persistence model, not the object model like in HQL queries. The Person property named firstName is mapped to the first_name column in the database and you must refer to that in the sqlRestriction string.

Also note that the SQL used here is not necessarily portable across databases.

Using Scrollable Results

You can use Hibernate's ScrollableResults feature by calling the scroll method:

def results = crit.scroll {
    maxResults(10)
}
def f = results.first()
def l = results.last()
def n = results.next()
def p = results.previous()
def future = results.scroll(10)
def accountNumber = results.getLong('number')

To quote the documentation of Hibernate ScrollableResults:

A result iterator that allows moving around within the results by arbitrary increments. The Query / ScrollableResults pattern is very similar to the JDBC PreparedStatement/ ResultSet pattern and the semantics of methods of this interface are similar to the similarly named methods on ResultSet.

Contrary to JDBC, columns of results are numbered from zero.

Setting properties in the Criteria instance

If a node within the builder tree doesn't match a particular criterion it will attempt to set a property on the Criteria object itself. This allows full access to all the properties in this class. This example calls setMaxResults and setFirstResult on the Criteria instance:

import org.hibernate.FetchMode as FM
…
def results = c.list {
    maxResults(10)
    firstResult(50)
    fetchMode("aRelationship", FM.JOIN)
}

Querying with Eager Fetching

In the section on Eager and Lazy Fetching we discussed how to declaratively specify fetching to avoid the N+1 SELECT problem. However, this can also be achieved using a criteria query:

def criteria = Task.createCriteria()
def tasks = criteria.list{
    eq "assignee.id", task.assignee.id
    join 'assignee'
    join 'project'
    order 'priority', 'asc'
}

Notice the usage of the join method: it tells the criteria API to use a JOIN to fetch the named associations with the Task instances. It's probably best not to use this for one-to-many associations though, because you will most likely end up with duplicate results. Instead, use the 'select' fetch mode:

import org.hibernate.FetchMode as FM
…
def results = Airport.withCriteria {
    eq "region", "EMEA"
    fetchMode "flights", FM.SELECT
}

fetchMode and join are general settings of the query and can only be specified at the top-level, i.e. you cannot use them inside projections or association constraints.

An important point to bear in mind is that if you include associations in the query constraints, those associations will automatically be eagerly loaded. For example, in this query:

def results = Airport.withCriteria {
    eq "region", "EMEA"
    flights {
        like "number", "BA%"
    }
}

Method Reference

If you invoke the builder with no method name such as:

c { … }

The build defaults to listing all the results and hence the above is equivalent to:

c.list { … }

6.4.4 Detached Criteria

Building Detached Criteria Queries

The primary point of entry for using the Detached Criteria is the grails.gorm.DetachedCriteria class which accepts a domain class as the only argument to its constructor:

import grails.gorm.*
…
def criteria = new DetachedCriteria(Person)

Once you have obtained a reference to a detached criteria instance you can execute where queries or criteria queries to build up the appropriate query. To build a normal criteria query you can use the build method:

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}

Note that methods on the DetachedCriteria instance do not mutate the original object but instead return a new query. In other words, you have to use the return value of the build method to obtain the mutated criteria object:

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}
def bartQuery = criteria.build {
    eq 'firstName', 'Bart'
}

Executing Detached Criteria Queries

Unlike regular criteria, Detached Criteria are lazy, in that no query is executed at the point of definition. Once a Detached Criteria query has been constructed then there are a number of useful query methods which are summarized in the table below:

As an example the following code will list the first 4 matching records sorted by the firstName property:

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}
def results = criteria.list(max:4, sort:"firstName")

You can also supply additional criteria to the list method:

def results = criteria.list(max:4, sort:"firstName") {
    gt 'age', 30
}

To retrieve a single result you can use the get or find methods (which are synonyms):

Person p = criteria.find() // or criteria.get()

The DetachedCriteria class itself also implements the Iterable interface which means that it can be treated like a list:

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}
criteria.each {
    println it.firstName
}

In this case the query is only executed when the each method is called. The same applies to all other Groovy collection iteration methods.

You can also execute dynamic finders on DetachedCriteria just like on domain classes. For example:

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}
def bart = criteria.findByFirstName("Bart")

Using Detached Criteria for Subqueries

Within the context of a regular criteria query you can use DetachedCriteria to execute subquery. For example if you want to find all people who are older than the average age the following query will accomplish that:

def results = Person.withCriteria {
     gt "age", new DetachedCriteria(Person).build {
         projections {
             avg "age"
         }
     }
     order "firstName"
 }

Notice that in this case the subquery class is the same as the original criteria query class (ie. Person) and hence the query can be shortened to:

def results = Person.withCriteria {
     gt "age", {
         projections {
             avg "age"
         }
     }
     order "firstName"
 }

If the subquery class differs from the original criteria query then you will have to use the original syntax.

