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The Grails Framework - Reference Documentation

Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown

Version: 1.2.1

Table of Contents

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 technology like Spring & Hibernate.

Grails is a full stack framework and attempts to solve as many pieces of the web development puzzle through the core technology and it's associated plug-ins. 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.

2. Getting Started

2.1 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
• Now you need to add the bin directory to your PATH variable:
• On Unix/Linux base system this can be done by doing a export PATH="$PATH:$GRAILS_HOME/bin"
• 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 in the terminal window and see output similar to the below:

Welcome to Grails 1.0 - http://grails.org/
Licensed under Apache Standard License 2.0
Grails home is set to: /Developer/grails-1.0
No script name specified. Use 'grails help' for more info

2.2 Upgrading from previous versions of Grails

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 you need to change the above to:

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

The same rules apply to the javascript and render

Tag and Body return values

Tags no longer return java.lang.String instances but instead return a StreamCharBuffer instance. The StreamCharBuffer class implements all the same methods as String, however code like this may break:

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

In these cases you should use 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 to earlier versions of Grails. However, if your application depends on the older semantics you can still use the now deprecated implementation by settings 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) is executing a query then this can cause an addition flush resulting in a StackOverflowError. 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):

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 is written in Groovy 1.5 you will need to recompile it against Groovy 1.6 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.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

Since 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( "Bootstrap.groovy" )

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

3) The root directory of the project is no long on the classpath, the result is that 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 may want to drop the 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")

2.3 Creating an Application

grails [command name]

In this case the command you need to execute is create-app:

grails create-app helloworld

This will create a new directory inside the current one that contains the project. You should now navigate to this directory in terminal:

cd helloworld

2.4 A Hello World Example

grails create-controller hello  

This will create a new controller (Refer to the section on Controllers for more information) in the grails-app/controllers directory called HelloController.groovy.

Controllers are capable of dealing with web requests and to fulfil the "hello world!" use case our implementation needs to look like the following:

class HelloController {
	def world = {
		render "Hello World!"
	}
}

Job done. Now start-up the container with another new command called run-app:

grails run-app

This will start-up a server on port 8080 and you should now be able to access your application with the URL: http://localhost:8080/helloworld

The result will look something like the following screenshot:

This is the Grails intro page which is rendered by the web-app/index.gsp file. You will note it has a detected the presence of your controller and clicking on the link to our controller we can see the text "Hello World!" printed to the browser window.

2.5 Getting Set-up in an IDE

IntelliJ IDEA

IntelliJ IDEA and the JetGroovy plug-in offer good support for Groovy & Grails developer. Refer to the section on Groovy and Grails support on the JetBrains website for a feature overview.

To integrate Grails 1.2 to with IntelliJ run the following command to generate appropriate project files:

grails integrate-with --intellij

NetBeans

A good Open Source alternative is Sun's NetBeans, which 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 Sun's Glassfish server. For an overview of features see the NetBeans Integration guide on the Grails website which was written by the NetBeans team.

Eclipse

We recommend that users of Eclipse looking to develop Grails application take a look at SpringSource 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.

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 1.2 to 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 .

2.6 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.
• views - Groovy Server Pages.
• scripts - Gant scripts.
• src - Supporting sources
• groovy - Other Groovy sources
• java - Other Java sources
• test - Unit and integration tests.

2.7 Running an Application

grails run-app

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

grails -Dserver.port=8090 run-app

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

2.8 Testing an Application

grails test-app

Grails also automatically generates an Ant build.xml which can also run the tests by delegating to Grails' test-app command:

ant test

This is useful when you need to build Grails applications as part of a continuous integration platform such as CruiseControl.

2.9 Deploying an Application

grails war

This will produce a WAR file in the root of your project which can then be deployed as per your containers instructions.

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 implication

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

2.10 Creating Artefacts

These are merely 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.

2.10 Supported Java EE Containers

• Tomcat 5.5
• Tomcat 6.0
• SpringSource tc Server
• SpringSource dm Server 1.0
• GlassFish v1 (Sun AS 9.0)
• GlassFish v2 (Sun AS 9.1)
• GlassFish v3 Prelude
• Sun App Server 8.2
• Websphere 6.1
• Websphere 5.1
• Resin 3.2
• Oracle AS
• JBoss 4.2
• Jetty 6.1
• Jetty 5
• Weblogic 7/8/9/10

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.

2.11 Generating an Application

grails generate-all Book

3. Configuration

With Grails' default settings you can actually develop and application without doing any configuration whatsoever. Grails ships with an embedded container and in-memory HSQLDB meaning there isn't even a database to set-up.

However, typically you want to set-up a real database at some point and the way you do that is described in the following section.

3.1 Basic Configuration

You can add your own configuration in here, for example:

foo.bar.hello = "world"

Then later in your application you can access these settings in one of two ways. The most common is via the GrailsApplication object, which is available as a variable in controllers and tag libraries:

assert "world" == grailsApplication.config.foo.bar.hello

The other way involves getting a reference to the ConfigurationHolder class that holds a reference to the configuration object:

import org.codehaus.groovy.grails.commons.*
…
def config = ConfigurationHolder.config
assert "world" == config.foo.bar.hello

3.1.1 Built in options

• grails.config.locations - The location of properties files or addition Grails Config files that should be merged with main configuration
• grails.enable.native2ascii - Set this to false if you do not require native2ascii conversion of Grails i18n properties files
• 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 is 'utf-8')
• grails.mime.file.extensions - Whether to use the file extension to dictate the mime type in Content Negotiation
• 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.

War generation

• grails.war.destFile - Sets the location where the war command should place the generated WAR file
• grails.war.dependencies - A closure containing Ant builder syntax or a list of JAR filenames. Allows you to customise what libaries are included in the WAR file.
• grails.war.java5.dependencies - A list of the JARs that should be included in the WAR file for JDK 1.5 and above.
• grails.war.copyToWebApp - A closure containing Ant builder syntax that is legal inside an Ant copy, for example "fileset()". Allows you to 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 pre-warring stuff it needs to.

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

3.1.2 Logging

Logging Basics

Grails uses its common configuration mechanism to configure the underlying Log4j log system. To configure logging you must modify the file Config.groovy located in the grails-app/conf directory.

This single Config.groovy file allows you to specify separate logging configurations for development, test, and production environments. Grails processes the Config.groovy and configures Log4j appropriately.

Since 1.1 Grails provides a Log4j DSL, that you can use to configure Log4j an example of which can be seen below:

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

Essentially, each method translates into a log level and you can pass the names of the packages you want to log at that level as arguments to the method.

Some useful loggers include:

• org.codehaus.groovy.grails.commons - Core artefact 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
• org.springframework - See what Spring is doing
• org.hibernate - See what Hibernate is doing

The Root Logger

The Root logger is the logger that all other loggers inherit from. You can configure the Root logger using the root method:

root {
    error()
    additivity = true
}

The above example configures the root logger to log messages at the error level and above to the default standard out appender. You can also configure the root logger to log to one or more named appenders:

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

Here the root logger will log to two appenders - the default 'stdout' appender and a 'file' appender.

You can also configure the root logger from the argument passed into the Log4J closure:

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

Custom Appenders

Using the Log4j you can define custom appenders. The following appenders are available by default:

• jdbc - An appender that logs to a JDBC connection
• console - An appender that logs to standard out
• file - An appender that logs to a file
• rollingFile - An appender that logs to rolling set of files

For example to configure a rolling file appender you can do:

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

Each argument passed to the appender maps to a property of underlying Appender class. The example above sets the name, maxFileSize and file properties of the RollingFileAppender class.

If you prefer to simply create the appender programmatically yourself, or you have your own appender implementation then you can simply call the appender method and appender instance:

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

You can then log to a particular appender by passing the name as a key to one of the log level methods from the previous section:

error myAppender:"org.codehaus.groovy.grails.commons"

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')
    }
}

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 written to the StackTrace logger. This logs to a file called stacktrace.log - but you can change this in your Config.groovy to do anything you like. For example if you prefer full stack traces to go to standard out you can add this line:

error stdout:"StackTrace"

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

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

Logging by Convention

All application artefacts have a dynamically added log property. This includes domain classes, controllers, tag libraries and so on. Below is an example of its usage:

def foo = "bar"
log.debug "The value of foo is $foo"

Logs are named using the convention grails.app.<artefactType>.ClassName. Below is an example of how to configure logs for different Grails artefacts:

log4j = {
	// Set level for all application artefacts
	info "grails.app"
	// Set for a specific controller
	debug "grails.app.controller.YourController"
	// Set for a specific domain class
	debug "grails.app.domain.Book"
	// Set for all taglibs
	info "grails.app.tagLib"
}

The artefacts names are dictated by convention, some of the common ones are listed below:

• bootstrap - For bootstrap classes
• dataSource - For data sources
• tagLib - For tag libraries
• service - For service classes
• controller - For controllers
• domain - For domain entities

3.1.3 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.

Enable failOnError for all domain classes…

grails.gorm.failOnError=true

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 do something like save(flush: true).

3.2 Environments

Per Environment Configuration

Grails supports the concept of per environment configuration. Both the Config.groovy file and the DataSource.groovy file within the grails-app/conf directory can take advantage of 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.hsqldb.jdbcDriver"	
    username = "sa"
    password = ""				
}
environments {
    development {
        dataSource {
            dbCreate = "create-drop" // one of 'create', 'createeate-drop','update'
            url = "jdbc:hsqldb:mem:devDB"
        }
    }   
    test {
        dataSource {
            dbCreate = "update"
            url = "jdbc:hsqldb:mem:testDb"
        }
    }   
    production {
        dataSource {
            dbCreate = "update"
            url = "jdbc:hsqldb:file:prodDb;shutdown=true"
        }
    }
}

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. This syntax can also be used within Config.groovy.

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 could do:

grails test war

If you have other environments that you need to target 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

Its 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
	}
}

3.3 The DataSource

Essentially, if you are using another database other than HSQLDB you need to have a JDBC driver. For example for MySQL you would need Connector/J

Drivers typically come in the form of a JAR archive. Drop the JAR into your projects lib directory.

Once you have the JAR in place 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 or not
• pooled - Whether to use a pool of connections (defaults to true)
• logSql - Enable SQL logging
• 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.
• 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"
	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 local variable
	…
}

Example of advanced configuration using extra properties:

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

3.3.1 DataSources and Environments

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

dataSource {
	// common settings here
}                     
environments {
  production {
     dataSource {
          url = "jdbc:mysql://liveip.com/liveDb"					
     }			
  }
}

3.3.2 JNDI DataSources

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 remains the same.

3.3.3 Automatic Database Migration

• create-drop - Drop and re-create the database when Grails is run
• create - Create the database if it doesn't exist, but don't modify it if it does. Deletes existing data.
• update - Create the database if it doesn't exist, and modify it if it does exist

Both create-drop and create will destroy all existing data hence use with caution!

In development mode dbCreate is by default set to "create-drop":

dataSource {
	dbCreate = "create-drop" // one of 'create', 'create-drop','update'
}

What this does is automatically drop and re-create the database tables on each restart of the application. Obviously this may not be what you want in production.

Although Grails does not currently support Rails-style Migrations out of the box, there are currently three plugins that provide similar capabilities to Grails: Autobase (http://wiki.github.com/RobertFischer/autobase), The LiquiBase plugin and the DbMigrate plugin both of which are available via the grails list-plugins command

3.4 Externalized Configuration

In order to support deployment scenarios such as these the configuration can be externalized. To do so you need to point Grails at the locations of the configuration files Grails should be using by adding a grails.config.locations setting in Config.groovy:

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.

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

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

3.5 Versioning

Versioning Basics

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

To change the version of your application you can 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']

If it is the version of Grails you need you can use:

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

or the GrailsUtil class:

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

3.6 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. Firstly you need to create a src/docs/guide directory and then have numbered text files using the gdoc format. For example:

+ src/docs/guide/1. Introduction.gdoc
+ src/docs/guide/2. Getting Started.gdoc

The title of each chapter is taken from the file name. The order is dictated by the numerical value at the beginning of the file name.

Creating reference items

Reference items appear in the left menu on the documentation and are useful for quick reference documentation. Each reference item belongs to a category and a category is a directory located in the src/docs/ref directory. For example say you defined a new method called renderPDF, that belongs to a category called Controllers this can be done by creating a gdoc text file at the following location:

+ src/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.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 name of the documentation and the version are pulled from your project itself.

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 docs

This command will output an docs/manual/index.html which can be opened 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!

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:

[Intro|guide:1. Introduction]

The documentation engine will warn you if any links to sections in your guide break. Sometimes though it is preferable not to hard code the actual names of guide sections since you may move them around. To get around this you can create an alias inside grails-app/conf/Config.groovy:

grails.doc.alias.intro="1. Introduction"

And then the link becomes:

[Intro|guide:intro]

This is useful since if you linked the to "1. Introduction" chapter many times you would have to change all of those links.

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. If you want 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://www.hibernate.org/hib_docs/v3/api"

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:

Name	Number
Albert	46
Wilma	1348
James	12

{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}

3.7 Dependency Resolution

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

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

The default configuration looks like the following:

grails.project.dependency.resolution = {
    inherits "global" // inherit Grails' default dependencies
    log "warn" // log level of Ivy resolver, either 'error', 'warn', 'info', 'debug' or 'verbose'
    repositories {
        grailsHome()
        // uncomment the below to enable remote dependency resolution
        // from public Maven repositories
        //mavenCentral()
        //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 'com.mysql:mysql-connector-java:5.1.5'
    }
}

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

3.7.1 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 as such:

runtime 'com.mysql:mysql-connector-java:5.1.5'

The above uses the string syntax which is group:name:version. You can also use a map-based syntax:

runtime group:'com.mysql', name:'mysql-connector-java', version:'5.1.5'

Multiple dependencies can be specified by passing multiple arguments:

runtime 'com.mysql:mysql-connector-java:5.1.5',
		'net.sf.ehcache:ehcache:1.6.1'
// Or
runtime(
 	[group:'com.mysql', name:'mysql-connector-java', version:'5.1.5'],
	[group:'net.sf.ehcache', name:'ehcache', version:'1.6.1']
)

3.7.2 Dependency Repositories

Remote Repositories

Grails, when installed, does not use any remote public repositories. There is a default grailsHome() repository that will locate the JAR files Grails needs from your Grails installation. If you want to take advantage of a public repository you need to specify as such inside 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"
}

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'
}

Custom Resolvers

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

repositories {
	resolver new URLResolver(...)
}

Authentication

If your repository requires some form of authentication you can specify as such using a credentials block:

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

The above can also 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"
	}	
}

3.7.3 Debugging Resolution

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

3.7.4 Inherited Dependencies

inherits "global"

Inside the BuildConfig.groovy file. If you wish exclude certain inherited dependencies then you can do so using the excludes method:

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

3.7.5 Dependency Reports

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

grails dependency-report

This will output a report to the target/dependency-report directory by default. You can specify which configuration (scope) you want a report for by passing an argument containing the configuration name:

grails dependency-report runtime

3.7.6 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.

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

compile( 'org.hibernate:hibernate-core:3.3.1.GA') {
	exported = false
}

In this can the hibernate-core dependency will be available only to the plugin and not resolved as an application dependency.

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 the plugin method:

plugin("hibernate") {
    compile( 'org.hibernate:hibernate-core:3.3.1.GA') {
		excludes 'ehcache', 'xml-apis', 'commons-logging'
	}
    compile 'org.hibernate:hibernate-annotations:3.4.0.GA',
			'org.hibernate:hibernate-commons-annotations:3.3.0.ga'
	runtime 'javassist:javassist:3.4.GA'	
}

In this case the application is controlling how the hibernate plugin resolves dependencies and not the hibernate plugin. If you wish to simply exclude a single dependency resolved by a plugin then you can do so:

plugin("hibernate") {
    runtime "cglib:cglib-nodep:2.1_3"
	excludes 'javassist:javassist:3.4.GA'	
}

3.7.7 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. The Command Line

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

grails [command name]

• 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 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 left menu of the reference guide for more information about individual commands

4.1 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 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's source files.") {
	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 allows you to 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.

4.2 Re-using Grails scripts

The bootstrap script for example allows you to 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': "Load the Grails interactive shell") {
	depends( configureProxy, packageApp, classpath, loadApp, configureApp )
	Connection c 
	try {
		// do something with connection
		c = appCtx.getBean('dataSource').getConnection()
	}
	finally {
		c?.close()
	}
}

Pulling in targets from other scripts

Gant allows you to 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 you want to include, without any path information. Here is a list of Grails scripts that you may want to re-use:

Script	Description
_GrailsSettings	You really should include this! Fortunately, it is included automatically by all other Grails scripts bar one (_GrailsProxy), so you usually don't have to include it explicitly.
_GrailsEvents	If you want to fire events, you need to include this. Adds an event(String eventName, List args) method. Again, included by almost all other Grails scripts.
_GrailsClasspath	Sets up compilation, test, and runtime classpaths. If you want to use or play with them, include this script. Again, included by almost all other Grails scripts.
_GrailsProxy	If you want to access the internet, include this script so that you don't run into problems with proxies.
_GrailsArgParsing	Provides a parseArguments target that does what it says on the tin: parses the arguments provided by the user when they run your script. Adds them to the argsMap property.
_GrailsTest	Contains all the shared test code. Useful if you want to add any extra tests.
_GrailsRun	Provides all you need to run the application in the configured servlet container, either normally (runApp/runAppHttps) or from a WAR file (runWar/runWarHttps).

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 re-use.

