17 Plugins - Reference Documentation
Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari
Version: 3.0.17
Table of Contents
17 Plugins
Grails is first and foremost a web application framework, but it is also a platform. By exposing a number of extension points that let you extend anything from the command line interface to the runtime configuration engine, Grails can be customised to suit almost any needs. To hook into this platform, all you need to do is create a plugin.Extending the platform may sound complicated, but plugins can range from trivially simple to incredibly powerful. If you know how to build a Grails application, you'll know how to create a plugin for sharing a data model or some static resources.17.1 Creating and Installing Plugins
Creating Plugins
Creating a Grails plugin is a simple matter of running the command:grails create-plugin [PLUGIN NAME]
grails create-plugin example
would create a new plugin project called example
.In Grails 3.0 you should consider whether the plugin you create requires a web environment or whether the plugin can be used with other profiles. If your plugin does not require a web environment then use the "plugin" profile instead of the "web-plugin" profile:grails create-plugin [PLUGIN NAME] --profile=plugin
src/main/groovy
directory under the plugin package structure you will find a plugin descriptor class (a class that ends in "GrailsPlugin").Being a regular Grails project has a number of benefits in that you can immediately test your plugin by running (if the plugin targets the "web" profile):grails run-app
Plugin projects don't provide an index.gsp by default since most plugins don't need it. So, if you try to view the plugin running in a browser right after creating it, you will receive a page not found error. You can easily create a grails-app/views/index.gsp
for your plugin if you'd like.
The plugin descriptor name ends with the convention GrailsPlugin
and is found in the root of the plugin project. For example:class ExampleGrailsPlugin { … }
src/main/groovy
directory, otherwise they are not regarded as a plugin. The plugin class defines metadata about the plugin, and optionally various hooks into plugin extension points (covered shortly).You can also provide additional information about your plugin using several special properties:
title
- short one-sentence description of your plugingrailsVersion
- The version range of Grails that the plugin supports. eg. "1.2 > *" (indicating 1.2 or higher)author
- plugin author's nameauthorEmail
- plugin author's contact e-maildescription
- full multi-line description of plugin's featuresdocumentation
- URL of the plugin's documentationlicense
- License of the pluginissueManagement
- Issue Tracker of the pluginscm
- Source code management location of the plugin
class QuartzGrailsPlugin { def grailsVersion = "1.1 > *" def author = "Sergey Nebolsin" def authorEmail = "nebolsin@gmail.com" def title = "Quartz Plugin" def description = '''\ The Quartz plugin allows your Grails application to schedule jobs\ to be executed using a specified interval or cron expression. The\ underlying system uses the Quartz Enterprise Job Scheduler configured\ via Spring, but is made simpler by the coding by convention paradigm.\ ''' def documentation = "http://grails.org/plugin/quartz" … }
Installing Local Plugins
To make your plugin available for use in a Grails application run theinstall
command:grails install
build.gradle
file:compile "org.grails.plugins:quartz:0.1"
In Grails 2.x plugins were packaged as ZIP files, however in Grails 3.x plugins are simple JAR files that can be added to the classpath of the IDE.
Notes on excluded Artefacts
Although the create-plugin command creates certain files for you so that the plugin can be run as a Grails application, not all of these files are included when packaging a plugin. The following is a list of artefacts created, but not included by package-plugin:grails-app/build.gradle
(although it is used to generatedependencies.groovy
)grails-app/conf/application.yml
(renamed to plugin.yml)grails-app/conf/spring/resources.groovy
grails-app/conf/logback.groovy
- Everything within
/src/test/**
- SCM management files within
**/.svn/**
and**/CVS/**
Customizing the plugin contents
When developing a plugin you may create test classes and sources that are used during the development and testing of the plugin but should not be exported to the application.To exclude test sources you need to modify thepluginExcludes
property of the plugin descriptor AND exclude the resources inside your build.gradle
file. For example say you have some classes under the com.demo
package that are in your plugin source tree but should not be packaged in the application. In your plugin descriptor you should exclude these:// resources that should be loaded by the plugin once installed in the application def pluginExcludes = [ '**/com/demo/**' ]
build.gradle
you should exclude the compiled classes from the JAR file:jar {
exclude "com/demo/**/**"
}
Inline Plugins in Grails 3.0
In Grails 2.x it was possible to specify inline plugins inBuildConfig
, in Grails 3.x this functionality has been replaced by Gradle's multi-project build feature.To set up a multi project build create an appliation and a plugin in a parent directory:$ grails create-app myapp $ grails create-plugin myplugin
settings.gradle
file in the parent directory specifying the location of your application and plugin:include 'myapp', 'myplugin'
build.gradle
on the plugin:compile project(':myplugin')
17.2 Plugin Repositories
Distributing Plugins in the Grails Central Plugin Repository
The preferred way to distribute plugin is to publish to the official Grails Central Plugin Repository. This will make your plugin visible to the list-plugins command:grails list-plugins
grails plugin-info [plugin-name]
If you have created a Grails plugin and want it to be hosted in the central repository, you'll find instructions for getting an account on the plugin portal website.
