gradle Gradle Plugins How to write a standalone plugin

Example

To create a custom standalone Gradle plug-in using java (you can also use Groovy) you have to create a structure like this:

plugin
|-- build.gradle
|-- settings.gradle
|-- src
    |-- main
    |   |-- java
    |   |-- resources
    |       |-- META-INF
    |           |-- gradle-plugins
    |-- test

Setup gradle configuration

In the build.gradle file you define your project.

apply plugin: 'java'
apply plugin: 'maven'

dependencies {
    compile gradleApi()
} 

The java plugin will be used to write java code.
The gradleApi() dependency will give us all method and propertiess needed to create a Gradle plugin.

In the settings.gradle file:

rootProject.name = 'myplugin' 

It will define the artifact id in Maven.
If settings.gradle file is not present in the plugin directory the default value will be the name of the directory.

Create the Plugin

Define a class in the src/main/java/org/sample/MyPlugin.java implementing the Plugin interface.

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

public class MyPlugin implements Plugin<Project> {

    @Override
    public void apply(Project project) {
         project.getTasks().create("myTask", MyTask.class);
    }

} 

Define the task extending the DefaultTask class:

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

public class MyTask extends DefaultTask {

    @TaskAction
    public void myTask() {
        System.out.println("Hello World");
    }
}

Plugin Class declaration

In the META-INF/gradle-plugins folder you have to create a properties file defining the implementation-class property that identifies the Plugin implementation class.

In the META-INF/gradle-plugins/testplugin.properties

implementation-class=org.sample.MyPlugin.java

Notice that the properties filename matches the plugin id.

How to build and publish it

Change the build.gradle file adding some info to upload the plugin in a maven repo:

apply plugin: 'java'
apply plugin: 'maven'

dependencies {
    compile gradleApi()
}

repositories {
    jcenter()
}


group = 'org.sample'
version = '1.0'

uploadArchives {
    repositories {
        mavenDeployer {
        repository(url: mavenLocal().url)
        }
    }
} 

You can build and publish the Gradle plug-in to the Maven repo defined in the plugin/build.gradle file using the following command.

$ ./gradlew clean uploadArchives 

How to use it

To use the plugin add in the build.gradle of your project:

buildscript {
     repositories {
         mavenLocal()
     }
 dependencies {
    classpath group: 'org.sample',    // Defined in the build.gradle of the plugin
              name: 'myplugin',       // Defined by the rootProject.name 
              version: '1.0'
    }
 }

apply plugin: 'testplugin'            // Defined by the properties filename

Then you can call the task using:

 $ ./gradlew myTask