(Quick Reference)

17 Plugins - Reference Documentation

Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari

Version: 3.0.0.RC3

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]

This will create a plugin project for the name you specify. For example running 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 you 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

Make sure the plugin name does not contain more than one capital in a row, or it won't work. Camel case is fine, though.

The structure of a Grails plugin is very nearly the same as a Grails application project's except that in the 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 {
   …
}

All plugins must have this class under the 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 plugin
  • grailsVersion - The version of version range of Grails that the plugin supports. eg. "1.2 > *" (indicating 1.2 or higher)
  • author - plugin author's name
  • authorEmail - plugin author's contact e-mail
  • description - full multi-line description of plugin's features
  • documentation - URL of the plugin's documentation

Here is an example from the Quartz Grails 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 the install command:

grails install

This will install the plugin into your local Maven cache. Then to use the plugin within an application declare a dependency on the plugin in your 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 generate dependencies.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 the pluginExcludes 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/**'
  ]

And in your 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 in BuildConfig, 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

Then create a settings.gradle file in the parent directory specifying the location of your application and plugin:

include 'myapp', 'myplugin'

Finally add a dependency in your application's build.gradle on the plugin:

compile project(':myplugin')

Using this technique you have achieved the equivalent of inline plugins from Grails 2.x.

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

which lists all plugins that are in the central repository. Your plugin will also be available to the plugin-info command:

grails plugin-info [plugin-name]

which prints extra information about it, such as its description, who wrote, etc.

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. The create-script command will create the script in the src/main/scripts directory:

+ src/main/scripts     <-- additional scripts here
 + grails-app
      + controllers
      + services
      + etc.

Code generation scripts can be used to create artefacts within the project tree and automate interactions with Gradle.

If you want to create a new shell command that interacts with a loaded Grails application instance then you should use the create-command command:

$ grails create-command MyExampleCommand

This will create a file called 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 } }

An 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

Or

$ gradle myExample

The Grails version is all lower case hyphen separated and excludes the "Command" suffix.

The main difference between code generation scripts and 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 the grails-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's grails-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"/>

Note the usage of the 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 to plugin.yml)
  • grails-app/conf/spring/resources.groovy
  • Everything within /src/test/**
  • SCM management files within **/.svn/** and **/CVS/**

In addition, the default 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"
]

This is useful for example to include demo or test resources in the plugin repository, but not include them in the final distribution.

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 implicit application 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
}

The dynamic method conventions are as follows:

  • *Classes - Retrieves all the classes for a particular artefact name. For example application.controllerClasses.
  • get*Class - Retrieves a named class for a particular artefact. For example application.getControllerClass("PersonController")
  • is*Class - Returns true if the given class is of the given artefact type. For example application.isControllerClass(PersonController)

The 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 class
  • hasProperty - Returns true if the class has the specified property
  • newInstance - Creates a new instance of this class.
  • getName - Returns the logical name of the class in the application without the trailing convention part if applicable
  • getShortName - Returns the short name of the class without package prefix
  • getFullName - Returns the full name of the class in the application with the trailing convention part and with the package name
  • getPropertyName - Returns the name of the class as a property name
  • getLogicalPropertyName - Returns the logical property name of the class in the application without the trailing convention part if applicable
  • getNaturalName - Returns the name of the property in natural terms (e.g. 'lastName' becomes 'Last Name')
  • getPackageName - Returns the package name

For a full reference refer to the javadoc API.

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 the doWithSpring 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) }} }

This plugin configures the Grails 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 generated web.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 the doWithSpring method:

Closure doWithSpring() {{->
  myServlet(MyServlet)
}}

If you need to customize the servlet you can use Spring Boot's ServletRegistrationBean:

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)
}}

However, if you want to control the order of filter registrations you will need to use Spring Boot's FilterRegistrationBean:

myFilter(FilterRegistrationBean) {
    filter = bean(MyFilter)
    urlPatterns = ['/*']
    order = Ordered.HIGHEST_PRECEDENCE
}