In pre-1.1 versions of Grails, the "_Grails..." scripts were not available. Instead, you typically include the corresponding command script, for example "Init.groovy" or "Bootstrap.groovy".

Also, in pre-1.0.4 versions of Grails you cannot use the grailsScript() method. Instead, you must use includeTargets << new File(...) and specify the script's location in full (i.e. $GRAILS_HOME/scripts).

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 re-use. 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. Say 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.

4.3 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.

In versions of Grails prior to 1.0.4, the script was called Events.groovy, that is without the leading underscore.

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:

Event	Parameters	Description
StatusUpdate	message	Passed a string indicating current script status/progress
StatusError	message	Passed a string indicating an error message from the current script
StatusFinal	message	Passed a string indicating the final script status message, i.e. when completing a target, even if the target does not exit the scripting environment
CreatedArtefact	artefactType,artefactName	Called when a create-xxxx script has completed and created an artefact
CreatedFile	fileName	Called whenever a project source filed is created, not including files constantly managed by Grails
Exiting	returnCode	Called when the scripting environment is about to exit cleanly
PluginInstalled	pluginName	Called after a plugin has been installed
CompileStart	kind	Called when compilation starts, passing the kind of compile - source or tests
CompileEnd	kind	Called when compilation is finished, passing the kind of compile - source or tests
DocStart	kind	Called when documentation generation is about to start - javadoc or groovydoc
DocEnd	kind	Called when documentation generation has ended - javadoc or groovydoc
SetClasspath	rootLoader	Called during classpath initialization so plugins can augment the classpath with rootLoader.addURL(...). Note that this augments the classpath after event scripts are loaded so you cannot use this to load a class that your event script needs to import, although you can do this if you load the class by name.
PackagingEnd	none	Called at the end of packaging (which is called prior to the Tomcat server being started and after web.xml is generated)

4.4 Customising the build

The defaults

In order to customise a build, you first need to know what you can customise. 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:

Property	Config option	Default value
grailsWorkDir	grails.work.dir	$USER_HOME/.grails/<grailsVersion>
projectWorkDir	grails.project.work.dir	<grailsWorkDir>/projects/<baseDirName>
classesDir	grails.project.class.dir	<projectWorkDir>/classes
testClassesDir	grails.project.test.class.dir	<projectWorkDir>/test-classes
testReportsDir	grails.project.test.reports.dir	<projectWorkDir>/test/reports
resourcesDir	grails.project.resource.dir	<projectWorkDir>/resources
projectPluginsDir	grails.plugins.dir	<projectWorkDir>/plugins
globalPluginsDir	grails.global.plugins.dir	<grailsWorkDir>/global-plugins

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

Property	Description
baseDir	The location of the project.
userHome	The user's home directory.
grailsHome	The location of the Grails installation in use (may be null).
grailsVersion	The version of Grails being used by the project.
grailsEnv	The current Grails environment.
compileDependencies	A list of compile-time project dependencies as File instances.
testDependencies	A list of test-time project dependencies as File instances.
runtimeDependencies	A list of runtime-time project dependencies as File instances.

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 via 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

Name	Description
grails.server.port.http	Port to run the embedded servlet container on ("run-app" and "run-war"). Integer.
grails.server.port.https	Port to run the embedded servlet container on for HTTPS ("run-app https" and "run-war https"). Integer.
grails.config.base.webXml	Path to a custom web.xml file to use for the application (alternative to using the web.xml template).
grails.compiler.dependencies	Legacy approach to adding extra dependencies to the compiler classpath. Set it to a closure containing "fileset()" entries.
grails.testing.patterns	A list of Ant path patterns that allow you to control which files are included in the tests. The patterns should not include the test case suffix, which is set by the next property.
grails.testing.nameSuffix	By default, tests are assumed to have a suffix of "Tests". You can change it to anything you like but setting this option. For example, another common suffix is "Test".
grails.war.destFile	A string containing the file path of the generated WAR file, along with its full name (include extension). For example, "target/my-app.war".
grails.war.dependencies	A closure containing "fileset()" entries that allows you complete control over what goes in the WAR's "WEB-INF/lib" directory.
grails.war.copyToWebApp	A closure containing "fileset()" entries that allows you complete control over what goes in the root of the WAR. It overrides the default behaviour of including everything under "web-app".
grails.war.resources	A closure that takes the location of the staging directory as its first argument. You can use any Ant tasks to do anything you like. It is typically used to remove files from the staging directory before that directory is jar'd up into a WAR.
grails.project.web.xml	The location to generate Grails' web.xml to

4.5 Ant and Maven

Ant Integration

When you create a Grails application via the create-app command, Grails automatically creates an Apache Ant build.xml file for you 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 all geared up 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 Hudson

It uses the Grails Ant task to hook into the existing Grails build system. The task allows you to 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:

Attribute	Description	Required
home	The location of the Grails installation directory to use for the build.	Yes, unless classpath is specified.
classpathref	Classpath to load Grails from. Must include the "grails-bootstrap" artifact and should include "grails-scripts".	Yes, unless home is set or you use a classpath element.
script	The name of the Grails script to run, e.g. "TestApp".	Yes.
args	The arguments to pass to the script, e.g. "-unit -xml".	No. Defaults to "".
environment	The Grails environment to run the script in.	No. Defaults to the script default.
includeRuntimeClasspath	Advanced setting: adds the application's runtime classpath to the build classpath if true.	No. Defaults to true.

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 are using 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

From 1.1 onwards, Grails provides integration with Maven 2 via a Maven plugin. The current Maven plugin is based on, but effectively supercedes, the version created by Octo, who did a great job.

Preparation

In order to use the new 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.

To make life easier for you, we do recommend that you add a plugin group for Grails to your Maven settings file ( $USER_HOME/.m2/settings.xml ):

<settings>
  …
  <pluginGroups>
    <pluginGroup>org.grails</pluginGroup>
  </pluginGroups>
</settings>

In addition, if you have the Octo Maven Tools for Grails set up then you'll need to remove the com.octo.mtg plugin group.

Creating a Grails Maven Project

To create a Mavenized Grails project simple run the following command:

mvn archetype:generate -DarchetypeGroupId=org.grails \
    -DarchetypeArtifactId=grails-maven-archetype \
    -DarchetypeVersion=1.0 \
    -DarchetypeRepository=http://snapshots.repository.codehaus.org \
    -DgroupId=example -DartifactId=my-app

Choose whichever 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:

cd my-app
mvn initialize

Now you have a Grails application all ready to go. The plugin integrates into the standard build cycle, so you can use the standard Maven phases to build and package your app: mvn clean , mvn compile , mvn test , mvn package .

You can also take advantage of 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-plugin - Calls the install-plugin 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
• grails:uninstall-plugin - Calls the uninstall-plugin command

Mavenizing an existing project

Creating a new project is great way to start, but what if you already have one? You don't want to create a new project and then copy the contents of the old one over. The solution is to create a POM for the existing project using this Maven command:

mvn grails:create-pom -DgroupId=com.mycompany

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>1.0-SNAPSHOT</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 via the args property:

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

5. Object Relational Mapping (GORM)

GORM is Grails' object relational mapping (ORM) implementation. Under the hood it uses Hibernate 3 (an extremely popular and flexible open source ORM solution) but because of the dynamic nature of Groovy, the fact that it supports both static and dynamic typing, and the convention of Grails there is 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 Grails 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()

5.1 Quick Start Guide

grails create-domain-class Person

This will create a class at the location grails-app/domain/Person.groovy such as the one below:

class Person {	
}

If you have the dbCreate property set to "update", "create" or "create-drop" on your DataSource, Grails will automatically generated/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 via the shell or console by typing:

grails console

This loads an interactive GUI where you can type Groovy commands.

5.1.1 Basic CRUD

Create

To create a domain class use the Groovy new operator, 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 db. 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.

Update

To update an instance, set some properties and then simply 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()

5.2 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 can run the create-domain-class target as follows:

grails create-domain-class Book

The result will be a class at grails-app/domain/Book.groovy:

class Book {	
}

If you wish to use packages you can move the Book.groovy class into a sub directory under the domain directory and add the appropriate package declaration as per Groovy (and Java's) packaging rules.

The above 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:

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 via Constraints or the ORM DSL.

5.2.1 Association in GORM

5.2.1.1 One-to-one

Example A

class Face {
    Nose nose
}
class Nose {	
}

In this case we have unidirectional many-to-one relationship from Face to Nose. To make it a true one-to-one you should make nose unique:

class Face {
    Nose nose
	static constraints = {
		nose unique:true
	}
}
class Nose {	
}

To make this relationship bidirectional define the other side as follows:

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 and save it and the database updates/inserts will be cascaded down to 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

Another important implication of belongsTo is that if you delete a Face instance the Nose will be deleted too:

def f = Face.get(1)
f.delete() // both Face and Nose deleted

In the previous example the foreign key associated the Face with the Nose is stored in the parent as column called nose_id. If you want the foreign key to be stored in the child you need a hasOne association:

Example C

class Face {
    static hasOne = [nose:Nose]
}
class Nose {	
	Face face
}

In this example you get a bidirectional one-to-one where the foreign key column is stored in the nose table inside a column called face_id.

5.2.1.2 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)
a.books.each {
	println it.title
}

The default fetch strategy used by Grails is "lazy", which means that the collection will be lazily initialized. 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
}

5.2.1.3 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 the below 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

5.2.1.4 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             |
---------------------------------------------

5.2.2 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

5.2.3 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 sub classes (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 via the ORM DSL

However, excessive use of inheritance and table-per-subclass can result in poor query performance due to the excessive use of 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 sub classes of Content:

def content = Content.list() // list all blog entries, books and pod casts
content = Content.findAllByAuthor('Joe Bloggs') // find all by author
def podCasts = PodCast.list() // list only pod casts

5.2.4 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. The problem with this is there is no ordering when accessing the collection, which may not be what you want. To get custom ordering you can say that the set is a SortedSet:

class Author {
   SortedSet books
   static hasMany = [books:Book]
}

In this case a java.util.SortedSet implementation is used which means you have to 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 collections of the Author class will be ordered by their release date.

Lists of objects

If you simply want to be able 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 in order to retain this order at the db 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()

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 is a collection that doesn't allow duplicates. In order 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 in-order 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()

5.3 Persistence Basics

Essentially, Grails automatically binds a Hibernate session to the currently executing request. This allows you to use the save and delete methods as well as other GORM methods transparently.

5.3.1 Saving and Updating

def p = Person.get(1)
p.save()

A major difference with Hibernate is when you call save it does not necessarily perform any SQL operations at that point. Hibernate typically batches up SQL statements and executes them at the end. This is typically done for you automatically by Grails, which manages your Hibernate session.

There are occasions, however, when you may 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 will be synchronized with the db. This also allows you to catch any exceptions thrown, which is typically useful in highly concurrent scenarios involving optimistic locking:

def p = Person.get(1)
try {
	p.save(flush:true)
}
catch(Exception e) {
	// deal with exception
}

5.3.2 Deleting Objects

def p = Person.get(1)
p.delete()

By default Grails will use transactional write behind to perform the delete, if you want to perform the delete in place then you can use the flush argument:

def p = Person.get(1)
p.delete(flush:true)

Using the flush argument will also allow you to catch any errors that may potentially 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"])

5.3.3 Understanding Cascading Updates and Deletes

Whether it is a one-to-one, one-to-many or many-to-many if you define belongsTo updates and deletes will cascade from the owning class to its possessions (the other side of the relationship).

If you do not define belongsTo then no cascades will happen and you will have to manually save each object.

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.

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.

5.3.4 Eager and Lazy Fetching

class Airport {
	String name
	static hasMany = [flights:Flight]
}
class Flight {
	String number
	static belongsTo = [airport:Airport]
}

Given the above domain classes and the following code:

def airport = Airport.findByName("Gatwick")
airport.flights.each {
	println it.name
}

GORM will execute a single SQL query to fetch the Airport instance and then 1 extra query for each iteration over the flights association. In other words you get N+1 queries.

This can sometimes be optimal depending on the frequency of use of the association as you may have logic that dictates the associations is only accessed on certain occasions.

Configuring Eager Fetching

An alternative is to use eager fetching which can specified as follows:

class Airport {
	String name
	static hasMany = [flights:Flight]
	static mapping = {
		flight fetch:"join"
	}
}

In this case the association will be Airport instance and the flights association will be loaded all at once (depending on the mapping). This has the benefit of requiring fewer queries, however should be used carefully as you could load your entire database into memory with too many eager associations.

Associations can also be declared non-lazy using 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. Essentially, you can configure Hibernate to lazily fetch results in "batches". For example:

class Airport {
	String name
	static hasMany = [flights:Flight]
	static mapping = {
		flight 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
	}
}

5.3.5 Pessimistic and Optimistic Locking

Optimistic Locking

By default GORM classes are configured for optimistic locking. Optimistic locking essentially is a feature of Hibernate which involves storing a version number in a special version column in the database.

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 and the transaction will be rolled back.

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.

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 via 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.

Though Grails, through Hibernate, supports pessimistic locking, the embedded HSQLDB shipped with Grails which is used as the default in-memory database does not. If you need to test pessimistic locking you will need to do so against a database that does have support such as MySQL.

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
}

5.4 Querying with GORM

Groovy's ability to manipulate collections via 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

If you simply need to obtain all the instances of a given class you can use the list method:

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)

5.4.1 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 the 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 require an argument)
• IsNull - Is a null value (doesn't require an argument)

Notice that the last 3 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 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)

At the moment, you can only use dynamic finders with a maximum of two criteria, i.e. the method name can only have one boolean operator. If you need to use more, you should consider using either Criteria or the HQL.

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 & 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"])

5.4.2 Criteria

Criteria can be used either via 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. Example Usage:

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 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 ordery by holderLastName.

Conjunctions and Disjunctions

As demonstrated in the previous example you can group criteria in a logical OR using a 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 {
    …
    def hasMany = [transactions:Transaction]
    Set transactions
    …
}

We can query this association by using the property name transaction 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. To use projections you need to define a "projections" node within the criteria builder tree. 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')
	}
}

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"
}

Note that the parameter there is SQL. The first_name attribute referenced in the example relates to the persistence model, not the object model. The Person class may have a property named firstName which is mapped to a column in the database named first_name.

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. Thus allowing full access to all the properties in this class. The below 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.EAGER)
	}

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. This method indicates the criteria API that a JOIN query should be used to obtain the results.

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 { … }

Method	Description
list	This is the default method. It returns all matching rows.
get	Returns a unique result set, i.e. just one row. The criteria has to be formed that way, that it only queries one row. This method is not to be confused with a limit to just the first row.
scroll	Returns a scrollable result set.
listDistinct	If subqueries or associations are used, one may end up with the same row multiple times in the result set, this allows listing only distinct entities and is equivalent to DISTINCT_ROOT_ENTITY of the CriteriaSpecification class.
count	Returns the number of matching rows.

5.4.3 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

If you need to separate the query across multiple lines you can use a line continuation character:

def results = Book.findAll("\
from Book as b, \
     Author as a \
where b.author = a and a.surname = ?", ['Smith'])

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])

5.5 Advanced GORM Features

5.5.1 Events and Auto Timestamping

• beforeInsert - Executed before an object is initially persisted to the databse
• beforeUpdate - Executed before an object is updated
• beforeDelete - Executed before an object is deleted
• 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 updated
• 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 db

class Person {
   Date dateCreated
   def beforeInsert() {
       dateCreated = new Date()
   }
}

The beforeUpdate event

Fired before an existing object is updated

class Person {
   Date dateCreated
   Date lastUpdated
   def beforeInsert() {
       dateCreated = new Date()
   }
   def beforeUpdate() {
       lastUpdated = new Date()
   }
}

The beforeDelete event

Fired before an object is deleted.

class Person {
   String name
   Date dateCreated
   Date lastUpdated
   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 allows you to share the same transactional JDBC connection even though you're using a different underlying Session.

The onLoad event

Fired when an object is loaded from the db:

class Person {
   String name
   Date dateCreated
   Date lastUpdated
   def onLoad() {
      name = "I'm loaded"
   }
}

Automatic timestamping

The examples above demonstrated using events to update a lastUpdated and dateCreated property to keep track of updates to objects. However, this is actually not necessary. By merely defining a lastUpdated and dateCreated property these will be automatically updated for you by GORM.

If this is not the behaviour you want you can disable this feature with:

class Person {
   Date dateCreated
   Date lastUpdated
   static mapping = {
      autoTimestamp false
   }
}

5.5.2 Custom ORM Mapping

None if this is necessary if you are happy to stick to the conventions defined by GORM for table, column names and so on. You only needs this functionality if you need to in anyway tailor the way GORM maps onto legacy schemas or performs caching

Custom mappings are defined using a a static mapping block defined within your domain class:

class Person {
  ..
  static mapping = {
  }
}

5.5.2.1 Table and Column Names

Table names

The database table name which the class maps to can be customized using a call to table:

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 if its the name you want to change you can do:

class Person {
  String firstName
  static mapping = {
      table 'people'
      firstName column:'First_Name'
  }
}

In this case we define method calls that match each property name (in this case firstName). We then use the named parameter column, to specify the column name to map onto.

Column type

GORM supports configuration of Hibernate types via the DSL using the type attribute. This includes specifing user types that subclass the Hibernate org.hibernate.usertype.UserType class, 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 SQL TEXT or CLOB type depending on which database is being used.

See the Hibernate documentation regarding Basic Types for further information.

