Creating a Gradle Custom Plugin with Java

Question

I'm creating a build process with Gradle and I want to provide a plugin that uses Java code. The Gradle plugin documentation says this is possible:

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.

However, after multiple hours of Googling and reading, I have yet to find any explanation of how to create a Gradle custom plugin with Java. It seems like you could create the code for it in a directory like:

<rootProjectDir>/buildSrc/src/main/java/
  MyGradlePlugin.java
  MyGradleTasks.java

But the question then becomes:

How to implement the plugin class and tasks in Java to be compatible with Gradle?
How to get Gradle to recognize the Java classes and tasks so you can use them in a build?

Can you just reference the plugin class like you do for the Groovy equivalent?

src/main/resources/META-INF/gradle-plugins/myjavaplugin.properties implementation-class=org.me.MyGradlePlugin

I realize that I could just call the Java code with project.javaexec or JavaExec, but I'm concerned this will make my build process less portable.

Joe Fernandez · Accepted Answer · 2017-12-07 16:45:57Z

69

Here's a basic, stand-alone, Java-based Gradle plugin and the steps to get it working:

Make sure Gradle 1.6 or higher and Java JDK is installed
Create these files below with the directory structure indicated
Change directories to the <projectRoot>/plugin directory
Execute the plugin build: $ gradle uploadArchives This (very important) step compiles the Java code and puts it in your local Maven repo (../repo).
Now execute the consumer script by changing directories to <projectRoot>/consumer
Execute the script that depends on the plugin: $ gradle checkitout

Java Classes

projectRoot/plugin/src/main/java/org/joefernandez/gradle/MyJavaPlugin.java

package org.joefernandez.gradle;

import org.gradle.api.Project;
import org.gradle.api.Plugin;

public class MyJavaPlugin implements Plugin<Project> {

    @Override
    public void apply(Project target) {
        target.task("javaTask");
    }

}

projectRoot/plugin/src/main/java/org/joefernandez/gradle/MyJavaTask.java

package org.joefernandez.gradle;

import org.gradle.api.DefaultTask;
import org.gradle.api.tasks.TaskAction;

public class MyJavaTask extends DefaultTask {

    @TaskAction
    public void javaTask() {
        System.out.println("Hello from MyJavaTask");
    }

}

Plugin Class declaration

projectRoot/plugin/src/main/resources/META-INF/gradle-plugins/test-plugin.properties

implementation-class=org.joefernandez.gradle.MyJavaPlugin

Plugin Build Script

Note the uploadArchives task: You must run this task to make the plugin available to the consumer script.

projectRoot/plugin/build.gradle

apply plugin: 'java'

dependencies {
    compile gradleApi()
}

apply plugin: 'maven'

repositories {
    mavenCentral()
}

dependencies {
    testCompile 'junit:junit:4.11'
}

group = 'org.joefernandez'
version = '1.0-SNAPSHOT'

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: uri('../repo'))
        }
    }
}

Settings for the Plugin

projectRoot/plugin/settings.gradle

rootProject.name = 'MyJavaPlugin'

Root script

projectRoot/build.gradle

apply plugin: 'java'

dependencies {
    compile gradleApi()
}

Consumer script

projectRoot/consumer/build.gradle

buildscript {
    repositories {
        maven {
            url uri('../repo')
        }
    }
    dependencies {
        classpath group: 'org.joefernandez',
                   name: 'MyJavaPlugin',
                version: '1.0-SNAPSHOT'
    }
}
apply plugin: 'test-plugin'

task checkitout(type: org.joefernandez.gradle.MyJavaTask) {
    println("running consumer task!")
}

edited Dec 7, 2017 at 16:45

answered Jul 18, 2013 at 4:49

Joe Fernandez

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3 Comments

Brian Attwell Over a year ago

Specifying repository(url: uri('../repo')) is optional. You could instead export to the default local repo. And import from mavenLocal().

Brian Attwell Over a year ago

I created an Android Studio project based on Joe's descriptions at github.com/AttwellBrian/SamplePlugin/tree/master. Feel free to fork or copy-pasta.

spy Over a year ago

Not working for me. Gradle probably changed something

Corey Scott · Accepted Answer · 2013-12-18 18:04:23Z

I would like to propose a few amendments to Joe's great answer.

projectRoot/plugin/src/main/java/org/joefernandez/gradle/MyJavaPlugin.java

This change wires the task into the supplied project without the need to sub-class it.

package org.joefernandez.gradle;

import org.gradle.api.Project;
import org.gradle.api.Plugin;

public class MyJavaPlugin implements Plugin<Project> {

    @Override
    public void apply(Project target) {
        // CHANGE HERE
        target.getTasks().create("javaTask", MyJavaTask.class);
    }

}

Consumer script

projectRoot/consumer/build.gradle

This change removes the now unnecessary task definition

buildscript {
    repositories {
        maven {
            url uri('../repo')
        }
    }
    dependencies {
        classpath group: 'org.joefernandez',
                   name: 'MyJavaPlugin',
                version: '1.0-SNAPSHOT'
    }
}
apply plugin: 'test-plugin'

// CHANGE HERE

This them changes step 6, to:

Execute the script that depends on the plugin: $ gradle javaTask

Peter Niederwieser · Accepted Answer · 2013-07-15 23:01:53Z

3

Plugin classes must implement the org.gradle.api.Plugin interface. Task classes typically extend the org.gradle.api.DefaultTask class. It doesn't matter which (JVM) language is used for this. The Gradle codebase contains many plugin and task classes implemented in Java.

How to build and ship a plugin is another question, again mostly unrelated to which language is used. See the Gradle User Guide and samples/customPlugin in the full Gradle distribution.

answered Jul 15, 2013 at 23:01

Peter Niederwieser

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1 Comment

Joe Fernandez Over a year ago

Thanks, Peter! This got me on the right track. I'll post my solution shortly.

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Creating a Gradle Custom Plugin with Java

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