Package org.gradle.api.provider

## Interface Provider<T>

- Type Parameters:
    `T` - Type of value represented by provider
  All Known Subinterfaces:
    `BinaryProvider<T>`, `DirectoryProperty`, `FileSystemLocationProperty<T>`, `ListProperty<T>`, `MapProperty<K,​V>`, `NamedDomainObjectProvider<T>`, `Property<T>`, `RegularFileProperty`, `SetProperty<T>`, `TaskProvider<T>`

      @NonExtensible
      public interface Provider<T>

  A container object that provides a value of a specific type. The value can be retrieved using one of the query methods such as `get()` or `getOrNull()`.

  A provider may not always have a value available, for example when the value may not yet be known but will be known at some point in the future. When a value is not available, `isPresent()` returns `false` and retrieving the value will fail with an exception.

  A provider may not always provide the same value. Although there are no methods on this interface to change the value, the provider implementation may be mutable or use values from some changing source. A provider may also provide a value that is mutable and that changes over time.

  A provider may represent a task output. Such a provider carries information about the task producing its value. When this provider is attached to an input of another task, Gradle will automatically determine the task dependencies based on this connection.

  A typical use of a provider is to pass values from one Gradle model element to another, e.g. from a project extension to a task, or between tasks. Providers also allow expensive computations to be deferred until their value is actually needed, usually at task execution time.

  There are a number of ways to create a `Provider` instance. Some common methods:

  - A number of Gradle types, such as `Property`, extend `Provider` and can be used directly as a provider.
  - Calling `map(Transformer)` to create a new provider from an existing provider.
  - Using the return value of `TaskContainer.register(String)`, which is a provider that represents the task instance.
  - Using the methods on `Directory` and `DirectoryProperty` to produce file providers.
  - By calling `ProviderFactory.provider(Callable)` or `Project.provider(Callable)` to create a new provider from a `Callable`.

  For a provider whose value can be mutated, see `Property` and the methods on `ObjectFactory`.

  Note: This interface is not intended for implementation by build script or plugin authors.

  Since:
    4.0

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `<S> Provider<S>` | `flatMap​(Transformer<? extends Provider<? extends S>,​? super T> transformer)` | Returns a new `Provider` from the value of this provider transformed using the given function.
    `Provider<T>` | `forUseAtConfigurationTime()` | Returns a view of this `Provider` which can be safely read at configuration time.
    `T` | `get()` | Returns the value of this provider if it has a value present, otherwise throws `java.lang.IllegalStateException`.
    `T` | `getOrElse​(T defaultValue)` | Returns the value of this provider if it has a value present.
    `T` | `getOrNull()` | Returns the value of this provider if it has a value present.
    `boolean` | `isPresent()` | Returns `true` if there is a value present, otherwise `false`.
    `<S> Provider<S>` | `map​(Transformer<? extends S,​? super T> transformer)` | Returns a new `Provider` whose value is the value of this provider transformed using the given function.
    `Provider<T>` | `orElse​(Provider<? extends T> provider)` | Returns a `Provider` whose value is the value of this provider, if present, otherwise uses the value from the given provider, if present.
    `Provider<T>` | `orElse​(T value)` | Returns a `Provider` whose value is the value of this provider, if present, otherwise the given default value.
    `<B,​R> Provider<R>` | `zip​(Provider<B> right, BiFunction<T,​B,​R> combiner)` | Returns a provider which value will be computed by combining this provider value with another provider value using the supplied combiner function.

- - ### Method Detail

    - #### get

          T get()

      Returns the value of this provider if it has a value present, otherwise throws `java.lang.IllegalStateException`.
      Returns:
        the current value of this provider.
      Throws:
        `IllegalStateException` - if there is no value present

    - #### getOrNull

          @Nullable
          T getOrNull()

      Returns the value of this provider if it has a value present. Returns `null` a value is not available.
      Returns:
        the value or `null`

    - #### getOrElse

          T getOrElse​(T defaultValue)

      Returns the value of this provider if it has a value present. Returns the given default value if a value is not available.
      Returns:
        the value or the default value.
      Since:
        4.3

    - #### map

          <S> Provider<S> map​(Transformer<? extends S,​? super T> transformer)

      Returns a new `Provider` whose value is the value of this provider transformed using the given function.