One-to-One Mapping

In the case of associations it is also possible to change the foreign keys used to map associations. In the case of a one-to-one association this is exactly the same as any regular column. For example consider the below:

class Person {
  String firstName
  Address address
  static mapping = {
      table 'people'
      firstName column:'First_Name'
	  address column:'Person_Adress_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 simple 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 association 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 the below 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. If you need 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'
   }	
}

You can also specify the name of the join table to use:

class Group {
   …
   static mapping = {
       people column:'Group_Person_Id',joinTable:'PERSON_GROUP_ASSOCIATIONS'
   }	
}
class Person {
   …
   static mapping = {
       groups column:'Group_Group_Id',joinTable:'PERSON_GROUP_ASSOCIATIONS'
   }	
}

5.5.2.2 Caching Strategy

Setting up caching

Hibernate features a second-level cache with a customizable cache provider. This needs to be configured in the grails-app/conf/DataSource.groovy file as follows:

hibernate {
    cache.use_second_level_cache=true
    cache.use_query_cache=true
    cache.provider_class='org.hibernate.cache.EhCacheProvider'
}

You can of course customize these settings how you desire, for example if you want to use a distributed caching mechanism.

For further reading on caching and in particular Hibernate's second-level cache, refer to the Hibernate documentation on the subject.

Caching instances

In your mapping block to enable caching with the default settings use a call to the cache method:

class Person {
  ..
  static mapping = {
      table 'people'
      cache true
  }
}

This will configure a 'read-write' cache that includes both lazy and non-lazy properties. If you need to customize this further you can do:

class Person {
  ..
  static mapping = {
      table 'people'
      cache usage:'read-only', include:'non-lazy'
  }
}

Caching associations

As well as the ability to use Hibernate's second level cache to cache instances you can also cache collections (associations) of objects. For example:

class Person {
  String firstName
  static hasMany = [addresses:Address]
  static mapping = {
      table 'people'
      version false
      addresses column:'Address', cache:true
  }
}
class Address {
   String number
   String postCode
}

This will enable a 'read-write' caching mechanism on the addresses collection. You can also use:

cache:'read-write' // or 'read-only' or 'transactional'

To further configure the cache usage.

Caching Queries

You can cache queries such as dynamic finders and criteria. To do so using a dynamic finder you can pass the cache argument:

def person = Person.findByFirstName("Fred", [cache:true])

Note that in order for the results of the query to be cached, you still need to enable caching in your mapping as discussed in the previous section.

You can also cache criteria queries:

def people = Person.withCriteria {
	like('firstName', 'Fr%')
	cache true
}

Cache usages

Below is a description of the different cache settings and their usages:

• read-only - If your application needs to read but never modify instances of a persistent class, a read-only cache may be used.
• read-write - If the application needs to update data, a read-write cache might be appropriate.
• nonstrict-read-write - If the application only occasionally needs to update data (ie. if it is extremely unlikely that two transactions would try to update the same item simultaneously) and strict transaction isolation is not required, a nonstrict-read-write cache might be appropriate.
• transactional - The transactional cache strategy provides support for fully transactional cache providers such as JBoss TreeCache. Such a cache may only be used in a JTA environment and you must specify hibernate.transaction.manager_lookup_class in the grails-app/conf/DataSource.groovy file's hibernate config.

5.5.2.3 Inheritance Strategies

class Payment {
    Long id
    Long version
    Integer amount
    static mapping = {
        tablePerHierarchy false
    }
}
class CreditCardPayment extends Payment  {
    String cardNumber
}

The mapping of the root Payment class specifies that it will not be using table-per-hierarchy mapping for all child classes.

5.5.2.4 Custom Database Identity

To deal with this Hibernate defines the concept of an id generator. You can customize the id generator and the column it maps to as follows:

class Person {
  ..
  static mapping = {
      table 'people'
      version false
      id generator:'hilo', params:[table:'hi_value',column:'next_value',max_lo:100]
  }
}

In this case we're using one of Hibernate's built in 'hilo' generators that uses a separate table to generate ids.

For more information on the different Hibernate generators refer to the Hibernate reference documentation

Note that if you want to merely customise the column that the id lives on you can do:

class Person {
  ..
  static mapping = {
      table 'people'
      version false
      id column:'person_id'
  }
}

5.5.2.5 Composite Primary Keys

class Person {
  String firstName
  String lastName
  static mapping = {
      id composite:['firstName', 'lastName']
  }
}

The above will create a composite id of the firstName and lastName properties of the Person class. When you later need to retrieve an instance by id you have to use a prototype of the object itself:

def p = Person.get(new Person(firstName:"Fred", lastName:"Flintstone"))
println p.firstName

5.5.2.6 Database Indices

class Person {
  String firstName
  String address
  static mapping = {
      table 'people'
      version false
      id column:'person_id'
      firstName column:'First_Name', index:'Name_Idx'
      address column:'Address', index:'Name_Idx, Address_Index'
  }
}

5.5.2.7 Optimistic Locking and Versioning

If you're mapping to a legacy schema this can be problematic, so you can disable this feature by doing the following:

class Person {
  ..
  static mapping = {
      table 'people'
      version false
  }
}

If you disable optimistic locking you are essentially on your own with regards to concurrent updates and are open to the risk of users losing (due to data overriding) data unless you use pessimistic locking

5.5.2.8 Eager and Lazy Fetching

Lazy Collections

As discussed in the section on Eager and Lazy fetching, by default GORM collections use lazy fetching and is is configurable through the fetchMode setting. However, if you prefer to group all your mappings together inside the mappings block you can also use the ORM DSL to configure fetching:

class Person {
  String firstName
  static hasMany = [addresses:Address]
  static mapping = {
      addresses lazy:false
  }
}
class Address {
  String street
  String postCode
}

Lazy Single-Ended Associations

In GORM, one-to-one and many-to-one associations are by default lazy. Non-lazy single ended associations can be problematic in cases when you are loading many entities which have an association to another entity as a new SELECT statement is executed for each loaded entity.

You can make one-to-one and many-to-one associations non-lazy using the same technique as for lazy collections:

class Person {
	String firstName
	static belongsTo = [address:Address]
	static mapping = {
		address lazy:false
	}
}
class Address {
	String street
	String postCode
}

Here we set the address property of the Person class will be eagerly fetched.

Lazy Single-Ended Associations and Proxies

In order to facilitate single-ended lazy associations Hibernate uses runtime generated proxies. The way this works is that Hibernate dynamically subclasses the proxied entity to create the proxy.

In the previous example Hibernate would create a subclass of Address and return that as a proxy to the real entity. When you call any of the getters or setters Hibernate will initialize the the entity from the database.

Unfortunately this technique can produce surprising results. Consider the following example classes:

class Animal {}
class Mammal extends Animal {}
class Dog extends Mammal {
	String name
}
class Owner {
	Animal pet
}

Given you have an Owner with a pet association that is a Dog consider the following code:

def owner = Owner.get(1)
def pet = Animal.get(owner.petId)
if(pet instanceof Dog) {
	// doesn't work!
}

Now you may think this code will work, but in fact it will not. The reason is Hibernate creates a dynamic proxy by subclassing Animal for the owner.pet association and caches it in the first level cache. So even if the actual proxied class is a Dog it won't be an instance of the Dog class due to the way proxies work.

The get around this problem GORM provides an instanceOf method that should always be used:

def owner = Owner.get(1)
def pet = Animal.get(owner.petId)
if(pet?.instanceof( Dog )) {
	// this works
}

However, there are cases where this particular Hibernate abstraction may still leak through. For example:

def owner = Owner.get(1)
Dog pet = Animal.get(owner.petId)

In this case you will get a ClassCastException because the proxied Animal is not a Dog even though the actual instance is a Dog.

Our best advice is to be aware of Hibernate proxies and how to deal with them when you do run into issues.

5.5.2.9 Custom Cascade Behaviour

However, the ORM DSL gives you complete access to Hibernate's transitive persistence capabilities via the cascade attribute.

Valid settings for the cascade attribute include:

• create - cascades creation of new records from one association to another
• merge - merges the state of a detached association
• save-update - cascades only saves and updates to an association
• delete - cascades only deletes to an association
• lock - useful if a pessimistic lock should be cascaded to its associations
• refresh - cascades refreshes to an association
• evict - cascades evictions (equivalent to discard() in GORM) to associations if set
• all - cascade ALL operations to associations
• delete-orphan - Applies only to one-to-many associations and indicates that when a child is removed from an association then it should be automatically deleted

It is advisable to read the section in the Hibernate documentation on transitive persistence to obtain a better understanding of the different cascade styles and recommendation for their usage

To specific the cascade attribute simply define one or many (comma-separated) of the aforementioned settings as its value:

class Person {
  String firstName
  static hasMany = [addresses:Address]
  static mapping = {
      addresses cascade:"all-delete-orphan"
  }
}
class Address {
  String street
  String postCode
}

5.5.2.10 Custom Hibernate Types

The Hibernate reference manual has some information on custom types, but here we will focus on how to map them in Grails. Let's start by taking a look at a simple domain class that uses an old-fashioned (pre-Java 1.5) type-safe enum class:

class Book {
  String title
  String author
  Rating rating
  static mapping = {
      rating type: RatingUserType
  }
}

All we have done is declare the rating field the enum type and set the property's type in the custom mapping to the corresponding UserType implementation. That's all you have to do to start using your custom type. If you want, you can also use the other column settings such as "column" to change the column name and "index" to add it to an index.

Custom types aren't limited to just a single column - they can be mapped to as many columns as you want. In such cases you need to explicitly define in the mapping what columns to use, since Hibernate can only use the property name for a single column. Fortunately, Grails allows you to map multiple columns to a property using this syntax:

class Book {
  String title
  Name author
  Rating rating
  static mapping = {
      name type: NameUserType, {
          column name: "first_name"
          column name: "last_name"
      }
      rating type: RatingUserType
  }
}

The above example will create "first_name" and "last_name" columns for the author property. You'll be pleased to know that you can also use some of the normal column/property mapping attributes in the column definitions. For example:

column name: "first_name", index: "my_idx", unique: true

The column definitions do not support the following attributes: type, cascade, lazy, cache, and joinTable.

One thing to bear in mind with custom types is that they define the SQL types for the corresponding database columns. That helps take the burden of configuring them yourself, but what happens if you have a legacy database that uses a different SQL type for one of the columns? In that case, you need to override column's SQL type using the sqlType attribute:

class Book {
  String title
  Name author
  Rating rating
  static mapping = {
      name type: NameUserType, {
          column name: "first_name", sqlType: "text"
          column name: "last_name", sqlType: "text"
      }
      rating type: RatingUserType, sqlType: "text"
  }
}

Mind you, the SQL type you specify needs to still work with the custom type. So overriding a default of "varchar" with "text" is fine, but overriding "text" with "yes_no" isn't going to work.

5.5.3 Default Sort Order

def airports = Airport.list(sort:'name')

However, you can also declare the sort order declaratively:

class Airport {
	…
	static mapping = {
		sort "name"
	}
}

You can also configure the sort order if necessary:

class Airport {
	…
	static mapping = {
		sort name:"desc"
	}
}

Alternatively, you can configure sort order at the association level:

class Airport {
	…
	static hasMany = [flights:Flight]
	static mapping = {
		flights sort:'number'
	}
}

5.6 Programmatic Transactions

A typical usage scenario is as follows:

def transferFunds = {
	Account.withTransaction { status ->
		def source = Account.get(params.from)
		def dest = Account.get(params.to)
		def amount = params.amount.toInteger()
		if(source.active) {
			source.balance -= amount
			if(dest.active) {
				dest.amount += amount
			}
			else {
				status.setRollbackOnly()
			}
		}
		
	}
}

In this example we rollback the transactions if the destination account is not active and if any exception are thrown during the process the transaction will automatically be rolled back.

You can also use "save points" to rollback a transaction to a particular point in time if you don't want to rollback the entire transaction. This can be achieved through the use of Spring's SavePointManager interface.

The withTransaction method deals with the begin/commit/rollback logic for you within the scope of the block.

5.7 GORM and Constraints

Where feasible, Grails uses a domain class's constraints to influence the database columns generated for the corresponding domain class properties.

Consider the following example. Suppose we have a domain model with the following property.

String name
String description

By default, in MySQL, Grails would define these columns as...

column name | data type 
 description | varchar(255)

But perhaps the business rules for this domain class state that a description can be up to 1000 characters in length. If that were the case, we'd likely define the column as follows if we were creating the table via an SQL script.

column name | data type 
 description | TEXT

Chances are we'd also want to have some application-based validation to make sure we don't exceed that 1000 character limit before we persist any records. In Grails, we achieve this validation via constraints. We'd add the following constraint declaration to the domain class.

static constraints = {
        description(maxSize:1000)
}

This constraint would provide both the application-based validation we want and it would also cause the schema to be generated as shown above. Below is a description of the other constraints that influence schema generation.

Constraints Affecting String Properties

• inList
• maxSize
• size

If either the maxSize or the size constraint is defined, Grails sets the maximum column length based on the constraint value.

In general, it's not advisable to use both constraints on the same domain class property. However, if both the maxSize constraint and the size constraint are defined, then Grails sets the column length to the minimum of the maxSize constraint and the upper bound of the size constraint. (Grails uses the minimum of the two, because any length that exceeds that minimum will result in a validation error.)

If the inList constraint is defined (and the maxSize and the size constraints are not defined), then Grails sets the maximum column length based on the length of the longest string in the list of valid values. For example, given a list including values "Java", "Groovy", and "C++", Grails would set the column length to 6 (i.e., the number of characters in the string "Groovy").

Constraints Affecting Numeric Properties

• min
• max
• range

If the max constraint, the min constraint, or the range constraint is defined, Grails attempts to set the column precision based on the constraint value. (The success of this attempted influence is largely dependent on how Hibernate interacts with the underlying DBMS.)

In general, it's not advisable to combine the pair min/max and range constraints together on the same domain class property. However, if both of these constraints is defined, then Grails uses the minimum precision value from the constraints. (Grails uses the minimum of the two, because any length that exceeds that minimum precision will result in a validation error.)

• scale

If the scale constraint is defined, then Grails attempts to set the column scale based on the constraint value. This rule only applies to floating point numbers (i.e., java.lang.Float, java.Lang.Double, java.lang.BigDecimal, or subclasses of java.lang.BigDecimal). (The success of this attempted influence is largely dependent on how Hibernate interacts with the underlying DBMS.)

The constraints define the minimum/maximum numeric values, and Grails derives the maximum number of digits for use in the precision. Keep in mind that specifying only one of min/max constraints will not affect schema generation (since there could be large negative value of property with max:100, for example), unless specified constraint value requires more digits than default Hibernate column precision is (19 at the moment). For example...

someFloatValue(max:1000000, scale:3)

would yield:

someFloatValue DECIMAL(19, 3) // precision is default

but

someFloatValue(max:12345678901234567890, scale:5)

would yield:

someFloatValue DECIMAL(25, 5) // precision = digits in max + scale

and

someFloatValue(max:100, min:-100000)

would yield:

someFloatValue DECIMAL(8, 2) // precision = digits in min + default scale

6. The Web Layer

6.1 Controllers

The default URL Mapping setup ensures that the first part of your controller name is mapped to a URI and each action defined within your controller maps to URI within the controller name URI.

6.1.1 Understanding Controllers and Actions

Creating a controller

Controllers can be created with the create-controller target. For example try running the following command from the root of a Grails project:

grails create-controller book

The command will result in the creation of a controller at the location grails-app/controllers/BookController.groovy:

class BookController { … }

BookController by default maps to the /book URI (relative to your application root).

The create-controller command is merely for convenience and you can just as easily create controllers using your favorite text editor or IDE

Creating Actions

A controller can have multiple properties that are each assigned a block of code. Each of these properties maps to a URI:

class BookController {
    def list = {
        // do controller logic
        // create model
        return model
    }
}

This example maps to the /book/list URI by default thanks to the property being named list.

The Default Action

A controller has the concept of a default URI that maps to the root URI of the controller. By default the default URI in this case is /book. The default URI is dictated by the following rules:

• If only one action is present the default URI for a controller maps to that action.
• If you define an index action which is the action that handles requests when no action is specified in the URI /book
• Alternatively you can set it explicitly with the defaultAction property:

def defaultAction = "list"

6.1.2 Controllers and Scopes

Available Scopes

Scopes are essentially hash like objects that allow you to store variables. The following scopes are available to controllers:

• servletContext - Also known as application scope, this scope allows you to share state across the entire web application. The servletContext is an instance of javax.servlet.ServletContext
• session - The session allows associating state with a given user and typically uses cookies to associate a session with a client. The session object is an instance of HttpSession
• request - The request object allows the storage of objects for the current request only. The request object is an instance of HttpServletRequest
• params - Mutable map of incoming request (CGI) parameters
• flash - See below.

Accessing Scopes

Scopes can be accessed using the variable names above in combination with Groovy's array index operator even on classes provided by the Servlet API such as the HttpServletRequest:

class BookController {
    def find = {
        def findBy = params["findBy"]
        def appContext = request["foo"]
        def loggedUser = session["logged_user"]
    }
}

You can even access values within scopes using the de-reference operator making the syntax even clearer:

class BookController {
    def find = {
        def findBy = params.findBy
        def appContext = request.foo
        def loggedUser = session.logged_user
    }
}

This is one of the ways that Grails unifies access to the different scopes.

