# Authoring Tasks

In the introductory tutorial you learned how to create simple tasks. You also learned how to add additional behavior to these tasks later on, and you learned how to create dependencies between tasks. This was all about simple tasks, but Gradle takes the concept of tasks further. Gradle supports enhanced tasks, which are tasks that have their own properties and methods. This is really different from what you are used to with Ant targets. Such enhanced tasks are either provided by you or built into Gradle.

## Task outcomes

When Gradle executes a task, it can label the task with different outcomes in the console UI and via the Tooling API. These labels are based on if a task has actions to execute, if it should execute those actions, if it did execute those actions and if those actions made any changes.

`(no label)` or `EXECUTED`

  Task executed its actions.

  - Task has actions and Gradle has determined they should be executed as part of a build.

  - Task has no actions and some dependencies, and any of the dependencies are executed. See also Lifecycle Tasks.

`UP-TO-DATE`

  Task’s outputs did not change.

  - Task has outputs and inputs and they have not changed. See Incremental Builds.

  - Task has actions, but the task tells Gradle it did not change its outputs.

  - Task has no actions and some dependencies, but all of the dependencies are up-to-date, skipped or from cache. See also Lifecycle Tasks.

  - Task has no actions and no dependencies.

`FROM-CACHE`

  Task’s outputs could be found from a previous execution.

  - Task has outputs restored from the build cache. See Build Cache.

`SKIPPED`

  Task did not execute its actions.

  - Task has been explicitly excluded from the command-line. See Excluding tasks from execution.

  - Task has an `onlyIf` predicate return false. See Using a predicate.

`NO-SOURCE`

  Task did not need to execute its actions.

  - Task has inputs and outputs, but no sources. For example, source files are `.java` files for JavaCompile.

## Defining tasks

We have already seen how to define tasks using strings for task names in this chapter. There are a few variations on this style, which you may need to use in certain situations.

✨ | The task configuration APIs are described in more detail in the task configuration avoidance chapter.

Example 1. Defining tasks using strings for task names
build.gradle

    task('hello') {
        doLast {
            println "hello"
        }
    }

    task('copy', type: Copy) {
        from(file('srcDir'))
        into(buildDir)
    }

build.gradle.kts

    tasks.register("hello") {
        doLast {
            println("hello")
        }
    }

    tasks.register<Copy>("copy") {
        from(file("srcDir"))
        into(buildDir)
    }

There is an alternative syntax for defining tasks, which you may prefer to use:

Example 2. Defining tasks using the `tasks` container
build.gradle

    tasks.create('hello') {
        doLast {
            println "hello"
        }
    }

    tasks.create('copy', Copy) {
        from(file('srcDir'))
        into(buildDir)
    }

build.gradle.kts

    tasks.register("hello") {
        doLast {
            println("hello")
        }
    }

    tasks {
        register<Copy>("copy") {
            from(file("srcDir"))
            into(buildDir)
        }
    }

Here we add tasks to the `tasks` collection. Have a look at TaskContainer for more variations of the `register()` method.

And finally, there are language specific syntaxes for the Groovy and Kotlin DSL:

Example 3. Defining tasks using a DSL specific syntax
build.gradle

    // Using Groovy dynamic keywords

    task(hello) {
        doLast {
            println "hello"
        }
    }

    task(copy, type: Copy) {
        from(file('srcDir'))
        into(buildDir)
    }

build.gradle.kts

    // Using Kotlin delegated properties

    val hello by tasks.registering {
        doLast {
            println("hello")
        }
    }

    val copy by tasks.registering(Copy::class) {
        from(file("srcDir"))
        into(buildDir)
    }

Note that the Kotlin delegated properties syntax is particularly useful if you need the created task for further reference.

## Locating tasks

You often need to locate the tasks that you have defined in the build file, for example, to configure them or use them for dependencies. There are a number of ways of doing this. Firstly, just like with defining tasks there are language specific syntaxes for the Groovy and Kotlin DSL:

Example 4. Accessing tasks using a DSL specific syntax
build.gradle

    task hello
    task copy(type: Copy)

    // Access tasks using Groovy dynamic properties on Project

    println hello.name
    println project.hello.name

    println copy.destinationDir
    println project.copy.destinationDir

build.gradle.kts

    task("hello")
    task<Copy>("copy")

    // Access tasks using Kotlin delegated properties

    val hello by tasks.getting
    println(hello.name)

    val copy by tasks.getting(Copy::class)
    println(copy.destinationDir)

Tasks are also available through the `tasks` collection.

Example 5. Accessing tasks via tasks collection
build.gradle

    task hello
    task copy(type: Copy)

    println tasks.hello.name
    println tasks.named('hello').get().name

    println tasks.copy.destinationDir
    println tasks.named('copy').get().destinationDir

build.gradle.kts

    tasks.register("hello")
    tasks.register<Copy>("copy")

    println(tasks["hello"].name)
    println(tasks.named("hello").get().name)

    println(tasks.getByName<Copy>("copy").destinationDir)
    println(tasks.named<Copy>("copy").get().destinationDir)

You can access tasks from any project using the task’s path using the `tasks.getByPath()` method. You can call the `getByPath()` method with a task name, or a relative path, or an absolute path.

Example 6. Accessing tasks by path
build.gradle

    project(':projectA') {
        task hello
    }

    task hello

    println tasks.getByPath('hello').path
    println tasks.getByPath(':hello').path
    println tasks.getByPath('projectA:hello').path
    println tasks.getByPath(':projectA:hello').path

build.gradle.kts

    project(":projectA") {
        tasks.register("hello")
    }

    tasks.register("hello")

    println(tasks.getByPath("hello").path)
    println(tasks.getByPath(":hello").path)
    println(tasks.getByPath("projectA:hello").path)
    println(tasks.getByPath(":projectA:hello").path)

Output of `gradle -q hello`

    > gradle -q hello
    :hello
    :hello
    :projectA:hello
    :projectA:hello

Have a look at TaskContainer for more options for locating tasks.

## Configuring tasks

As an example, let’s look at the `Copy` task provided by Gradle. To create a `Copy` task for your build, you can declare in your build script:

Example 7. Creating a copy task
build.gradle

    task myCopy(type: Copy)

build.gradle.kts

    tasks.register<Copy>("myCopy")

This creates a copy task with no default behavior. The task can be configured using its API (see Copy). The following examples show several different ways to achieve the same configuration.

Just to be clear, realize that the name of this task is “`myCopy`”, but it is of type “`Copy`”. You can have multiple tasks of the same type, but with different names. You’ll find this gives you a lot of power to implement cross-cutting concerns across all tasks of a particular type.

Example 8. Configuring a task using the API
build.gradle

    Copy myCopy = tasks.getByName("myCopy")
    myCopy.from 'resources'
    myCopy.into 'target'
    myCopy.include('**/*.txt', '**/*.xml', '**/*.properties')

build.gradle.kts

    val myCopy = tasks.named<Copy>("myCopy")
    myCopy {
        from("resources")
        into("target")
        include("**/*.txt", "**/*.xml", "**/*.properties")
    }

This is similar to the way we would configure objects in Java. You have to repeat the context (`myCopy`) in the configuration statement every time. This is a redundancy and not very nice to read.

There is another way of configuring a task. It also preserves the context and it is arguably the most readable. It is usually our favorite.

Example 9. Configuring a task using a DSL specific syntax
build.gradle

    // Configure task using Groovy dynamic task configuration block
    myCopy {
       from 'resources'
       into 'target'
    }
    myCopy.include('**/*.txt', '**/*.xml', '**/*.properties')

build.gradle.kts

    // Configure task using Kotlin delegated properties and a lambda
    val myCopy by tasks.existing(Copy::class) {
        from("resources")
        into("target")
    }
    myCopy { include("**/*.txt", "**/*.xml", "**/*.properties") }

This works for any task. Task access is just a shortcut for the `tasks.named()` method. It is important to note that if you pass a block to the `named()` method, this block is applied to configure the task, not when the task executes.

Have a look at TaskContainer for more options for configuring tasks.

You can also use a configuration block when you define a task.

Example 10. Defining a task with a configuration block
build.gradle

    task copy(type: Copy) {
       from 'resources'
       into 'target'
       include('**/*.txt', '**/*.xml', '**/*.properties')
    }

build.gradle.kts

    tasks.register<Copy>("copy") {
       from("resources")
       into("target")
       include("**/*.txt", "**/*.xml", "**/*.properties")
    }

💡 | Don’t forget about the build phases A task has both configuration and actions. When using the `doLast`, you are simply using a shortcut to define an action. Code defined in the configuration section of your task will get executed during the configuration phase of the build regardless of what task was targeted. See Build Lifecycle for more details about the build lifecycle.