Grails' internal registered filters (GrailsWebRequestFilter, HiddenHttpMethodFilter etc.) are defined by incrementing HIGHEST_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 a doWithApplicationContext 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 in src/main/groovy:

package myplugin

trait DateTrait { Date currentDate() { return new Date() } }

Once you have a trait you must tell Grails which artefacts you want to inject the trait into at compile time. To do this you implement a TraitInjector:

package myplugin

@CompileStatic class ControllerTraitInjector implements TraitInjector {

@Override Class getTrait() { DateTrait }

@Override String[] getArtefactTypes() { ['Controller'] as String[] } }

The above 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 a doWithDynamicMethods 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" }
        }
    }
}

In this case we use the implicit application object to get a reference to all of the controller classes' MetaClass instances and add a new method called 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

The doWithDynamicMethods 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.HibernateTemplate

class 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) } } } }

Also because of the autowiring and dependency injection capability of the Spring container you can implement more powerful dynamic constructors that use the application context to wire dependencies into your object at runtime:

class MyConstructorPlugin {

void doWithDynamicMethods() for (domainClass in grailsApplication.domainClasses) { domainClass.metaClass.constructor = {-> return applicationContext.getBean(domainClass.name) } } } }

Here we actually replace the default constructor with one that looks up prototyped Spring beans instead!

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 Grails ServicesPlugin:

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 } } } } }

First it defines 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 reloaded Class or a Spring Resource
  • event.ctx - The Spring ApplicationContext instance
  • event.plugin - The plugin object that manages the resource (usually this)
  • event.application - The GrailsApplication instance
  • event.manager - The GrailsPluginManager instance

These objects are available to help you apply the appropriate changes based on what changed. In the "Services" example above, a new service bean is re-registered with the 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 the ServicesGrailsPlugin:

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"]

In this case when a controller is changed you will also receive the event chained from the controllers plugin.

It is also possible for a plugin to observe all loaded plugins by using a wildcard:

def observe = ["*"]

The Logging plugin does exactly this so that it can add the 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 called dependsOn. 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"] }

The Hibernate plugin is dependent on the presence of four plugins: the 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 > *"]

When the wildcard * character is used it denotes "any" version. The expression syntax also excludes any suffixes such as -BETA, -ALPHA etc. so for example the expression "1.0 > 1.1" would match any of the following versions:

  • 1.1
  • 1.0
  • 1.0.1
  • 1.0.3-SNAPSHOT
  • 1.1-BETA2

Controlling Load Order

Using dependsOn 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']

Here the plugin will be loaded after the 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
}

Here the Hibernate plugin will only register an 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']

In this example, the plugin will only load in the 'development' and 'test' environments. Nor will it be packaged into the WAR file, because it's excluded from the 'war' phase. This allows 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 tests
  • functional-test - when running functional tests
  • run - for run-app and run-war
  • war - when packaging the application as a WAR file
  • all - plugin applies to all scopes (default)

Both properties can be one of:

  • 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

For example,

def environments = "test"

will only include the plugin in the test environment, whereas

def environments = ["development", "test"]

will include it in both the development and test environments. Finally,

def environments = [includes: ["development", "test"]]

will do the same thing.

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 any searchable 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) {
    …
}

In this case, artefactType is the property name form of the artefact type. With core Grails you have:

  • domain
  • controller
  • tagLib
  • service
  • codec
  • bootstrap
  • urlMappings

So for example, if you want to iterate over all the domain classes, you use:

for (cls in grailsApplication.domainClasses) {
    …
}

and for URL mappings:

for (cls in grailsApplication.urlMappingsClasses) {
    …
}

You need to be aware that the objects returned by these properties are not instances of Class. Instead, they are instances of GrailsClass that has some particularly useful properties and methods, including one for the underlying Class:

  • shortName - the class name of the artefact without the package (equivalent of Class.simpleName).
  • logicalPropertyName - the artefact name in property form without the 'type' suffix. So MyGreatController 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.

The artefact API also allows you to fetch classes by name and check whether a class is an artefact:

  • get<type>Class(String name)
  • is<type>Class(Class clazz)

The first method will retrieve the 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 an ArtefactHandler implementation and register it in your main plugin class:

class MyGrailsPlugin {
    def artefacts = [ org.somewhere.MyArtefactHandler ]
    …
}

The 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.