Using Flash Scope

Grails supports the concept of flash scope is a temporary store for attributes which need to be available for this request and the next request only. Afterwards the attributes are cleared. This is useful for setting a message directly before redirection, for example:

def delete = {
    def b = Book.get( params.id )
    if(!b) {
        flash.message = "User not found for id ${params.id}"
        redirect(action:list)
    }
    … // remaining code
}

6.1.3 Models and Views

Returning the Model

A model is essentially a map that the view uses when rendering. The keys within that map translate to variable names accessible by the view. There are a couple of ways to return a model, the first way is to explicitly return a map instance:

def show = {
 	[ book : Book.get( params.id ) ]
}

If no explicit model is returned the controller's properties will be used as the model thus allowing you to write code like this:

class BookController {
    List books
    List authors
    def list = {
           books = Book.list()
           authors = Author.list()
    }
}

This is possible due to the fact that controllers are prototype scoped. In other words a new controller is created for each request. Otherwise code such as the above would not be thread safe.

In the above example the books and authors properties will be available in the view.

A more advanced approach is to return an instance of the Spring ModelAndView class:

import org.springframework.web.servlet.ModelAndView
def index = {
    def favoriteBooks = … // get some books just for the index page, perhaps your favorites
    // forward to the list view to show them
    return new ModelAndView("/book/list", [ bookList : favoriteBooks ])
}

Selecting the View

In both of the previous two examples there was no code that specified which view to render. So how does Grails know which view to pick? The answer lies in the conventions. For the action:

class BookController {
	def show = {
	 	[ book : Book.get( params.id ) ]
	}	
}

Grails will automatically look for a view at the location grails-app/views/book/show.gsp (actually Grails will try to look for a JSP first, as Grails can equally be used with JSP).

If you wish to render another view, then the render method there to help:

def show = {
  	def map = [ book : Book.get( params.id ) ]
    render(view:"display", model:map)
}

In this case Grails will attempt to render a view at the location grails-app/views/book/display.gsp. Notice that Grails automatically qualifies the view location with the book folder of the grails-app/views directory. This is convenient, but if you have some shared views you need to access instead use:

def show = {
  	def map = [ book : Book.get( params.id ) ]
    render(view:"/shared/display", model:map)
}

In this case Grails will attempt to render a view at the location grails-app/views/shared/display.gsp.

Rendering a Response

Sometimes its easier (typically with Ajax applications) to render snippets of text or code to the response directly from the controller. For this, the highly flexible render method can be used:

render "Hello World!"

The above code writes the text "Hello World!" to the response, other examples include:

// write some markup
render {
   for(b in books) {
      div(id:b.id, b.title)
   }
}
// render a specific view
render(view:'show')
// render a template for each item in a collection
render(template:'book_template', collection:Book.list())
// render some text with encoding and content type
render(text:"<xml>some xml</xml>",contentType:"text/xml",encoding:"UTF-8")

If you plan on using Groovy's MarkupBuilder to generate html for use with the render method becareful of naming clashes between html elements and Grails tags. e.g.

def login = {
        StringWriter w = new StringWriter()
        def builder = new groovy.xml.MarkupBuilder(w)
        builder.html{
            head{
                title 'Log in'
            }
            body{
                h1 'Hello'
                form{
                }
            }
        }
        def html = w.toString()
        render html
    }

Will actually call the form tag (which will return some text that will be ignored by the MarkupBuilder). To correctly output a <form> elemement, use the following:

def login = {
        // …
        body{
            h1 'Hello'
            builder.form{
            }
        }
        // …
   }

6.1.4 Redirects and Chaining

Redirects

Actions can be redirected using the redirect method present in all controllers:

class OverviewController {
                     def login = {}
                     def find = {
                         if(!session.user)
                            redirect(action:login)
                         …
                     }
                 }

Internally the redirect method uses the HttpServletResonse object's sendRedirect method.

The redirect method expects either:

• Another closure within the same controller class:

// Call the login action within the same class
                 redirect(action:login)

• The name of a controller and action:

// Also redirects to the index action in the home controller
                 redirect(controller:'home',action:'index')

• A URI for a resource relative the application context path:

// Redirect to an explicit URI
                 redirect(uri:"/login.html")

• Or a full URL:

// Redirect to a URL
                 redirect(url:"http://grails.org")

Parameters can be optionally passed from one action to the next using the params argument of the method:

redirect(action:myaction, params:[myparam:"myvalue"])

These parameters are made available through the params dynamic property that also accesses request parameters. If a parameter is specified with the same name as a request parameter the request parameter is overridden and the controller parameter used.

Since the params object is also a map, you can use it to pass the current request parameters from one action to the next:

redirect(action:"next", params:params)

Finally, you can also include a fragment in the target URI:

redirect(controller: "test", action: "show", fragment: "profile")

will (depending on the URL mappings) redirect to something like "/myapp/test/show#profile".

h4. Chaining

Actions can also be chained. Chaining allows the model to be retained from one action to the next. For example calling the first action in the below action:

class ExampleChainController {
                     def first = {
                         chain(action:second,model:[one:1])
                     }
                     def second  = {
                         chain(action:third,model:[two:2])
                     }
                     def third = {
                          [three:3])
                     }
                 }

Results in the model:

[one:1, two:2, three:3]

The model can be accessed in subsequent controller actions in the chain via the chainModel map. This dynamic property only exists in actions following the call to the chain method:

class ChainController {
                     def nextInChain = {
                         def model = chainModel.myModel
                         …
                     }
                 }

Like the redirect method you can also pass parameters to the chain method:

chain(action:"action1", model:[one:1], params:[myparam:"param1"])

6.1.5 Controller Interceptors

If your interceptor is likely to apply to more than one controller, you are almost certainly better off writing a Filter. Filters can be applied to multiple controllers or URIs, without the need to change the logic of each controller

Before Interception

The beforeInterceptor intercepts processing before the action is executed. If it returns false then the intercepted action will not be executed. The interceptor can be defined for all actions in a controller as follows:

def beforeInterceptor = {
       println "Tracing action ${actionUri}"
}

The above is declared inside the body of the controller definition. It will be executed before all actions and does not interfere with processing. A common use case is however for authentication:

def beforeInterceptor = [action:this.&auth,except:'login']
// defined as a regular method so its private
def auth() {
     if(!session.user) {
            redirect(action:'login')
            return false
     }
}
def login = {
     // display login page
}

The above code defines a method called auth. A method is used so that it is not exposed as an action to the outside world (i.e. it is private). The beforeInterceptor then defines an interceptor that is used on all actions 'except' the login action and is told to execute the 'auth' method. The 'auth' method is referenced using Groovy's method pointer syntax, within the method itself it detects whether there is a user in the session otherwise it redirects to the login action and returns false, instruction the intercepted action not to be processed.

After Interception

To define an interceptor that is executed after an action use the afterInterceptor property:

def afterInterceptor = { model ->
       println "Tracing action ${actionUri}"
}

The after interceptor takes the resulting model as an argument and can hence perform post manipulation of the model or response.

An after interceptor may also modify the Spring MVC ModelAndView object prior to rendering. In this case, the above example becomes:

def afterInterceptor = { model, modelAndView ->
       println "Current view is ${modelAndView.viewName}"
       if(model.someVar) modelAndView.viewName = "/mycontroller/someotherview"
       println "View is now ${modelAndView.viewName}"
}

This allows the view to be changed based on the model returned by the current action. Note that the modelAndView may be null if the action being intercepted called redirect or render.

Interception Conditions

Rails users will be familiar with the authentication example and how the 'except' condition was used when executing the interceptor (interceptors are called 'filters' in Rails, this terminology conflicts with the servlet filter terminology in Java land):

def beforeInterceptor = [action:this.&auth,except:'login']

This executes the interceptor for all actions except the specified action. A list of actions can also be defined as follows:

def beforeInterceptor = [action:this.&auth,except:['login','register']]

The other supported condition is 'only', this executes the interceptor for only the specified actions:

def beforeInterceptor = [action:this.&auth,only:['secure']]

6.1.6 Data Binding

Grails uses Spring's underlying data binding capability to perform data binding.

Binding Request Data to the Model

There are two ways to bind request parameters onto the properties of a domain class. The first involves using a domain classes' implicit constructor:

def save = {
  def b = new Book(params)
  b.save()
}

The data binding happens within the code new Book(params). By passing the params object to the domain class constructor Grails automatically recognizes that you are trying to bind from request parameters. So if we had an incoming request like:

/book/save?title=The%20Stand&author=Stephen%20King

Then the title and author request parameters would automatically get set on the domain class. If you need to perform data binding onto an existing instance then you can use the properties property:

def save = {
  def b = Book.get(params.id)
  b.properties = params
  b.save()
}

This has exactly the same effect as using the implicit constructor.

Data binding and Single-ended Associations

If you have a one-to-one or many-to-one association you can use Grails' data binding capability to update these relationships too. For example if you have an incoming request such as:

/book/save?author.id=20

Grails will automatically detect the .id suffix on the request parameter and look-up the Author instance for the given id when doing data binding such as:

def b = new Book(params)

Data Binding and Many-ended Associations

If you have a one-to-many or many-to-many association there are different techniques for data binding depending of the association type.

If you have a Set based association (default for a hasMany) then the simplest way to populate an association is to simply send a list of identifiers. For example consider the usage of <g:select> below:

<g:select name="books"
          from="${Book.list()}"
          size="5" multiple="yes" optionKey="id"
          value="${author?.books}" />

This produces a select box that allows you to select multiple values. In this case if you submit the form Grails will automatically use the identifiers from the select box to populate the books association.

However, if you have a scenario where you want to update the properties of the associated objects the this technique won't work. Instead you have to use the subscript operator:

<g:textField name="books[0].title" value="the Stand" />
<g:textField name="books[1].title" value="the Shining" />

However, with Set based association it is critical that you render the mark-up in the same order that you plan to do the update in. This is because a Set has no concept of order, so although we're referring to books0 and books1 it is not guaranteed that the order of the association will be correct on the server side unless you apply some explicit sorting yourself.

This is not a problem if you use List based associations, since a List has a defined order and an index you can refer to. This is also true of Map based associations.

Note also that if the association you are binding to has a size of 2 and you refer to an element that is outside the size of association:

<g:textField name="books[0].title" value="the Stand" />
<g:textField name="books[1].title" value="the Shining" />
<g:textField name="books[2].title" value="Red Madder" />

Then Grails will automatically create a new instance for you at the defined position. If you "skipped" a few elements in the middle:

<g:textField name="books[0].title" value="the Stand" />
<g:textField name="books[1].title" value="the Shining" />
<g:textField name="books[5].title" value="Red Madder" />

Then Grails will automatically create instances in between. For example in the above case Grails will create 4 additional instances if the association being bound had a size of 2.

Data binding with Multiple domain classes

It is possible to bind data to multiple domain objects from the params object.

For example so you have an incoming request to:

/book/save?book.title=The%20Stand&author.name=Stephen%20King

You'll notice the difference with the above request is that each parameter has a prefix such as author. or book. which is used to isolate which parameters belong to which type. Grails' params object is like a multi-dimensional hash and you can index into to isolate only a subset of the parameters to bind.

def b = new Book(params['book'])

Notice how we use the prefix before the first dot of the book.title parameter to isolate only parameters below this level to bind. We could do the same with an Author domain class:

def a = new Author(params['author'])

Data binding and type conversion errors

Sometimes when performing data binding it is not possible to convert a particular String into a particular target type. What you get is a type conversion error. Grails will retain type conversion errors inside the errors property of a Grails domain class. Take this example:

class Book {
    …
    URL publisherURL
}

Here we have a domain class Book that uses the Java concrete type java.net.URL to represent URLs. Now say we had an incoming request such as:

/book/save?publisherURL=a-bad-url

In this case it is not possible to bind the string a-bad-url to the publisherURL property os a type mismatch error occurs. You can check for these like this:

def b = new Book(params)
if(b.hasErrors()) {
   println "The value ${b.errors.getFieldError('publisherURL').rejectedValue} is not a valid URL!"
}

Although we have not yet covered error codes (for more information see the section on Validation), for type conversion errors you would want a message to use for the error inside the grails-app/i18n/messages.properties file. You can use a generic error message handler such as:

typeMismatch.java.net.URL=The field {0} is not a valid URL

Or a more specific one:

typeMismatch.Book.publisherURL=The publisher URL you specified is not a valid URL

Data Binding and Security concerns

When batch updating properties from request parameters you need to be careful not to allow clients to bind malicious data to domain classes that end up being persisted to the database. You can limit what properties are bound to a given domain class using the subscript operator:

def p = Person.get(1)
p.properties['firstName','lastName'] = params

In this case only the firstName and lastName properties will be bound.

Another way to do this is instead of using domain classes as the target of data binding you could use Command Objects. Alternatively there is also the flexible bindData method.

The bindData method allows the same data binding capability, but to arbitrary objects:

def p = new Person()
bindData(p, params)

However, the bindData method also allows you to exclude certain parameters that you don't want updated:

def p = new Person()
bindData(p, params, [exclude:'dateOfBirth'])

Or include only certain properties:

def p = new Person()
bindData(p, params, [include:['firstName','lastName]])

6.1.7 XML and JSON Responses

Using the render method to output XML

Grails' supports a few different ways to produce XML and JSON responses. The first one covered is via the render method.

The render method can be passed a block of code to do mark-up building in XML:

def list = {
	def results = Book.list()
	render(contentType:"text/xml") {
		books {
			for(b in results) {
				book(title:b.title)
			}
		}	
	}
}

The result of this code would be something like:

<books>
	  <book title="The Stand" />
	  <book title="The Shining" />	
</books>

Note that you need to be careful to avoid naming conflicts when using mark-up building. For example this code would produce an error:

def list = {
	def books = Book.list()  // naming conflict here
	render(contentType:"text/xml") {
		books {
			for(b in results) {
				book(title:b.title)
			}
		}	
	}
}

The reason is that there is local variable books which Groovy attempts to invoke as a method.

Using the render method to output JSON

The render method can also be used to output JSON:

def list = {
	def results = Book.list()
	render(contentType:"text/json") {
		books = array {
			for(b in results) {
				book title:b.title
			}
		}	
	}
}

In this case the result would be something along the lines of:

[
	{title:"The Stand"}, 
	{title:"The Shining"}
]

Again the same dangers with naming conflicts apply to JSON building.

Automatic XML Marshalling

Grails also supports automatic marshaling of domain classes to XML via special converters.

To start off with import the grails.converters package into your controller:

import grails.converters.*

Now you can use the following highly readable syntax to automatically convert domain classes to XML:

render Book.list() as XML

The resulting output would look something like the following::

<?xml version="1.0" encoding="ISO-8859-1"?>
<list>
  <book id="1">
    <author>Stephen King</author>
    <title>The Stand</title>
  </book>
  <book id="2">
    <author>Stephen King</author>
    <title>The Shining</title>
  </book>
</list>

An alternative to using the converters is to use the codecs feature of Grails. The codecs feature provides encodeAsXML and encodeAsJSON methods:

def xml = Book.list().encodeAsXML()
render xml

For more information on XML marshaling see the section on REST

Automatic JSON Marshalling

Grails also supports automatic marshaling to JSON via the same mechanism. Simply substitute XML with JSON:

render Book.list() as JSON

The resulting output would look something like the following:

[
	{"id":1,
	 "class":"Book",
	 "author":"Stephen King",
	 "title":"The Stand"},
	{"id":2,
	 "class":"Book",
	 "author":"Stephen King",
	 "releaseDate":new Date(1194127343161),
	 "title":"The Shining"}
 ]

Again as an alternative you can use the encodeAsJSON to achieve the same effect.

6.1.8 More on JSONBuilder

JSONBuilder and Grails versions

JSONBuilder behaves different depending on the version of Grails you use. For version below 1.2 there deprecated grails.util.JSonBuilder class is used. This section covers the usage of the Grails 1.2 JSONBuilder

For backwards compatibility the old JSonBuilder class is used with the render method for older applications, if you want to use the newer/better JSONBuilder class then you can do so by setting the following in Config.groovy:

grails.json.legacy.builder=false

Rendering Simple Objects

To render a simple JSON object just set properties within the context of the closure:

render(contentType:"text/json") {
	hello = "world"
}

The above will produce the JSON:

{"hello":"world"}

Rendering JSON Arrays

To render a list of objects simple assign a list:

render(contentType:"text/json") {
	categories = ['a', 'b', 'c']
}

This will produce:

{"categories":["a","b","c"]}

You can also render lists of complex objects, for example:

render(contentType:"text/json") {
	categories = [ { a = "A" }, { b = "B" } ]
}

This will produce:

{"categories":[ {"a":"A"} , {"b":"B"}] }

If you want to return a list as the root then you have to use the special element method:

render(contentType:"text/json") {
	element 1
	element 2
	element 3		
}

The above code produces:

[1,2,3]

Rendering Complex Objects

Rendering complex objects can be done with closures. For example:

render(contentType:"text/json") {
    categories = ['a', 'b', 'c']
    title ="Hello JSON"
    information = {
        pages = 10
    }
}

The above will produce the JSON:

{"categories":["a","b","c"],"title":"Hello JSON","information":{"pages":10}}

Arrays of Complex Objects

As mentioned previously you can nest complex objects within arrays using closures:

render(contentType:"text/json") {
	categories = [ { a = "A" }, { b = "B" } ]
}

However, if you need to build them up dynamically then you may want to use the array method:

def results = Book.list()
render(contentType:"text/json") {
	books = array {
		for(b in results) {
			book title:b.title
		}
	}	
}

Direct JSONBuilder API Access

If you don't have access to the render method, but still want to produce JSON you can use the API directly:

def builder = new JSONBuilder()
def result = builder.build {
    categories = ['a', 'b', 'c']
    title ="Hello JSON"
    information = {
        pages = 10
    }
}
// prints the JSON text
println result.toString()
def sw = new StringWriter()
result.render sw

6.1.9 Uploading Files

Programmatic File Uploads

Grails supports file uploads via Spring's MultipartHttpServletRequest interface. To upload a file the first step is to create a multipart form like the one below:

Upload Form: <br />
	<g:form action="upload" method="post" enctype="multipart/form-data">
		<input type="file" name="myFile" />
		<input type="submit" />
	</g:form>

There are then a number of ways to handle the file upload. The first way is to work with the Spring MultipartFile instance directly:

def upload = {
    def f = request.getFile('myFile')
    if(!f.empty) {
      f.transferTo( new File('/some/local/dir/myfile.txt') )
      response.sendError(200,'Done');
    }    
    else {
       flash.message = 'file cannot be empty'
       render(view:'uploadForm')
    }
}

This is clearly handy for doing transfers to other destinations and manipulating the file directly as you can obtain an InputStream and so on via the MultipartFile interface.