## Passing arguments to a task constructor

As opposed to configuring the mutable properties of a `Task` after creation, you can pass argument values to the `Task` class’s constructor. In order to pass values to the `Task` constructor, you must annotate the relevant constructor with `@javax.inject.Inject`.

Example 11. Task class with `@Inject` constructor
build.gradle

    class CustomTask extends DefaultTask {
        final String message
        final int number

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

build.gradle.kts

    open class CustomTask @Inject constructor(
        private val message: String,
        private val number: Int
    ) : DefaultTask()

You can then create a task, passing the constructor arguments at the end of the parameter list.

Example 12. Creating a task with constructor arguments using TaskContainer
build.gradle

    tasks.create('myTask', CustomTask, 'hello', 42)

build.gradle.kts

    tasks.register<CustomTask>("myTask", "hello", 42)

You can also create the task using a `constructorArgs` Map argument using the Project API:

Example 13. Creating a task with constructor arguments using Map
build.gradle

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

build.gradle.kts

    task("myTask", "type" to CustomTask::class.java, "constructorArgs" to listOf("hello", 42))

✨ | Prefer creating a task with constructor arguments using the TaskContainer It’s recommended to use the Task Configuration Avoidance APIs to improve configuration time.

In all circumstances, the values passed as constructor arguments must be non-null. If you attempt to pass a `null` value, Gradle will throw a `NullPointerException` indicating which runtime value is `null`.

## Adding dependencies to a task

There are several ways you can define the dependencies of a task. In Task dependencies you were introduced to defining dependencies using task names. Task names can refer to tasks in the same project as the task, or to tasks in other projects. To refer to a task in another project, you prefix the name of the task with the path of the project it belongs to. The following is an example which adds a dependency from `projectA:taskX` to `projectB:taskY`:

Example 14. Adding dependency on task from another project
build.gradle

    project('projectA') {
        task taskX {
            dependsOn ':projectB:taskY'
            doLast {
                println 'taskX'
            }
        }
    }

    project('projectB') {
        task taskY {
            doLast {
                println 'taskY'
            }
        }
    }

build.gradle.kts

    project("projectA") {
        tasks.register("taskX") {
            dependsOn(":projectB:taskY")
            doLast {
                println("taskX")
            }
        }
    }

    project("projectB") {
        tasks.register("taskY") {
            doLast {
                println("taskY")
            }
        }
    }

Output of `gradle -q taskX`

    > gradle -q taskX
    taskY
    taskX

Instead of using a task name, you can define a dependency using a `Task` object, as shown in this example:

Example 15. Adding dependency using task object
build.gradle

    task taskX {
        doLast {
            println 'taskX'
        }
    }

    task taskY {
        doLast {
            println 'taskY'
        }
    }

    taskX.dependsOn taskY

build.gradle.kts

    val taskX by tasks.registering {
        doLast {
            println("taskX")
        }
    }

    val taskY by tasks.registering {
        doLast {
            println("taskY")
        }
    }

    taskX {
        dependsOn(taskY)
    }

Output of `gradle -q taskX`

    > gradle -q taskX
    taskY
    taskX

For more advanced uses, you can define a task dependency using a lazy block. When evaluated, the block is passed the task whose dependencies are being calculated. The lazy block should return a single `Task` or collection of `Task` objects, which are then treated as dependencies of the task. The following example adds a dependency from `taskX` to all the tasks in the project whose name starts with `lib`:

Example 16. Adding dependency using a lazy block
build.gradle

    task taskX {
        doLast {
            println 'taskX'
        }
    }

    // Using a Groovy Closure
    taskX.dependsOn {
        tasks.findAll { task -> task.name.startsWith('lib') }
    }

    task lib1 {
        doLast {
            println 'lib1'
        }
    }

    task lib2 {
        doLast {
            println 'lib2'
        }
    }

    task notALib {
        doLast {
            println 'notALib'
        }
    }

build.gradle.kts

    val taskX by tasks.registering {
        doLast {
            println("taskX")
        }
    }

    // Using a Gradle Provider
    taskX {
        dependsOn(provider {
            tasks.filter { task -> task.name.startsWith("lib") }
        })
    }

    tasks.register("lib1") {
        doLast {
            println("lib1")
        }
    }

    tasks.register("lib2") {
        doLast {
            println("lib2")
        }
    }

    tasks.register("notALib") {
        doLast {
            println("notALib")
        }
    }

Output of `gradle -q taskX`

    > gradle -q taskX
    lib1
    lib2
    taskX

For more information about task dependencies, see the Task API.

## Ordering tasks

In some cases it is useful to control the order in which 2 tasks will execute, without introducing an explicit dependency between those tasks. The primary difference between a task ordering and a task dependency is that an ordering rule does not influence which tasks will be executed, only the order in which they will be executed.

Task ordering can be useful in a number of scenarios:

- Enforce sequential ordering of tasks: e.g. 'build' never runs before 'clean'.

- Run build validations early in the build: e.g. validate I have the correct credentials before starting the work for a release build.

- Get feedback faster by running quick verification tasks before long verification tasks: e.g. unit tests should run before integration tests.

- A task that aggregates the results of all tasks of a particular type: e.g. test report task combines the outputs of all executed test tasks.

There are two ordering rules available: “must run after” and “should run after”.

When you use the “must run after” ordering rule you specify that `taskB` must always run after `taskA`, whenever both `taskA` and `taskB` will be run. This is expressed as `taskB.mustRunAfter(taskA)`. The “should run after” ordering rule is similar but less strict as it will be ignored in two situations. Firstly if using that rule introduces an ordering cycle. Secondly when using parallel execution and all dependencies of a task have been satisfied apart from the “should run after” task, then this task will be run regardless of whether its “should run after” dependencies have been run or not. You should use “should run after” where the ordering is helpful but not strictly required.

With these rules present it is still possible to execute `taskA` without `taskB` and vice-versa.

Example 17. Adding a 'must run after' task ordering
build.gradle

    task taskX {
        doLast {
            println 'taskX'
        }
    }
    task taskY {
        doLast {
            println 'taskY'
        }
    }
    taskY.mustRunAfter taskX

build.gradle.kts

    val taskX by tasks.registering {
        doLast {
            println("taskX")
        }
    }
    val taskY by tasks.registering {
        doLast {
            println("taskY")
        }
    }
    taskY { mustRunAfter(taskX) }

Output of `gradle -q taskY taskX`

    > gradle -q taskY taskX
    taskX
    taskY

Example 18. Adding a 'should run after' task ordering
build.gradle

    task taskX {
        doLast {
            println 'taskX'
        }
    }
    task taskY {
        doLast {
            println 'taskY'
        }
    }
    taskY.shouldRunAfter taskX

build.gradle.kts

    val taskX by tasks.registering {
        doLast {
            println("taskX")
        }
    }
    val taskY by tasks.registering {
        doLast {
            println("taskY")
        }
    }
    taskY { shouldRunAfter(taskX) }

Output of `gradle -q taskY taskX`

    > gradle -q taskY taskX
    taskX
    taskY

In the examples above, it is still possible to execute `taskY` without causing `taskX` to run:

Example 19. Task ordering does not imply task execution
Output of `gradle -q taskY`

    > gradle -q taskY
    taskY

To specify a “must run after” or “should run after” ordering between 2 tasks, you use the Task.mustRunAfter(java.lang.Object...) and Task.shouldRunAfter(java.lang.Object...) methods. These methods accept a task instance, a task name or any other input accepted by Task.dependsOn(java.lang.Object...).

Note that “`B.mustRunAfter(A)`” or “`B.shouldRunAfter(A)`” does not imply any execution dependency between the tasks:

- It is possible to execute tasks `A` and `B` independently. The ordering rule only has an effect when both tasks are scheduled for execution.

- When run with `--continue`, it is possible for `B` to execute in the event that `A` fails.

As mentioned before, the “should run after” ordering rule will be ignored if it introduces an ordering cycle:

Example 20. A 'should run after' task ordering is ignored if it introduces an ordering cycle
build.gradle

    task taskX {
        doLast {
            println 'taskX'
        }
    }
    task taskY {
        doLast {
            println 'taskY'
        }
    }
    task taskZ {
        doLast {
            println 'taskZ'
        }
    }
    taskX.dependsOn taskY
    taskY.dependsOn taskZ
    taskZ.shouldRunAfter taskX

build.gradle.kts

    val taskX by tasks.registering {
        doLast {
            println("taskX")
        }
    }
    val taskY by tasks.registering {
        doLast {
            println("taskY")
        }
    }
    val taskZ by tasks.registering {
        doLast {
            println("taskZ")
        }
    }
    taskX { dependsOn(taskY) }
    taskY { dependsOn(taskZ) }
    taskZ { shouldRunAfter(taskX) }

Output of `gradle -q taskX`

    > gradle -q taskX
    taskZ
    taskY
    taskX

## Adding a description to a task

You can add a description to your task. This description is displayed when executing `gradle tasks`.