In the previous example the projection ensured that only a single result was returned (the average age). If your subquery returns multiple results then there are different criteria methods that need to be used to compare the result. For example to find all the people older than the ages 18 to 65 a gtAll query can be used:

def results = Person.withCriteria {
    gtAll "age", {
        projections {
            property "age"
        }
        between 'age', 18, 65
    }
    order "firstName"
}

The following table summarizes criteria methods for operating on subqueries that return multiple results:

Batch Operations with Detached Criteria

The DetachedCriteria class can be used to execute batch operations such as batch updates and deletes. For example, the following query will update all people with the surname "Simpson" to have the surname "Bloggs":

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}
int total = criteria.updateAll(lastName:"Bloggs")

Note that one limitation with regards to batch operations is that join queries (queries that query associations) are not allowed within the DetachedCriteria instance.

To batch delete records you can use the deleteAll method:

def criteria = new DetachedCriteria(Person).build {
    eq 'lastName', 'Simpson'
}
int total = criteria.deleteAll()

6.4.5 Hibernate Query Language (HQL)

GORM provides a number of methods that work with HQL including find, findAll and executeQuery. An example of a query can be seen below:

def results =
      Book.findAll("from Book as b where b.title like 'Lord of the%'")

Positional and Named Parameters

In this case the value passed to the query is hard coded, however you can equally use positional parameters:

def results =
      Book.findAll("from Book as b where b.title like ?", ["The Shi%"])

def author = Author.findByName("Stephen King")
def books = Book.findAll("from Book as book where book.author = ?",
                         [author])

Or even named parameters:

def results =
      Book.findAll("from Book as b " +
                   "where b.title like :search or b.author like :search",
                   [search: "The Shi%"])

def author = Author.findByName("Stephen King")
def books = Book.findAll("from Book as book where book.author = :author",
                         [author: author])

Multiline Queries

Use the line continuation character to separate the query across multiple lines:

def results = Book.findAll("\
from Book as b, \
     Author as a \
where b.author = a and a.surname = ?", ['Smith'])

Triple-quoted Groovy multiline Strings will NOT work with HQL queries.

Pagination and Sorting

You can also perform pagination and sorting whilst using HQL queries. To do so simply specify the pagination options as a Map at the end of the method call and include an "ORDER BY" clause in the HQL:

def results =
      Book.findAll("from Book as b where " +
                   "b.title like 'Lord of the%' " +
                   "order by b.title asc",
                   [max: 10, offset: 20])

6.5 Advanced GORM Features

6.5.1 Events and Auto Timestamping

• beforeInsert - Executed before an object is initially persisted to the database
• beforeUpdate - Executed before an object is updated
• beforeDelete - Executed before an object is deleted
• beforeValidate - Executed before an object is validated
• afterInsert - Executed after an object is persisted to the database
• afterUpdate - Executed after an object has been updated
• afterDelete - Executed after an object has been deleted
• onLoad - Executed when an object is loaded from the database

To add an event simply register the relevant closure with your domain class.

Do not attempt to flush the session within an event (such as with obj.save(flush:true)). Since events are fired during flushing this will cause a StackOverflowError.

Event types

The beforeInsert event

Fired before an object is saved to the database

class Person {
   private static final Date NULL_DATE = new Date(0)
   String firstName
   String lastName
   Date signupDate = NULL_DATE
   def beforeInsert() {
      if (signupDate == NULL_DATE) {
         signupDate = new Date()
      }
   }
}

The beforeUpdate event

Fired before an existing object is updated

class Person {
   def securityService
   String firstName
   String lastName
   String lastUpdatedBy
   static constraints = {
      lastUpdatedBy nullable: true
   }
   def beforeUpdate() {
      lastUpdatedBy = securityService.currentAuthenticatedUsername()
   }
}

The beforeDelete event

Fired before an object is deleted.

class Person {
   String name
   def beforeDelete() {
      ActivityTrace.withNewSession {
         new ActivityTrace(eventName: "Person Deleted", data: name).save()
      }
   }
}

Notice the usage of withNewSession method above. Since events are triggered whilst Hibernate is flushing using persistence methods like save() and delete() won't result in objects being saved unless you run your operations with a new Session.

Fortunately the withNewSession method lets you share the same transactional JDBC connection even though you're using a different underlying Session.

The beforeValidate event

Fired before an object is validated.

class Person {
   String name
   static constraints = {
       name size: 5..45
   }
   def beforeValidate() {
       name = name?.trim()
   }
}

The beforeValidate method is run before any validators are run.

Validation may run more often than you think. It is triggered by the validate() and save() methods as you'd expect, but it is also typically triggered just before the view is rendered as well. So when writing beforeValidate() implementations, make sure that they can handle being called multiple times with the same property values.

