scala

# concurrent

#### package concurrent

This package object contains primitives for concurrent and parallel programming.

#### Guide

A more detailed guide to Futures and Promises, including discussion and examples can be found at http://docs.scala-lang.org/overviews/core/futures.html.

#### Common Imports

When working with Futures, you will often find that importing the whole concurrent package is convenient, furthermore you are likely to need an implicit ExecutionContext in scope for many operations involving Futures and Promises:

    import scala.concurrent._
    import ExecutionContext.Implicits.global

#### Specifying Durations

Operations often require a duration to be specified. A duration DSL is available to make defining these easier:

    import scala.concurrent.duration._
    val d: Duration = 10.seconds

#### Using Futures For Non-blocking Computation

Basic use of futures is easy with the factory method on Future, which executes a provided function asynchronously, handing you back a future result of that function without blocking the current thread. In order to create the Future you will need either an implicit or explicit ExecutionContext to be provided:

    import scala.concurrent._
    import ExecutionContext.Implicits.global  // implicit execution context

    val firstZebra: Future[Int] = Future {
      val source = scala.io.Source.fromFile("/etc/dictionaries-common/words")
      source.toSeq.indexOfSlice("zebra")
    }

#### Avoid Blocking

Although blocking is possible in order to await results (with a mandatory timeout duration):

    import scala.concurrent.duration._
    Await.result(firstZebra, 10.seconds)

and although this is sometimes necessary to do, in particular for testing purposes, blocking in general is discouraged when working with Futures and concurrency in order to avoid potential deadlocks and improve performance. Instead, use callbacks or combinators to remain in the future domain:

    val animalRange: Future[Int] = for {
      aardvark <- firstAardvark
      zebra <- firstZebra
    } yield zebra - aardvark

    animalRange.onSuccess {
      case x if x > 500000 => println("It's a long way from Aardvark to Zebra")
    }

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### Type Members

1. #### trait Awaitable[+T] extends AnyRef

   An object that may eventually be completed with a result value of type `T` which may be awaited using blocking methods.

   An object that may eventually be completed with a result value of type `T` which may be awaited using blocking methods.

   The Await object provides methods that allow accessing the result of an `Awaitable` by blocking the current thread until the `Awaitable` has been completed or a timeout has occurred.

2. #### trait BlockContext extends AnyRef

   A context to be notified by `scala.concurrent.blocking` when a thread is about to block.

   A context to be notified by `scala.concurrent.blocking` when a thread is about to block. In effect this trait provides the implementation for `scala.concurrent.Await`. `scala.concurrent.Await.result()` and `scala.concurrent.Await.ready()` locates an instance of `BlockContext` by first looking for one provided through `BlockContext.withBlockContext()` and failing that, checking whether `Thread.currentThread` is an instance of `BlockContext`. So a thread pool can have its `java.lang.Thread` instances implement `BlockContext`. There's a default `BlockContext` used if the thread doesn't implement `BlockContext`.

   Typically, you'll want to chain to the previous `BlockContext`, like this:

       val oldContext = BlockContext.current
       val myContext = new BlockContext {
         override def blockOn[T](thunk: =>T)(implicit permission: CanAwait): T = {
           // you'd have code here doing whatever you need to do
           // when the thread is about to block.
           // Then you'd chain to the previous context:
           oldContext.blockOn(thunk)
         }
       }
       BlockContext.withBlockContext(myContext) {
         // then this block runs with myContext as the handler
         // for scala.concurrent.blocking
       }

3. #### sealed trait CanAwait extends AnyRef

   This marker trait is used by Await to ensure that Awaitable.ready and Awaitable.result are not directly called by user code.

   This marker trait is used by Await to ensure that Awaitable.ready and Awaitable.result are not directly called by user code. An implicit instance of this trait is only available when user code is currently calling the methods on Await.

   Annotations
     @implicitNotFound( msg = ... )

4. #### type CancellationException = java.util.concurrent.CancellationException

5. #### class Channel[A] extends AnyRef

   This class provides a simple FIFO queue of data objects, which are read by one or more reader threads.

   This class provides a simple FIFO queue of data objects, which are read by one or more reader threads.

   A

     type of data exchanged

   Version

     1.0, 10/03/2003

6. #### class DelayedLazyVal[T] extends AnyRef

   A `DelayedLazyVal` is a wrapper for lengthy computations which have a valid partially computed result.

   A `DelayedLazyVal` is a wrapper for lengthy computations which have a valid partially computed result.

   The first argument is a function for obtaining the result at any given point in time, and the second is the lengthy computation. Once the computation is complete, the `apply` method will stop recalculating it and return a fixed value from that point forward.

   Version

     2.8

7. #### trait ExecutionContext extends AnyRef

   An `ExecutionContext` can execute program logic asynchronously, typically but not necessarily on a thread pool.

   An `ExecutionContext` can execute program logic asynchronously, typically but not necessarily on a thread pool.

   A general purpose `ExecutionContext` must be asynchronous in executing any `Runnable` that is passed into its `execute`-method. A special purpose `ExecutionContext` may be synchronous but must only be passed to code that is explicitly safe to be run using a synchronously executing `ExecutionContext`.

   APIs such as `Future.onComplete` require you to provide a callback and an implicit `ExecutionContext`. The implicit `ExecutionContext` will be used to execute the callback.

   It is possible to simply import `scala.concurrent.ExecutionContext.Implicits.global` to obtain an implicit `ExecutionContext`. This global context is a reasonable default thread pool.