Example 21. Adding a description to a task
build.gradle

    task copy(type: Copy) {
       description 'Copies the resource directory to the target directory.'
       from 'resources'
       into 'target'
       include('**/*.txt', '**/*.xml', '**/*.properties')
    }

build.gradle.kts

    tasks.register<Copy>("copy") {
       description = "Copies the resource directory to the target directory."
       from("resources")
       into("target")
       include("**/*.txt", "**/*.xml", "**/*.properties")
    }

## Replacing tasks

Sometimes you want to replace a task. For example, if you want to exchange a task added by the Java plugin with a custom task of a different type. You can achieve this with:

Example 22. Overwriting a task
build.gradle

    task copy(type: Copy)

    task copy(overwrite: true) {
        doLast {
            println('I am the new one.')
        }
    }

build.gradle.kts

    tasks.register<Copy>("copy")

    task("copy", "overwrite" to true).apply {
        doLast {
            println("I am the new one.")
        }
    }

Output of `gradle -q copy`

    > gradle -q copy
    I am the new one.

This will replace a task of type `Copy` with the task you’ve defined, because it uses the same name. When you define the new task, you have to set the `overwrite` property to true. Otherwise Gradle throws an exception, saying that a task with that name already exists.

## Skipping tasks

Gradle offers multiple ways to skip the execution of a task.

### Using a predicate

You can use the `onlyIf()` method to attach a predicate to a task. The task’s actions are only executed if the predicate evaluates to true. You implement the predicate as a closure. The closure is passed the task as a parameter, and should return true if the task should execute and false if the task should be skipped. The predicate is evaluated just before the task is due to be executed.

Example 23. Skipping a task using a predicate
build.gradle

    task hello {
        doLast {
            println 'hello world'
        }
    }

    hello.onlyIf { !project.hasProperty('skipHello') }

build.gradle.kts

    val hello by tasks.registering {
        doLast {
            println("hello world")
        }
    }

    hello {
        onlyIf { !project.hasProperty("skipHello") }
    }

Output of `gradle hello -PskipHello`

    > gradle hello -PskipHello
    > Task :hello SKIPPED

    BUILD SUCCESSFUL in 0s

### Using StopExecutionException

If the logic for skipping a task can’t be expressed with a predicate, you can use the StopExecutionException. If this exception is thrown by an action, the further execution of this action as well as the execution of any following action of this task is skipped. The build continues with executing the next task.

Example 24. Skipping tasks with StopExecutionException
build.gradle

    task compile {
        doLast {
            println 'We are doing the compile.'
        }
    }

    compile.doFirst {
        // Here you would put arbitrary conditions in real life.
        // But this is used in an integration test so we want defined behavior.
        if (true) { throw new StopExecutionException() }
    }
    task myTask {
        dependsOn('compile')
        doLast {
            println 'I am not affected'
        }
    }

build.gradle.kts

    val compile by tasks.registering {
        doLast {
            println("We are doing the compile.")
        }
    }

    compile {
        doFirst {
            // Here you would put arbitrary conditions in real life.
            // But this is used in an integration test so we want defined behavior.
            if (true) {
                throw StopExecutionException()
            }
        }
    }
    tasks.register("myTask") {
        dependsOn(compile)
        doLast {
            println("I am not affected")
        }
    }

Output of `gradle -q myTask`

    > gradle -q myTask
    I am not affected

This feature is helpful if you work with tasks provided by Gradle. It allows you to add conditional execution of the built-in actions of such a task.[1]

### Enabling and disabling tasks

Every task has an `enabled` flag which defaults to `true`. Setting it to `false` prevents the execution of any of the task’s actions. A disabled task will be labelled SKIPPED.

Example 25. Enabling and disabling tasks
build.gradle

    task disableMe {
        doLast {
            println 'This should not be printed if the task is disabled.'
        }
    }
    disableMe.enabled = false

build.gradle.kts

    val disableMe by tasks.registering {
        doLast {
            println("This should not be printed if the task is disabled.")
        }
    }
    disableMe {
        enabled = false
    }

Output of `gradle disableMe`

    > gradle disableMe
    > Task :disableMe SKIPPED

    BUILD SUCCESSFUL in 0s

### Task timeouts

Every task has a `timeout` property which can be used to limit its execution time. When a task reaches its timeout, its task execution thread is interrupted. The task will be marked as failed. Finalizer tasks will still be run. If `--continue` is used, other tasks can continue running after it. Tasks that don’t respond to interrupts can’t be timed out. All of Gradle’s built-in tasks respond to timeouts in a timely manner.

Example 26. Specifying task timeouts
build.gradle

    task hangingTask() {
        doLast {
            Thread.sleep(100000)
        }
        timeout = Duration.ofMillis(500)
    }

build.gradle.kts

    import java.time.Duration

    tasks {
        register("hangingTask") {
            doLast {
                Thread.sleep(100000)
            }
            timeout.set(Duration.ofMillis(500))
        }
    }

## Up-to-date checks (AKA Incremental Build)

An important part of any build tool is the ability to avoid doing work that has already been done. Consider the process of compilation. Once your source files have been compiled, there should be no need to recompile them unless something has changed that affects the output, such as the modification of a source file or the removal of an output file. And compilation can take a significant amount of time, so skipping the step when it’s not needed saves a lot of time.

Gradle supports this behavior out of the box through a feature it calls incremental build. You have almost certainly already seen it in action: it’s active nearly every time the `UP-TO-DATE` text appears next to the name of a task when you run a build. Task outcomes are described in Task outcomes.

How does incremental build work? And what does it take to make use of it in your own tasks? Let’s take a look.

### Task inputs and outputs

In the most common case, a task takes some inputs and generates some outputs. If we use the compilation example from earlier, we can see that the source files are the inputs and, in the case of Java, the generated class files are the outputs. Other inputs might include things like whether debug information should be included.

Figure 1. Example task inputs and outputs

An important characteristic of an input is that it affects one or more outputs, as you can see from the previous figure. Different bytecode is generated depending on the content of the source files and the minimum version of the Java runtime you want to run the code on. That makes them task inputs. But whether compilation has 500MB or 600MB of maximum memory available, determined by the `memoryMaximumSize` property, has no impact on what bytecode gets generated. In Gradle terminology, `memoryMaximumSize` is just an internal task property.

As part of incremental build, Gradle tests whether any of the task inputs or outputs have changed since the last build. If they haven’t, Gradle can consider the task up to date and therefore skip executing its actions. Also note that incremental build won’t work unless a task has at least one task output, although tasks usually have at least one input as well.

What this means for build authors is simple: you need to tell Gradle which task properties are inputs and which are outputs. If a task property affects the output, be sure to register it as an input, otherwise the task will be considered up to date when it’s not. Conversely, don’t register properties as inputs if they don’t affect the output, otherwise the task will potentially execute when it doesn’t need to. Also be careful of non-deterministic tasks that may generate different output for exactly the same inputs: these should not be configured for incremental build as the up-to-date checks won’t work.

Let’s now look at how you can register task properties as inputs and outputs.

#### Custom task types

If you’re implementing a custom task as a class, then it takes just two steps to make it work with incremental build:

1. Create typed properties (via getter methods) for each of your task inputs and outputs

2. Add the appropriate annotation to each of those properties

✨ | Annotations must be placed on getters or on Groovy properties. Annotations placed on setters, or on a Java field without a corresponding annotated getter are ignored.

Gradle supports three main categories of inputs and outputs:

- Simple values

  Things like strings and numbers. More generally, a simple value can have any type that implements `Serializable`.

- Filesystem types

  These consist of the standard `File` class but also derivatives of Gradle’s FileCollection type and anything else that can be passed to either the Project.file(java.lang.Object) method - for single file/directory properties - or the Project.files(java.lang.Object...), ProjectLayout.files(java.lang.Object…​), and ProjectLayout.configurableFiles(java.lang.Object…​) methods.

- Nested values

  Custom types that don’t conform to the other two categories but have their own properties that are inputs or outputs. In effect, the task inputs or outputs are nested inside these custom types.

As an example, imagine you have a task that processes templates of varying types, such as FreeMarker, Velocity, Moustache, etc. It takes template source files and combines them with some model data to generate populated versions of the template files.