File Uploads through Data Binding

File uploads can also be performed via data binding. For example say you have an Image domain class as per the below example:

class Image {
   byte[] myFile
}

Now if you create an image and pass in the params object such as the below example, Grails will automatically bind the file's contents as a byte to the myFile property:

def img = new Image(params)

It is also possible to set the contents of the file as a string by changing the type of the myFile property on the image to a String type:

class Image {
   String myFile
}

6.1.10 Command Objects

Declaring Command Objects

Command objects are typically declared in the same source file as a controller directly below the controller class definition. For example:

class UserController {
	…
}
class LoginCommand {
   String username
   String password
   static constraints = {
           username(blank:false, minSize:6)
           password(blank:false, minSize:6)
   }
}

As the previous example demonstrates you can supply constraints to command objects just as you can with domain classes.

Using Command Objects

To use command objects, controller actions may optionally specify any number of command object parameters. The parameter types must be supplied so that Grails knows what objects to create, populate and validate.

Before the controller action is executed Grails will automatically create an instance of the command object class, populate the properties of the command object with request parameters having corresponding names and the command object will be validated. For Example:

class LoginController {
  def login = { LoginCommand cmd ->
         if(cmd.hasErrors()) {
                redirect(action:'loginForm')
         }
         else {
            // do something else
        }
  }
}

Command Objects and Dependency Injection

Command objects can participate in dependency injection. This is useful if your command object has some custom validation logic which may need to interact with Grails services:

class LoginCommand {
    def loginService
	String username
	String password
	static constraints = {
		username(validator: { val, obj ->
			obj.loginService.canLogin(obj.username, obj.password)
		})
	}
}

In this example the command object interacts with a bean injected by name from the Spring ApplicationContext.

6.1.11 Handling Duplicate Form Submissions

<g:form useToken="true" ...>

Then in your controller code you can use the withForm method to handle valid and invalid requests:

withForm {
   // good request
}.invalidToken {
   // bad request
}

If you only provide the withForm method and not the chained invalidToken method then by default Grails will store the invalid token in a flash.invalidToken variable and redirect the request back to the original page. This can then be checked in the view:

<g:if test="${flash.invalidToken}">
  Don't click the button twice!
</g:if>

The withForm tag makes use of the session and hence requires session affinity if used in a cluster.

6.1.12 Simple Type Converters

Type Conversion Methods

If you prefer to avoid the overhead of Data Binding and simply want to convert incoming parameters (typically Strings) into another more appropriate type the params object has a number of convenience methods for each type:

def total = params.int('total')

The above example uses the int method, there are also methods for boolean, long, char, short and so on. Each of these methods are null safe and safe from any parsing errors so you don't have to perform any addition checks on the parameters.

These same type conversion methods are also available on the tagLibraries parameter of GSP tags.

Handling Multi Parameters

A common use case is dealing with multiple request parameters of the same name. For example you could get a query string such as ?name=Bob&name=Judy.

In this case dealing with 1 parameter and dealing with many has different semantics since Groovy's iteration mechanics for String iterate over each character. To avoid this problem the params object provides a list method that always returns a list:

for(name in params.list('name')) {
	println name
}

6.2 Groovy Server Pages

In Grails GSPs live in the grails-app/views directory and are typically rendered automatically (by convention) or via the render method such as:

render(view:"index")

A GSP is typically a mix of mark-up and GSP tags which aid in view rendering.

Although it is possible to have Groovy logic embedded in your GSP and doing this will be covered in this document the practice is strongly discouraged. Mixing mark-up and code is a bad thing and most GSP pages contain no code and needn't do so.

A GSP typically has a "model" which is a set of variables that are used for view rendering. The model is passed to the GSP view from a controller. For example consider the following controller action:

def show = {
	[book: Book.get(params.id)]
}

This action will look-up a Book instance and create a model that contains a key called book. This key can then be reference within the GSP view using the name book:

<%=book.title%>

6.2.1 GSP Basics

GSP supports the usage of <% %> blocks to embed Groovy code (again this is discouraged):

<html>
   <body>
     <% out << "Hello GSP!" %>
   </body>
</html>

As well as this syntax you can also use the <%= %> syntax to output values:

<html>
   <body>
     <%="Hello GSP!" %>
   </body>
</html>

GSP also supports JSP-style server-side comments as the following example demonstrates:

<html>
   <body>
	 <%-- This is my comment --%>
     <%="Hello GSP!" %>
   </body>
</html>

6.2.1.1 Variables and Scopes

<% now = new Date() %>

And then re-use those variables further down the page:

<%=now%>

However, within the scope of a GSP there are a number of pre-defined variables including:

• application - The javax.servlet.ServletContext instance
• applicationContext The Spring ApplicationContext instance
• flash - The flash object
• grailsApplication - The GrailsApplication instance
• out - The response writer for writing to the output stream
• params - The params object for retrieving request parameters
• request - The HttpServletRequest instance
• response - The HttpServletResponse instance
• session - The HttpSession instance
• webRequest - The GrailsWebRequest instance

6.2.1.2 Logic and Iteration

<html>
   <body>
      <% [1,2,3,4].each { num -> %>
         <p><%="Hello ${num}!" %></p>
      <%}%>
   </body>
</html>

As well as logical branching:

<html>
   <body>
      <% if(params.hello == 'true' )%>	
      <%="Hello!"%>
      <% else %>
      <%="Goodbye!"%>
   </body>
</html>

6.2.1.3 Page Directives

The import directive allows you to import classes into the page. However, it is rarely needed due to Groovy's default imports and GSP Tags:

<%@ page import="java.awt.*" %>

GSP also supports the contentType directive:

<%@ page contentType="text/json" %>

The contentType directive allows using GSP to render other formats.

6.2.1.4 Expressions

<html>
  <body>
    Hello ${params.name}
  </body>
</html>

However, unlike JSP EL you can have any Groovy expression within the ${..} parenthesis. Variables within the ${..} are not escaped by default, so any HTML in the variable's string is output directly to the page. To reduce the risk of Cross-site-scripting (XSS) attacks, you can enable automatic HTML escaping via the grails.views.default.codec setting in grails-app/conf/Config.groovy:

grails.views.default.codec='html'

Other possible values are 'none' (for no default encoding) and 'base64'.

6.2.2 GSP Tags

The section on Tag Libraries covers how to add your own custom tag libraries.

All built-in GSP tags start with the prefix g:. Unlike JSP, you don't need to specify any tag library imports. If a tag starts with g: it is automatically assumed to be a GSP tag. An example GSP tag would look like:

<g:example />

GSP tags can also have a body such as:

<g:example>
   Hello world
</g:example>

Expressions can be passed into GSP tag attributes, if an expression is not used it will be assumed to be a String value:

<g:example attr="${new Date()}">
   Hello world
</g:example>

Maps can also be passed into GSP tag attributes, which are often used for a named parameter style syntax:

<g:example attr="${new Date()}" attr2="[one:1, two:2, three:3]">
   Hello world
</g:example>

Note that within the values of attributes you must use single quotes for Strings:

<g:example attr="${new Date()}" attr2="[one:'one', two:'two']">
   Hello world
</g:example>

With the basic syntax out the way, the next sections look at the tags that are built into Grails by default.

6.2.2.1 Variables and Scopes

<g:set var="now" value="${new Date()}" />

Here we assign a variable called now to the result of a GSP expression (which simply constructs a new java.util.Date instance). You can also use the body of the <g:set> tag to define a variable:

<g:set var="myHTML">
   Some re-usable code on: ${new Date()}
</g:set>

Variables can also be placed in one of the following scopes:

• page - Scoped to the current page (default)
• request - Scoped to the current request
• flash - Placed within flash scope and hence available for the next request
• session - Scoped for the user session
• application - Application-wide scope.

To select which scope a variable is placed into use the scope attribute:

<g:set var="now" value="${new Date()}" scope="request" />

6.2.2.2 Logic and Iteration

<g:if test="${session.role == 'admin'}">
   <%-- show administrative functions --%>
</g:if>
<g:else>
   <%-- show basic functions --%>
</g:else>

For iteration GSP has the each and while tags:

<g:each in="${[1,2,3]}" var="num">
   <p>Number ${num}</p>
</g:each>
<g:set var="num" value="${1}" />
<g:while test="${num < 5 }">
    <p>Number ${num++}</p>
</g:while>

6.2.2.3 Search and Filtering

Stephen King's Books:
<g:findAll in="${books}" expr="it.author == 'Stephen King'">
     <p>Title: ${it.title}</p>
</g:findAll>

The expr attribute contains a Groovy expression that can be used as a filter. Speaking of filters the grep tag does a similar job such as filter by class:

<g:grep in="${books}" filter="NonFictionBooks.class">
     <p>Title: ${it.title}</p>
</g:grep>

Or using a regular expression:

<g:grep in="${books.title}" filter="~/.*?Groovy.*?/">
     <p>Title: ${it}</p>
</g:grep>

The above example is also interesting due to its usage of GPath. GPath is Groovy's equivalent to an XPath like language. Essentially the books collection is a collection of Book instances. However assuming each Book has a title, you can obtain a list of Book titles using the expression books.title. Groovy will auto-magically go through the list of Book instances, obtain each title, and return a new list!

6.2.2.4 Links and Resources

<g:link action="show" id="1">Book 1</g:link>
<g:link action="show" id="${currentBook.id}">${currentBook.name}</g:link>
<g:link controller="book">Book Home</g:link>
<g:link controller="book" action="list">Book List</g:link>
<g:link url="[action:'list',controller:'book']">Book List</g:link>
<g:link action="list" params="[sort:'title',order:'asc',author:currentBook.author]">
     Book List
</g:link>

6.2.2.5 Forms and Fields

Form Basics

GSP supports many different tags for aiding in dealing with HTML forms and fields, the most basic of which is the form tag. The form tag is a controller/action aware version of the regular HTML form tag. The url attribute allows you to specify which controller and action to map to:

<g:form name="myForm" url="[controller:'book',action:'list']">...</g:form>

In this case we create a form called myForm that submits to the BookController's list action. Beyond that all of the usual HTML attributes apply.

Form Fields

As well as easy construction of forms GSP supports custom tags for dealing with different types of fields including:

• textField - For input fields of type 'text'
• checkBox - For input fields of type 'checkbox'
• radio - For input fields of type 'radio'
• hiddenField - For input fields of type 'hidden'
• select - For dealing with HTML select boxes

Each of these allow GSP expressions as the value:

<g:textField name="myField" value="${myValue}" />

GSP also contains extended helper versions of the above tags such as radioGroup (for creating groups of radio tags), localeSelect, currencySelect and timeZoneSelect (for selecting locale's, currencies and time zone's respectively).

Multiple Submit Buttons

The age old problem of dealing with multiple submit buttons is also handled elegantly with Grails via the actionSubmit tag. It is just like a regular submit, but allows you to specify an alternative action to submit to:

<g:actionSubmit value="Some update label" action="update" />

6.2.2.6 Tags as Method Calls

Tags as method calls from GSPs

When called as methods tags return their results as a String instead of writing directly to the response. So for example the createLinkTo tag can equally be called as a method:

Static Resource: ${createLinkTo(dir:"images", file:"logo.jpg")}

This is particularly useful when you need to use a tag within an attribute:

<img src="${createLinkTo(dir:'images', file:'logo.jpg')}" />

In view technologies that don't support this feature you have to nest tags within tags, which becomes messy quickly and often has an adverse effect of WYSWIG tools such as Dreamweaver that attempt to render the mark-up as it is not well-formed:

<img src="<g:createLinkTo dir="images" file="logo.jpg" />" />

Tags as method calls from Controllers and Tag Libraries

You can also invoke tags from controllers and tag libraries. Tags within the default g: namespace can be invoked without the prefix and a String result is returned:

def imageLocation = createLinkTo(dir:"images", file:"logo.jpg")

However, you can also prefix the namespace to avoid naming conflicts:

def imageLocation = g.createLinkTo(dir:"images", file:"logo.jpg")

If you have a custom namespace you can use that prefix instead (Example using the FCK Editor plugin):

def editor = fck.editor()

6.2.3 Views and Templates

Template Basics

Grails uses the convention of placing an underscore before the name of a view to identify it as a template. For example a you may have a template that deals with rendering Books located at grails-app/views/book/_bookTemplate.gsp:

<div class="book" id="${book?.id}">
   <div>Title: ${book?.title}</div>
   <div>Author: ${book?.author?.name}</div>
</div>

To render this template from one of the views in grails-app/views/book you can use the render tag:

<g:render template="bookTemplate" model="[book:myBook]" />

Notice how we pass into a model to use using the model attribute of the render tag. If you have multiple Book instances you can also render the template for each Book using the render tag:

<g:render template="bookTemplate" var="book" collection="${bookList}" />

Shared Templates

In the previous example we had a template that was specific to the BookController and its views at grails-app/views/book. However, you may want to share templates across your application.

In this case you can place them in the root views directory at grails-app/views or any subdirectory below that location and then with the template attribute use a / before the template name to indicate the relative template path. For example if you had a template called grails-app/views/shared/_mySharedTemplate.gsp, you could reference it as follows:

<g:render template="/shared/mySharedTemplate" />

You can also use this technique to reference templates in any directory from any view or controller:

<g:render template="/book/bookTemplate" model="[book:myBook]" />

The Template Namespace

Since templates are used so frequently there is template namespace, called tmpl, available that makes using templates easier. Consider for example the following usage pattern:

<g:render template="bookTemplate" model="[book:myBook]" />

This can be expressed with the tmpl namespace as follows:

<tmpl:bookTemplate book="${myBook}" />

Templates in Controllers and Tag Libraries

You can also render templates from controllers using the render method found within controllers, which is useful for Ajax applications:

def show = {
    def b = Book.get(params.id)
	render(template:"bookTemplate", model:[book:b])
}

The render method within controllers writes directly to the response, which is the most common behaviour. If you need to instead obtain the result of template as a String you can use the render tag:

def show = {
    def b = Book.get(params.id)
	String content = g.render(template:"bookTemplate", model:[book:b])
	render content
}

Notice the usage of the g. namespace which tells Grails we want to use the tag as method call instead of the render method.

6.2.4 Layouts with Sitemesh

Creating Layouts

Grails leverages Sitemesh, a decorator engine, to support view layouts. Layouts are located in the grails-app/views/layouts directory. A typical layout can be seen below:

<html>
      <head>
          <title><g:layoutTitle default="An example decorator" /></title>
          <g:layoutHead />
      </head>
      <body onload="${pageProperty(name:'body.onload')}">
            <div class="menu"><!--my common menu goes here--></menu>
                 <div class="body">
                      <g:layoutBody />
                 </div>
            </div>
      </body>
</html>

The key elements are the layoutHead, layoutTitle and layoutBody tag usages, here is what they do:

• layoutTitle - outputs the target page's title
• layoutHead - outputs the target pages head tag contents
• layoutBody - outputs the target pages body tag contents

The previous example also demonstrates the pageProperty tag which can be used to inspect and return aspects of the target page.

Triggering Layouts

There are a few ways to trigger a layout. The simplest is to add a meta tag to the view:

<html>
    <head>
	    <title>An Example Page</title>
        <meta name="layout" content="main"></meta>
    </head>
    <body>This is my content!</body>
</html>

In this case a layout called grails-app/views/layouts/main.gsp will be used to layout the page. If we were to use the layout from the previous section the output would resemble the below:

<html>
      <head>
          <title>An Example Page</title>
      </head>
      <body onload="">
        <div class="menu"><!--my common menu goes here--></div>
                 <div class="body">
					This is my content!
                 </div>
      </body>
</html>

Specifying A Layout In A Controller

Another way to specify a layout is to specify the name of the layout by assigning a value to the "layout" property in a controller. For example, if you have a controller such as:

class BookController {
    static layout = 'customer'
    def list = {  … }
}

You can create a layout called grails-app/views/layouts/customer.gsp which will be applied to all views that the BookController delegates to. The value of the "layout" property may contain a directory structure relative to the grails-app/views/layouts/ directory. For example:

class BookController {
    static layout = 'custom/customer'
    def list = {  … }
}

Views rendered from that controller would be decorated with the grails-app/views/layouts/custom/customer.gsp template.

Layout by Convention

Another way to associate layouts is to use "layout by convention". For example, if you have a controller such as:

class BookController {
    def list = {  … }
}

You can create a layout called grails-app/views/layouts/book.gsp, by convention, which will be applied to all views that the BookController delegates to.

Alternatively, you can create a layout called grails-app/views/layouts/book/list.gsp which will only be applied to the list action within the BookController.