17.3 Providing Basic Artefacts
Add Command Line Commands
A plugin can add new commands to the Grails 3.0 interactive shell in one of two ways. First, using the create-script you can create a code generation script which will become available to the application. Thecreate-script
command will create the script in the src/main/scripts
directory:+ src/main/scripts <-- additional scripts here + grails-app + controllers + services + etc.
create-command
command:$ grails create-command MyExampleCommand
grails-app/commands/PACKAGE_PATH/MyExampleCommand.groovy
that extends ApplicationCommand:import grails.dev.commands.*class MyExampleCommand implements ApplicationCommand { boolean handle(ExecutionContext ctx) { println "Hello World" return true } }
ApplicationCommand
has access to the GrailsApplication
instance and is subject to autowiring like any other Spring bean.For each ApplicationCommand
present Grails will create a shell command and a Gradle task to invoke the ApplicationCommand
. In the above example you can invoke the MyExampleCommand
class using either:$ grails my-example
$ gradle myExample
ApplicationCommand
instances is that the latter has full access to the Grails application state and hence can be used to perform tasks that interactive with the database, call into GORM etc.In Grails 2.x Gant scripts could be used to perform both these tasks, in Grails 3.x code generation and interacting with runtime application state has been cleanly separated.Adding a new grails-app artifact (Controller, Tag Library, Service, etc.)
A plugin can add new artifacts by creating the relevant file within thegrails-app
tree.+ grails-app + controllers <-- additional controllers here + services <-- additional services here + etc. <-- additional XXX here
Providing Views, Templates and View resolution
When a plugin provides a controller it may also provide default views to be rendered. This is an excellent way to modularize your application through plugins. Grails' view resolution mechanism will first look for the view in the application it is installed into and if that fails will attempt to look for the view within the plugin. This means that you can override views provided by a plugin by creating corresponding GSPs in the application'sgrails-app/views
directory.For example, consider a controller called BookController
that's provided by an 'amazon' plugin. If the action being executed is list
, Grails will first look for a view called grails-app/views/book/list.gsp
then if that fails it will look for the same view relative to the plugin.However if the view uses templates that are also provided by the plugin then the following syntax may be necessary:<g:render template="fooTemplate" plugin="amazon"/>
plugin
attribute, which contains the name of the plugin where the template resides. If this is not specified then Grails will look for the template relative to the application.Excluded Artefacts
By default Grails excludes the following files during the packaging process:grails-app/conf/logback.groovy
grails-app/conf/application.yml
(renamed toplugin.yml
)grails-app/conf/spring/resources.groovy
- Everything within
/src/test/**
- SCM management files within
**/.svn/**
and**/CVS/**
UrlMappings.groovy
file is excluded to avoid naming conflicts, however you are free to add a UrlMappings definition under a different name which will be included. For example a file called grails-app/conf/BlogUrlMappings.groovy
is fine.The list of excludes is extensible with the pluginExcludes
property:// resources that are excluded from plugin packaging
def pluginExcludes = [
"grails-app/views/error.gsp"
]
17.4 Evaluating Conventions
Before looking at providing runtime configuration based on conventions you first need to understand how to evaluate those conventions from a plugin. Every plugin has an implicitapplication
variable which is an instance of the GrailsApplication interface.The GrailsApplication
interface provides methods to evaluate the conventions within the project and internally stores references to all artifact classes within your application.Artifacts implement the GrailsClass interface, which represents a Grails resource such as a controller or a tag library. For example to get all GrailsClass
instances you can do:for (grailsClass in application.allClasses) {
println grailsClass.name
}
GrailsApplication
has a few "magic" properties to narrow the type of artefact you are interested in. For example to access controllers you can use:for (controllerClass in application.controllerClasses) {
println controllerClass.name
}
*Classes
- Retrieves all the classes for a particular artefact name. For exampleapplication.controllerClasses
.get*Class
- Retrieves a named class for a particular artefact. For exampleapplication.getControllerClass("PersonController")