This task will have three inputs and one output:

- Template source files

- Model data

- Template engine

- Where the output files are written

When you’re writing a custom task class, it’s easy to register properties as inputs or outputs via annotations. To demonstrate, here is a skeleton task implementation with some suitable inputs and outputs, along with their annotations:

Example 27. Custom task class
buildSrc/src/main/java/org/example/ProcessTemplates.java

    package org.example;

    import java.io.File;
    import java.util.HashMap;
    import org.gradle.api.*;
    import org.gradle.api.file.*;
    import org.gradle.api.tasks.*;

    public class ProcessTemplates extends DefaultTask {
        private TemplateEngineType templateEngine;
        private FileCollection sourceFiles;
        private TemplateData templateData;
        private File outputDir;

        @Input
        public TemplateEngineType getTemplateEngine() {
            return this.templateEngine;
        }

        @InputFiles
        public FileCollection getSourceFiles() {
            return this.sourceFiles;
        }

        @Nested
        public TemplateData getTemplateData() {
            return this.templateData;
        }

        @OutputDirectory
        public File getOutputDir() { return this.outputDir; }

        // + setter methods for the above - assume we’ve defined them

        @TaskAction
        public void processTemplates() {
            // ...
        }
    }

buildSrc/src/main/java/org/example/TemplateData.java

    package org.example;

    import java.util.HashMap;
    import java.util.Map;
    import org.gradle.api.tasks.Input;

    public class TemplateData {
        private String name;
        private Map<String, String> variables;

        public TemplateData(String name, Map<String, String> variables) {
            this.name = name;
            this.variables = new HashMap<>(variables);
        }

        @Input
        public String getName() { return this.name; }

        @Input
        public Map<String, String> getVariables() {
            return this.variables;
        }
    }

Output of `gradle processTemplates`

    > gradle processTemplates
    > Task :processTemplates

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

Output of `gradle processTemplates` (run again)

    > gradle processTemplates
    > Task :processTemplates UP-TO-DATE

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 up-to-date

There’s plenty to talk about in this example, so let’s work through each of the input and output properties in turn:

- `templateEngine`

  Represents which engine to use when processing the source templates, e.g. FreeMarker, Velocity, etc. You could implement this as a string, but in this case we have gone for a custom enum as it provides greater type information and safety. Since enums implement `Serializable` automatically, we can treat this as a simple value and use the `@Input` annotation, just as we would with a `String` property.

- `sourceFiles`

  The source templates that the task will be processing. Single files and collections of files need their own special annotations. In this case, we’re dealing with a collection of input files and so we use the `@InputFiles` annotation. You’ll see more file-oriented annotations in a table later.

- `templateData`

  For this example, we’re using a custom class to represent the model data. However, it does not implement `Serializable`, so we can’t use the `@Input` annotation. That’s not a problem as the properties within `TemplateData` - a string and a hash map with serializable type parameters - are serializable and can be annotated with `@Input`. We use `@Nested` on `templateData` to let Gradle know that this is a value with nested input properties.

- `outputDir`

  The directory where the generated files go. As with input files, there are several annotations for output files and directories. A property representing a single directory requires `@OutputDirectory`. You’ll learn about the others soon.

These annotated properties mean that Gradle will skip the task if none of the source files, template engine, model data or generated files have changed since the previous time Gradle executed the task. This will often save a significant amount of time. You can learn how Gradle detects changes later.

This example is particularly interesting because it works with collections of source files. What happens if only one source file changes? Does the task process all the source files again or just the modified one? That depends on the task implementation. If the latter, then the task itself is incremental, but that’s a different feature to the one we’re discussing here. Gradle does help task implementers with this via its incremental task inputs feature.

Now that you have seen some of the input and output annotations in practice, let’s take a look at all the annotations available to you and when you should use them. The table below lists the available annotations and the corresponding property type you can use with each one.

Table 1. Incremental build property type annotations
Annotation | Expected property type | Description
--- | --- | ---
`@Input` | Any `Serializable` type | A simple input value
`@InputFile` | `File`* | A single input file (not directory)
`@InputDirectory` | `File`* | A single input directory (not file)
`@InputFiles` | `Iterable<File>`* | An iterable of input files and directories
`@Classpath` | `Iterable<File>`* | An iterable of input files and directories that represent a Java classpath. This allows the task to ignore irrelevant changes to the property, such as different names for the same files. It is similar to annotating the property `@PathSensitive(RELATIVE)` but it will ignore the names of JAR files directly added to the classpath, and it will consider changes in the order of the files as a change in the classpath. Gradle will inspect the contents of jar files on the classpath and ignore changes that do not affect the semantics of the classpath (such as file dates and entry order). See also Using the classpath annotations. Note: The `@Classpath` annotation was introduced in Gradle 3.2. To stay compatible with earlier Gradle versions, classpath properties should also be annotated with `@InputFiles`.
`@CompileClasspath` | `Iterable<File>`* | An iterable of input files and directories that represent a Java compile classpath. This allows the task to ignore irrelevant changes that do not affect the API of the classes in classpath. See also Using the classpath annotations. The following kinds of changes to the classpath will be ignored: Changes to the path of jar or top level directories. Changes to timestamps and the order of entries in Jars. Changes to resources and Jar manifests, including adding or removing resources. Changes to private class elements, such as private fields, methods and inner classes. Changes to code, such as method bodies, static initializers and field initializers (except for constants). Changes to debug information, for example when a change to a comment affects the line numbers in class debug information. Changes to directories, including directory entries in Jars.
✨ | The `@CompileClasspath` annotation was introduced in Gradle 3.4. To stay compatible with Gradle 3.3 and 3.2, compile classpath properties should also be annotated with `@Classpath`. For compatibility with Gradle versions before 3.2 the property should also be annotated with `@InputFiles`.
`@OutputFile` | `File`* | A single output file (not directory)
`@OutputDirectory` | `File`* | A single output directory (not file)
`@OutputFiles` | `Map<String, File>`** or `Iterable<File>`* | An iterable or map of output files. Using a file tree turns caching off for the task.
`@OutputDirectories` | `Map<String, File>`** or `Iterable<File>`* | An iterable of output directories. Using a file tree turns caching off for the task.
`@Destroys` | `File` or `Iterable<File>`* | Specifies one or more files that are removed by this task. Note that a task can define either inputs/outputs or destroyables, but not both.
`@LocalState` | `File` or `Iterable<File>`* | Specifies one or more files that represent the local state of the task. These files are removed when the task is loaded from cache.
`@Nested` | Any custom type | A custom type that may not implement `Serializable` but does have at least one field or property marked with one of the annotations in this table. It could even be another `@Nested`.
`@Console` | Any type | Indicates that the property is neither an input nor an output. It simply affects the console output of the task in some way, such as increasing or decreasing the verbosity of the task.
`@Internal` | Any type | Indicates that the property is used internally but is neither an input nor an output.
`@SkipWhenEmpty` | `File`* | Used with `@InputFiles` or `@InputDirectory` to tell Gradle to skip the task if the corresponding files or directory are empty, along with all other input files declared with this annotation. Tasks that have been skipped due to all of their input files that were declared with this annotation being empty will result in a distinct “no source” outcome. For example, `NO-SOURCE` will be emitted in the console output.
`@Optional` | Any type | Used with any of the property type annotations listed in the Optional API documentation. This annotation disables validation checks on the corresponding property. See the section on validation for more details.
`@PathSensitive` | `File`* | Used with any input file property to tell Gradle to only consider the given part of the file paths as important. For example, if a property is annotated with `@PathSensitive(PathSensitivity.NAME_ONLY)`, then moving the files around without changing their contents will not make the task out-of-date.

✨ |
* | In fact, `File` can be any type accepted by Project.file(java.lang.Object) and `Iterable<File>` can be any type accepted by Project.files(java.lang.Object…​), ProjectLayout.files(java.lang.Object...), or ProjectLayout.configurableFiles(java.lang.Object...). This includes instances of `Callable`, such as closures, allowing for lazy evaluation of the property values. Be aware that the types `FileCollection` and `FileTree` are `Iterable<File>`s.
** | Similar to the above, `File` can be any type accepted by Project.file(java.lang.Object). The `Map` itself can be wrapped in `Callable`s, such as closures.

Annotations are inherited from all parent types including implemented interfaces. Property type annotations override any other property type annotation declared in a parent type. This way an `@InputFile` property can be turned into an `@InputDirectory` property in a child task type.

Annotations on a property declared in a type override similar annotations declared by the superclass and in any implemented interfaces. Superclass annotations take precedence over annotations declared in implemented interfaces.