If you have both the above mentioned layouts in place the layout specific to the action will take precedence when the list action is executed.

Inline Layouts

Grails' also supports Sitemesh's concept of inline layouts with the applyLayout tag. The applyLayout tag can be used to apply a layout to a template, URL or arbitrary section of content. Essentially, this allows to even further modularize your view structure by "decorating" your template includes.

Some examples of usage can be seen below:

<g:applyLayout name="myLayout" template="bookTemplate" collection="${books}" />
<g:applyLayout name="myLayout" url="http://www.google.com" />
<g:applyLayout name="myLayout">
The content to apply a layout to
</g:applyLayout>

Server-Side Includes

While the applyLayout tag is useful for applying layouts to external content, if you simply want to include external content in the current page you can do so with the include:

<g:include controller="book" action="list"></g:include>

You can even combine the include tag and the applyLayout tag for added flexibility:

<g:applyLayout name="myLayout">
   <g:include controller="book" action="list"></g:include>
</g:applyLayout>

Finally, you can also call the include tag from a controller or tag library as a method:

def content = include(controller:"book", action:"list")

The resulting content will be provided via the return value of the include tag.

6.2.5 Sitemesh Content Blocks

<content tag="navbar">
… draw the navbar here…
</content>
<content tag="header">
… draw the header here…
</content>
<content tag="footer">
… draw the footer here…
</content>
<content tag="body">
… draw the body here…
</content>

Then within the layout you can reference these components and apply individual layouts to each:

<html>
	<body>
		<div id="header">
			<g:applyLayout name="headerLayout"><g:pageProperty name="page.header"></g:applyLayout>
		</div>
		<div id="nav">
			<g:applyLayout name="navLayout"><g:pageProperty name="page.navbar"></g:applyLayout>
		</div>
		<div id="body">
			<g:applyLayout name="bodyLayout"><g:pageProperty name="page.body"></g:applyLayout>
		</div>
		<div id="footer">
			<g:applyLayout name="footerLayout"><g:pageProperty name="page.footer"></g:applyLayout>			
		</div>
	</body>
</html>

6.3 Tag Libraries

Quite simply, to create a tag library create a Groovy class that ends with the convention TagLib and place it within the grails-app/taglib directory:

class SimpleTagLib {
}

Now to create a tag simply create property that is assigned a block of code that takes two arguments: The tag attributes and the body content:

class SimpleTagLib {
	def simple = { attrs, body ->
    }
}

The attrs argument is a simple map of the attributes of the tag, whilst the body argument is another invokable block of code that returns the body content:

class SimpleTagLib {
	def emoticon = { attrs, body ->
	   out << body() << (attrs.happy == 'true' ? " :-)" : " :-(")	
    }
}

As demonstrated above there is an implicit out variable that refers to the output Writer which you can use to append content to the response. Then you can simply reference the tag inside your GSP, no imports necessary:

<g:emoticon happy="true">Hi John</g:emoticon>

6.3.1 Variables and Scopes

• actionName - The currently executing action name
• controllerName - The currently executing controller name
• flash - The flash object
• grailsApplication - The GrailsApplication instance
• out - The response writer for writing to the output stream
• pageScope - A reference to the pageScope object used for GSP rendering (i.e. the binding)
• params - The params object for retrieving request parameters
• pluginContextPath - The context path to the plugin that contains the tag library
• request - The HttpServletRequest instance
• response - The HttpServletResponse instance
• servletContext - The javax.servlet.ServletContext instance
• session - The HttpSession instance

6.3.2 Simple Tags

def dateFormat = { attrs, body ->
	out << new java.text.SimpleDateFormat(attrs.format).format(attrs.date)
}

The above uses Java's SimpleDateFormat class to format a date and then write it to the response. The tag can then be used within a GSP as follows:

<g:dateFormat format="dd-MM-yyyy" date="${new Date()}" />

With simple tags sometimes you need to write HTML mark-up to the response. One approach would be to embed the content directly:

def formatBook = { attrs, body ->
    out << "<div id="${attrs.book.id}">"	
    out << "Title : ${attrs.book.title}"	
	out << "</div>"
}

Although this approach may be tempting it is not very clean. A better approach would be to re-use the render tag:

def formatBook = { attrs, body ->
    out << render(template:"bookTemplate", model:[book:attrs.book])	
}

And then have a separate GSP template that does the actual rendering.

6.3.3 Logical Tags

def isAdmin = { attrs, body ->
     def user = attrs['user']
     if(user != null && checkUserPrivs(user)) {
           out << body()
     }
}

The tag above checks if the user is an administrator and only outputs the body content if he/she has the correct set of access privileges:

<g:isAdmin user="${myUser}">
    // some restricted content
</g:isAdmin>

6.3.4 Iterative Tags

def repeat = { attrs, body ->
    attrs.times?.toInteger().times { num ->
        out << body(num)
    }
}

In this example we check for a times attribute and if it exists convert it to a number then use Groovy's times method to iterate by the number of times specified by the number:

<g:repeat times="3">
<p>Repeat this 3 times! Current repeat = ${it}</p>
</g:repeat>

Notice how in this example we use the implicit it variable to refer to the current number. This works because when we invoked the body we passed in the current value inside the iteration:

out << body(num)

That value is then passed as the default variable it to the tag. However, if you have nested tags this can lead to conflicts, hence you should should instead name the variables that the body uses:

def repeat = { attrs, body ->
	def var = attrs.var ? attrs.var : "num"
    attrs.times?.toInteger().times { num ->
        out << body((var):num)
    }
}

Here we check if there is a var attribute and if there is use that as the name to pass into the body invocation on this line:

out << body((var):num)

Note the usage of the parenthesis around the variable name. If you omit these Groovy assumes you are using a String key and not referring to the variable itself.

Now we can change the usage of the tag as follows:

<g:repeat times="3" var="j">
<p>Repeat this 3 times! Current repeat = ${j}</p>
</g:repeat>

Notice how we use the var attribute to define the name of the variable j and then we are able to reference that variable within the body of the tag.

6.3.5 Tag Namespaces

class SimpleTagLib {
    static namespace = "my"
    def example = { attrs ->
        …
    }
}

Here we have specified a namespace of my and hence the tags in this tag lib must then be referenced from GSP pages like this:

<my:example name="..." />

Where the prefix is the same as the value of the static namespace property. Namespaces are particularly useful for plugins.

Tags within namespaces can be invoked as methods using the namespace as a prefix to the method call:

out << my.example(name:"foo")

This works from GSP, controllers or tag libraries

6.3.6 Using JSP Tag Libraries

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

Then you can use it like any other tag:

<fmt:formatNumber value="${10}" pattern=".00"/>

With the added bonus that you can invoke JSP tags like methods:

${fmt.formatNumber(value:10, pattern:".00")}

6.3.7 Tag return value

Tag libraries can also return direct object values to the caller since Grails 1.2.. Object returning tag names are listed in a static returnObjectForTags property in the tag library class.

Example:

class ObjectReturningTagLib {
	static namespace = "cms"
	static returnObjectForTags = ['content']
	def content = { attrs, body ->
		CmsContent.findByCode(attrs.code)?.content	
    }
}

6.4 URL Mappings

The UrlMappings class contains a single property called mappings that has been assigned a block of code:

class UrlMappings {
    static mappings = {
    }	
}

6.4.1 Mapping to Controllers and Actions

"/product"(controller:"product", action:"list")

In this case we've mapped the URL /product to the list action of the ProductController. You could of course omit the action definition to map to the default action of the controller:

"/product"(controller:"product")

An alternative syntax is to assign the controller and action to use within a block passed to the method:

"/product" {
	controller = "product"
	action = "list"
}

Which syntax you use is largely dependent on personal preference. If you simply want to rewrite on URI onto another explicit URI (rather than a controller/action pair) this can be achieved with the following example:

"/hello"(uri:"/hello.dispatch")

Rewriting specific URIs is often useful when integrating with other frameworks.

6.4.2 Embedded Variables

Simple Variables

The previous section demonstrated how to map trivial URLs with concrete "tokens". In URL mapping speak tokens are the sequence of characters between each slash / character. A concrete token is one which is well defined such as as /product. However, in many circumstances you don't know what the value of a particular token will be until runtime. In this case you can use variable placeholders within the URL for example:

static mappings = {
  "/product/$id"(controller:"product")
}

In this case by embedding a $id variable as the second token Grails will automatically map the second token into a parameter (available via the params object) called id. For example given the URL /product/MacBook, the following code will render "MacBook" to the response:

class ProductController {
     def index = { render params.id }
}

You can of course construct more complex examples of mappings. For example the traditional blog URL format could be mapped as follows:

static mappings = {
   "/$blog/$year/$month/$day/$id"(controller:"blog", action:"show")
}

The above mapping would allow you to do things like:

/graemerocher/2007/01/10/my_funky_blog_entry

The individual tokens in the URL would again be mapped into the params object with values available for year, month, day, id and so on.

Dynamic Controller and Action Names

Variables can also be used to dynamically construct the controller and action name. In fact the default Grails URL mappings use this technique:

static mappings = {
    "/$controller/$action?/$id?"()
}

Here the name of the controller, action and id are implicitly obtained from the variables controller, action and id embedded within the URL.

You can also resolve the controller name and action name to execute dynamically using a closure:

static mappings = {
    "/$controller" {
	   action = { params.goHere }
    }
}

Optional Variables

Another characteristic of the default mapping is the ability to append a ? at the end of a variable to make it an optional token. In a further example this technique could be applied to the blog URL mapping to have more flexible linking:

static mappings = {
   "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
}

With this mapping all of the below URLs would match with only the relevant parameters being populated in the params object:

/graemerocher/2007/01/10/my_funky_blog_entry
/graemerocher/2007/01/10
/graemerocher/2007/01
/graemerocher/2007
/graemerocher

Arbitrary Variables

You can also pass arbitrary parameters from the URL mapping into the controller by merely setting them in the block passed to the mapping:

"/holiday/win" {
     id = "Marrakech"
     year = 2007
}

This variables will be available within the params object passed to the controller.

Dynamically Resolved Variables

The hard coded arbitrary variables are useful, but sometimes you need to calculate the name of the variable based on runtime factors. This is also possible by assigning a block to the variable name:

"/holiday/win" {
     id = { params.id } 
     isEligible = { session.user != null } // must be logged in
}

In the above case the code within the blocks is resolved when the URL is actually matched and hence can be used in combination with all sorts of logic.

6.4.3 Mapping to Views

static mappings = {
      "/"(view:"/index")  // map the root URL
}

Alternatively if you need a view that is specific to a given controller you could use:

static mappings = {
   "/help"(controller:"site",view:"help") // to a view for a controller
}

6.4.4 Mapping to Response Codes

static mappings = {
   "500"(controller:"errors", action:"serverError")
   "404"(controller:"errors", action:"notFound")
   "403"(controller:"errors", action:"forbidden")
}

Or alternatively if you merely want to provide custom error pages:

static mappings = {
   "500"(view:"/errors/serverError")
   "404"(view:"/errors/notFound")
   "403"(view:"/errors/forbidden")
}

6.4.5 Mapping to HTTP methods

As an example the following mappings provide a RESTful API URL mappings for the ProductController:

static mappings = {
   "/product/$id"(controller:"product"){
       action = [GET:"show", PUT:"update", DELETE:"delete", POST:"save"]
   }	
}

6.4.6 Mapping Wildcards

static mappings = {
	"/images/*.jpg"(controller:"image")
}

This mapping will match all paths to images such as /image/logo.jpg. Of course you can achieve the same effect with a variable:

static mappings = {
	"/images/$name.jpg"(controller:"image")
}

However, you can also use double wildcards to match more than one level below:

static mappings = {
	"/images/**.jpg"(controller:"image")
}

In this cases the mapping will match /image/logo.jpg as well as /image/other/logo.jpg. Even better you can use a double wildcard variable:

static mappings = {
	// will match /image/logo.jpg and /image/other/logo.jpg 
	"/images/$name**.jpg"(controller:"image")
}

In this case it will store the path matched by the wildcard inside a name parameter obtainable from the params object:

def name = params.name
println name // prints "logo" or "other/logo"

If you are using wildcard URL mappings then you may want to exclude certain URIs from Grails' URL mapping process. To do this you can provide an excludes setting inside the UrlMappings.groovy class:

class UrlMappings = {
	static excludes = ["/images/**", "/css/**"]
	static mappings = {
		…
	}
}

In this case Grails won't attempt to match any URIs that start with /images or /css.

6.4.7 Automatic Link Re-Writing

This is done through a URL re-writing technique that reverse engineers the links from the URL mappings. So given a mapping such as the blog one from an earlier section:

static mappings = {
   "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
}

If you use the link tag as follows:

<g:link controller="blog" action="show" params="[blog:'fred', year:2007]">My Blog</g:link>
<g:link controller="blog" action="show" params="[blog:'fred', year:2007, month:10]">My Blog - October 2007 Posts</g:link>

Grails will automatically re-write the URL in the correct format:

<a href="/fred/2007">My Blog</a>
<a href="/fred/2007/10">My Blog - October 2007 Posts</a>

6.4.8 Applying Constraints

static mappings = {
   "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
}

This allows URLs such as:

/graemerocher/2007/01/10/my_funky_blog_entry

However, it would also allow:

/graemerocher/not_a_year/not_a_month/not_a_day/my_funky_blog_entry

This is problematic as it forces you to do some clever parsing in the controller code. Luckily, URL Mappings can be constrained to further validate the URL tokens:

"/$blog/$year?/$month?/$day?/$id?" {
     controller = "blog"
     action = "show"
     constraints {
          year(matches:/\d{4}/)
          month(matches:/\d{2}/)
          day(matches:/\d{2}/)
     }
}

In this case the constraints ensure that the year, month and day parameters match a particular valid pattern thus relieving you of that burden later on.

6.4.9 Named URL Mappings

The syntax for defining a named mapping is as follows:

static mappings = {
   name <mapping name>: <url pattern> {
      // …
   }
}

An example:

static mappings = {
    name personList: "/showPeople" {
        controller = 'person'
        action = 'list'
    }
    name accountDetails: "/details/$acctNumber" {
        controller = 'product'
        action = 'accountDetails'
    }
}

The mapping may be referenced in a link tag in a GSP.

<g:link mapping="personList">List People</g:link>

That would result in:

<a href="/showPeople">List People</a>

Parameters may be specified using the params attribute.

<g:link mapping="accountDetails" params="[acctNumber:'8675309']">Show Account</g:link>

That would result in:

<a href="/details/8675309">Show Account</a>

Alternatively you may reference a named mapping using the link namespace.

<link:personList>List People</link:personList>

That would result in:

<a href="/showPeople">List People</a>

The link namespace approach allows parameters to be specified as attributes.

<link:accountDetails acctNumber="8675309">Show Account</link:accountDetails>

That would result in:

<a href="/details/8675309">Show Account</a>

6.5 Web Flow

Overview

Grails supports the creation of web flows built on the Spring Web Flow project. A web flow is a conversation that spans multiple requests and retains state for the scope of the flow. A web flow also has a defined start and end state.

Web flows don't require an HTTP session, but instead store their state in a serialized form, which is then restored using a flow execution key that Grails passes around as a request parameter. This makes flows far more scalable than other forms of stateful application that use the HttpSession and its inherit memory and clustering concerns.

Web flow is essentially an advanced state machine that manages the "flow" of execution from one state to the next. Since the state is managed for you, you don't have to be concerned with ensuring that users enter an action in the middle of some multi step flow, as web flow manages that for you. This makes web flow perfect for use cases such as shopping carts, hotel booking and any application that has multi page work flows.

Creating a Flow

To create a flow create a regular Grails controller and then add an action that ends with the convention Flow. For example:

class BookController {
   def index = {
      redirect(action:"shoppingCart")
   }
   def shoppingCartFlow = {
        …
   }
}

Notice when redirecting or referring to the flow as an action we omit the Flow suffix. In other words the name of the action of the above flow is shoppingCart.

6.5.1 Start and End States

class BookController {
   …
   def shoppingCartFlow = {
       showCart {
           on("checkout").to "enterPersonalDetails"           
           on("continueShopping").to "displayCatalogue"
       }
       …
       displayCatalogue {
            redirect(controller:"catalogue", action:"show")
       }
       displayInvoice()
   }
}

Here the showCart node is the start state of the flow. Since the showCart state doesn't define an action or redirect it is assumed be a view state that, by convention, refers to the view grails-app/views/book/shoppingCart/showCart.gsp.

Notice that unlike regular controller actions, the views are stored within a directory that matches the name of the flow: grails-app/views/book/shoppingCart.

The shoppingCart flow also has two possible end states. The first is displayCatalogue which performs an external redirect to another controller and action, thus exiting the flow. The second is displayInvoice which is an end state as it has no events at all and will simply render a view called grails-app/views/book/shoppingCart/displayInvoice.gsp whilst ending the flow at the same time.

Once a flow has ended it can only be resumed from the start state, in this case showCart, and not from any other state.