   However, application developers should carefully consider where they want to set policy; ideally, one place per application (or per logically-related section of code) will make a decision about which `ExecutionContext` to use. That is, you might want to avoid hardcoding `scala.concurrent.ExecutionContext.Implicits.global` all over the place in your code. One approach is to add `(implicit ec: ExecutionContext)` to methods which need an `ExecutionContext`. Then import a specific context in one place for the entire application or module, passing it implicitly to individual methods.

   A custom `ExecutionContext` may be appropriate to execute code which blocks on IO or performs long-running computations. `ExecutionContext.fromExecutorService` and `ExecutionContext.fromExecutor` are good ways to create a custom `ExecutionContext`.

   The intent of `ExecutionContext` is to lexically scope code execution. That is, each method, class, file, package, or application determines how to run its own code. This avoids issues such as running application callbacks on a thread pool belonging to a networking library. The size of a networking library's thread pool can be safely configured, knowing that only that library's network operations will be affected. Application callback execution can be configured separately.

   Annotations
     @implicitNotFound( msg = ... )

8. #### trait ExecutionContextExecutor extends ExecutionContext with Executor

   An ExecutionContext that is also a Java Executor.

9. #### trait ExecutionContextExecutorService extends ExecutionContextExecutor with ExecutorService

   An ExecutionContext that is also a Java ExecutorService.

10. #### type ExecutionException = java.util.concurrent.ExecutionException

11. #### trait Future[+T] extends Awaitable[T]

    The trait that represents futures.

    The trait that represents futures.

    Asynchronous computations that yield futures are created with the `Future` call:

        val s = "Hello"
        val f: Future[String] = Future {
          s + " future!"
        }
        f onSuccess {
          case msg => println(msg)
        }

    See also

      Futures and Promises

12. #### trait OnCompleteRunnable extends AnyRef

    A marker indicating that a `java.lang.Runnable` provided to `scala.concurrent.ExecutionContext` wraps a callback provided to `Future.onComplete`.

    A marker indicating that a `java.lang.Runnable` provided to `scala.concurrent.ExecutionContext` wraps a callback provided to `Future.onComplete`. All callbacks provided to a `Future` end up going through `onComplete`, so this allows an `ExecutionContext` to special-case callbacks that were executed by `Future` if desired.

13. #### trait Promise[T] extends AnyRef

    Promise is an object which can be completed with a value or failed with an exception.

14. #### class SyncChannel[A] extends AnyRef

    A `SyncChannel` allows one to exchange data synchronously between a reader and a writer thread.

    A `SyncChannel` allows one to exchange data synchronously between a reader and a writer thread. The writer thread is blocked until the data to be written has been read by a corresponding reader thread.

    Version

      2.0, 04/17/2008

15. #### class SyncVar[A] extends AnyRef

    A class to provide safe concurrent access to a mutable cell.

    A class to provide safe concurrent access to a mutable cell. All methods are synchronized.

    A

      type of the contained value

    Version

      1.0, 10/03/2003

16. #### type TimeoutException = java.util.concurrent.TimeoutException

17. #### class Lock extends AnyRef

    This class ...

    This class ...

    Annotations
      @deprecated
    Deprecated

      (Since version 2.11.2) Use java.util.concurrent.locks.Lock

    Version

      1.0, 10/03/2003

### Value Members

1. #### object Await

   `Await` is what is used to ensure proper handling of blocking for `Awaitable` instances.

   `Await` is what is used to ensure proper handling of blocking for `Awaitable` instances.

   While occasionally useful, e.g. for testing, it is recommended that you avoid Await when possible in favor of callbacks and combinators like onComplete and use in for comprehensions. Await will block the thread on which it runs, and could cause performance and deadlock issues.

2. #### object BlockContext

3. #### object ExecutionContext

   Contains factory methods for creating execution contexts.

4. #### object Future

   Future companion object.

5. #### object JavaConversions

   The `JavaConversions` object provides implicit conversions supporting interoperability between Scala and Java concurrency classes.

6. #### object Promise

7. #### def blocking[T](body: ⇒ T): T

   Used to designate a piece of code which potentially blocks, allowing the current BlockContext to adjust the runtime's behavior.

   Used to designate a piece of code which potentially blocks, allowing the current BlockContext to adjust the runtime's behavior. Properly marking blocking code may improve performance or avoid deadlocks.

   Blocking on an Awaitable should be done using Await.result instead of `blocking`.

   body

     A piece of code which contains potentially blocking or long running calls.

   Annotations
     @throws( clazz = classOf[Exception] )
   Exceptions thrown

     `CancellationException` if the computation was cancelled

     `InterruptedException` in the case that a wait within the blocking `body` was interrupted

8. #### package duration

### Deprecated Value Members

1. #### def future[T](body: ⇒ T)(implicit executor: ExecutionContext): Future[T]

   Starts an asynchronous computation and returns a `Future` object with the result of that computation.

   Starts an asynchronous computation and returns a `Future` object with the result of that computation.

   The result becomes available once the asynchronous computation is completed.

   T

     the type of the result

   body

     the asynchronous computation

   executor

     the execution context on which the future is run

   returns

     the `Future` holding the result of the computation

   Annotations
     @deprecated
   Deprecated

     (Since version 2.11.0) Use `Future { ... }` instead.

2. #### def promise[T](): Promise[T]

   Creates a promise object which can be completed with a value or an exception.

   Creates a promise object which can be completed with a value or an exception.

   T

     the type of the value in the promise

   returns

     the newly created `Promise` object

   Annotations
     @deprecated
   Deprecated

     (Since version 2.11.0) Use `Promise[T]()` instead.

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