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(Quick Reference) # 15 Plugins - Reference Documentation Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari Version: 2.3.8 ## Table of Contents # 15 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. ## 15.1 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`. 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 root of the plugin directory you will find a plugin Groovy file called the "plugin descriptor". The only plugins included in a new plugin project are Tomcat and Release. Hibernate is not included by default. Being a regular Grails project has a number of benefits in that you can immediately test your plugin by running:
    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 {
       def version = "0.1"   …
    }
All plugins must have this class in the root of their directory structure. The plugin class defines the version of the plugin and other metadata, 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 `version` - The version of your plugin. Valid values include example "0.1", "0.2-SNAPSHOT", "1.1.4" etc. `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 version = "0.1"
        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 `maven-install` command:
    grails maven-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 `grails-app/conf/BuildConfig.groovy` file:
    compile ":quartz:0.1"
#### 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/conf/BootStrap.groovy` `grails-app/conf/BuildConfig.groovy` (although it is used to generate `dependencies.groovy`) `grails-app/conf/Config.groovy` `grails-app/conf/DataSource.groovy` (and any other `*DataSource.groovy`) `grails-app/conf/UrlMappings.groovy` `grails-app/conf/spring/resources.groovy` Everything within `/web-app/WEB-INF` Everything within `/web-app/plugins/**` Everything within `/test/**` SCM management files within `**/.svn/**` and `**/CVS/**` If you need artefacts within `WEB-INF` it is recommended you use the `_Install.groovy` script (covered later), which is executed when a plugin is installed, to provide such artefacts. In addition, although `UrlMappings.groovy` is excluded you are allowed to include a `UrlMappings` definition with a different name, such as `MyPluginUrlMappings.groovy`. #### Customizing the plugin contents You can specify what to exclude in addition to the default excludes by adding elements to the `pluginExcludes` descriptor property (described below). In addition, there are two ways to configure the contents of the plugin ZIP or JAR file. One is to create an event handler for the `CreatePluginArchiveStart` event, which is fired after all of the plugin files have been copied to the staging directory. By adding an event handler you can add, modify, or delete files as needed. Add the handler to `_Events.groovy` in the `scripts` directory, for example
    eventCreatePluginArchiveStart = { stagingDir ->
       // update staging directory contents here
    }
You can customize the location of the staging directory with the `grails.project.plugin.staging.dir` attribute in `BuildConfig.groovy` or as as system property. Note that there is also a `CreatePluginArchiveEnd` event which is fired after the ZIP or JAR is packaged. You can also do this work in a Closure in `BuildConfig.groovy` with the property `grails.plugin.resources` which is analogous to the `grails.war.resources` property, e.g.
    grails.plugin.resources = { stagingDir ->
       // update staging directory contents here
    }
#### Specifying Plugin Locations An application can load plugins from anywhere on the file system, even if they have not been installed. Specify the location of the (unpacked) plugin in the application's `grails-app/conf/BuildConfig.groovy` file:

    // Useful to test plugins you are developing.
    grails.plugin.location.shiro =
            "/home/dilbert/dev/plugins/grails-shiro"// Useful for modular applications where all plugins and
    // applications are in the same directory.
    grails.plugin.location.'grails-ui' = "../grails-grails-ui"
This is particularly useful in two cases: You are developing a plugin and want to test it in a real application without packaging and installing it first. You have split an application into a set of plugins and an application, all in the same "super-project" directory. The Artifactory repository for Grails now includes all the dependencies for published plugins. So, if you are using inline plugins that have dependencies, it is necessary to do a secondary resolve because these dependencies might not be in the repository. Therefore, you should set `legacyResolve` to `true` in your `BuildConfig.groovy` if you are using inline plugins with dependencies. ## 15.2 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 this wiki page. When you have access to the Grails Plugin repository, install the Release Plugin by declaring it as a 'build' scoped dependency in `grails-app/conf/BuildConfig.groovy` file:
    grails.project.dependency.resolution = {
        …
        plugins {
            build ':release:3.0.0'
        }
    }
And execute the `publish-plugin` command to release your plugin:
    grails publish-plugin
This will automatically publish the plugin to the central repository. If the command is successful, it will immediately be available on the plugin portal at http://grails.org/plugin/<pluginName>. You can find out more about the Release plugin and its other features in its user guide. #### Configuring Additional Repositories The process for configuring repositories in Grails differs between versions. For version of Grails 1.2 and earlier please refer to the Grails 1.2 documentation on the subject. The following sections cover Grails 1.3 and above. Grails 1.3 and above use Ivy under the hood to resolve plugin dependencies. The mechanism for defining additional plugin repositories is largely the same as defining repositories for JAR dependencies. For example you can define a remote Maven repository that contains Grails plugins using the following syntax in `grails-app/conf/BuildConfig.groovy`:

    repositories {
        mavenRepo "http://repository.codehaus.org"    // ...or with a name
        mavenRepo name: "myRepo",
                  root: "http://myserver:8081/artifactory/plugins-snapshots-local"
    }
You can also define a SVN-based Grails repository (such as the one hosted at http://plugins.grails.org) using the `grailsRepo` method:

    repositories {
        grailsRepo "http://myserver/mygrailsrepo"    // ...or with a name
        grailsRepo "http://myserver/svn/grails-plugins", "mySvnRepo"
    }
There is a shortcut to setup the Grails central repository:
    repositories {
        grailsCentral()
    }
The order in which plugins are resolved is based on the ordering of the repositories. So in this case the Grails central repository will be searched last:
    repositories {
        grailsRepo "http://myserver/mygrailsrepo"
        grailsCentral()
    }
#### Publishing to Maven Compatible Repositories In general it is recommended for Grails 1.3 and above to use standard Maven-style repositories to self host plugins. The benefits of doing so include the ability for existing tooling and repository managers to interpret the structure of a Maven repository. You use the Release plugin to publish a plugin to a Maven repository. Please refer to the section of the Maven deployment user guide on the subject. ## 15.3 Understanding a Plugin's Structure As as mentioned previously, a plugin is basically a regular Grails application with a plugin descriptor. However when installed, the structure of a plugin differs slightly. For example, take a look at this plugin directory structure:
    + grails-app
         + controllers
         + domain
         + taglib
         etc.
     + lib
     + src
         + java
         + groovy
     + web-app
         + js
         + css
When a plugin is installed the contents of the `grails-app` directory will go into a directory such as `plugins/example-1.0/grails-app`. They will not be copied into the main source tree. A plugin never interferes with a project's primary source tree. Dealing with static resources is slightly different. When developing a plugin, just like an application, all static resources go in the `web-app` directory. You can then link to static resources just like in an application. This example links to a JavaScript source:
    <g:resource dir="js" file="mycode.js" />
When you run the plugin in development mode the link to the resource will resolve to something like `/js/mycode.js`. However, when the plugin is installed into an application the path will automatically change to something like `/plugin/example-0.1/js/mycode.js` and Grails will deal with making sure the resources are in the right place. There is a special `pluginContextPath` variable that can be used whilst both developing the plugin and when in the plugin is installed into the application to find out what the correct path to the plugin is. At runtime the `pluginContextPath` variable will either evaluate to an empty string or `/plugins/example` depending on whether the plugin is running standalone or has been installed in an application Java and Groovy code that the plugin provides within the lib and `src/java` and `src/groovy` directories will be compiled into the main project's `web-app/WEB-INF/classes` directory so that they are made available at runtime. ## 15.4 Providing Basic Artefacts #### Adding a new Script A plugin can add a new script simply by providing the relevant Gant script in its scripts directory:
    + MyPlugin.groovy
       + scripts     <-- additional scripts here
       + grails-app
            + controllers
            + services
            + etc.
        + lib
#### 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. Note that the plugin is loaded from where it is installed and not copied into the main application tree.
    + ExamplePlugin.groovy
       + scripts
       + grails-app
            + controllers  <-- additional controllers here
            + services <-- additional services here
            + etc.  <-- additional XXX here
        + lib
#### 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/BootStrap.groovy` `grails-app/conf/BuildConfig.groovy` (although it is used to generate `dependencies.groovy`) `grails-app/conf/Config.groovy` `grails-app/conf/DataSource.groovy` (and any other `*DataSource.groovy`) `grails-app/conf/UrlMappings.groovy` `grails-app/conf/spring/resources.groovy` Everything within `/web-app/WEB-INF` Everything within `/web-app/plugins/**` Everything within `/test/**` SCM management files within `**/.svn/**` and `**/CVS/**` If your plugin requires files under the `web-app/WEB-INF` directory it is recommended that you modify the plugin's `scripts/_Install.groovy` Gant script to install these artefacts into the target project's directory tree. 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. ## 15.5 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 (eg. 'lastName' becomes 'Last Name') `getPackageName` - Returns the package name For a full reference refer to the javadoc API. ## 15.6 Hooking into Build Events #### Post-Install Configuration and Participating in Upgrades Grails plugins can do post-install configuration and participate in application upgrade process (the upgrade command). This is achieved using two specially named scripts under the `scripts` directory of the plugin - `_Install.groovy` and `_Upgrade.groovy`. `_Install.groovy` is executed after the plugin has been installed and `_Upgrade.groovy` is executed each time the user upgrades the application (but not the plugin) with upgrade command. These scripts are Gant scripts, so you can use the full power of Gant. An addition to the standard Gant variables there is also a `pluginBasedir` variable which points at the plugin installation basedir. As an example this `_Install.groovy` script will create a new directory type under the `grails-app` directory and install a configuration template:

    ant.mkdir(dir: "${basedir}/grails-app/jobs")ant.copy(file: "${pluginBasedir}/src/samples/SamplePluginConfig.groovy",
             todir: "${basedir}/grails-app/conf")
The `pluginBasedir` variable is not available in custom scripts, but you can use `fooPluginDir`, where `foo` is the name of your plugin. #### Scripting events It is also possible to hook into command line scripting events. These are events triggered during execution of Grails target and plugin scripts. For example, you can hook into status update output (i.e. "Tests passed", "Server running") and the creation of files or artefacts. A plugin just has to provide an `_Events.groovy` script to listen to the required events. Refer the documentation on Hooking into Events for further information. ## 15.7 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 by providing a property called `doWithSpring` which is assigned a block of code. 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.ReloadableResourceBundleMessageSourceclass I18nGrailsPlugin {    def version = "0.1"    def 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. #### Participating in web.xml Generation Grails generates the `WEB-INF/web.xml` file at load time, and although plugins cannot change this file directly, they can participate in the generation of the file. A plugin can provide a `doWithWebDescriptor` property that is assigned a block of code that gets passed the `web.xml` as an `XmlSlurper` `GPathResult`. ##### Add `servlet` and `servlet-mapping` Consider this example from the `ControllersPlugin`:

    def doWithWebDescriptor = { webXml ->    def mappingElement = webXml.'servlet-mapping'    def lastMapping = mappingElement[mappingElement.size() - 1]
        lastMapping + {
            'servlet-mapping' {
                'servlet-name'("grails")
                'url-pattern'("*.dispatch")
            }
        }
    }
Here the plugin gets a reference to the last `<servlet-mapping>` element and appends Grails' servlet after it using XmlSlurper's ability to programmatically modify XML using closures and blocks. ##### Add `filter` and `filter-mapping` Adding a filter with its mapping works a little differently. The location of the `<filter>` element doesn't matter since order is not important, so it's simplest to insert your custom filter definition immediately after the last `<context-param>` element. Order is important for mappings, but the usual approach is to add it immediately after the last `<filter>` element like so:

    def doWithWebDescriptor = { webXml ->    def contextParam = webXml.'context-param'    contextParam[contextParam.size() - 1] + {
            'filter' {
                'filter-name'('springSecurityFilterChain')
                'filter-class'(DelegatingFilterProxy.name)
            }
        }    def filter = webXml.'filter'
        filter[filter.size() - 1] + {
            'filter-mapping'{
                'filter-name'('springSecurityFilterChain')
                'url-pattern'('/*')
            }
        }
    }
In some cases you need to ensure that your filter comes after one of the standard Grails filters, such as the Spring character encoding filter or the SiteMesh filter. Fortunately you can insert filter mappings immediately after the standard ones (more accurately, any that are in the template web.xml file) like so:

    def doWithWebDescriptor = { webXml ->
        ...    // Insert the Spring Security filter after the Spring
        // character encoding filter.
        def filter = webXml.'filter-mapping'.find {
            it.'filter-name'.text() == "charEncodingFilter"
        }    filter + {
            'filter-mapping'{
                'filter-name'('springSecurityFilterChain')
                'url-pattern'('/*')
            }
        }
    }
#### 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 {    def name = "simple"
        def version = "1.1"    def doWithApplicationContext = { appCtx ->
            def sessionFactory = appCtx.sessionFactory
            // do something here with session factory
        }
    }
## 15.8 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` closure. For Grails-managed classes like controllers, tag libraries and so forth you can add methods, constructors etc. using the ExpandoMetaClass mechanism by accessing each controller's MetaClass:
    class ExamplePlugin {
        def doWithDynamicMethods = { applicationContext ->
            for (controllerClass in application.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 {    def doWithDynamicMethods = { applicationContext ->
            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.HibernateTemplateclass ExampleHibernatePlugin {   def doWithDynamicMethods = { applicationContext ->       for (domainClass in application.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 {    def doWithDynamicMethods = { applicationContext ->
             for (domainClass in application.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! ## 15.9 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 {
        …
        def watchedResources = "file:./grails-app/services/*Service.groovy"    …
        def onChange = { event ->
            if (event.source) {
                def serviceClass = application.addServiceClass(event.source)
                def serviceName = "${serviceClass.propertyName}"
                def beans = beans {
                    "$serviceName"(serviceClass.getClazz()) { bean ->
                        bean.autowire =  true
                    }
                }
                if (event.ctx) {
                    event.ctx.registerBeanDefinition(
                            serviceName,
                            beans.getBeanDefinition(serviceName))
                }
            }
        }
    }
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. ## 15.10 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. ## 15.11 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. ## 15.11.1 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. ## 15.11.2 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: GrailsJobClass DefaultGrailsJobClass JobArtefactHandler Another example is the Shiro plugin which adds a realm artefact. ## 15.12 Binary Plugins Regular Grails plugins are packaged as zip files containing the full source of the plugin. This has some advantages in terms of being an open distribution system (anyone can see the source), in addition to avoiding problems with the source compatibility level used for compilation. As of Grails 2.0 you can pre-compile Grails plugins into regular JAR files known as "binary plugins". This has several advantages (and some disadvantages as discussed in the advantages of source plugins above) including: Binary plugins can be published as standard JAR files to a Maven repository Binary plugins can be declared like any other JAR dependency Commercial plugins are more viable since the source isn't published IDEs have a better understanding since binary plugins are regular JAR files containing classes #### Packaging To package a plugin in binary form you can use the package-plugin command and the `--binary` flag:
    grails package-plugin --binary
Supported artefacts include: Grails artifact classes such as controllers, domain classes and so on I18n Message bundles GSP Views, layouts and templates You can also specify the packaging in the plugin descriptor:
    def packaging = "binary"
in which case the packaging will default to binary. #### Using Binary Plugins The packaging process creates a JAR file in the `target` directory of the plugin, for example `target/foo-plugin-0.1.jar`. There are two ways to incorporate a binary plugin into an application. One is simply placing the plugin JAR file in your application's `lib` directory. The other is to publish the plugin JAR to a compatible Maven repository and declare it as a dependency in `grails-app/conf/BuildConfig.groovy`:
    dependencies {
        compile "mycompany:myplugin:0.1"
    }
Since binary plugins are packaged as JAR files, they are declared as dependencies in the `dependencies` block, not in the `plugins` block as you may be naturally inclined to do. The `plugins` block is used for declaring traditional source plugins packaged as zip files | Quick Reference (hide)