6.5.2 Action States and View States

View states

A view state is a one that doesn't define an action or a redirect. So for example the below is a view state:

enterPersonalDetails {
   on("submit").to "enterShipping"
   on("return").to "showCart"
}

It will look for a view called grails-app/views/book/shoppingCart/enterPersonalDetails.gsp by default. Note that the enterPersonalDetails state defines two events: submit and return. The view is responsible for triggering these events. If you want to change the view to be rendered you can do so with the render method:

enterPersonalDetails {
   render(view:"enterDetailsView")
   on("submit").to "enterShipping"
   on("return").to "showCart"
}

Now it will look for grails-app/views/book/shoppingCart/enterDetailsView.gsp. If you want to use a shared view, start with a / in view argument:

enterPersonalDetails {
   render(view:"/shared/enterDetailsView")
   on("submit").to "enterShipping"
   on("return").to "showCart"
}

Now it will look for grails-app/views/shared/enterDetailsView.gsp

Action States

An action state is a state that executes code but does not render any view. The result of the action is used to dictate flow transition. To create an action state you need to define an action to to be executed. This is done by calling the action method and passing it a block of code to be executed:

listBooks {
   action { 
	  [ bookList:Book.list() ]
   }
   on("success").to "showCatalogue"
   on(Exception).to "handleError"
}

As you can see an action looks very similar to a controller action and in fact you can re-use controller actions if you want. If the action successfully returns with no errors the success event will be triggered. In this case since we return a map, this is regarded as the "model" and is automatically placed in flow scope.

In addition, in the above example we also use an exception handler to deal with errors on the line:

on(Exception).to "handleError"

What this does is make the flow transition to a state called handleError in the case of an exception.

You can write more complex actions that interact with the flow request context:

processPurchaseOrder  {
     action {
         def a =  flow.address
         def p = flow.person
         def pd = flow.paymentDetails
         def cartItems = flow.cartItems
         flow.clear()
         def o = new Order(person:p, shippingAddress:a, paymentDetails:pd)
         o.invoiceNumber = new Random().nextInt(9999999)
         cartItems.each { o.addToItems(it) }
         o.save()
         [order:o]
     }
     on("error").to "confirmPurchase"
     on(Exception).to "confirmPurchase"
     on("success").to "displayInvoice"
}

Here is a more complex action that gathers all the information accumulated from the flow scope and creates an Order object. It then returns the order as the model. The important thing to note here is the interaction with the request context and "flow scope".

Transition Actions

Another form of action is what is known as a transition action. A transition action is executed directly prior to state transition once an event has been triggered. A trivial example of a transition action can be seen below:

enterPersonalDetails {
   on("submit") {
       log.trace "Going to enter shipping"	
   }.to "enterShipping"
   on("return").to "showCart"
}

Notice how we pass a block of the code to submit event that simply logs the transition. Transition states are extremely useful for data binding and validation, which is covered in a later section.

6.5.3 Flow Execution Events

Triggering Events from a View State

As discussed previously the start state of the flow in a previous code listing deals with two possible events. A checkout event and a continueShopping event:

def shoppingCartFlow = {
    showCart {
        on("checkout").to "enterPersonalDetails"           
        on("continueShopping").to "displayCatalogue"
    }
    …
}

Since the showCart event is a view state it will render the view grails-app/book/shoppingCart/showCart.gsp. Within this view you need to have components that trigger flow execution. On a form this can be done use the submitButton tag:

<g:form action="shoppingCart">
    <g:submitButton name="continueShopping" value="Continue Shopping"></g:submitButton>
    <g:submitButton name="checkout" value="Checkout"></g:submitButton>
</g:form>

The form must submit back to the shoppingCart flow. The name attribute of each submitButton tag signals which event will be triggered. If you don't have a form you can also trigger an event with the link tag as follows:

<g:link action="shoppingCart" event="checkout" />

Triggering Events from an Action

To trigger an event from an action you need to invoke a method. For example there is the built in error() and success() methods. The example below triggers the error() event on validation failure in a transition action:

enterPersonalDetails {
   on("submit") {
         def p = new Person(params)
         flow.person = p
         if(!p.validate())return error()
   }.to "enterShipping"
   on("return").to "showCart"
}

In this case because of the error the transition action will make the flow go back to the enterPersonalDetails state.

With an action state you can also trigger events to redirect flow:

shippingNeeded {
   action {
       if(params.shippingRequired) yes()
       else no()
   }
   on("yes").to "enterShipping"
   on("no").to "enterPayment"
}

6.5.4 Flow Scopes

Scope Basics

You'll notice from previous examples that we used a special object called flow to store objects within "flow scope". Grails flows have 5 different scopes you can utilize:

• request - Stores an object for the scope of the current request
• flash - Stores the object for the current and next request only
• flow - Stores objects for the scope of the flow, removing them when the flow reaches an end state
• conversation - Stores objects for the scope of the conversation including the root flow and nested subflows
• session - Stores objects inside the users session

Grails service classes can be automatically scoped to a web flow scope. See the documentation on Services for more information.

Also returning a model map from an action will automatically result in the model being placed in flow scope. For example, using a transition action, you can place objects within flow scope as follows:

enterPersonalDetails {
   on("submit") {
         [person:new Person(params)]
   }.to "enterShipping"
   on("return").to "showCart"
}

Be aware that a new request is always created for each state, so an object placed in request scope in an action state (for example) will not be available in a subsequent view state. Use one of the other scopes to pass objects from one state to another. Also note that Web Flow:

1. Moves objects from flash scope to request scope upon transition between states;
2. Merges objects from the flow and conversation scopes into the view model before rendering (so you shouldn't include a scope prefix when referencing these objects within a view, e.g. GSP pages).

Flow Scopes and Serialization

When placing objects in flash, flow or conversation scope they must implement java.io.Serializable otherwise you will get an error. This has an impact on domain classes in that domain classes are typically placed within a scope so that they can be rendered in a view. For example consider the following domain class:

class Book {
	String title
}

In order to place an instance of the Book class in a flow scope you will need to modify it as follows:

class Book implements Serializable {
	String title
}

This also impacts associations and closures you declare within a domain class. For example consider this:

class Book implements Serializable {
	String title
	Author author
}

Here if the Author association is not Serializable you will also get an error. This also impacts closures used in GORM events such as onLoad, onSave and so on. The following domain class will cause an error if an instance is placed in a flow scope:

class Book implements Serializable {
	String title
	def onLoad = {
		println "I'm loading"
	}
}

The reason is that the assigned block on the onLoad event cannot be serialized. To get around this you should declare all events as transient:

class Book implements Serializable {
	String title
	transient onLoad = {
		println "I'm loading"
	}
}

6.5.5 Data Binding and Validation

enterPersonalDetails {
   on("submit").to "enterShipping"
   on("return").to "showCart"
}

The view contains a form with two submit buttons that either trigger the submit event or the return event:

<g:form action="shoppingCart">
    <!-- Other fields -->
    <g:submitButton name="submit" value="Continue"></g:submitButton>
    <g:submitButton name="return" value="Back"></g:submitButton>
</g:form>

However, what about the capturing the information submitted by the form? To to capture the form info we can use a flow transition action:

enterPersonalDetails {
   on("submit") {
         flow.person = new Person(params)
         !flow.person.validate() ? error() : success()
   }.to "enterShipping"
   on("return").to "showCart"
}

Notice how we perform data binding from request parameters and place the Person instance within flow scope. Also interesting is that we perform validation and invoke the error() method if validation fails. This signals to the flow that the transition should halt and return to the enterPersonalDetails view so valid entries can be entered by the user, otherwise the transition should continue and go to the enterShipping state.

Like regular actions, flow actions also support the notion of Command Objects by defining the first argument of the closure:

enterPersonalDetails {
   on("submit") { PersonDetailsCommand cmd ->	     
          flow.personDetails = cmd
         !flow.personDetails.validate() ? error() : success()
   }.to "enterShipping"
   on("return").to "showCart"
}

6.5.6 Subflows and Conversations

def searchFlow = {
            displaySearchForm {
                on("submit").to "executeSearch"
            }
            executeSearch {
                action {
                    [results:searchService.executeSearch(params.q)]
                }
                on("success").to "displayResults"
                on("error").to "displaySearchForm"
            }
            displayResults {
                on("searchDeeper").to "extendedSearch"
                on("searchAgain").to "displaySearchForm"
            }
            extendedSearch {
                subflow(extendedSearchFlow)   // <--- extended search subflow
                on("moreResults").to "displayMoreResults"
                on("noResults").to "displayNoMoreResults"
            }
            displayMoreResults()
            displayNoMoreResults()
}

It references a subflow in the extendedSearch state. The subflow is another flow entirely:

def extendedSearchFlow = {
       startExtendedSearch {
           on("findMore").to "searchMore"
           on("searchAgain").to "noResults"
       }
       searchMore {
           action {
              def results = searchService.deepSearch(ctx.conversation.query)
              if(!results)return error()
              conversation.extendedResults = results
           }
           on("success").to "moreResults"
           on("error").to "noResults"
       }
       moreResults()
       noResults()
}

Notice how it places the extendedResults in conversation scope. This scope differs to flow scope as it allows you to share state that spans the whole conversation not just the flow. Also notice that the end state (either moreResults or noResults of the subflow triggers the events in the main flow:

extendedSearch {
         subflow(extendedSearchFlow)   // <--- extended search subflow
         on("moreResults").to "displayMoreResults"
         on("noResults").to "displayNoMoreResults"
}

6.6 Filters

6.6.1 Applying Filters

class ExampleFilters {
   def filters = {
        // your filters here
   }
}

Each filter you define within the filters block has a name and a scope. The name is the method name and the scope is defined using named arguments. For example if you need to define a filter that applies to all controllers and all actions you can use wildcards:

sampleFilter(controller:'*', action:'*') {
  // interceptor definitions
}

The scope of the filter can be one of the following things:

• A controller and/or action name pairing with optional wildcards
• A URI, with Ant path matching syntax

Filter rule attributes:

• controller - controller matching pattern, by default * is replaced with .* and a regex is compiled
• action - action matching pattern, by default * is replaced with .* and a regex is compiled
• regex (true/false) - use regex syntax (don't replace '*' with '.*')
• uri - a uri to match, expressed with as Ant style path (e.g. /book/**)
• find (true/false) - rule matches with partial match (see java.util.regex.Matcher.find())
• invert (true/false) - invert the rule (NOT rule)

Some examples of filters include:

• All controllers and actions

all(controller:'*', action:'*') {
}

• Only for the BookController

justBook(controller:'book', action:'*') {
}

• All controllers except the BookController

notBook(controller:'book', invert:true) {
}

• All actions containing 'save' in the action name

saveInActionName(action:'save', find:true) {
}

• Applied to a URI space

someURIs(uri:'/book/**') {
}

• Applied to all URIs

allURIs(uri:'/**') {
}

In addition, the order in which you define the filters dictates the order in which they are executed.

6.6.2 Filter Types

• before - Executed before the action. Can return false to indicate all future filters and the action should not execute
• after - Executed after an action. Takes a first argument as the view model
• afterView - Executed after view rendering

For example to fulfill the common authentication use case you could define a filter as follows:

class SecurityFilters {
   def filters = {
       loginCheck(controller:'*', action:'*') {
           before = {
              if(!session.user && !actionName.equals('login')) {
                  redirect(action:'login')
                  return false
               }
           }
       }
   }
}

Here the loginCheck filter uses a before interceptor to execute a block of code that checks if a user is in the session and if not redirects to the login action. Note how returning false ensure that the action itself is not executed.

6.6.3 Variables and Scopes

• request - The HttpServletRequest object
• response - The HttpServletResponse object
• session - The HttpSession object
• servletContext - The ServletContext object
• flash - The flash object
• params - The request parameters object
• actionName - The action name that is being dispatched to
• controllerName - The controller name that is being dispatched to
• grailsApplication - The Grails application currently running
• applicationContext - The ApplicationContext object

However, filters only support a subset of the methods available to controllers and tag libraries. These include:

• redirect - For redirects to other controllers and actions
• render - For rendering custom responses

6.7 Ajax

6.7.1 Ajax using Prototype

This section covers Grails' support for Prototype. To get started you need to add this line to the <head> tag of your page:

<g:javascript library="prototype" />

This uses the javascript tag to automatically place the correct references in place for Prototype. If you require Scriptaculous too you can do the following instead:

<g:javascript library="scriptaculous" />

This works because of Grails' support for adaptive tag libraries. Thanks to Grails' plugin system there is support for a number of different Ajax libraries including (but not limited to):

• prototype
• dojo
• yui
• mootools

6.7.1.1 Remoting Linking

<g:remoteLink action="delete" id="1">Delete Book</g:remoteLink>

The above link sends an asynchronous request to the delete action of the current controller with an id of 1.

6.7.1.2 Updating Content

def delete = {
      def b = Book.get( params.id )
      b.delete()
      render "Book ${b.id} was deleted"
}

GSP code:

<div id="message"></div>
<g:remoteLink action="delete" id="1" update="message">Delete Book</g:remoteLink>

The above example will call the action and set the contents of the message div to the response in this case "Book 1 was deleted". This is done by the update attribute on the tag, which can also take a map to indicate what should be updated on failure:

<div id="message"></div>
<div id="error"></div>
<g:remoteLink action="delete" id="1"
              update="[success:'message',failure:'error']">Delete Book</g:remoteLink>

Here the error div will be updated if the request failed.

6.7.1.3 Remote Form Submission

<g:formRemote url="[controller:'book',action:'delete']" update="[success:'message',failure:'error']">
       <input type="hidden" name="id" value="1" />
       <input type="submit" value="Delete Book!" />
</g:formRemote >

Or alternatively you can use the submitToRemote tag to create a submit button. This allows some buttons to submit remotely and some not depending on the action:

<form action="delete">
       <input type="hidden" name="id" value="1" />
       <g:submitToRemote action="delete" update="[success:'message',failure:'error']" />
</form>

6.7.1.4 Ajax Events

<g:remoteLink action="show" 
              id="1" 
              update="success" 
              onLoading="showProgress()" 
              onComplete="hideProgress()">Show Book 1</g:remoteLink>

The above code will execute the "showProgress()" function which may show a progress bar or whatever is appropriate. Other events include:

• onSuccess - The javascript function to call if successful
• onFailure - The javascript function to call if the call failed
• on_ERROR_CODE - The javascript function to call to handle specified error codes (eg on404="alert('not found!')")
• onUninitialized - The javascript function to call the a ajax engine failed to initialise
• onLoading - The javascript function to call when the remote function is loading the response
• onLoaded - The javascript function to call when the remote function is completed loading the response
• onComplete - The javascript function to call when the remote function is complete, including any updates

If you need a reference to the XmlHttpRequest object you can use the implicit event parameter e to obtain it:

<g:javascript>
   function fireMe(e) {
	   alert("XmlHttpRequest = " + e)
   }
}
</g:javascript>
<g:remoteLink action="example" 
              update="success" 
              onSuccess="fireMe(e)">Ajax Link</g:remoteLink>

6.7.2 Ajax with Dojo

grails install-plugin dojo

This will download the current supported version of Dojo and install it into your Grails project. With that done you can add the following reference to the top of your page:

<g:javascript library="dojo" />

Now all of Grails tags such as remoteLink, formRemote and submitToRemote work with Dojo remoting.

6.7.3 Ajax with GWT

6.7.4 Ajax on the Server

• Content Centric Ajax - Where you merely use the HTML result of a remote call to update the page
• Data Centric Ajax - Where you actually send an XML or JSON response from the server and programmatically update the page
• Script Centric Ajax - Where the server sends down a stream of Javascript to be evaluated on the fly

Most of the examples in the Ajax section cover Content Centric Ajax where you are updating the page, but you may also want to use Data Centric or Script Centric. This guide covers the different styles of Ajax.

Content Centric Ajax

Just to re-cap, content centric Ajax involves sending some HTML back from the server and is typically done by rendering a template with the render method:

def showBook = {
	def b = Book.get(params.id)
	render(template:"bookTemplate", model:[book:b])
}

Calling this on the client involves using the remoteLink tag:

<g:remoteLink action="showBook" id="${book.id}" update="book${book.id}">Update Book</g:remoteLink>
<div id="book${book.id}">
   <!--existing book mark-up -->
</div>

Data Centric Ajax with JSON

Data Centric Ajax typically involves evaluating the response on the client and updating programmatically. For a JSON response with Grails you would typically use Grails' JSON marshaling capability:

import grails.converters.*
def showBook = {
	def b = Book.get(params.id)
    render b as JSON	
}

And then on the client parse the incoming JSON request using an Ajax event handler:

<g:javascript>
function updateBook(e) {
	var book = eval("("+e.responseText+")") // evaluate the JSON
	$("book"+book.id+"_title").innerHTML = book.title
}
<g:javascript>
<g:remoteLink action="test" update="foo" onSuccess="updateBook(e)">Update Book</g:remoteLink>
<g:set var="bookId">book${book.id}</g:set>
<div id="${bookId}">
	<div id="${bookId}_title">The Stand</div>
</div>

Data Centric Ajax with XML

On the server side using XML is equally trivial:

import grails.converters.*
def showBook = {
	def b = Book.get(params.id)
    render b as XML	
}

However, since DOM is involved the client gets more complicated:

<g:javascript>
function updateBook(e) {
	var xml = e.responseXML
	var id = xml.getElementsByTagName("book").getAttribute("id")
	$("book"+id+"_title")=xml.getElementsByTagName("title")[0].textContent
}
<g:javascript>
<g:remoteLink action="test" update="foo" onSuccess="updateBook(e)">Update Book</g:remoteLink>
<g:set var="bookId">book${book.id}</g:set>
<div id="${bookId}">
	<div id="${bookId}_title">The Stand</div>
</div>

Script Centric Ajax with JavaScript

Script centric Ajax involves actually sending Javascript back that gets evaluated on the client. An example of this can be seen below:

def showBook = {
	def b = Book.get(params.id)
    response.contentType = "text/javascript"
    String title = b.title.encodeAsJavascript()
    render "$('book${b.id}_title')='${title}'"
}

The important thing to remember is to set the contentType to text/javascript. If you are using Prototype on the client the returned Javascript will automatically be evaluated due to this contentType setting.