The Console and Internal annotations in the table are special cases as they don’t declare either task inputs or task outputs. So why use them? It’s so that you can take advantage of the Java Gradle Plugin Development plugin to help you develop and publish your own plugins. This plugin checks whether any properties of your custom task classes lack an incremental build annotation. This protects you from forgetting to add an appropriate annotation during development.

##### Using the classpath annotations

Besides `@InputFiles`, for JVM-related tasks Gradle understands the concept of classpath inputs. Both runtime and compile classpaths are treated differently when Gradle is looking for changes.

As opposed to input properties annotated with `@InputFiles`, for classpath properties the order of the entries in the file collection matter. On the other hand, the names and paths of the directories and jar files on the classpath itself are ignored. Timestamps and the order of class files and resources inside jar files on a classpath are ignored, too, thus recreating a jar file with different file dates will not make the task out of date.

Runtime classpaths are marked with `@Classpath`, and they offer further customization via classpath normalization.

Input properties annotated with `@CompileClasspath` are considered Java compile classpaths. Additionally to the aforementioned general classpath rules, compile classpaths ignore changes to everything but class files. Gradle uses the same class analysis described in Java compile avoidance to further filter changes that don’t affect the class' ABIs. This means that changes which only touch the implementation of classes do not make the task out of date.

##### Nested inputs

When analyzing `@Nested` task properties for declared input and output sub-properties Gradle uses the type of the actual value. Hence it can discover all sub-properties declared by a runtime sub-type.

When adding `@Nested` to a `Provider`, the value of the `Provider` is treated as a nested input.

When adding `@Nested` to an iterable, each element is treated as a separate nested input. Each nested input in the iterable is assigned a name, which by default is the dollar sign followed by the index in the iterable, e.g. `$2`. If an element of the iterable implements `Named`, then the name is used as property name. The ordering of the elements in the iterable is crucial for for reliable up-to-date checks and caching if not all of the elements implement `Named`. Multiple elements which have the same name are not allowed.

When adding `@Nested` to a map, then for each value a nested input is added, using the key as name.

The type and classpath of nested inputs is tracked, too. This ensures that changes to the implementation of a nested input causes the build to be out of date. By this it is also possible to add user provided code as an input, e.g. by annotating an `@Action` property with `@Nested`. Note that any inputs to such actions should be tracked, either by annotated properties on the action or by manually registering them with the task.

Using nested inputs allows richer modeling and extensibility for tasks, as e.g. shown by Test.getJvmArgumentProviders().

This allows us to model the JaCoCo Java agent, thus declaring the necessary JVM arguments and providing the inputs and outputs to Gradle:

JacocoAgent.java

    class JacocoAgent implements CommandLineArgumentProvider {
        private final JacocoTaskExtension jacoco;

        public JacocoAgent(JacocoTaskExtension jacoco) {
            this.jacoco = jacoco;
        }

        @Nested
        @Optional
        public JacocoTaskExtension getJacoco() {
            return jacoco.isEnabled() ? jacoco : null;
        }

        @Override
        public Iterable<String> asArguments() {
            return jacoco.isEnabled() ? ImmutableList.of(jacoco.getAsJvmArg()) : Collections.<String>emptyList();
        }
    }

    test.getJvmArgumentProviders().add(new JacocoAgent(extension));

For this to work, `JacocoTaskExtension` needs to have the correct input and output annotations.

The approach works for Test JVM arguments, since `Test.getJvmArgumentProviders()` is an `Iterable` annotated with `@Nested`.

There are other task types where this kind of nested inputs are available:

- JavaExec.getArgumentProviders() - model e.g. custom tools

- JavaExec.getJvmArgumentProviders() - used for Jacoco Java agent

- CompileOptions.getCompilerArgumentProviders() - model e.g annotation processors

- Exec.getArgumentProviders() - model e.g custom tools

In the same way, this kind of modelling is available to custom tasks.

#### Runtime API

Custom task classes are an easy way to bring your own build logic into the arena of incremental build, but you don’t always have that option. That’s why Gradle also provides an alternative API that can be used with any tasks, which we look at next.

When you don’t have access to the source for a custom task class, there is no way to add any of the annotations we covered in the previous section. Fortunately, Gradle provides a runtime API for scenarios just like that. It can also be used for ad-hoc tasks, as you’ll see next.

##### Using it for ad-hoc tasks

This runtime API is provided through a couple of aptly named properties that are available on every Gradle task:

- Task.getInputs() of type TaskInputs

- Task.getOutputs() of type TaskOutputs

- Task.getDestroyables() of type TaskDestroyables

These objects have methods that allow you to specify files, directories and values which constitute the task’s inputs and outputs. In fact, the runtime API has almost feature parity with the annotations. All it lacks is an equivalent for `@Nested`.

Let’s take the template processing example from before and see how it would look as an ad-hoc task that uses the runtime API:

Example 28. Ad-hoc task
build.gradle

    task processTemplatesAdHoc {
        inputs.property("engine", TemplateEngineType.FREEMARKER)
        inputs.files(fileTree("src/templates"))
        inputs.property("templateData.name", "docs")
        inputs.property("templateData.variables", [year: 2013])
        outputs.dir("$buildDir/genOutput2")

        doLast {
            // Process the templates here
        }
    }

build.gradle.kts

    tasks.register("processTemplatesAdHoc") {
        inputs.property("engine", TemplateEngineType.FREEMARKER)
        inputs.files(fileTree("src/templates"))
        inputs.property("templateData.name", "docs")
        inputs.property("templateData.variables", mapOf("year" to "2013"))
        outputs.dir("$buildDir/genOutput2")

        doLast {
            // Process the templates here
        }
    }

Output of `gradle processTemplatesAdHoc`

    > gradle processTemplatesAdHoc
    > Task :processTemplatesAdHoc

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

As before, there’s much to talk about. To begin with, you should really write a custom task class for this as it’s a non-trivial implementation that has several configuration options. In this case, there are no task properties to store the root source folder, the location of the output directory or any of the other settings. That’s deliberate to highlight the fact that the runtime API doesn’t require the task to have any state. In terms of incremental build, the above ad-hoc task will behave the same as the custom task class.

All the input and output definitions are done through the methods on `inputs` and `outputs`, such as `property()`, `files()`, and `dir()`. Gradle performs up-to-date checks on the argument values to determine whether the task needs to run again or not. Each method corresponds to one of the incremental build annotations, for example `inputs.property()` maps to `@Input` and `outputs.dir()` maps to `@OutputDirectory`.

The files that a task removes can be specified through `destroyables.register()`.

Example 29. Ad-hoc task declaring a destroyable
build.gradle

    task removeTempDir {
        destroyables.register("$projectDir/tmpDir")
        doLast {
            delete("$projectDir/tmpDir")
        }
    }

build.gradle.kts

    tasks.register("removeTempDir") {
        destroyables.register("$projectDir/tmpDir")
        doLast {
            delete("$projectDir/tmpDir")
        }
    }

One notable difference between the runtime API and the annotations is the lack of a method that corresponds directly to `@Nested`. That’s why the example uses two `property()` declarations for the template data, one for each `TemplateData` property. You should utilize the same technique when using the runtime API with nested values. Any given task can either declare destroyables or inputs/outputs, but cannot declare both.

##### Using it for custom task types

Another type of example involves adding input and output definitions to instances of a custom task class that lacks the requisite annotations. For example, imagine that the `ProcessTemplates` task is provided by a plugin and that it’s missing the incremental build annotations. In order to make up for that deficiency, you can use the runtime API:

Example 30. Using runtime API with custom task type
build.gradle

    task processTemplatesRuntime(type: ProcessTemplatesNoAnnotations) {
        templateEngine = TemplateEngineType.FREEMARKER
        sourceFiles = fileTree("src/templates")
        templateData = new TemplateData("test", [year: 2014])
        outputDir = file("$buildDir/genOutput3")

        inputs.property("engine", templateEngine)
        inputs.files(sourceFiles)
        inputs.property("templateData.name", templateData.name)
        inputs.property("templateData.variables", templateData.variables)
        outputs.dir(outputDir)
    }

build.gradle.kts

    tasks.register<ProcessTemplatesNoAnnotations>("processTemplatesRuntime") {
        templateEngine = TemplateEngineType.FREEMARKER
        sourceFiles = fileTree("src/templates")
        templateData = TemplateData("test", mapOf("year" to "2014"))
        outputDir = file("$buildDir/genOutput3")

        inputs.property("engine", templateEngine)
        inputs.files(sourceFiles)
        inputs.property("templateData.name", templateData.name)
        inputs.property("templateData.variables", templateData.variables)
        outputs.dir(outputDir)
    }

Output of `gradle processTemplatesRuntime`

    > gradle processTemplatesRuntime
    > Task :processTemplatesRuntime

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

Output of `gradle processTemplatesRuntime` (run again)

    > gradle processTemplatesRuntime
    > Task :processTemplatesRuntime UP-TO-DATE

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 up-to-date

As you can see, we can both configure the tasks properties and use those properties as arguments to the incremental build runtime API. Using the runtime API like this is a little like using `doLast()` and `doFirst()` to attach extra actions to a task, except in this case we’re attaching information about inputs and outputs. Note that if the task type is already using the incremental build annotations, the runtime API will add inputs and outputs rather than replace them.

