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Example 50.7. Testing a custom task

src/test/groovy/org/gradle/GreetingTaskTest.groovy

class GreetingTaskTest { @Test

public void canAddTaskToProject() {

Project project = ProjectBuilder.builder().build()

def task = project.task('greeting', type: GreetingTask) assertTrue(task instanceof GreetingTask)

}

}

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51

Writing Custom Plugins

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.

51.1. Packaging a plugin

There are several places where you can put the source for the plugin.

Build script

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 project

You can put the source for the plugin in the rootProjectDir/buildSrc/src/main/gro 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.

See Chapter 52, Organizing Build Logic for more details about the buildSrc project.

Standalone 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.

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51.2. Writing a simple plugin

To create a custom plugin, you need to write an implementation of Plugin. Gradle instantiates the plugin and calls the plugin instance'sPlugin.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 51.1. A custom plugin

build.gradle

apply plugin: GreetingPlugin

class GreetingPlugin implements Plugin<Project> { 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.

51.3. Getting input from the build

Most plugins need to obtain some configuration from the build script. One method for doing this is to use extension objects. The Gradle Project has an associated ExtensionContainer object that helps keep track of all the settings and properties being passed to plugins. You can capture user input by telling the extension container about your plugin. To capture input, simply add a Java Bean compliant class into the extension container's list of extensions. Groovy is a good languag 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 extension object to the project. Here we add agreeting extension object to the project, which allows you to configure the greeting.

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Example 51.2. A custom plugin extension

build.gradle

apply plugin: GreetingPlugin

greeting.message = 'Hi from Gradle'

class GreetingPlugin implements Plugin<Project> { void apply(Project project) {

//Add the 'greeting' extension object project.extensions.create("greeting", GreetingPluginExtension)

//Add a task that uses the configuration

project.task('hello') << {

println project.greeting.message

}

}

}

class GreetingPluginExtension {

def String message = 'Hello from GreetingPlugin'

}

Output of gradle -q hello

> gradle -q hello Hi from Gradle

In this example, GreetingPluginExtension is a plain old Groovy object with a field called mess

. The extension object is added to the plugin list with the name greeting. This object then becomes available as a project property with the same name as the extension object.

Oftentimes, you have several related properties you need to specify on a single plugin. Gradle adds a configuration closure block for each extension object, so you can group settings together. The following example shows you how this works.

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Example 51.3. A custom plugin with configuration closure

build.gradle

apply plugin: GreetingPlugin

greeting {

message = 'Hi' greeter = 'Gradle'

}

class GreetingPlugin implements Plugin<Project> { void apply(Project project) {

project.extensions.create("greeting", GreetingPluginExtension) project.task('hello') << {

println "${project.greeting.message} from ${project.greeting.greet

}

}

}

class GreetingPluginExtension { String message

String greeter

}

Output of gradle -q hello

> gradle -q hello Hi from Gradle

In this example, several settings can be grouped together within the greeting closure. The name of the closure block in the build script (greeting) needs to match the extension object name. Then, when the closure is executed, the fields on the extension object will be mapped to the variables within the closure based on the standard Groovy closure delegate feature.

51.4. Working with files in custom tasks and plugins

When developing custom tasks and plugins, it's a good idea to be very flexible when acceptin input configuration for file locations. To do this, you can leverage the Project.file() method to resolve values to files as late as possible.

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Example 51.4. Evaluating file properties lazily

build.gradle

class GreetingToFileTask extends DefaultTask {

def destination

File getDestination() { project.file(destination)

}

@TaskAction def greet() {

def file = getDestination() file.parentFile.mkdirs() file.write "Hello!"

}

}

task greet(type: GreetingToFileTask) { destination = { project.greetingFile }

}

task sayGreeting(dependsOn: greet) << { println file(greetingFile).text

}

greetingFile = "$buildDir/hello.txt"

Output of gradle -q sayGreeting

> gradle -q sayGreeting Hello!

