Learn the basics of writing and applying Gradle plugins.

In this section, you will:

  • Add a custom task to the Plugin

  • Apply the Plugin to a subproject

  • Use the Plugin

Step 0. Before you Begin

  1. You initialized your Java app in part 1.

  2. You understand the Gradle Build Lifecycle from part 2.

  3. You added a subproject and a separate Build in part3.

  4. You viewed a Settings file in part 4.

  5. You authored a Build script in part 5.

  6. You authored a Task in part 6.

Step 1. Develop the Plugin

Let’s tie our custom LicenseTask to our plugin.

Update the LicensePlugin with the code below:

gradle/license-plugin/plugin/src/main/kotlin/com/gradle/LicensePlugin.kt
class LicensePlugin: Plugin<Project> {
    override fun apply(project: Project) {
        project.tasks.register("license", LicenseTask::class.java) { task ->
            task.description = "add a license header to source code"   // Add description
            task.group = "from license plugin"                          // Add group
        }
    }
}

Step 2. Add a license.txt file

Add a file called license.txt to the root directory of the project and add the following text to it:

license.txt
/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

Step 3. Apply the Plugin

Apply the plugin to the app subproject:

app/build.gradle.kts
plugins {
    application
    id("com.gradle.license") // Apply custom plugin
}

Make sure the plugin is correctly applied by listing the available tasks in the app subproject:

$ ./gradlew :app:tasks

------------------------------------------------------------
Tasks runnable from project ':app'
------------------------------------------------------------

...

From license plugin tasks
-------------------------
license - add a license header to source code

Step 4. Run the custom Task

Finally, it’s time to run the new task.

First, let’s inspect some source code:

app/src/main/java/authoring/tutorial/App.java
package authoring.tutorial;

import com.gradle.CustomLib;

public class App {
    public String getGreeting() {
        return "CustomLib identifier is: " + CustomLib.identifier;
    }

    public static void main(String[] args) {
        System.out.println(new App().getGreeting());
    }
}

Next, let’s run the task with ./gradlew :app:license:

$ ./gradlew :app:license

> Task :license-plugin:plugin:compileKotlin UP-TO-DATE
> Task :license-plugin:plugin:compileJava NO-SOURCE
> Task :license-plugin:plugin:pluginDescriptors UP-TO-DATE
> Task :license-plugin:plugin:processResources UP-TO-DATE
> Task :license-plugin:plugin:classes UP-TO-DATE
> Task :license-plugin:plugin:jar UP-TO-DATE

> Configure project :app

> Task :app:license

BUILD SUCCESSFUL in 410ms
5 actionable tasks: 1 executed, 4 up-to-date

Now inspect the same source code, which should include a license header:

app/src/main/java/authoring/tutorial/App.java
/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package authoring.tutorial;

import com.gradle.CustomLib;

public class App {
    public String getGreeting() {
        return "CustomLib identifier is: " + CustomLib.identifier;
    }

    public static void main(String[] args) {
        System.out.println(new App().getGreeting());
    }
}