Gradle Task

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External

Internal

Overview

A task is a core Gradle concept. It represents a single atomic piece of work for a build, such as compiling classes or generating javadoc. Each task belongs to a specific project. Each task has a name, which can be used to refer to the task within its project, and a fully qualified path, which is unique across all projects. The path is the concatenation of the owning project's path and the task name, separated by the ":" character.

A build consists in executing a sequence of tasks in succession. Gradle computes the Directed Acyclic Graph of to be executed in order to fulfill the tasks specified on command line, and then executes the task actions, honoring inter-task dependencies and insuring the fact that a task is executed only once. Gradle builds the complete dependency graph before any tasks is executed.


Behavior must be declared in a task action to be executed at the execution phase. Anything declared in a task that is not an action is executed at the configuration phase. For more details see Explicit Task Declaration.

All tasks known to a build can be displayed with:

gradle tasks

The tasks known to a specific project can be displayed with:

gradle :<project-path>:tasks

Task Action

Each task consists of a sequence of Action objects, which are executed in the order of their declaration when the tasks is executed. Actions can be added to a task by calling Task's doFirst() or doLast() methods with an action closure. When the action is executed, the closure is called with the task as parameter.

doFirst|doLast { action-closure }

The last doFirst() invocation will configure the action that is executed first. The last doLast() invocation will configure the action that is executed last. The execution order is easy to figure out if you think of an action list to which doFirst() and doLast() add actions.

Actions can be anonymous or named.

Task Failure

Any task failure stops the build, with two exceptions:

  1. If the task action throws StopActionException, it aborts the current action, and continues at the next action.
  2. If the task action throws StopExecutionException, it aborts the current action, and continues at the next task.

Task Execution Order

https://docs.gradle.org/current/dsl/org.gradle.api.Task.html#N1790C

A task may have dependencies on other tasks or might be scheduled to always run after another task. Gradle ensures that all task dependencies and ordering rules are honored during the execution phase, so that the task is executed after all of its dependencies and any "must run after" tasks have been executed.

Dependencies can be declared directly inside tasks:

task a {

    // ...
}

task b {

    // ....

    dependsOn 'a'
}

Functions that can be used to declare dependencies between tasks are:

dependsOn '<task-name>', '<task-name>', ... 
mustRunAfter '<task-name>', ...

Tasks names as String can be used, but also the following:

  • An absolute and relative path, as String.
  • A Task.
  • A closure. The closure may task a task as a parameter. It may return any of the types returned here, and its return value is recursively converted into a task. null is treated as an empty collection.
  • A TaskDependency.
  • A Buildable.
  • A RegularFileProperty or DirectoryProperty.
  • An Iterable, Collection, Map or array that contains the types listed here.
  • A Callable.
setDependsOn

Parallel Execution

TODO https://docs.gradle.org/current/dsl/org.gradle.api.Task.html#N179C7

Conditional Execution

onlyIf { closure }
onlyIf ( onlyIfSpec )

Task Properties

TODO when needed https://docs.gradle.org/current/dsl/org.gradle.api.Task.html#N17986

React to a Task's Life Cycle Events

build.gradle allows declaring closures to react to various events related to tasks lifecycle. Also see Task Container.

Task Sources

Default Tasks

Any build comes with a number of pre-defined default tasks, regardless on whether plugins have been applied or not. They are available below:

gradle dependencies

Displays all dependencies (classpath) for various dependency configurations of the root project or a sub-project. More details in:

Inspecting Dependencies
gradle dependencyInsight

Displays the insight into a specific dependency. More details in:

Inspecting Dependencies
gradle dependentComponents

Displays the dependent components of components in project ':subproject-A'.

gradle model

Displays the configuration model of project ':subproject-A'.

gradle projects

Displays the sub-projects of project ':subproject-A'.

gradle properties 

Displays the properties of project ':subproject-A'.

gradle tasks 

Displays the tasks runnable from project ':subproject-A'.

gradle buildEnvironment

Displays all buildscript dependencies declared in project ':subproject-A'.

gradle components

Displays the components produced by project ':subproject-A'.

Tasks From Plugins

Plugins that have been applied to the project may come with their own tasks. This is a non-exhaustive list:

Explicit Task Declaration

Tasks may be explicitly declared in build.gradle using the "task" keyword (Project's task() method). There are four forms to declare custom tasks:

task <task-name> // for empty tasks
task <task-name> { configuration-closure }
task <task-name>(type: <SomeTaskType>) // empty task of a specific type
task <task-name>(type: <SomeTaskType>)  { configuration-closure }

The configuration closures [#Task_Action|should add actions]], as described above.

As an example, a task can be declared as follows:

task sample {

    println 'this is not an action, but a statement to be executed when the task is declared, in the build configuration phase'

    doFirst {

         println 'action executed second'
    }

    doFirst {

         println 'action executed first'
    }
 
    doLast {

         println 'action executed last'
    }
}

Note that the statements that are not part of a task action, declared outside doFirst {} and doLast {} in the task configuration closure will be executed in the build configuration phase, when the task is declared, not in the build execution phase.

Task Types

https://docs.gradle.org/current/dsl/#N10376

Copy

Copy copies files into a destination directory, optionally renaming and filtering files. The task implements CopySpec for specifying what to copy.

Delete

Delete deletes files or directories.

Exec

Exec executes a command line process.

task runMain(type: Exec) {
    commandLine 'echo', 'something'
}

JavaExec

JavaExec executes a Java application in a child process.

task runMain(type: JavaExec) {
  classpath = 'something'
  main = 'io.example.Main'
}

JavaExec does not require implicitly the Java plugin, but it needs a classpath, and including the Java plugin allows us to express it as a project variable:

apply plugin: 'java'

task runMain(type: JavaExec) {
  classpath = sourceSets.main.runtimeClasspath
  main = 'io.example.Main'
}

The Java plugin adds "main" to the sourceSets, and initializes its runtimeClasspath.

GenerateMavenPom

GenerateMavenPom generates a Maven module descriptor (POM) file.

PublishToMavenRepository

PublishToMavenRepository publishes a Maven publication to a Maven artifact repository.

Archive Tasks

https://docs.gradle.org/current/dsl/org.gradle.api.tasks.bundling.AbstractArchiveTask.html

Zip

Zip assembles a ZIP archive. The default is to compress the content of the ZIP.

task dist(type: Zip) {
    dependsOn spiJar
    from 'src/dist'
    into('libs') {
        from spiJar.archivePath
        from configurations.runtime
    }
}

Jar

Jar assembles a JAR archive.

task myJar(type: Jar) {
   ....
}
task srcJar(type: Jar) {
   from sourceSets.main.allJava
}