In this example, we configure the greet task destination property as a closure, which is evaluated with the Project.file() method to turn the return value of the closure into a file object at the last minute. You will notice that in the above example we specify the greetingFile property value after we have configured to use it for the task. This kind of lazy evaluation is a key benefit of accepting any value when setting a file property, then resolving that value when reading the property.

51.5. A standalone project

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.

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Example 51.5. A build for a custom plugin

build.gradle

apply plugin: 'groovy'

dependencies {

compile gradleApi() groovy localGroovy()

}

Note: The code for this example can be found at samples/customPlugin/plugin 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'sMETA-INF/gradle-plugins directory that matches the name of your plugin.

Example 51.6. 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 resource folder, and that the implementation-class property identifies the Plugin implementation class.

51.5.1. Using your plugin in another project

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 52.5, “Extern 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 51.7. Using a custom plugin in another project

build.gradle

buildscript { repositories {

maven {

url uri('../repo')

}

}

dependencies {

classpath group: 'org.gradle', name: 'customPlugin', version: '1.0-SNA

}

}

buildscript { configurations.classpath.resolutionStrategy.cacheChangingModulesFor 0, 'sec

}

apply plugin: 'greeting'

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51.5.2. Writing tests for your plugin

You can use the ProjectBuilder class to create Project instances to use when you test your

plugin implementation.

Example 51.8. 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)

}

}

51.6. Maintaining multiple domain objects

Gradle provides some utility classes for maintaining collections of object, which work well with the Gradle build language.

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Example 51.9. 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> { void apply(Project project) {

def books = project.container(Book) books.all {

sourceFile = project.file("src/docs/$name")

}

project.extensions.books = books

}

}

class Book {

final String name File sourceFile

Book(String name) { this.name = name

}

}

Output of gradle -q books

> gradle -q books

developerGuide -> /home/user/gradle/samples/userguide/organizeBuildLogic/custo quickStart -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPlu userGuide -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPlug

The Project.container() methods create instances of NamedDomainObjectContainer, that have many useful methods for managing and configuring the objects. In order to use a type with any of the project.container methods, it MUST expose a property named “name” as the unique, and constant, name for the object. The project.container(Class) variant of the

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container method creates new instances by attempting to invoke the constructor of the class that takes a single string argument, which is the desired name of the object. See the above link for proj method variants that allow custom instantiation strategies.

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52

Organizing Build Logic

Gradle offers a variety of ways to organize your build logic. First of all you can put your build logic directly in the action closure of a task. If a couple of tasks share the same logic you can extract this logic into a method. If multiple projects of a multi-project build share some logic you can define this method in the parent project. If the build logic gets too complex for being properly modeled by methods you want have an OO Model. Gradle makes this very easy. Just drop your classes in a certain directory and Gradle automatically compiles them and puts them in the classpath of your build script.

Here is a summary of the ways you can organise your build logic:

POGOs. You can declare and use plain old Groovy objects (POGOs) directly in your build script. The build script is written in Groovy, after all, and Groovy provides you with lots of excellent ways to organize code.

Inherited properties and methods. In a multi-project build, sub-projects inherit the properties and methods of their parent project.

Configuration injection. In a multi-project build, a project (usually the root project) can inject properties and methods into another project.

buildSrc project. Drop the source for your build classes into a certain directory and Gradle automatically compiles them and includes them in the classpath of your build script.

Shared scripts. Define common configuration in an external build, and apply the script to multiple projects, possibly across different builds.

Custom tasks. Put your build logic into a custom task, and reuse that task in multiple places.

Custom plugins. Put your build logic into a custom plugin, and apply that plugin to multiple projects. The plugin must be in the classpath of your build script. You can achieve this either by using build sources or by adding an external library that contains the plugin.

Execute an external build. Execute another Gradle build from the current build.

External libraries. Use external libraries directly in your build file.

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