# Creating and Registering Tasks

version 9.3.1

The work that Gradle can do on a project is defined by one or more tasks.

A task represents some independent unit of work that a build performs. This might be compiling some classes, creating a JAR, generating Javadoc, or publishing some archives to a repository.

When a user runs `./gradlew build` in the command line, Gradle will execute the `build` task along with any other tasks it depends on.

## Task Types

A task type defines what kind of work a task can do. It’s like a blueprint or class.

Gradle includes many built-in task types, such as `Copy`, `Jar`, and `Test`, and you can also define your own. By default, a task is of type `DefaultTask` .

Let’s start with a simple custom task that prints a message:

build.gradle.kts

    tasks.register("hello") {
        doLast {
            println("Hello world!")
        }
    }

build.gradle

    tasks.register('hello') {
        doLast {
            println 'Hello world!'
        }
    }

You just registered a task called `hello` of type `DefaultTask` and gave it an action using `doLast{}`.

A task is created in the build script using the `TaskContainer.register()` method, which allows it to be then used in the build logic.

When you run the `hello` task in the command-line using `./gradlew hello`, it prints your message:

    $ ./gradlew hello
    Hello world!

When you register (i.e. create) a task in your build script, you can:

- Use the default task type (`DefaultTask`) and define the behavior inline.

- Use a built-in task type, like `Copy`, to take advantage of pre-defined behavior.

- Create and use a custom task type if you need reusable behavior across tasks.

This example registers a task called `copyTask` which copies `\*.war` files from the `source` directory to the `target` directory using the `Copy` built-in task type:

build.gradle.kts

    tasks.register<Copy>("copyTask") {
        from("source")
        into("target")
        include("*.war")
    }

build.gradle

    tasks.register('copyTask', Copy) {
        from("source")
        into("target")
        include("*.war")
    }

## Built-in Task Types

Gradle provides many built-in task types with common and popular functionality, such as copying or deleting files.

This registers a Gradle task named `removeOutput` of type `Delete`. When the task runs, it will delete the file `build/outputs/1.txt` relative to the project directory.

build.gradle.kts

    tasks.register<Delete>("removeOutput") {
        delete(layout.buildDirectory.file("outputs/1.txt"))
    }

build.gradle

    tasks.register('removeOutput', Delete) {
        delete layout.buildDirectory.file("outputs/1.txt")
    }

There are many task types developers can take advantage of, including `GroovyDoc`, `Zip`, `Jar`, `JacocoReport`, `Sign`, or `Delete`, which are detailed in the DSL.

## Custom Task Types

Gradle tasks are a subclass of `Task`.

In the example below, the `HelloTask` class, a custom task type, is created by extending `DefaultTask` (our default task type):

build.gradle.kts

    // Extend the DefaultTask class to create a HelloTask class
    abstract class HelloTask : DefaultTask() {
        @TaskAction
        fun hello() {
            println("hello from HelloTask")
        }
    }

    // Register the hello Task with type HelloTask
    tasks.register<HelloTask>("hello") {
        group = "Custom tasks"
        description = "A lovely greeting task."
    }

build.gradle

    // Extend the DefaultTask class to create a HelloTask class
    class HelloTask extends DefaultTask {
        @TaskAction
        void hello() {
            println("hello from HelloTask")
        }
    }

    // Register the hello Task with type HelloTask
    tasks.register("hello", HelloTask) {
        group = "Custom tasks"
        description = "A lovely greeting task."
    }

The `hello` task is registered with the new type `HelloTask`.

Executing our new `hello` task results in the following:

    $ ./gradlew hello

    > Task :app:hello
    hello from HelloTask

The Gradle `help` task can reveal the specifications of the `hello` task:

    $ ./gradlew help --task hello

    > Task :help
    Detailed task information for hello

    Path
    :app:hello

    Type
    HelloTask (Build_gradle$HelloTask)

    Options
    --rerun     Causes the task to be re-run even if up-to-date.

    Description
    A lovely greeting task.

    Group
    Custom tasks

## Task Input and Outputs

For a custom task to do useful work, it typically needs some inputs which it uses to produce outputs.

A task can declare those inputs (files, values) and outputs (files it creates). Ideally, these inputs and outputs leverage Gradle managed types. This helps Gradle skip work when nothing has changed:

build.gradle.kts

    abstract class CreateAFileTask : DefaultTask() {
        @get:Input
        abstract val fileText: Property<String>

        @Input
        val fileName = "myfile.txt"

        @OutputFile
        val myFile: File = File(fileName)

        @TaskAction
        fun action() {
            myFile.createNewFile()
            myFile.writeText(fileText.get())
        }
    }

build.gradle

    abstract class CreateAFileTask extends DefaultTask {
        @Input
        abstract Property<String> getFileText()

        @Input
        final String fileName = "myfile.txt"

        @OutputFile
        final File myFile = new File(fileName)

        @TaskAction
        void action() {
            myFile.createNewFile()
            myFile.text = fileText.get()
        }
    }

Now Gradle knows what the task needs and what it produces. If nothing changes, the task is skipped.