is*Class
- Returnstrue
if the given class is of the given artefact type. For exampleapplication.isControllerClass(PersonController)
GrailsClass
interface has a number of useful methods that let you further evaluate and work with the conventions. These include:
getPropertyValue
- Gets the initial value of the given property on the classhasProperty
- Returnstrue
if the class has the specified propertynewInstance
- Creates a new instance of this class.getName
- Returns the logical name of the class in the application without the trailing convention part if applicablegetShortName
- Returns the short name of the class without package prefixgetFullName
- Returns the full name of the class in the application with the trailing convention part and with the package namegetPropertyName
- Returns the name of the class as a property namegetLogicalPropertyName
- Returns the logical property name of the class in the application without the trailing convention part if applicablegetNaturalName
- Returns the name of the property in natural terms (e.g. 'lastName' becomes 'Last Name')getPackageName
- Returns the package name
17.5 Hooking into Runtime Configuration
Grails provides a number of hooks to leverage the different parts of the system and perform runtime configuration by convention.Hooking into the Grails Spring configuration
First, you can hook in Grails runtime configuration overriding thedoWithSpring
method from the Plugin class and returning a closure that defines additional beans. For example the following snippet is from one of the core Grails plugins that provides i18n support:import org.springframework.web.servlet.i18n.CookieLocaleResolver import org.springframework.web.servlet.i18n.LocaleChangeInterceptor import org.springframework.context.support.ReloadableResourceBundleMessageSource import grails.plugins.*class I18nGrailsPlugin extends Plugin { def version = "0.1" Closure doWithSpring() {{-> messageSource(ReloadableResourceBundleMessageSource) { basename = "WEB-INF/grails-app/i18n/messages" } localeChangeInterceptor(LocaleChangeInterceptor) { paramName = "lang" } localeResolver(CookieLocaleResolver) }} }
messageSource
bean and a couple of other beans to manage Locale resolution and switching. It using the Spring Bean Builder syntax to do so.Customizing the Servlet Environment
In previous versions of Grails it was possible to dynamically modify the generatedweb.xml
. In Grails 3.x there is no web.xml
file and it is not possible to programmatically modify the web.xml
file anymore.However, it is possible to perform the most commons tasks of modifying the Servlet environment in Grails 3.x.Adding New Servlets
If you want to add a new Servlet instance the simplest way is simply to define a new Spring bean in thedoWithSpring
method:Closure doWithSpring() {{-> myServlet(MyServlet) }}
Closure doWithSpring() {{-> myServlet(ServletRegistrationBean, new MyServlet(), "/myServlet/*") { loadOnStartup = 2 } }}
Adding New Servlet Filters
Just like Servlets, the simplest way to configure a new filter is to simply define a Spring bean:Closure doWithSpring() {{-> myFilter(MyFilter) }}
myFilter(FilterRegistrationBean) { filter = bean(MyFilter) urlPatterns = ['/*'] order = Ordered.HIGHEST_PRECEDENCE }
Grails' internal registered filters (GrailsWebRequestFilter
,HiddenHttpMethodFilter
etc.) are defined by incrementingHIGHEST_PRECEDENCE
by 10 thus allowing several filters to be inserted before or between Grails' filters.
Doing Post Initialisation Configuration
Sometimes it is useful to be able do some runtime configuration after the Spring ApplicationContext has been built. In this case you can define adoWithApplicationContext
closure property.class SimplePlugin extends Plugin{ def name = "simple" def version = "1.1" @Override void doWithApplicationContext() { def sessionFactory = applicationContext.sessionFactory // do something here with session factory } }
17.6 Adding Methods at Compile Time
Grails 3.0 makes it easy to add new traits to existing artefact types from a plugin. For example say you wanted to add methods for manipulating dates to controllers. This can be done by first defining a trait insrc/main/groovy
:package myplugintrait DateTrait { Date currentDate() { return new Date() } }
package myplugin@CompileStatic class ControllerTraitInjector implements TraitInjector { @Override Class getTrait() { DateTrait } @Override String[] getArtefactTypes() { ['Controller'] as String[] } }
TraitInjector
will add the DateTrait
to all controllers. The getArtefactTypes
method defines the types of artefacts that the trait should be applied to.
17.7 Adding Dynamic Methods at Runtime
The Basics
Grails plugins let you register dynamic methods with any Grails-managed or other class at runtime. This work is done in adoWithDynamicMethods
method.