GORM supports an overloaded version of beforeValidate which accepts a List parameter which may include the names of the properties which are about to be validated. This version of beforeValidate will be called when the validate method has been invoked and passed a List of property names as an argument.

class Person {
   String name
   String town
   Integer age
   static constraints = {
       name size: 5..45
       age range: 4..99
   }
   def beforeValidate(List propertiesBeingValidated) {
      // do pre validation work based on propertiesBeingValidated
   }
}
def p = new Person(name: 'Jacob Brown', age: 10)
p.validate(['age', 'name'])

Note that when validate is triggered indirectly because of a call to the save method that the validate method is being invoked with no arguments, not a List that includes all of the property names.

Either or both versions of beforeValidate may be defined in a domain class. GORM will prefer the List version if a List is passed to validate but will fall back on the no-arg version if the List version does not exist. Likewise, GORM will prefer the no-arg version if no arguments are passed to validate but will fall back on the List version if the no-arg version does not exist. In that case, null is passed to beforeValidate.

The onLoad/beforeLoad event

Fired immediately before an object is loaded from the database:

class Person {
   String name
   Date dateCreated
   Date lastUpdated
   def onLoad() {
      log.debug "Loading ${id}"
   }
}

beforeLoad() is effectively a synonym for onLoad(), so only declare one or the other.

The afterLoad event

Fired immediately after an object is loaded from the database:

class Person {
   String name
   Date dateCreated
   Date lastUpdated
   def afterLoad() {
      name = "I'm loaded"
   }
}

Custom Event Listeners

As of Grails 2.0 there is a new API for plugins and applications to register and listen for persistence events. This API is not tied to Hibernate and also works for other persistence plugins such as the MongoDB plugin for GORM.

To use this API you need to subclass AbstractPersistenceEventListener (in package org.grails.datastore.mapping.engine.event ) and implement the methods onPersistenceEvent and supportsEventType. You also must provide a reference to the datastore to the listener. The simplest possible implementation can be seen below:

public MyPersistenceListener(final Datastore datastore) {
    super(datastore)
}
@Override
protected void onPersistenceEvent(final AbstractPersistenceEvent event) {
    switch(event.eventType) {
        case PreInsert:
            println "PRE INSERT ${event.entityObject}"
        break
        case PostInsert:
            println "POST INSERT ${event.entityObject}"
        break
        case PreUpdate:
            println "PRE UPDATE ${event.entityObject}"
        break;
        case PostUpdate:
            println "POST UPDATE ${event.entityObject}"
        break;
        case PreDelete:
            println "PRE DELETE ${event.entityObject}"
        break;
        case PostDelete:
            println "POST DELETE ${event.entityObject}"
        break;
        case PreLoad:
            println "PRE LOAD ${event.entityObject}"
        break;
        case PostLoad:
            println "POST LOAD ${event.entityObject}"
        break;
    }
}
@Override
public boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
    return true
}

The AbstractPersistenceEvent class has many subclasses (PreInsertEvent, PostInsertEvent etc.) that provide further information specific to the event. A cancel() method is also provided on the event which allows you to veto an insert, update or delete operation.

Once you have created your event listener you need to register it with the ApplicationContext. This can be done in BootStrap.groovy:

def init = {
    application.mainContext.eventTriggeringInterceptor.datastores.each { k, datastore ->
        applicationContext.addApplicationListener new MyPersistenceListener(datastore)
    }
}

or use this in a plugin:

def doWithApplicationContext = { applicationContext ->
    application.mainContext.eventTriggeringInterceptor.datastores.each { k, datastore ->
        applicationContext.addApplicationListener new MyPersistenceListener(datastore)
    }
}

Hibernate Events

It is generally encouraged to use the non-Hibernate specific API described above, but if you need access to more detailed Hibernate events then you can define custom Hibernate-specific event listeners.

You can also register event handler classes in an application's grails-app/conf/spring/resources.groovy or in the doWithSpring closure in a plugin descriptor by registering a Spring bean named hibernateEventListeners. This bean has one property, listenerMap which specifies the listeners to register for various Hibernate events.

The values of the Map are instances of classes that implement one or more Hibernate listener interfaces. You can use one class that implements all of the required interfaces, or one concrete class per interface, or any combination. The valid Map keys and corresponding interfaces are listed here:

For example, you could register a class AuditEventListener which implements PostInsertEventListener, PostUpdateEventListener, and PostDeleteEventListener using the following in an application:

beans = {
   auditListener(AuditEventListener)
   hibernateEventListeners(HibernateEventListeners) {
      listenerMap = ['post-insert': auditListener,
                     'post-update': auditListener,
                     'post-delete': auditListener]
   }
}

or use this in a plugin:

def doWithSpring = {
   auditListener(AuditEventListener)
   hibernateEventListeners(HibernateEventListeners) {
      listenerMap = ['post-insert': auditListener,
                     'post-update': auditListener,
                     'post-delete': auditListener]
   }
}

Automatic timestamping

If you define a dateCreated property it will be set to the current date for you when you create new instances. Likewise, if you define a lastUpdated property it will be automatically be updated for you when you change persistent instances.

