Gradle Task TODEPLETE

TO DEPLETE INTO

Gradle Task

Any build ends up executing a sequence of tasks in succession, or in parallel if explicitly configured, after all tasks have been arranged in a directed acyclic graph. Understanding how the directed acyclic graph is built and how tasks are scheduled for execution is key to understanding how gradle works.

Tasks can be explicitly declared in a build configuration file, or can be exposed by plugins.

Each task belongs to a specific project. The association is made when the task is declared in build.gradle or when the plugin that declares the task is applied to the project in build.gradle. A project is essentially a collection of tasks. The tasks of a project can be accessed in a build script with:

project-reference.tasks

This will return the project's TaskContainer.

Example:

println "" + project.tasks

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:

:sub-project-A:child-B:taskC

A build consists in executing a sequence of tasks in succession. Gradle computes the Directed Acyclic Graph of tasks necessary to be executed 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 does not belong to an action is executed at the configuration phase. For more details see Explicit Task Declaration.

Displaying Tasks

Some of the common tasks for a project can be displayed with:

gradle [:project-path:]tasks

In case of the root project, the project path is empty:

gradle tasks

For a sub-project:

gradle :sub-project-A:tasks

Custom tasks declared in the project's build.gradle are not present in the output.

All tasks of a project, including the custom tasks and the tasks of its sub-projects, if any, can be displayed with:

gradle [:project-path:]tasks --all

For the root project:

gradle tasks --all

Details about a specific task can be obtained with:

gradle help --task <task-name>

Display Tasks in Task Execution Graph in Execution Order

To display all tasks in the task execution graph, use:

println "Tasks in Task Execution Graph, in execution order: " + gradle.taskGraph.allTasks

To hook this into a default task execution:

task debugPublish {
  dependsOn 'publish'
  doFirst {
     println "Tasks in Task Execution Graph, in execution order: " + gradle.taskGraph.allTasks
  }
}

Dry Run

Display the tasks that would be executed, without actually doing anything

gradle -m|--dry-run ...

This is useful in understanding task execution order.

Obtaining a Task Reference

Gradle Programming - Obtaining a Task Reference

Task Execution Order

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.

Command Line to Task Execution Graph

The execution graph is built starting from task names specified on command line.

The names specified on command line are looked up in all projects of the build, and if they exist in any project, they will be added to the execution graph, for as many times they appear. For example, if tasks "t" was declared in the root project and "sub-project-A" and "sub-project-B", then the execution graph looks like:

task ':t', task ':subproject-A:t', task ':subproject-B:t'

More tasks are added to the execution graph if the initial tasks have dependencies on other tasks.

Declaring Dependencies between Tasks

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

Dependencies can be declared directly inside new task declarations:

task a {

    // ...
}

task b {

    // ....

    dependsOn 'a'
}

The order of the declaration matters. The dependency task must be declared before the dependent task.

Dependencies can also be declared inside the script block that configures an existing task:

docker {

  dependsOn build
  ...
}

Dependencies can be declared between tasks that do not belong to the same project. This makes sense as Gradle manages a global task dependency graph. In "project-B":

docker {

  dependsOn rootProject.project(':project-A').getTasksByName("build", false).asList().get(0)
  ...
}

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

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

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

Unfortunately, there's no "mustRunBefore".

finalizedBy '<task-name>', ...

Describe 'mustRunAfter' and 'finalizedBy'.

The task name may be a fully qualified path, thus allowing us to declare dependencies on tasks from another projects, in a multi-project build.

If a task declares a dependency on a task that was declared in multiple projects, only the dependency task that was declared in the same project as the dependent task will be executed, and not all of them.

Aside from task names, the following can be used:

• A closure. The closure may take a task as a parameter. It may return any of the types mentioned below, and its return value is recursively converted into a task. null is treated as an empty collection. For example, the closure shown below returns a set of tasks. Note that the closure is executed after the configuration phase, at the time the closure is executed, all build configuration script have been evaluated.

dependsOn {

  Set<Task> tasks = new HashSet<>();
  subprojects.each {
      tasks.add(it.tasks.findByName('t'))
  }
  return tasks;
}

• An absolute and relative path, as String.
• A Task.
• A TaskDependency.
• A Buildable.
• A RegularFileProperty or DirectoryProperty.
• An Iterable, Collection, Map or array that contains the types listed here.
• A Callable.

Gradle will detect circular dependencies, if they exist, and fail:

FAILURE: Build failed with an exception.

* What went wrong:
Circular dependency between the following tasks:
:x
\--- :subproject-A:t
     \--- :x (*)

(*) - details omitted (listed previously)

Examples of Task Dependency Declarations

Root Project Task Depends on Specific Sub-Project Tasks

task topLevelTask {

    dependsOn {

        Set<Task> tasks = new HashSet<>();

        subprojects.each {

            tasks.add(it.tasks.findByName('someName'))
        }

        return tasks;
   }
}

Update this if a better way is found.

The same example can be written by declaring the closure first and then invoking it:

def allSubProjectTasksNamedSomeName = { Task t ->

    Set<Task> tasks = new HashSet<>();

    subprojects.each {

        tasks.add(it.tasks.findByName('someName'))
    }

    return tasks;
}

task topLevelTask {

    dependsOn allSubProjectTasksNamedSomeName
}

Update this if a better way is found.

Declaring a Dependency between an Existing Task and a Custom Task

Declare the custom task with:

task myTask {
  doFirst {
    ...
  }
}

Then configure the existing task to depend on the custom task:

assemble {
  dependsOn 'myTask'
}

Parallel Execution

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

By default, tasks are not executed in parallel unless a task is waiting on asynchronous work and another task, which is not dependent, is ready to execute. Parallel execution can be enabled by the --parallel flag when the build is initiated. In parallel mode, the tasks of different projects of a multi-project build able to be executed in parallel.

Default Task

A default task can be declared in the build script, so when gradle is invoked without any argument, that task is executed.

defaultTasks 'distZip'

More than one tasks can be declared as "default".

Task Action

Each task consists of a sequence of Action objects, which are executed in the order of their declaration. 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.

task someTask {
  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 queue to which doFirst() and doLast() add actions at the head or at the tail.

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.

Conditional Execution

onlyIf { closure }

onlyIf ( onlyIfSpec )

Task Properties

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

Task Container

https://docs.gradle.org/4.7/javadoc/org/gradle/api/tasks/TaskContainer.html

The project TaskContainer can be accessed from a build script with:

 project-reference.tasks

The TaskContainer can then queried for tasks. Also see Gradle Programming - Obtaining a Task Reference.

Configuring Existing Tasks

TODO more, organize.

All tasks of the same type can be configured at once:

tasks.withType(Test) { 
 // configure the task
}

Also see Get the Reference of a Task by Name above.

I believe this also works:

test {
 // configure the task
}

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:

• Java Plugin Tasks

Task Types

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

Copy

Copy Task

Delete

Delete Task

Archive Tasks

Archive Tasks

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.

CreateStartScripts

CreateStartScripts

clean

Under unclear circumstances, some Gradle deployments complain about not finding the task "clean", though others work well with the same configuration. This can be fixed by applying the "base" plugin:

apply plugin:'base'

React to a Task's Life Cycle Events

TODO: Research necessary.

Task Creation

tasks.whenTaskAdded { task ->
      task.ext.srcDir = 'src/main/java'
}

Task Execution

gradle.taskGraph.beforeTask { Task task ->
      println "executing $task ..."
}
gradle.taskGraph.afterTask { Task task, TaskState state ->
      if (state.failure) {
          println "FAILED"
      }
      else {
          println "done"
      }
}