Note that Grails 3.x features newer features such as traits that are usable from code compiled with CompileStatic
. It is recommended that dynamic behavior is only added for cases that are not possible with traits.
class ExamplePlugin extends Plugin { void doWithDynamicMethods() { for (controllerClass in grailsApplication.controllerClasses) { controllerClass.metaClass.myNewMethod = {-> println "hello world" } } } }
myNewMethod
to each controller. If you know beforehand the class you wish the add a method to you can simply reference its metaClass
property.For example we can add a new method swapCase
to java.lang.String
:class ExamplePlugin extends Plugin { @Override void doWithDynamicMethods() { String.metaClass.swapCase = {-> def sb = new StringBuilder() delegate.each { sb << (Character.isUpperCase(it as char) ? Character.toLowerCase(it as char) : Character.toUpperCase(it as char)) } sb.toString() } assert "UpAndDown" == "uPaNDdOWN".swapCase() } }
Interacting with the ApplicationContext
ThedoWithDynamicMethods
closure gets passed the Spring ApplicationContext
instance. This is useful as it lets you interact with objects within it. For example if you were implementing a method to interact with Hibernate you could use the SessionFactory
instance in combination with a HibernateTemplate
:import org.springframework.orm.hibernate3.HibernateTemplateclass ExampleHibernatePlugin extends Plugin{ void doWithDynamicMethods() { for (domainClass in grailsApplication.domainClasses) { domainClass.metaClass.static.load = { Long id-> def sf = applicationContext.sessionFactory def template = new HibernateTemplate(sf) template.load(delegate, id) } } } }
class MyConstructorPlugin { void doWithDynamicMethods() for (domainClass in grailsApplication.domainClasses) { domainClass.metaClass.constructor = {-> return applicationContext.getBean(domainClass.name) } } } }
17.8 Participating in Auto Reload Events
Monitoring Resources for Changes
Often it is valuable to monitor resources for changes and perform some action when they occur. This is how Grails implements advanced reloading of application state at runtime. For example, consider this simplified snippet from the GrailsServicesPlugin
:class ServicesGrailsPlugin extends Plugin { … def watchedResources = "file:./grails-app/services/*Service.groovy" … void onChange( Map<String, Object> event) { if (event.source) { def serviceClass = grailsApplication.addServiceClass(event.source) def serviceName = "${serviceClass.propertyName}" beans { "$serviceName"(serviceClass.getClazz()) { bean -> bean.autowire = true } } } } }
watchedResources
as either a String or a List of strings that contain either the references or patterns of the resources to watch. If the watched resources specify a Groovy file, when it is changed it will automatically be reloaded and passed into the onChange
closure in the event
object.The event
object defines a number of useful properties:
event.source
- The source of the event, either the reloadedClass
or a SpringResource
event.ctx
- The SpringApplicationContext
instanceevent.plugin
- The plugin object that manages the resource (usuallythis
)event.application
- TheGrailsApplication
instanceevent.manager
- TheGrailsPluginManager
instance
ApplicationContext
when one of the service classes changes.Influencing Other Plugins
In addition to reacting to changes, sometimes a plugin needs to "influence" another.Take for example the Services and Controllers plugins. When a service is reloaded, unless you reload the controllers too, problems will occur when you try to auto-wire the reloaded service into an older controller Class.To get around this, you can specify which plugins another plugin "influences". This means that when one plugin detects a change, it will reload itself and then reload its influenced plugins. For example consider this snippet from theServicesGrailsPlugin
:def influences = ['controllers']
Observing other plugins
If there is a particular plugin that you would like to observe for changes but not necessary watch the resources that it monitors you can use the "observe" property:def observe = ["controllers"]
def observe = ["*"]
log
property back to any artefact that changes while the application is running.