Obviously in this case it is critical that you have an agreed client-side API as you don't want changes on the client breaking the server. This is one of the reasons Rails has something like RJS. Although Grails does not currently have a feature such as RJS there is a Dynamic JavaScript Plug-in that offers similar capabilities.

6.8 Content Negotiation

Configuring Mime Types

Before you can start dealing with content negotiation you need to tell Grails what content types you wish to support. By default Grails comes configured with a number of different content types within grails-app/conf/Config.groovy using the grails.mime.types setting:

grails.mime.types = [ xml: ['text/xml', 'application/xml'],
                      text: 'text-plain',
                      js: 'text/javascript',
                      rss: 'application/rss+xml',
                      atom: 'application/atom+xml',
                      css: 'text/css',
                      cvs: 'text/csv',
                      all: '*/*',
                      json: 'text/json',
                      html: ['text/html','application/xhtml+xml']
                    ]

The above bit of configuration allows Grails to detect to format of a request containing either the 'text/xml' or 'application/xml' media types as simply 'xml'. You can add your own types by simply adding new entries into the map.

Content Negotiation using the Accept header

Every incoming HTTP request has a special Accept header that defines what media types (or mime types) a client can "accept". In older browsers this is typically:

*/*

Which simply means anything. However, on newer browser something all together more useful is sent such as (an example of a Firefox Accept header):

text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5

Grails parses this incoming format and adds a property to the request object that outlines the preferred request format. For the above example the following assertion would pass:

assert 'html' == request.format

Why? The text/html media type has the highest "quality" rating of 0.9, therefore is the highest priority. If you have an older browser as mentioned previously the result is slightly different:

assert 'all' == request.format

In this case 'all' possible formats are accepted by the client. To deal with different kinds of requests from Controllers you can use the withFormat method that acts as kind of a switch statement:

import grails.converters.*
class BookController {
	def books
	def list = {
		this.books = Book.list()
		withFormat {
			html bookList:books
			js { render "alert('hello')" } 
			xml { render books as XML }
		}
	}
}

What happens here is that if the preferred format is html then Grails will execute the html() call only. What this is does is make Grails look for a view called either grails-app/views/books/list.html.gsp or grails-app/views/books/list.gsp. If the format is xml then the closure will be invoked and an XML response rendered.

How do we handle the "all" format? Simply order the content-types within your withFormat block so that whichever one you want executed comes first. So in the above example, "all" will trigger the html handler.

When using withFormat make sure it is the last call in your controller action as the return value of the withFormat method is used by the action to dictate what happens next.

Content Negotiation with the format Request Parameter

If fiddling with request headers if not your favorite activity you can override the format used by specifying a format request parameter:

/book/list?format=xml

You can also define this parameter in the URL Mappings definition:

"/book/list"(controller:"book", action:"list") {
	format = "xml"
}

Content Negotiation with URI Extensions

Grails also supports content negotiation via URI extensions. For example given the following URI:

/book/list.xml

Grails will shave off the extension and map it to /book/list instead whilst simultaneously setting the content format to xml based on this extension. This behaviour is enabled by default, so if you wish to turn it off, you must set the grails.mime.file.extensions property in grails-app/conf/Config.groovy to false:

grails.mime.file.extensions = false

Testing Content Negotiation

To test content negotiation in an integration test (see the section on Testing) you can either manipulate the incoming request headers:

void testJavascriptOutput() {
	def controller = new TestController()
	controller.request.addHeader "Accept", "text/javascript, text/html, application/xml, text/xml, */*"
	controller.testAction()
	assertEquals "alert('hello')", controller.response.contentAsString
}

Or you can set the format parameter to achieve a similar effect:

void testJavascriptOutput() {
	def controller = new TestController()
	controller.params.format = 'js'
	controller.testAction()
	assertEquals "alert('hello')", controller.response.contentAsString
}

7. Validation

Constraints in Grails are a way to declaratively specify validation rules. Most commonly they are applied to domain classes, however URL Mappings and Command Objects also support constraints.

7.1 Declaring Constraints

class User {
    String login
    String password
    String email
    Integer age
    static constraints = {
	  …
    }
}

You then use method calls that match the property name for which the constraint applies in combination with named parameters to specify constraints:

class User {
    ...
    static constraints = {
        login(size:5..15, blank:false, unique:true)
        password(size:5..15, blank:false)
        email(email:true, blank:false)
        age(min:18, nullable:false)
    }
}

In this example we've declared that the login property must be between 5 and 15 characters long, it cannot be blank and must be unique. We've all applied other constraints to the password, email and age properties.

A complete reference for the available constraints can be found on the reference guide

7.2 Validating Constraints

Validation Basics

To validate a domain class you can call the validate method on any instance:

def user =  new User(params)
if(user.validate()) {
    // do something with user
}
else {
    user.errors.allErrors.each {
        println it
    }
}

The errors property on domain classes is an instance of the Spring Errors interface. The Errors interface provides methods to navigate the validation errors and also retrieve the original values.

Validation Phases

Within Grails there are essentially 2 phases of validation, the first phase is data binding which occurs when you bind request parameters onto an instance such as:

def user = new User(params)

At this point you may already have errors in the errors property due to type conversion (such as converting Strings to Dates). You can check these and obtain the original input value using the Errors API:

if(user.hasErrors()) {
	if(user.errors.hasFieldErrors("login")) {
		println user.errors.getFieldError("login").rejectedValue
	}
}

The second phase of validation happens when you call validate or save. This is when Grails will validate the bound values againts the constraints you defined. For example, by default the persistent save method calls validate before executing hence allowing you to write code like:

if(user.save()) {
    return user
}
else {
    user.errors.allErrors.each {
        println it
    }
}

7.3 Validation on the Client

Displaying Errors

Typically if you get a validation error you want to redirect back to the view for rendering. Once there you need some way of rendering errors. Grails supports a rich set of tags for dealing with errors. If you simply want to render the errors as a list you can use renderErrors:

<g:renderErrors bean="${user}" />

If you need more control you can use hasErrors and eachError:

<g:hasErrors bean="${user}">
  <ul>
   <g:eachError var="err" bean="${user}">
       <li>${err}</li> 
   </g:eachError>
  </ul>
</g:hasErrors>

Highlighting Errors

It is often useful to highlight using a red box or some indicator when a field has been incorrectly input. This can also be done with the hasErrors by invoking it as a method. For example:

<div class='value ${hasErrors(bean:user,field:'login','errors')}'>
   <input type="text" name="login" value="${fieldValue(bean:user,field:'login')}"/>
</div>

What this code does is check if the login field of the user bean has any errors and if it does adds an errors CSS class to the div thus allowing you to use CSS rules to highlight the div.

Retrieving Input Values

Each error is actually an instance of the FieldError class in Spring, which retains the original input value within it. This is useful as you can use the error object to restore the value input by the user using the fieldValue tag:

<input type="text" name="login" value="${fieldValue(bean:user,field:'login')}"/>

This code will look if there is an existing FieldError in the User bean and if there is obtain the originally input value for the login field.

7.4 Validation and Internationalization

Constraints and Message Codes

The codes themselves are dictated by a convention. For example consider the constraints we looked at earlier:

package com.mycompany.myapp
class User {
    ...
    static constraints = {
        login(size:5..15, blank:false, unique:true)
        password(size:5..15, blank:false)
        email(email:true, blank:false)
        age(min:18, nullable:false)
    }
}

If the blank constraint was violated Grails will, by convention, look for a message code in the form:

[Class Name].[Property Name].[Constraint Code]

In the case of the blank constraint this would be user.login.blank so you would need a message such as the following in your grails-app/i18n/messages.properties file:

user.login.blank=Your login name must be specified!

The class name is looked for both with and without a package, with the packaged version taking precedence. So for example, com.mycompany.myapp.User.login.blank will be used before user.login.blank. This allows for cases where you domain class encounters message code clashes with plugins.

For a reference on what codes are for which constraints refer to the reference guide for each constraint.

Displaying Messages

The renderErrors tag will automatically deal with looking up messages for you using the message tag. However, if you need more control of rendering you will need to do this yourself:

<g:hasErrors bean="${user}">
  <ul>
   <g:eachError var="err" bean="${user}">
       <li><g:message error="${err}" /></li> 
   </g:eachError>
  </ul>
</g:hasErrors>

In this example within the body of the eachError tag we use the message tag in combination with its error argument to read the message for the given error.

7.5 Validation Non Domain and Command Object Classes

The Validateable Annotation

Classes which define the static constraints property and are marked with the @Validateable annotation may be made validateable by the framework. Consider this example:

// src/groovy/com/mycompany/myapp/User.groovy
package com.mycompany.myapp
import org.codehaus.groovy.grails.validation.Validateable
@Validateable
class User {
    ...
    static constraints = {
        login(size:5..15, blank:false, unique:true)
        password(size:5..15, blank:false)
        email(email:true, blank:false)
        age(min:18, nullable:false)
    }
}

You need to tell the framework which packages to search for @Validateable classes by assigning a list of Strings to the grails.validateable.packages property in Config.groovy.

// grails-app/conf/Config.groovy
...
grails.validateable.packages = ['com.mycompany.dto', 'com.mycompany.util']
...

The framework will only search those packages (and child packages of those) for classes marked with @Validateable.

Registering Validateable Classes

If a class is not marked with @Validateable, it may still be made validateable by the framework. The steps required to do this are to define the static constraints property in the class (as described above) and then telling the framework about the class by assigning a value to the grails.validateable.classes property in Config.groovy.

// grails-app/conf/Config.groovy
...
grails.validateable.classes = [com.mycompany.myapp.User, com.mycompany.dto.Account]
...

8. The Service Layer

Services in Grails are seen as the place to put the majority of the logic in your application, leaving controllers responsible for handling request flow via redirects and so on.

Creating a Service

You can create a Grails service by running the create-service command from the root of your project in a terminal window:

grails create-service simple

The above example will create a service at the location grails-app/services/SimpleService.groovy. A service's name ends with the convention Service, other than that a service is a plain Groovy class:

class SimpleService {	
}

8.1 Declarative Transactions

Default Declarative Transactions

Services are typically involved with co-ordinating logic between domain classes, and hence often involved with persistence that spans large operations. Given the nature of services they frequently require transactional behaviour. You can of course use programmatic transactions with the withTransaction method, however this is repetitive and doesn't fully leverage the power of Spring's underlying transaction abstraction.

Services allow the enablement of transaction demarcation, which is essentially a declarative way of saying all methods within this service are to be made transactional. All services have transaction demarcation enabled by default - to disable it, simply set the transactional property to false:

class CountryService {
    static transactional = false
}

You may also set this property to true in case the default changes in the future, or simply to make it clear that the service is intentionally transactional.

Warning: dependency injection is the only way that declarative transactions work. You will not get a transactional service if you use the new operator such as new BookService()

The result is all methods are wrapped in a transaction and automatic rollback occurs if an exception is thrown in the body of one of the methods. The propagation level of the transaction is by default set to PROPAGATION_REQUIRED.

Custom Transaction Configuration

Grails also fully supports Spring's Transactional annotation for cases where you need more fine-grained control over transactions at a per-method level or need specify an alternative propagation level:

import org.springframework.transaction.annotation.*
class BookService {
	@Transactional(readOnly = true) 
	def listBooks() { Book.list() }
	@Transactional def updateBook() { 
		// … 
	}
}

For more information refer to the section of the Spring user guide on Using @Transactional.

Unlike Spring you do not need any prior configuration to use Transactional, just specify the annotation as needed and Grails will pick them up automatically.

8.2 Scoped Services

You can change this behaviour by placing a service in a particular scope. The supported scopes are:

• prototype - A new service is created every time it is injected into another class
• request - A new service will be created per request
• flash - A new service will be created for the current and next request only
• flow - In web flows the service will exist for the scope of the flow
• conversation - In web flows the service will exist for the scope of the conversation. ie a root flow and its sub flows
• session - A service is created for the scope of a user session
• singleton (default) - Only one instance of the service ever exists

If your service is flash, flow or conversation scoped it will need to implement java.io.Serializable and can only be used in the context of a Web Flow

To enable one of the scopes, add a static scope property to your class whose value is one of the above:

static scope = "flow"

8.3 Dependency Injection and Services

Dependency Injection Basics

A key aspect of Grails services is the ability to take advantage of the Spring Framework's dependency injection capability. Grails supports "dependency injection by convention". In other words, you can use the property name representation of the class name of a service, to automatically inject them into controllers, tag libraries, and so on.

As an example, given a service called BookService, if you place a property called bookService within a controller as follows:

class BookController {
   def bookService
   …
}

In this case, the Spring container will automatically inject an instance of that service based on its configured scope. All dependency injection is done by name. You can also specify the type as follows:

class AuthorService {
	BookService bookService
}

NOTE: Normally the property name is generated by lower casing the first letter of the type. For example, an instance of the BookService class would map to a property named bookService.

To be consistent with standard JavaBean convetions, if the first 2 letters of the class name are upper case, the property name is the same as the class name. For example, an instance of the MYhelperService class would map to a property named MYhelperService.

See section 8.8 of the JavaBean specification for more information on de-capitalization rules.

Dependency Injection and Services

You can inject services in other services with the same technique. Say you had an AuthorService that needed to use the BookService, declaring the AuthorService as follows would allow that:

class AuthorService {
	def bookService
}

Dependency Injection and Domain Classes

You can even inject services into domain classes, which can aid in the development of rich domain models:

class Book {	
	…
	def bookService
	def buyBook() {
		bookService.buyBook(this)
	}
}

8.4 Using Services from Java

class BookService {
	void buyBook(Book book) {
		// logic
	}
}

However, this can be rectified by placing this class in a package, by moving the class into a sub directory such as grails-app/services/bookstore and then modifying the package declaration:

package bookstore
class BookService {
	void buyBook(Book book) {
		// logic
	}
}

An alternative to packages is to instead have an interface within a package that the service implements:

package bookstore;
interface BookStore {
	void buyBook(Book book);
}

And then the service:

class BookService implements bookstore.BookStore {
	void buyBook(Book b) {
		// logic
	}
}

This latter technique is arguably cleaner, as the Java side only has a reference to the interface and not to the implementation class. Either way, the goal of this exercise to enable Java to statically resolve the class (or interface) to use, at compile time. Now that this is done you can create a Java class within the src/java package, and provide a setter that uses the type and the name of the bean in Spring:

package bookstore;
// note: this is Java class
public class BookConsumer {
	private BookStore store;
	public void setBookStore(BookStore storeInstance) {
		this.store = storeInstance;
	}	
	…
}

Once this is done you can configure the Java class as a Spring bean in grails-app/conf/spring/resources.xml (For more information one this see the section on Grails and Spring):

<bean id="bookConsumer" class="bookstore.BookConsumer">
	<property name="bookStore" ref="bookService" />
</bean>

9. Testing

The first thing to be aware of is that all of the create-* commands actually end up creating integration tests automatically for you. For example say you run the create-controller command as follows:

grails create-controller simple

Not only will Grails create a controller at grails-app/controllers/SimpleController.groovy, but also an integration test at test/integration/SimpleControllerTests.groovy. What Grails won't do however is populate the logic inside the test! That is left up to you.

Running Tests

Test are run with the test-app command:

grails test-app

The above command will produce output such as:

-------------------------------------------------------
Running Unit Tests…
Running test FooTests...FAILURE
Unit Tests Completed in 464ms …
-------------------------------------------------------
Tests failed: 0 errors, 1 failures

Whilst reports will have been written out the target/test-reports directory.

You can force a clean before running tests by passing -clean to the test-app command.

Targeting Tests

You can selectively target the test(s) to be run in different ways. To run all tests for a controller named SimpleController you would run:

grails test-app SimpleController

This will run any tests for the class named SimpleController. Wildcards can be used...

grails test-app *Controller

This will test all classes ending in Controller. Package names can optionally be specified...

grails test-app some.org.*Controller

or to run all tests in a package...

grails test-app some.org.*

or to run all tests in a package including subpackages...

grails test-app some.org.**

You can also target particular test methods...

grails test-app SimpleController.testLogin

This will run the testLogin test in the SimpleController tests. You can specify as many patterns in combination as you like...

grails test-app some.org.* SimpleController.testLogin BookController

Targeting Test Types and/or Phases

In addition to targeting certain tests, you can also target test types and/or phases by using the phase:type syntax.

Grails organises tests by phase and by type. A test phase relates to the state of the Grails application during the tests, and the type relates to the testing mechanism.

Grails comes with support for 4 test phases (unit, integration, functional and other) and JUnit test types for the unit and integration phases. These test types have the same name as the phase.

Testing plugins may provide new test phases or new test types for existing phases. Refer to the plugin documentation.

To execute the JUnit integration tests you can run:

grails test-app integration:integration

Both phase and type are optional. Their absence acts as a wildcard. The following command will run all test types in the unit phase:

grails test-app unit:

The Grails Spock Plugin is one plugin that adds new test types to Grails. It adds a spock test type to the unit, integration and functional phases. To run all spock tests in all phases you would run the following:

grails test-app :spock

To run the all of the spock tests in the functional phase you would run...

grails test-app functional:spock

More than one pattern can be specified...

grails test-app unit:spock integration:spock

Targeting Tests in Types and/or Phases

Test and type/phase targetting can be applied at the same time:

grails test-app integration: unit: some.org.**

This would run all tests in the integration and unit phases that are in the page some.org or a subpackage of.

9.1 Unit Testing