## Task Action

Task actions are the blocks of code that define what the custom task does when it runs.

Every task can have one or more actions, and they’re executed during the execution phase of the Gradle build lifecycle.

A task action is typically added using `doLast {}` or `doFirst {}`:

build.gradle.kts

    tasks.register("hello") {
        doLast {
            println("Hello world!")
        }
    }

build.gradle

    tasks.register('hello') {
        doLast {
            println 'Hello world!'
        }
    }

In this example, the action is `println("Hello world!")`. It will run when the task is executed.

Similarly, in the example below, a custom task type is created called `GreetingTask`. The `@TaskAction` annotation marks a method that Gradle should call when the task of this type is executed:

build.gradle.kts

    abstract class GreetingTask : DefaultTask() {
        @TaskAction
        fun greet() {
            println("hello from GreetingTask")
        }
    }

    // Create a task using the task type
    tasks.register<GreetingTask>("hello")

build.gradle

    abstract class GreetingTask extends DefaultTask {
        @TaskAction
        def greet() {
            println 'hello from GreetingTask'
        }
    }

    // Create a task using the task type
    tasks.register('hello', GreetingTask)

## Task Dependencies

You can declare tasks that depend on other tasks:

build.gradle.kts

    tasks.register("hello") {
        doLast {
            println("Hello world!")
        }
    }
    tasks.register("intro") {
        dependsOn("hello")
        doLast {
            println("I'm Gradle")
        }
    }

build.gradle

    tasks.register('hello') {
        doLast {
            println 'Hello world!'
        }
    }
    tasks.register('intro') {
        dependsOn tasks.hello
        doLast {
            println "I'm Gradle"
        }
    }

    $ gradle -q intro
    Hello world!
    I'm Gradle

The dependency of `taskX` to `taskY` may be declared before `taskY` is defined:

build.gradle.kts

    tasks.register("taskX") {
        dependsOn("taskY")
        doLast {
            println("taskX")
        }
    }
    tasks.register("taskY") {
        doLast {
            println("taskY")
        }
    }

build.gradle

    tasks.register('taskX') {
        dependsOn 'taskY'
        doLast {
            println 'taskX'
        }
    }
    tasks.register('taskY') {
        doLast {
            println 'taskY'
        }
    }

    $ gradle -q taskX
    taskY
    taskX

The `hello` task from the previous example is updated to include a dependency:

build.gradle.kts

    tasks.register("hello") {
        group = "Custom"
        description = "A lovely greeting task."
        doLast {
            println("Hello world!")
        }
        dependsOn(tasks.assemble)
    }

build.gradle

    tasks.register('hello') {
        group = "Custom"
        description = "A lovely greeting task."
        doLast {
            println("Hello world!")
        }
        dependsOn(tasks.assemble)
    }

The `hello` task now depends on the `assemble` task, which means that Gradle must execute the `assemble` task before it can execute the `hello` task:

    $ ./gradlew :app:hello

    > Task :app:compileJava UP-TO-DATE
    > Task :app:processResources NO-SOURCE
    > Task :app:classes UP-TO-DATE
    > Task :app:jar UP-TO-DATE
    > Task :app:startScripts UP-TO-DATE
    > Task :app:distTar UP-TO-DATE
    > Task :app:distZip UP-TO-DATE
    > Task :app:assemble UP-TO-DATE

    > Task :app:hello
    Hello world!

## Task Configuration

Once registered, tasks can be accessed via the `TaskProvider` API for further configuration.

For instance, you can add behavior to an existing task:

build.gradle.kts

    tasks.register("hello") {
        doLast {
            println("Hello Earth")
        }
    }
    tasks.named("hello") {
        doFirst {
            println("Hello Venus")
        }
    }
    tasks.named("hello") {
        doLast {
            println("Hello Mars")
        }
    }
    tasks.named("hello") {
        doLast {
            println("Hello Jupiter")
        }
    }

build.gradle

    tasks.register('hello') {
        doLast {
            println 'Hello Earth'
        }
    }
    tasks.named('hello') {
        doFirst {
            println 'Hello Venus'
        }
    }
    tasks.named('hello') {
        doLast {
            println 'Hello Mars'
        }
    }
    tasks.named('hello') {
        doLast {
            println 'Hello Jupiter'
        }
    }

    $ gradle -q hello
    Hello Venus
    Hello Earth
    Hello Mars
    Hello Jupiter

| The calls `doFirst` and `doLast` can be executed multiple times. They add an action to the beginning or the end of the task’s actions list. When the task executes, the actions in the action list are executed in order.

A task is optionally configured in a build script using the `TaskCollection.named()` method.

## Task Classification

There are two classes of tasks that can be executed:

1. Actionable tasks have some action(s) attached to do work in your build: `compileJava`.

2. Lifecycle tasks are tasks with no actions attached: `assemble`, `build`.

Typically, a lifecycle tasks depends on many actionable tasks, and is used to execute many tasks at once.

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