# Lazy Configuration

As a build grows in complexity, knowing when and where a particular value is configured can become difficult to reason about. Gradle provides several ways to manage this complexity using lazy configuration.

## Lazy properties

The Provider API is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

Gradle provides lazy properties, which delay the calculation of a property’s value until it’s absolutely required. Lazy types are faster, more understandable and better instrumented than the internal convention mapping mechanisms. This provides two main benefits to build script and plugin authors:

1. Build authors can wire together Gradle models without worrying when a particular property’s value will be known. For example, when you want to map properties in an extension to task properties but the values aren’t known until the build script configures them.

2. Build authors can avoid resource intensive work during the configuration phase, which can have a direct impact on maximum build performance. For example, when a property value comes from parsing a file.

Gradle represents lazy properties with two interfaces:

- `Provider` are properties that can only be queried and cannot be changed.

  - Properties with these types are read-only.

  - The method `Provider.get()` returns the current value of the property.

  - A `Provider` can be created by the factory method `ProviderFactory.provider(java.util.concurrent.Callable)`.

- `Property` are properties that can be queried and overwritten.

  - Properties with these types are configurable.

  - `Property` implements the `Provider` interface.

  - The method `Property.set(T)` specifies a value for the property, overwriting whatever value may have been present.

  - The method `Property.set(org.gradle.api.provider.Provider)` specifies a `Provider` for the value for the property, overwriting whatever value may have been present. This allows you to wire together `Provider` and `Property` instances before the values are configured.

  - A `Property` can be created by the factory method `ObjectFactory.property(java.lang.Class)`.

Neither of these types nor their subtypes are intended to be implemented by a build script or plugin author. Gradle provides several factory methods to create instances of these types. See the Quick Reference for all of the types and factories available.

Lazy properties are intended to be passed around and only evaluated when required (usually, during the execution phase). For more information about the Gradle build phases, please see the section called “Build phases”.

The following demonstrates a task with a read-only property and a configurable property:

Example: Using a read-only and configurable property

`build.gradle`

    class Greeting extends DefaultTask {
        // Configurable by the user
        @Input
        final Property<String> message = project.objects.property(String)

        // Read-only property calculated from the message
        @Internal
        final Provider<String> fullMessage = message.map { it + " from Gradle" }

        @TaskAction
        void printMessage() {
            logger.quiet(fullMessage.get())
        }
    }

    task greeting(type: Greeting) {
        // Note that this is effectively calling Property.set()
        message = 'Hi'
    }

Output of `gradle greeting`

    > gradle greeting
    :greeting
    Hi from Gradle

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

The `Greeting` task has a `Property<String>` for the mutable part of the message and a `Provider<String>` for the calculated, read-only, message.

Note that Groovy Gradle DSL will generate setter methods for each `Property`-typed property in a task implementation. These setter methods allow you to configure the property using the assignment (`=`) operator as a convenience.

## Creating a Property or Provider

If provider types are not intended to be implemented directly by build script or plugin authors, how do you create a new one? Gradle provides various factory APIs to create new instances of both `Provider` and `Property`:

- `ProviderFactory.provider(java.util.concurrent.Callable)` instantiates a new `Provider`. An instance of the `ProviderFactory` can be referenced from `Project.getProviders()` or by injecting `ProviderFactory` through a constructor or method.

- `ObjectFactory.property(java.lang.Class)` instantiates a new `Property`. An instance of the `ObjectFactory` can be referenced from `Project.getObjects()` or by injecting `ObjectFactory` through a constructor or method.

`Project` does not provide a specific method signature for creating a provider from a `groovy.lang.Closure`. When writing a plugin with Groovy, you can use the method signature accepting a `java.util.concurrent.Callable` parameter. Groovy’s Closure to type coercion will take care of the rest.

## Working with files and Providers

In Working With Files, we introduced four collection types for `File`-like objects:

Collection of files recap

Read-only Type | Configurable Type
`FileCollection` | `ConfigurableFileCollection`
`FileTree` | `ConfigurableFileTree`

All of these types are also considered `Provider` types.

In this section, we are going to introduce more strongly typed models for a `FileSystemLocation`: `Directory` and `RegularFile`. These types shouldn’t be confused with the standard Java java.io.File type as they tell Gradle to expect more specific values (a directory or a non-directory, regular file).

Gradle provides two specialized `Property` subtypes for dealing with these types: `RegularFileProperty` and `DirectoryProperty`. `ProjectLayout` has methods to create these: `ProjectLayout.fileProperty()` and `ProjectLayout.directoryProperty()`.

A `DirectoryProperty` can also be used to create a lazily evaluated `Provider` for a `Directory` and `RegularFile` via `DirectoryProperty.dir(java.lang.String)` and `DirectoryProperty.file(java.lang.String)` respectively. These methods create paths that are relative to the location set for the original `DirectoryProperty`.

