Part 7: Writing Plugins

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

You initialized your Java app in part 1.
You understand the Gradle Build Lifecycle from part 2.
You added a subproject and a separate Build in part3.
You viewed a Settings file in part 4.
You authored a Build script in part 5.
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 for the Plugin below (don’t change anything else in the file):

gradle/license-plugin/plugin/src/main/kotlin/license/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
        }
    }
}

gradle/license-plugin/plugin/src/main/kotlin/license/LicensePlugin.groovy

class LicensePlugin implements Plugin<Project> {
    void apply(Project project) {
        project.tasks.register("license", LicenseTask) { task ->
            task.setDescription("add a license header to source code")  // Add description
            task.setGroup("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 (if not done already):

app/build.gradle.kts

plugins {
    application
    id("com.tutorial.license") // Apply custom plugin
}

app/build.gradle

plugins {
    id 'application'
    id('com.tutorial.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());
    }
}

Congratulations, you have completed the tutorial!

Step 4. Next steps

We recommend going through each section of the User Manual.

Next Step: Structuring Builds >>

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