If this is not the behaviour you want you can disable this feature with:

class Person {
   Date dateCreated
   Date lastUpdated
   static mapping = {
      autoTimestamp false
   }
}

If you have nullable: false constraints on either dateCreated or lastUpdated, your domain instances will fail validation - probably not what you want. Omit constraints from these properties unless you disable automatic timestamping.

6.5.2 Custom ORM Mapping

None of this is necessary if you are happy to stick to the conventions defined by GORM for table names, column names and so on. You only needs this functionality if you need to tailor the way GORM maps onto legacy schemas or configures caching

Custom mappings are defined using a static mapping block defined within your domain class:

class Person {
    …
    static mapping = {
        version false
        autoTimestamp false
    }
}

You can also configure global mappings in Config.groovy (or an external config file) using this setting:

grails.gorm.default.mapping = {
    version false
    autoTimestamp false
}

It has the same syntax as the standard mapping block but it applies to all your domain classes! You can then override these defaults within the mapping block of a domain class.

6.5.2.1 Table and Column Names

Table names

The database table name which the class maps to can be customized using the table method:

class Person {
    …
    static mapping = {
        table 'people'
    }
}

In this case the class would be mapped to a table called people instead of the default name of person.

Column names

It is also possible to customize the mapping for individual columns onto the database. For example to change the name you can do:

class Person {
    String firstName
    static mapping = {
        table 'people'
        firstName column: 'First_Name'
    }
}

Here firstName is a dynamic method within the mapping Closure that has a single Map parameter. Since its name corresponds to a domain class persistent field, the parameter values (in this case just "column") are used to configure the mapping for that property.

Column type

GORM supports configuration of Hibernate types with the DSL using the type attribute. This includes specifing user types that implement the Hibernate org.hibernate.usertype.UserType interface, which allows complete customization of how a type is persisted. As an example if you had a PostCodeType you could use it as follows:

class Address {
    String number
    String postCode
    static mapping = {
        postCode type: PostCodeType
    }
}

Alternatively if you just wanted to map it to one of Hibernate's basic types other than the default chosen by Grails you could use:

class Address {
    String number
    String postCode
    static mapping = {
        postCode type: 'text'
    }
}

This would make the postCode column map to the default large-text type for the database you're using (for example TEXT or CLOB).

See the Hibernate documentation regarding Basic Types for further information.

Many-to-One/One-to-One Mappings

In the case of associations it is also possible to configure the foreign keys used to map associations. In the case of a many-to-one or one-to-one association this is exactly the same as any regular column. For example consider the following:

class Person {
    String firstName
    Address address
    static mapping = {
        table 'people'
        firstName column: 'First_Name'
        address column: 'Person_Address_Id'
    }
}

By default the address association would map to a foreign key column called address_id. By using the above mapping we have changed the name of the foreign key column to Person_Adress_Id.

One-to-Many Mapping

With a bidirectional one-to-many you can change the foreign key column used by changing the column name on the many side of the association as per the example in the previous section on one-to-one associations. However, with unidirectional associations the foreign key needs to be specified on the association itself. For example given a unidirectional one-to-many relationship between Person and Address the following code will change the foreign key in the address table:

class Person {
    String firstName
    static hasMany = [addresses: Address]
    static mapping = {
        table 'people'
        firstName column: 'First_Name'
        addresses column: 'Person_Address_Id'
    }
}

If you don't want the column to be in the address table, but instead some intermediate join table you can use the joinTable parameter:

class Person {
    String firstName
    static hasMany = [addresses: Address]
    static mapping = {
        table 'people'
        firstName column: 'First_Name'
        addresses joinTable: [name: 'Person_Addresses',
                              key: 'Person_Id',
                              column: 'Address_Id']
    }
}

Many-to-Many Mapping

Grails, by default maps a many-to-many association using a join table. For example consider this many-to-many association:

class Group {
    …
    static hasMany = [people: Person]
}

class Person {
    …
    static belongsTo = Group
    static hasMany = [groups: Group]
}

In this case Grails will create a join table called group_person containing foreign keys called person_id and group_id referencing the person and group tables. To change the column names you can specify a column within the mappings for each class.

class Group {
   …
   static mapping = {
       people column: 'Group_Person_Id'
   }
}
class Person {
   …
   static mapping = {
       groups column: 'Group_Group_Id'
   }
}