Example: Using file and directory property

`build.gradle`

    class FooExtension {
        final DirectoryProperty someDirectory
        final RegularFileProperty someFile
        final ConfigurableFileCollection someFiles

        FooExtension(Project project) {
            someDirectory = project.layout.directoryProperty()
            someFile = project.layout.fileProperty()
            someFiles = project.files()
        }
    }

    project.extensions.create('foo', FooExtension, project)

    foo {
        someDirectory = project.layout.projectDirectory.dir('some-directory')
        someFile = project.layout.buildDirectory.file('some-file')
        someFiles.from project.files(someDirectory, someFile)
    }

    task print {
        doLast {
            def someDirectory = project.foo.someDirectory.get().asFile
            logger.quiet("foo.someDirectory = " + someDirectory)
            logger.quiet("foo.someFiles contains someDirectory? " + project.foo.someFiles.contains(someDirectory))

            def someFile = project.foo.someFile.get().asFile
            logger.quiet("foo.someFile = " + someFile)
            logger.quiet("foo.someFiles contains someFile? " + project.foo.someFiles.contains(someFile))
        }
    }

Output of `gradle print`

    > gradle print
    :print
    foo.someDirectory = /home/user/gradle/samples/providers/fileAndDirectoryProperty/some-directory
    foo.someFiles contains someDirectory? true
    foo.someFile = /home/user/gradle/samples/providers/fileAndDirectoryProperty/build/some-file
    foo.someFiles contains someFile? true

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

This example shows how `Provider` types can be used inside an extension. Lazy values for `Project.getBuildDir()` and `Project.getProjectDir()` can be accessed through `Project.getLayout()` with `ProjectLayout.getBuildDirectory()` and `ProjectLayout.getProjectDirectory()`.

## Working with task dependencies and Providers

Many builds have several tasks that depend on each other. This usually means that one task processes the outputs of another task as an input. For these outputs and inputs, we need to know their locations on the file system and appropriately configure each task to know where to look. This can be cumbersome if any of these values are configurable by a user or configured by multiple plugins.

To make this easier, Gradle offers convenient APIs for defining files or directories as task inputs and outputs in a descriptive way. As an example consider the following plugin with a producer and consumer task, which are wired together via inputs and outputs:

Example: Implicit task dependency

`build.gradle`

    class Producer extends DefaultTask {
        @OutputFile
        final RegularFileProperty outputFile = newOutputFile()

        @TaskAction
        void produce() {
            String message = 'Hello, World!'
            def output = outputFile.get().asFile
            output.text = message
            logger.quiet("Wrote '${message}' to ${output}")
        }
    }

    class Consumer extends DefaultTask {
        @InputFile
        final RegularFileProperty inputFile = newInputFile()

        @TaskAction
        void consume() {
            def input = inputFile.get().asFile
            def message = input.text
            logger.quiet("Read '${message}' from ${input}")
        }
    }

    task producer(type: Producer)
    task consumer(type: Consumer)

    // Wire property from producer to consumer task
    consumer.inputFile = producer.outputFile

    // Set values for the producer lazily
    // Note that the consumer does not need to be changed again.
    producer.outputFile = layout.buildDirectory.file('file.txt')

    // Change the base output directory.
    // Note that this automatically changes producer.outputFile and consumer.inputFile
    buildDir = 'output'

Output of `gradle consumer`

    > gradle consumer
    :producer
    Wrote 'Hello, World!' to /home/user/gradle/samples/providers/implicitTaskDependency/output/file.txt
    :consumer
    Read 'Hello, World!' from /home/user/gradle/samples/providers/implicitTaskDependency/output/file.txt

    BUILD SUCCESSFUL in 0s
    2 actionable tasks: 2 executed

In the example above, the task outputs and inputs are connected before any location is defined. This is possible because the input and output properties use the `Provider` API. The output property is created with `DefaultTask.newOutputFile()` and the input property is created with `DefaultTask.newInputFile()`. Values are only resolved when they are needed during execution. The setters can be called at any time before the task is executed and the change will automatically affect all related input and output properties.

Another thing to note is the absence of any explicit task dependency. Properties created via `newOutputFile()` and `newOutputDirectory()` bring knowledge about which task is generating them, so using them as task input will implicitly link tasks together.

## Working with collection Providers

In this section, we are going to explore lazy collections. They work exactly like any other `Provider` and, just like `FileSystemLocation` providers, they have additional modeling around them. There are two provider interfaces available, one for `List` values and another for `Set` values:

- For `List` values the interface is called `ListProperty`. You can create a new `ListProperty` using `ObjectFactory.listProperty(java.lang.Class)` and specifying the element’s type.