17.9 Understanding Plugin Load Order
Controlling Plugin Dependencies
Plugins often depend on the presence of other plugins and can adapt depending on the presence of others. This is implemented with two properties. The first is calleddependsOn
. For example, take a look at this snippet from the Hibernate plugin:class HibernateGrailsPlugin { def version = "1.0" def dependsOn = [dataSource: "1.0", domainClass: "1.0", i18n: "1.0", core: "1.0"] }
dataSource
, domainClass
, i18n
and core
plugins.The dependencies will be loaded before the Hibernate plugin and if all dependencies do not load, then the plugin will not load.The dependsOn
property also supports a mini expression language for specifying version ranges. A few examples of the syntax can be seen below:def dependsOn = [foo: "* > 1.0"] def dependsOn = [foo: "1.0 > 1.1"] def dependsOn = [foo: "1.0 > *"]
- 1.1
- 1.0
- 1.0.1
- 1.0.3-SNAPSHOT
- 1.1-BETA2
Controlling Load Order
UsingdependsOn
establishes a "hard" dependency in that if the dependency is not resolved, the plugin will give up and won't load. It is possible though to have a weaker dependency using the loadAfter
and loadBefore
properties:def loadAfter = ['controllers']
controllers
plugin if it exists, otherwise it will just be loaded. The plugin can then adapt to the presence of the other plugin, for example the Hibernate plugin has this code in its doWithSpring
closure:if (manager?.hasGrailsPlugin("controllers")) { openSessionInViewInterceptor(OpenSessionInViewInterceptor) { flushMode = HibernateAccessor.FLUSH_MANUAL sessionFactory = sessionFactory } grailsUrlHandlerMapping.interceptors << openSessionInViewInterceptor }
OpenSessionInViewInterceptor
if the controllers
plugin has been loaded. The manager
variable is an instance of the GrailsPluginManager interface and it provides methods to interact with other plugins.You can also use the loadBefore
property to specify one or more plugins that your plugin should load before:def loadBefore = ['rabbitmq']
Scopes and Environments
It's not only plugin load order that you can control. You can also specify which environments your plugin should be loaded in and which scopes (stages of a build). Simply declare one or both of these properties in your plugin descriptor:def environments = ['development', 'test', 'myCustomEnv'] def scopes = [excludes:'war']
development-only
plugins to not be packaged for production use.The full list of available scopes are defined by the enum BuildScope, but here's a summary:
test
- when running testsfunctional-test
- when running functional testsrun
- for run-app and run-warwar
- when packaging the application as a WAR fileall
- plugin applies to all scopes (default)
- a string - a sole inclusion
- a list - a list of environments or scopes to include
- a map - for full control, with 'includes' and/or 'excludes' keys that can have string or list values
def environments = "test"
def environments = ["development", "test"]
def environments = [includes: ["development", "test"]]
17.10 The Artefact API
You should by now understand that Grails has the concept of artefacts: special types of classes that it knows about and can treat differently from normal Groovy and Java classes, for example by enhancing them with extra properties and methods. Examples of artefacts include domain classes and controllers. What you may not be aware of is that Grails allows application and plugin developers access to the underlying infrastructure for artefacts, which means you can find out what artefacts are available and even enhance them yourself. You can even provide your own custom artefact types.17.10.1 Asking About Available Artefacts
As a plugin developer, it can be important for you to find out about what domain classes, controllers, or other types of artefact are available in an application. For example, the Searchable plugin needs to know what domain classes exist so it can check them for anysearchable
properties and index the appropriate ones. So how does it do it? The answer lies with the grailsApplication
object, and instance of GrailsApplication that's available automatically in controllers and GSPs and can be injected everywhere else.The grailsApplication
object has several important properties and methods for querying artefacts. Probably the most common is the one that gives you all the classes of a particular artefact type:for (cls in grailsApplication.<artefactType>Classes) {
…
}
artefactType
is the property name form of the artefact type. With core Grails you have:
- domain
- controller
- tagLib
- service
- codec
- bootstrap
- urlMappings
for (cls in grailsApplication.domainClasses) {
…
}
for (cls in grailsApplication.urlMappingsClasses) {
…
}
Class
:
shortName
- the class name of the artefact without the package (equivalent ofClass.simpleName
).logicalPropertyName
- the artefact name in property form without the 'type' suffix. SoMyGreatController
becomes 'myGreat'.isAbstract()
- a boolean indicating whether the artefact class is abstract or not.getPropertyValue(name)
- returns the value of the given property, whether it's a static or an instance one. This works best if the property is initialised on declaration, e.g.static transactional = true
.
- get<type>Class(String name)
- is<type>Class(Class clazz)
GrailsClass
instance for the given name, e.g. 'MyGreatController'. The second will check whether a class is a particular type of artefact. For example, you can use grailsApplication.isControllerClass(org.example.MyGreatController)
to check whether MyGreatController
is in fact a controller.
17.10.2 Adding Your Own Artefact Types
Plugins can easily provide their own artefacts so that they can easily find out what implementations are available and take part in reloading. All you need to do is create anArtefactHandler
implementation and register it in your main plugin class:class MyGrailsPlugin { def artefacts = [ org.somewhere.MyArtefactHandler ] … }
artefacts
list can contain either handler classes (as above) or instances of handlers.So, what does an artefact handler look like? Well, put simply it is an implementation of the ArtefactHandler interface. To make life a bit easier, there is a skeleton implementation that can readily be extended: ArtefactHandlerAdapter.In addition to the handler itself, every new artefact needs a corresponding wrapper class that implements GrailsClass. Again, skeleton implementations are available such as AbstractInjectableGrailsClass, which is particularly useful as it turns your artefact into a Spring bean that is auto-wired, just like controllers and services.The best way to understand how both the handler and wrapper classes work is to look at the Quartz plugin:
Another example is the Shiro plugin which adds a realm artefact.