      The resulting provider will be live, so that each time it is queried, it queries the original (this) provider and applies the transformation to the result. Whenever the original provider has no value, the new provider will also have no value and the transformation will not be called.

      When this provider represents a task or the output of a task, the new provider will be considered an output of the task and will carry dependency information that Gradle can use to automatically attach task dependencies to tasks that use the new provider for input values.

      Parameters:
        `transformer` - The transformer to apply to values. May return `null`, in which case the provider will have no value.
      Since:
        4.3

    - #### flatMap

          <S> Provider<S> flatMap​(Transformer<? extends Provider<? extends S>,​? super T> transformer)

      Returns a new `Provider` from the value of this provider transformed using the given function.

      While very similar in functionality to the regular `map` operation, this method offers a convenient way of connecting together task inputs and outputs. (For a deeper understanding of the topic see the Lazy Configuration section of the Gradle manual.)

      Task inputs and outputs often take the form of `providers` or `properties`, the latter being a special case of provider whose value can be changed at will. An example of using `flatMap` for connecting such properties would be following:

           class Producer extends DefaultTask {
               @OutputFile
               abstract RegularFileProperty getOutputFile()

               //irrelevant details omitted
           }

           class Consumer extends DefaultTask {
               @InputFile
               abstract RegularFileProperty getInputFile()

               //irrelevant details omitted
           }

           def producer = tasks.register("producer", Producer)
           def consumer = tasks.register("consumer", Consumer)

           consumer.configure {
               inputFile = producer.flatMap { it.outputFile }
           }

      An added benefit of connecting input and output properties like this is that Gradle can automatically detect task dependencies based on such connections. To make this happen at code level, any task details associated with this provider (the one on which `flatMap` is being called) are ignored. The new provider will use whatever task details are associated with the return value of the transformation.

      The new provider returned by `flatMap` will be live, so that each time it is queried, it queries this provider and applies the transformation to the result. Whenever this provider has no value, the new provider will also have no value and the transformation will not be called.

      Parameters:
        `transformer` - The transformer to apply to values. May return `null`, in which case the provider will have no value.
      Since:
        5.0

    - #### isPresent

          boolean isPresent()

      Returns `true` if there is a value present, otherwise `false`.
      Returns:
        `true` if there is a value present, otherwise `false`

    - #### orElse

          Provider<T> orElse​(T value)

      Returns a `Provider` whose value is the value of this provider, if present, otherwise the given default value.
      Parameters:
        `value` - The default value to use when this provider has no value.
      Since:
        5.6

    - #### orElse

          Provider<T> orElse​(Provider<? extends T> provider)

      Returns a `Provider` whose value is the value of this provider, if present, otherwise uses the value from the given provider, if present.
      Parameters:
        `provider` - The provider whose value should be used when this provider has no value.
      Since:
        5.6

    - #### forUseAtConfigurationTime

          Provider<T> forUseAtConfigurationTime()

      Returns a view of this `Provider` which can be safely read at configuration time.
      Since:
        6.5

    - #### zip

          <B,​R> Provider<R> zip​(Provider<B> right,
                                      BiFunction<T,​B,​R> combiner)

      Returns a provider which value will be computed by combining this provider value with another provider value using the supplied combiner function.

      If the supplied providers represents a task or the output of a task, the resulting provider will carry the dependency information.

      Type Parameters:
        `B` - the type of the second provider
        `R` - the type of the result of the combiner
      Parameters:
        `right` - the second provider to combine with
        `combiner` - the combiner of values
      Returns:
        a combined provider
      Since:
        6.6