- For `Set` values the interface is called `SetProperty`. You can create a new `SetProperty` using `ObjectFactory.setProperty(java.lang.Class)` and specifying the element’s type.

This type of property allows you to overwrite the entire collection value with `HasMultipleValues.set(java.lang.Iterable)` and `HasMultipleValues.set(org.gradle.api.provider.Provider)` or add new elements through the various `add` methods:

- `HasMultipleValues.add(T)`: Add a single concrete element to the collection

- `HasMultipleValues.add(org.gradle.api.provider.Provider)`: Add a lazily evaluated element to the collection

- `HasMultipleValues.addAll(org.gradle.api.provider.Provider)`: Add a lazily evaluated collection of elements to the list

Just like every `Provider`, the collection is calculated when `Provider.get()` is called. The following example show the `ListProperty` in action:

Example: List property

`build.gradle`

    task print {
        doLast {
            ListProperty<String> list = project.objects.listProperty(String)

            // Resolve the list
            logger.quiet('The list contains: ' + list.get())

            // Add elements to the empty list
            list.add(project.provider { 'element-1' })  // Add a provider element
            list.add('element-2')                       // Add a concrete element

            // Resolve the list
            logger.quiet('The list contains: ' + list.get())

            // Overwrite the entire list with a new list
            list.set(['element-3', 'element-4'])

            // Resolve the list
            logger.quiet('The list contains: ' + list.get())

            // Add more elements through a list provider
            list.addAll(project.provider { ['element-5', 'element-6'] })

            // Resolve the list
            logger.quiet('The list contains: ' + list.get())
        }
    }

Output of `gradle print`

    > gradle print
    :print
    The list contains: []
    The list contains: [element-1, element-2]
    The list contains: [element-3, element-4]
    The list contains: [element-3, element-4, element-5, element-6]

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

## Guidelines

This section will introduce guidelines to be successful with the Provider API. To see those guidelines in action, have a look at gradle-site-plugin, a Gradle plugin demonstrating established techniques and practices for plugin development.

- The `Property` and `Provider` types have all of the overloads you need to query or configure a value. For this reason, you should follow the following guidelines:

  - For configurable properties, expose the `Property` directly through a single getter.

  - For non-configurable properties, expose an `Provider` directly through a single getter.

- Avoid simplifying calls like `obj.getProperty().get()` and `obj.getProperty().set(T)` in your code by introducing additional getters and setters.

- When migrating your plugin to use providers, follow these guidelines:

  - If it’s a new property, expose it as a `Property` or `Provider` using a single getter.

  - If it’s incubating, change it to use a `Property` or `Provider` using a single getter.

  - If it’s a stable property, add a new `Property` or `Provider` and deprecate the old one. You should wire the old getter/setters into the new property as appropriate.

## Future development

Going forward, new properties will use the Provider API. The Groovy Gradle DSL adds convenience methods to make the use of Providers mostly transparent in build scripts. Existing tasks will have their existing "raw" properties replaced by Providers as needed and in a backwards compatible way. New tasks will be designed with the Provider API.

The Provider API is incubating. Please create new issues at gradle/gradle to report bugs or to submit use cases for new features.

## Provider API Quick Reference

Lazy properties summary

Description | Read-only | Configurable | Factory
A file on disk | `Provider`<`RegularFile`> | `RegularFileProperty` | `ProjectLayout.fileProperty()` `Directory.file(java.lang.String)` `DirectoryProperty.file(java.lang.String)`
A file used as a task input/output | `Provider`<`RegularFile`> | `RegularFileProperty` | `DefaultTask.newInputFile()` `DefaultTask.newOutputFile()`
A directory on disk | `Provider`<`Directory`> | `DirectoryProperty` | `ProjectLayout.directoryProperty()` `Directory.dir(java.lang.String)` `DirectoryProperty.dir(java.lang.String)`
A directory used as a task input/output | `Provider`<`Directory`> | `DirectoryProperty` | `DefaultTask.newInputDirectory()` `DefaultTask.newOutputDirectory()`
Collection of files | `FileCollection` | `ConfigurableFileCollection` | `Project.files(java.lang.Object[])`
Hierarchy of files | `FileTree` | `ConfigurableFileTree` | `Project.fileTree(java.lang.Object)`
List of any type | `Provider`<List<T>> | `ListProperty` | `ObjectFactory.listProperty(java.lang.Class)`
Set of any type | `Provider`<Set<T>> | `SetProperty` | `ObjectFactory.setProperty(java.lang.Class)`
Any other type | `Provider`<T> | `Property`<T> | `ObjectFactory.property(java.lang.Class)`