##### Fine-grained configuration

The runtime API methods only allow you to declare your inputs and outputs in themselves. However, the file-oriented ones return a builder - of type TaskInputFilePropertyBuilder - that lets you provide additional information about those inputs and outputs.

You can learn about all the options provided by the builder in its API documentation, but we’ll show you a simple example here to give you an idea of what you can do.

Let’s say we don’t want to run the `processTemplates` task if there are no source files, regardless of whether it’s a clean build or not. After all, if there are no source files, there’s nothing for the task to do. The builder allows us to configure this like so:

Example 31. Using skipWhenEmpty() via the runtime API
build.gradle

    task processTemplatesRuntimeConf(type: ProcessTemplatesNoAnnotations) {
        // ...
        sourceFiles = fileTree("src/templates") {
            include "**/*.fm"
        }

        inputs.files(sourceFiles).skipWhenEmpty()
        // ...
    }

build.gradle.kts

    tasks.register<ProcessTemplatesNoAnnotations>("processTemplatesRuntimeConf") {
        // ...
        sourceFiles = fileTree("src/templates") {
            include("**/*.fm")
        }

        inputs.files(sourceFiles).skipWhenEmpty()
        // ...
    }

Output of `gradle clean processTemplatesRuntimeConf`

    > gradle clean processTemplatesRuntimeConf
    > Task :processTemplatesRuntimeConf NO-SOURCE

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 up-to-date

The `TaskInputs.files()` method returns a builder that has a `skipWhenEmpty()` method. Invoking this method is equivalent to annotating to the property with `@SkipWhenEmpty`.

Now that you have seen both the annotations and the runtime API, you may be wondering which API you should be using. Our recommendation is to use the annotations wherever possible, and it’s sometimes worth creating a custom task class just so that you can make use of them. The runtime API is more for situations in which you can’t use the annotations.

#### Important beneficial side effects

Once you declare a task’s formal inputs and outputs, Gradle can then infer things about those properties. For example, if an input of one task is set to the output of another, that means the first task depends on the second, right? Gradle knows this and can act upon it.

We’ll look at this feature next and also some other features that come from Gradle knowing things about inputs and outputs.

##### Inferred task dependencies

Consider an archive task that packages the output of the `processTemplates` task. A build author will see that the archive task obviously requires `processTemplates` to run first and so may add an explicit `dependsOn`. However, if you define the archive task like so:

Example 32. Inferred task dependency via task outputs
build.gradle

    task packageFiles(type: Zip) {
        from processTemplates.outputs
    }

build.gradle.kts

    tasks.register<Zip>("packageFiles") {
        from(processTemplates.get().outputs)
    }

Output of `gradle clean packageFiles`

    > gradle clean packageFiles
    > Task :processTemplates
    > Task :packageFiles

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 2 executed, 1 up-to-date

Gradle will automatically make `packageFiles` depend on `processTemplates`. It can do this because it’s aware that one of the inputs of packageFiles requires the output of the processTemplates task. We call this an inferred task dependency.

The above example can also be written as

Example 33. Inferred task dependency via a task argument
build.gradle

    task packageFiles2(type: Zip) {
        from processTemplates
    }

build.gradle.kts

    tasks.register<Zip>("packageFiles2") {
        from(processTemplates)
    }

Output of `gradle clean packageFiles2`

    > gradle clean packageFiles2
    > Task :processTemplates
    > Task :packageFiles2

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 2 executed, 1 up-to-date

This is because the `from()` method can accept a task object as an argument. Behind the scenes, `from()` uses the `project.files()` method to wrap the argument, which in turn exposes the task’s formal outputs as a file collection. In other words, it’s a special case!

##### Input and output validation

The incremental build annotations provide enough information for Gradle to perform some basic validation on the annotated properties. In particular, it does the following for each property before the task executes:

- `@InputFile` - verifies that the property has a value and that the path corresponds to a file (not a directory) that exists.

- `@InputDirectory` - same as for `@InputFile`, except the path must correspond to a directory.

- `@OutputDirectory` - verifies that the path doesn’t match a file and also creates the directory if it doesn’t already exist.

Such validation improves the robustness of the build, allowing you to identify issues related to inputs and outputs quickly.

You will occasionally want to disable some of this validation, specifically when an input file may validly not exist. That’s why Gradle provides the `@Optional` annotation: you use it to tell Gradle that a particular input is optional and therefore the build should not fail if the corresponding file or directory doesn’t exist.

##### Continuous build

Another benefit of defining task inputs and outputs is continuous build. Since Gradle knows what files a task depends on, it can automatically run a task again if any of its inputs change. By activating continuous build when you run Gradle - through the `--continuous` or `-t` options - you will put Gradle into a state in which it continually checks for changes and executes the requested tasks when it encounters such changes.

You can find out more about this feature in Continuous build.

##### Task parallelism

One last benefit of defining task inputs and outputs is that Gradle can use this information to make decisions about how to run tasks when the "--parallel" option is used. For instance, Gradle will inspect the outputs of tasks when selecting the next task to run and will avoid concurrent execution of tasks that write to the same output directory. Similarly, Gradle will use the information about what files a task destroys (e.g. specified by the `Destroys` annotation) and avoid running a task that removes a set of files while another task is running that consumes or creates those same files (and vice versa). It can also determine that a task that creates a set of files has already run and that a task that consumes those files has yet to run and will avoid running a task that removes those files in between. By providing task input and output information in this way, Gradle can infer creation/consumption/destruction relationships between tasks and can ensure that task execution does not violate those relationships.

### How does it work?

Before a task is executed for the first time, Gradle takes a fingerprint of the inputs. This fingerprint contains the paths of input files and a hash of the contents of each file. Gradle then executes the task. If the task completes successfully, Gradle takes a fingerprint of the outputs. This fingerprint contains the set of output files and a hash of the contents of each file. Gradle persists both fingerprints for the next time the task is executed.

Each time after that, before the task is executed, Gradle takes a new fingerprint of the inputs and outputs. If the new fingerprints are the same as the previous fingerprints, Gradle assumes that the outputs are up to date and skips the task. If they are not the same, Gradle executes the task. Gradle persists both fingerprints for the next time the task is executed.

Gradle also considers the code of the task as part of the inputs to the task. When a task, its actions, or its dependencies change between executions, Gradle considers the task as out-of-date.

Gradle understands if a file property (e.g. one holding a Java classpath) is order-sensitive. When comparing the fingerprint of such a property, even a change in the order of the files will result in the task becoming out-of-date.

Note that if a task has an output directory specified, any files added to that directory since the last time it was executed are ignored and will NOT cause the task to be out of date. This is so unrelated tasks may share an output directory without interfering with each other. If this is not the behaviour you want for some reason, consider using TaskOutputs.upToDateWhen(groovy.lang.Closure)

The inputs for the task are also used to calculate the build cache key used to load task outputs when enabled. For more details see Task output caching.

✨ | For tracking the implementation of tasks, task actions and nested inputs, Gradle uses the class name and an identifier for the classpath which contains the implementation. There are some situations when Gradle is not able to track the implementation precisely: Unknown classloader When the classloader which loaded the implementation has not been created by Gradle, the classpath cannot be determined. Java lambda Java lambda classes are created at runtime with a non-deterministic classname. Therefore, the class name does not identify the implementation of the lambda and changes between different Gradle runs. When the implementation of a task, task action or a nested input cannot be tracked precisely, Gradle disables any caching for the task. That means that the task will never be up-to-date or loaded from the build cache.

### Advanced techniques

Everything you’ve seen so far in this section will cover most of the use cases you’ll encounter, but there are some scenarios that need special treatment. We’ll present a few of those next with the appropriate solutions.

#### Adding your own cached input/output methods

Have you ever wondered how the `from()` method of the `Copy` task works? It’s not annotated with `@InputFiles` and yet any files passed to it are treated as formal inputs of the task. What’s happening?

