A Gradle plugin packages up reusable pieces of build logic, which can be used across many different projects and builds. Gradle allows you to implement your own custom plugins, so you can reuse your build logic, and share it with others.
You can implement a custom plugin in any language you like, provided the implementation ends up compiled as bytecode. For the examples here, we are going to use Groovy as the implementation language. You could use Java or Scala instead, if you want.
There are several places where you can put the source for the plugin.
You can include the source for the plugin directly in the build script. This has the benefit that the plugin is automatically compiled and included in the classpath of the build script without you having to do anything. However, the plugin is not visible outside the build script, and so you cannot reuse the plugin outside the build script it is defined in.
buildSrc
projectYou can put the source for the plugin in the
directory.
Gradle will take care of compiling and testing the plugin and making it available on the
classpath of the build script. The plugin is visible to every build script used by the build.
However, it is not visible outside the build, and so you cannot reuse the plugin outside the
build it is defined in.
rootProjectDir
/buildSrc/src/main/groovy
See Chapter 41, Organizing Build Logic for more details about the buildSrc
project.
You can create a separate project for your plugin. This project produces and publishes a JAR which you can then use in multiple builds and share with others. Generally, this JAR might include some custom plugins, or bundle several related task classes into a single library. Or some combination of the two.
In our examples, we will start with the plugin in the build script, to keep things simple. Then we will look at creating a standalone project.
To create a custom plugin, you need to write an implementation of Plugin
.
Gradle instantiates the plugin and calls the plugin instance's Plugin.apply()
method when the
plugin is used with a project. The project
object is passed as a parameter, which the plugin can use to configure the project however it needs to.
The following sample contains a greeting plugin, which adds a hello
task to the project.
Example 40.1. A custom plugin
build.gradle
apply plugin: GreetingPlugin class GreetingPlugin implements Plugin<Project> { def void apply(Project project) { project.task('hello') << { println "Hello from the GreetingPlugin" } } }
Output of gradle -q hello
> gradle -q hello Hello from the GreetingPlugin
One thing to note is that a new instance of a given plugin is created for each project it is applied to.
Most plugins need to obtain some configuration from the build script. One method for doing this is to use convention objects.
The Gradle Project
has a Convention
object
that helps keep track of all the settings and properties being passed to plugins. You can capture user input by telling
the Project Convention about your plugin. To capture input, simply add a Java Bean compliant class into the Convention's list of
plugins. Groovy is a good language choice for a plugin because plain old Groovy objects contain all the getter and setter methods
that a Java Bean requires.
Let's add a simple convention object to the project. Here we add a greeting
property to the
project, which allows you to configure the greeting.
Example 40.2. A custom plugin convention
build.gradle
apply plugin: GreetingPlugin greeting = 'Hi from Gradle' class GreetingPlugin implements Plugin<Project> { def void apply(Project project) { project.convention.plugins.greet = new GreetingPluginConvention() project.task('hello') << { println project.convention.plugins.greet.greeting } } } class GreetingPluginConvention { def String greeting = 'Hello from GreetingPlugin' }
Output of gradle -q hello
> gradle -q hello Hi from Gradle
In this example, GreetingPluginConvention
is a plain old Groovy object with a field called greeting
.
The convention object is added to the plugin list with the name greet
. The name of the variable in
the build needs to match the name of the field in the convention object. The name you choose for your plugin
(greet
) is arbitrary and can be whatever you choose.
Oftentimes, you have several related properties you need to specify on a single plugin. With Groovy plugins it is easy to offer a configuration closure block to group settings together. The following example shows you how to do this.
Example 40.3. A custom plugin with closure convention
build.gradle
apply plugin: GreetingPlugin greet { message = 'Hi from Gradle' } class GreetingPlugin implements Plugin<Project> { def void apply(Project project) { project.convention.plugins.greet = new GreetingPluginConvention() project.task('hello') << { println project.convention.plugins.greet.message } } } class GreetingPluginConvention { String message def greet(Closure closure) { closure.delegate = this closure() } }
Output of gradle -q hello
> gradle -q hello Hi from Gradle
In this example, several convention settings can be grouped together within the greet
closure.
The name of the closure block in the build script (greet
) needs a matching method on
the convention object, and that method must take a closure as an argument. Then, when the closure is executed,
the fields on the convention object will be mapped to the variables within the closure based on the standard
Groovy closure delegate feature. This technique is possible in other JVM languages but may not be as convenient
as in Groovy.
Now we will move our plugin to a standalone project, so we can publish it and share it with others. This project is simply a Groovy project that produces a JAR containing the plugin classes. Here is a simple build script for the project. It applies the Groovy plugin, and adds the Gradle API as a compile-time dependency.
Example 40.4. A build for a custom plugin
build.gradle
apply plugin: 'groovy'
dependencies {
compile gradleApi()
}
Note: The code for this example can be found at samples/customPlugin
which is in both the binary and source distributions of Gradle.
So how does Gradle find the Plugin
implementation? The answer is you need to provide a properties file in the jar's
META-INF/gradle-plugins
directory that matches the name of your plugin.
Example 40.5. Wiring for a custom plugin
src/main/resources/META-INF/gradle-plugins/greeting.properties
implementation-class=org.gradle.GreetingPlugin
Notice that the properties filename matches the plugin's name and is placed in the resources folder, and
that the implementation-class
property identifies the Plugin
implementation class.
To use a plugin in a build script, you need to add the plugin classes to the build script's classpath. To
do this, you use a buildscript { }
block, as described in Section 41.5, “External dependencies for the build script”.
The following example shows how you might do this when the JAR containing the plugin has been published
to a local repository:
Example 40.6. Using a custom plugin in another project
usesCustomPlugin.gradle
buildscript { repositories { mavenRepo urls: uri('repo') } dependencies { classpath group: 'org.gradle', name: 'customPlugin', version: '1.0-SNAPSHOT' } } apply plugin: 'greeting'
You can use the ProjectBuilder
class to create
Project
instances to use when you test your plugin implementation.
Example 40.7. Testing a custom plugin
src/test/groovy/org/gradle/GreetingPluginTest.groovy
class GreetingPluginTest { @Test public void greeterPluginAddsGreetingTaskToProject() { Project project = ProjectBuilder.builder().build() project.apply plugin: 'greeting' assertTrue(project.tasks.hello instanceof GreetingTask) } }
Gradle provides some utility classes for maintaining collections of object, which work well with the Gradle build language.
Example 40.8. Managing domain objects
build.gradle
apply plugin: DocumentationPlugin books { quickStart { sourceFile = file('src/docs/quick-start') } userGuide { } developerGuide { } } task books << { books.each { book -> println "$book.name -> $book.sourceFile" } } class DocumentationPlugin implements Plugin<Project> { def void apply(Project project) { def books = project.container(Book) { name -> new Book(name) } books.all { sourceFile = project.file("src/docs/$name") } project.convention.plugins.documentation = new DocumentationPluginConvention(books) } } class Book { final String name File sourceFile Book(String name) { this.name = name } } class DocumentationPluginConvention { final NamedDomainObjectContainer<Book> books DocumentationPluginConvention(NamedDomainObjectContainer<Book> books) { this.books = books } def books(Closure cl) { books.configure(cl) } }
Output of gradle -q books
> gradle -q books developerGuide -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPluginWithDomainObjectContainer/src/docs/developerGuide quickStart -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPluginWithDomainObjectContainer/src/docs/quick-start userGuide -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPluginWithDomainObjectContainer/src/docs/userGuide