Extending Gradle with a Custom Enhanced Task

Internal

• Gradle Tasks | Enhanced Tasks

Overview

An enhanced task requires writing Groovy or Java code either in in-line in build.gradle or a script plugin file, or into a source code file (or files) maintained in the project's buildSrc or externally. The behavior is built into the task and the task exposes some properties that can be configured from the build script.

In-Line Custom Enhanced Task

An in-line custom enhanced task can be declared as follows, either in build.gradle or in a script plugin:

...
class CustomEnhancedTask extends DefaultTask {
    @TaskAction
    void impl() {
        System.out.println("this is an in-line enhanced task")
    }
}

task customTask(type: CustomEnhancedTask)
...

Example:

Custom enhanced Java task declared in-line in build.gradle

This approach is not recommended as we are losing testability and reusability of the task.

File-Based Custom Enhanced Task

The same task can be defined in a Java/Groovy/Kotlin file either in the project's buildSrc or in an external project:

package playground.gradle;

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

public class CustomEnhancedTask extends DefaultTask {
   @TaskAction
   void impl() {
      System.out.println("this is an custom enhanced task developed in buildSrc");
   }
}

If the task was defined in the project's buildSrc, then it can then be declared in build.gradle as follows. The task name is arbitrary, it does not have to match the class name, but matching names is intuitive and thus preferred.

task customEnhancedTask(type: playground.gradle.CustomEnhancedTask)

Example:

Custom enhanced task developed in buildSrc

If the task was defined in an external project, the external project must publish the JAR containing the task in a repository, and the Gradle project using the task must declare the JAR as dependency in the buildScript block of the build.gradle file and pull it from the repository it was published in:

buildscript {
    repositories {
        mavenLocal()
    }
    dependencies {
        classpath 'playground.gradle.standalone-task:06-enhanced-task-developed-in-standalone-project:0.1.0'
    }
}

task custEnhTask(type: playground.gradle.CustomEnhancedTask)

More details on configuring the build script classpath are available in:

Build Script Classpath

Examples:

Custom enhanced task developed in standalone project

Custom enhanced task imported from external project

Programming Custom Tasks

@TaskAction

org.gradle.api.tasks.TaskAction

Action Failure

If the action code fails in any way, we indicate that the action, and thus the task failed by throwing an unchecked exception. RuntimeException works. Gradle will display the exception message:

./gradlew example
> Task :example FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':example'.
> this is the RuntimeException message
[...]

Logging

Accessing Gradle Logging in Custom Java Tasks

Allowing Configuration via a Task Constructor

https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:passing_arguments_to_a_task_constructor

Configuration values can be passed to a Task's class constructor, if the respective constructor was annotated with @javax.inject.Inject:

class CustomTask extends DefaultTask {
  final String message
  final int number

  @Inject
  CustomTask(String message, int number) { 
    this.message = message
    this.number = number
  } 
}

The task can then be configured with constructor arguments using the task container, as shown below. This should be the preferred way of configuring tasks, as it improves the configuration time. For more details, see Gradle Tasks | Task Configuration Avoidance APIs.

tasks.create('myTask', CustomTask, 'hello', 42)
task myTask(type: CustomTask, constructorArgs: ['hello', 42])

In all circumstances, the values passed as constructor arguments must be non-null. If a null value is passed, Gradle will throw a NullPointerException.

Programmatic Access to Properties

Gradle Extra Properties | Programmatic Access to Extra Properties

TODO

The name of the method annotated with @TaskAction is arbitrary.

TODO:

• Gradle in Action Section 8.3.2 Using the CloudBees API from tasks.
• Gradle in Action Section 8.4 Writing custom task classes.
• https://docs.gradle.org/current/userguide/custom_tasks.html

Enhanced Task Testing

TODO https://docs.gradle.org/current/userguide/custom_tasks.html#sec:writing_tests_for_your_task_class

TODO/TODEPLETE

DEPLETE Gradle_Task_TODEPLETE#Explicit_Task_Declaration_.28Custom_Tasks.29