The implementation is quite simple and you can use the same technique for your own tasks to improve their APIs. Write your methods so that they add files directly to the appropriate annotated property. As an example, here’s how to add a `sources()` method to the custom `ProcessTemplates` class we introduced earlier:

Example 34. Declaring a method to add task inputs
build.gradle

    task processTemplates(type: ProcessTemplates) {
        templateEngine = TemplateEngineType.FREEMARKER
        templateData = new TemplateData("test", [year: 2012])
        outputDir = file("$buildDir/genOutput")

        sources fileTree("src/templates")
    }

build.gradle.kts

    tasks.register<ProcessTemplates>("processTemplates") {
        templateEngine = TemplateEngineType.FREEMARKER
        templateData = TemplateData("test", mapOf("year" to "2012"))
        outputDir = file("$buildDir/genOutput")

        sources(fileTree("src/templates"))
    }

ProcessTemplates.java

    public class ProcessTemplates extends DefaultTask {
        // ...
        private FileCollection sourceFiles = getProject().getLayout().files();

        @SkipWhenEmpty
        @InputFiles
        @PathSensitive(PathSensitivity.NONE)
        public FileCollection getSourceFiles() {
            return this.sourceFiles;
        }

        public void sources(FileCollection sourceFiles) {
            this.sourceFiles = this.sourceFiles.plus(sourceFiles);
        }

        // ...
    }

Output of `gradle processTemplates`

    > gradle processTemplates
    > Task :processTemplates

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

In other words, as long as you add values and files to formal task inputs and outputs during the configuration phase, they will be treated as such regardless from where in the build you add them.

If we want to support tasks as arguments as well and treat their outputs as the inputs, we can use the `project.layout.files()` method like so:

Example 35. Declaring a method to add a task as an input
build.gradle

    task copyTemplates(type: Copy) {
        into "$buildDir/tmp"
        from "src/templates"
    }

    task processTemplates2(type: ProcessTemplates) {
        // ...
        sources copyTemplates
    }

build.gradle.kts

    val copyTemplates by tasks.registering(Copy::class) {
        into("$buildDir/tmp")
        from("src/templates")
    }

    tasks.register<ProcessTemplates>("processTemplates2") {
        // ...
        sources(copyTemplates.get())
    }

ProcessTemplates.java

        // ...
        public void sources(Task inputTask) {
            this.sourceFiles = this.sourceFiles.plus(getProject().getLayout().files(inputTask));
        }
        // ...

Output of `gradle processTemplates2`

    > gradle processTemplates2
    > Task :copyTemplates
    > Task :processTemplates2

    BUILD SUCCESSFUL in 0s
    2 actionable tasks: 2 executed

This technique can make your custom task easier to use and result in cleaner build files. As an added benefit, our use of `getProject().getLayout().files()` means that our custom method can set up an inferred task dependency.

One last thing to note: if you are developing a task that takes collections of source files as inputs, like this example, consider using the built-in SourceTask. It will save you having to implement some of the plumbing that we put into `ProcessTemplates`.

#### Linking an `@OutputDirectory` to an `@InputFiles`

When you want to link the output of one task to the input of another, the types often match and a simple property assignment will provide that link. For example, a `File` output property can be assigned to a `File` input.

Unfortunately, this approach breaks down when you want the files in a task’s `@OutputDirectory` (of type `File`) to become the source for another task’s `@InputFiles` property (of type `FileCollection`). Since the two have different types, property assignment won’t work.

As an example, imagine you want to use the output of a Java compilation task - via the `destinationDir` property - as the input of a custom task that instruments a set of files containing Java bytecode. This custom task, which we’ll call `Instrument`, has a `classFiles` property annotated with `@InputFiles`. You might initially try to configure the task like so:

Example 36. Failed attempt at setting up an inferred task dependency
build.gradle

    plugins {
        id 'java'
    }

    task badInstrumentClasses(type: Instrument) {
        classFiles = fileTree(compileJava.destinationDir)
        destinationDir = file("$buildDir/instrumented")
    }

build.gradle.kts

    plugins {
        java
    }

    tasks.register<Instrument>("badInstrumentClasses") {
        classFiles = fileTree(tasks.compileJava.get().destinationDir)
        destinationDir = file("$buildDir/instrumented")
    }

Output of `gradle clean badInstrumentClasses`

    > gradle clean badInstrumentClasses
    > Task :clean UP-TO-DATE
    > Task :badInstrumentClasses NO-SOURCE

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 up-to-date

There’s nothing obviously wrong with this code, but you can see from the console output that the compilation task is missing. In this case you would need to add an explicit task dependency between `instrumentClasses` and `compileJava` via `dependsOn`. The use of `fileTree()` means that Gradle can’t infer the task dependency itself.

One solution is to use the `TaskOutputs.files` property, as demonstrated by the following example:

Example 37. Setting up an inferred task dependency between output dir and input files
build.gradle

    task instrumentClasses(type: Instrument) {
        classFiles = compileJava.outputs.files
        destinationDir = file("$buildDir/instrumented")
    }

build.gradle.kts

    tasks.register<Instrument>("instrumentClasses") {
        classFiles = tasks.compileJava.get().outputs.files
        destinationDir = file("$buildDir/instrumented")
    }

Output of `gradle clean instrumentClasses`

    > gradle clean instrumentClasses
    > Task :clean UP-TO-DATE
    > Task :compileJava
    > Task :instrumentClasses

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 2 executed, 1 up-to-date

Alternatively, you can get Gradle to access the appropriate property itself by using one of `project.files()`, `project.layout.files()` or `project.layout.configurableFiles()` in place of `project.fileTree()`:

Example 38. Setting up an inferred task dependency with `layout.files()`
build.gradle

    task instrumentClasses2(type: Instrument) {
        classFiles = layout.files(compileJava)
        destinationDir = file("$buildDir/instrumented")
    }

build.gradle.kts

    tasks.register<Instrument>("instrumentClasses2") {
        classFiles = layout.files(tasks.compileJava.get())
        destinationDir = file("$buildDir/instrumented")
    }

Output of `gradle clean instrumentClasses2`

    > gradle clean instrumentClasses2
    > Task :clean UP-TO-DATE
    > Task :compileJava
    > Task :instrumentClasses2

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 2 executed, 1 up-to-date

Remember that `files()`, `layout.files()` and `layout.configurableFiles()` can take tasks as arguments, whereas `fileTree()` cannot.

The downside of this approach is that all file outputs of the source task become the input files of the target - `instrumentClasses` in this case. That’s fine as long as the source task only has a single file-based output, like the `JavaCompile` task. But if you have to link just one output property among several, then you need to explicitly tell Gradle which task generates the input files using the `builtBy` method:

Example 39. Setting up an inferred task dependency with builtBy()
build.gradle

    task instrumentClassesBuiltBy(type: Instrument) {
        classFiles = fileTree(compileJava.destinationDir) {
            builtBy compileJava
        }
        destinationDir = file("$buildDir/instrumented")
    }

build.gradle.kts

    tasks.register<Instrument>("instrumentClassesBuiltBy") {
        classFiles = fileTree(tasks.compileJava.get().destinationDir) {
            builtBy(tasks.compileJava.get())
        }
        destinationDir = file("$buildDir/instrumented")
    }

Output of `gradle clean instrumentClassesBuiltBy`

    > gradle clean instrumentClassesBuiltBy
    > Task :clean UP-TO-DATE
    > Task :compileJava
    > Task :instrumentClassesBuiltBy

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 2 executed, 1 up-to-date

You can of course just add an explicit task dependency via `dependsOn`, but the above approach provides more semantic meaning, explaining why `compileJava` has to run beforehand.

#### Providing custom up-to-date logic

Gradle automatically handles up-to-date checks for output files and directories, but what if the task output is something else entirely? Perhaps it’s an update to a web service or a database table. Gradle has no way of knowing how to check whether the task is up to date in such cases.

That’s where the `upToDateWhen()` method on `TaskOutputs` comes in. This takes a predicate function that is used to determine whether a task is up to date or not. One use case is to disable up-to-date checks completely for a task, like so:

Example 40. Ignoring up-to-date checks
build.gradle

    task alwaysInstrumentClasses(type: Instrument) {
        classFiles = layout.files(compileJava)
        destinationDir = file("$buildDir/instrumented")
        outputs.upToDateWhen { false }
    }

build.gradle.kts

    tasks.register<Instrument>("alwaysInstrumentClasses") {
        classFiles = layout.files(tasks.compileJava.get())
        destinationDir = file("$buildDir/instrumented")
        outputs.upToDateWhen { false }
    }

Output of `gradle clean alwaysInstrumentClasses`

    > gradle clean alwaysInstrumentClasses
    > Task :compileJava
    > Task :alwaysInstrumentClasses

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 2 executed, 1 up-to-date

Output of `gradle alwaysInstrumentClasses`

    > gradle alwaysInstrumentClasses
    > Task :compileJava UP-TO-DATE
    > Task :alwaysInstrumentClasses

    BUILD SUCCESSFUL in 0s
    2 actionable tasks: 1 executed, 1 up-to-date

The `{ false }` closure ensures that `alwaysInstrumentClasses` will always be executed, irrespective of whether there is no change in the inputs or outputs.

