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:

1. How to implement the plugin class and tasks in Java to be compatible with Gradle?

2. 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.

Answer 1

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

1. Make sure Gradle 1.6 or higher and Java JDK is installed
2. Create these files below with the directory structure indicated
3. Change directories to the <projectRoot>/plugin directory
4. Execute the plugin build: $ gradle uploadArchives This (very important) step compiles the Java code and puts it in your local Maven repo (../repo).
5. Now execute the consumer script by changing directories to <projectRoot>/consumer
6. 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!") }