You can of course put more complex logic into the closure. You could check whether a particular record in a database table exists or has changed for example. Just be aware that up-to-date checks should save you time. Don’t add checks that cost as much or more time than the standard execution of the task. In fact, if a task ends up running frequently anyway, because it’s rarely up to date, then it may not be worth having an up-to-date check at all. Remember that your checks will always run if the task is in the execution task graph.

One common mistake is to use `upToDateWhen()` instead of `Task.onlyIf()`. If you want to skip a task on the basis of some condition unrelated to the task inputs and outputs, then you should use `onlyIf()`. For example, in cases where you want to skip a task when a particular property is set or not set.

#### Configure input normalization

For up to date checks and the build cache Gradle needs to determine if two task input properties have the same value. In order to do so, Gradle first normalizes both inputs and then compares the result. For example, for a compile classpath, Gradle extracts the ABI signature from the classes on the classpath and then compares signatures between the last Gradle run and the current Gradle run as described in Java compile avoidance.

It is possible to customize Gradle’s built-in strategy for runtime classpath normalization. All inputs annotated with `@Classpath` are considered to be runtime classpaths.

Let’s say you want to add a file `build-info.properties` to all your produced jar files which contains information about the build, e.g. the timestamp when the build started or some ID to identify the CI job that published the artifact. This file is only for auditing purposes, and has no effect on the outcome of running tests. Nonetheless, this file is part of the runtime classpath for the `test` task and changes on every build invocation. Therefore, the `test` would be never up-to-date or pulled from the build cache. In order to benefit from incremental builds again, you are able tell Gradle to ignore this file on the runtime classpath at the project level by using Project.normalization(org.gradle.api.Action):

Example 41. Runtime classpath normalization
build.gradle

    normalization {
        runtimeClasspath {
            ignore 'build-info.properties'
        }
    }

build.gradle.kts

    normalization {
        runtimeClasspath {
            ignore("build-info.properties")
        }
    }

The effect of this configuration would be that changes to `build-info.properties` would be ignored for up-to-date checks and build cache key calculations. Note that this will not change the runtime behavior of the `test` task - i.e. any test is still able to load `build-info.properties` and the runtime classpath is still the same as before.

### Stale task outputs

When the Gradle version changes, Gradle detects that outputs from tasks that ran with older versions of Gradle need to be removed to ensure that the newest version of the tasks are starting from a known clean state.

✨ | Automatic clean-up of stale output directories has only been implemented for the output of source sets (Java/Groovy/Scala compilation).

## Task rules

Sometimes you want to have a task whose behavior depends on a large or infinite number value range of parameters. A very nice and expressive way to provide such tasks are task rules:

Example 42. Task rule
build.gradle

    tasks.addRule("Pattern: ping<ID>") { String taskName ->
        if (taskName.startsWith("ping")) {
            task(taskName) {
                doLast {
                    println "Pinging: " + (taskName - 'ping')
                }
            }
        }
    }

build.gradle.kts

    tasks.addRule("Pattern: ping<ID>") {
        val taskName = this
        if (startsWith("ping")) {
            task(taskName) {
                doLast {
                    println("Pinging: " + (taskName.replace("ping", "")))
                }
            }
        }
    }

Output of `gradle -q pingServer1`

    > gradle -q pingServer1
    Pinging: Server1

The String parameter is used as a description for the rule, which is shown with `gradle tasks`.

Rules are not only used when calling tasks from the command line. You can also create dependsOn relations on rule based tasks:

Example 43. Dependency on rule based tasks
build.gradle

    tasks.addRule("Pattern: ping<ID>") { String taskName ->
        if (taskName.startsWith("ping")) {
            task(taskName) {
                doLast {
                    println "Pinging: " + (taskName - 'ping')
                }
            }
        }
    }

    task groupPing {
        dependsOn pingServer1, pingServer2
    }

build.gradle.kts

    tasks.addRule("Pattern: ping<ID>") {
        val taskName = this
        if (startsWith("ping")) {
            task(taskName) {
                doLast {
                    println("Pinging: " + (taskName.replace("ping", "")))
                }
            }
        }
    }

    task("groupPing") {
        dependsOn("pingServer1", "pingServer2")
    }

Output of `gradle -q groupPing`

    > gradle -q groupPing
    Pinging: Server1
    Pinging: Server2

If you run “`gradle -q tasks`” you won’t find a task named “`pingServer1`” or “`pingServer2`”, but this script is executing logic based on the request to run those tasks.

## Finalizer tasks

Finalizer tasks are automatically added to the task graph when the finalized task is scheduled to run.

Example 44. Adding a task finalizer
build.gradle

    task taskX {
        doLast {
            println 'taskX'
        }
    }
    task taskY {
        doLast {
            println 'taskY'
        }
    }

    taskX.finalizedBy taskY

build.gradle.kts

    val taskX by tasks.registering {
        doLast {
            println("taskX")
        }
    }
    val taskY by tasks.registering {
        doLast {
            println("taskY")
        }
    }

    taskX { finalizedBy(taskY) }

Output of `gradle -q taskX`

    > gradle -q taskX
    taskX
    taskY

Finalizer tasks will be executed even if the finalized task fails.

Example 45. Task finalizer for a failing task
build.gradle

    task taskX {
        doLast {
            println 'taskX'
            throw new RuntimeException()
        }
    }
    task taskY {
        doLast {
            println 'taskY'
        }
    }

    taskX.finalizedBy taskY

build.gradle.kts

    val taskX by tasks.registering {
        doLast {
            println("taskX")
            throw RuntimeException()
        }
    }
    val taskY by tasks.registering {
        doLast {
            println("taskY")
        }
    }

    taskX { finalizedBy(taskY) }

Output of `gradle -q taskX`

    > gradle -q taskX
    taskX
    taskY

    FAILURE: Build failed with an exception.

    * Where:
    Build file '/home/user/gradle/samples/groovy/build.gradle' line: 4

    * What went wrong:
    Execution failed for task ':taskX'.
    > java.lang.RuntimeException (no error message)

    * Try:
    Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

    * Get more help at https://help.gradle.org

    BUILD FAILED in 0s

On the other hand, finalizer tasks are not executed if the finalized task didn’t do any work, for example if it is considered up to date or if a dependent task fails.

Finalizer tasks are useful in situations where the build creates a resource that has to be cleaned up regardless of the build failing or succeeding. An example of such a resource is a web container that is started before an integration test task and which should be always shut down, even if some of the tests fail.

To specify a finalizer task you use the Task.finalizedBy(java.lang.Object…​) method. This method accepts a task instance, a task name, or any other input accepted by Task.dependsOn(java.lang.Object…​).

## Lifecycle tasks

Lifecycle tasks are tasks that do not do work themselves. They typically do not have any task actions. Lifecycle tasks can represent several concepts:

- a work-flow step (e.g., run all checks with `check`)

- a buildable thing (e.g., create a debug 32-bit executable for native components with `debug32MainExecutable`)

- a convenience task to execute many of the same logical tasks (e.g., run all compilation tasks with `compileAll`)

Many Gradle plug-ins define their own lifecycle tasks to make it convenient to do specific things. When developing your own plugins, you should consider using your own lifecycle tasks or hooking into some of the tasks already provided by Gradle. See the Java plugin tasks for an example.

Unless a lifecycle task has actions, its outcome is determined by its dependencies. If any of the task’s dependencies are executed, the lifecycle task will be considered executed. If all of the task’s dependencies are up-to-date, skipped or from cache, the lifecycle task will be considered up-to-date.

## Summary

If you are coming from Ant, an enhanced Gradle task like Copy seems like a cross between an Ant target and an Ant task. Although Ant’s tasks and targets are really different entities, Gradle combines these notions into a single entity. Simple Gradle tasks are like Ant’s targets, but enhanced Gradle tasks also include aspects of Ant tasks. All of Gradle’s tasks share a common API and you can create dependencies between them. These tasks are much easier to configure than an Ant task. They make full use of the type system, and are more expressive and easier to maintain.

1. You might be wondering why there is neither an import for the `StopExecutionException` nor do we access it via its fully qualified name. The reason is, that Gradle adds a set of default imports to your script (see Default imports).