/Scala 2.12 Library

Package scala.concurrent

package concurrent

This package object contains primitives for concurrent and parallel programming.


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:

import scala.concurrent._

When using things like Futures, it is often required to have an implicit ExecutionContext in scope. The general advice for these implicits are as follows.

If the code in question is a class or method definition, and no ExecutionContext is available, request one from the caller by adding an implicit parameter list:

def myMethod(myParam: MyType)(implicit ec: ExecutionContext) = …
class MyClass(myParam: MyType)(implicit ec: ExecutionContext) { … }

This allows the caller of the method, or creator of the instance of the class, to decide which ExecutionContext should be used.

For typical REPL usage and experimentation, importing the global ExecutionContext is often desired.

import scala.concurrent.ExcutionContext.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")

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")
Linear Supertypes

Type Members

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.

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.

trait BlockContext extends AnyRef

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:
BlockContext.withBlockContext(myContext) {
  // then this block runs with myContext as the handler
  // for scala.concurrent.blocking

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. An implicit instance of this trait is only available when user code is currently calling the methods on Await.

@implicitNotFound( msg = ... )

type CancellationException = java.util.concurrent.CancellationException

class Channel[A] extends AnyRef

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


type of data exchanged

class DelayedLazyVal[T] extends AnyRef

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.



trait ExecutionContext extends AnyRef

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.

While it is possible to simply import scala.concurrent.ExecutionContext.Implicits.global to obtain an implicit ExecutionContext, application developers should carefully consider where they want to set execution 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 will mostly want to avoid hardcoding, especially via an import, scala.concurrent.ExecutionContext.Implicits.global. The recommended approach is to add (implicit ec: ExecutionContext) to methods, or class constructor parameters, which need an ExecutionContext.

Then locally import a specific ExecutionContext in one place for the entire application or module, passing it implicitly to individual methods. Alternatively define a local implicit val with the required ExecutionContext.

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.

@implicitNotFound( msg = ... )

trait ExecutionContextExecutor extends ExecutionContext with Executor

trait ExecutionContextExecutorService extends ExecutionContextExecutor with ExecutorService

type ExecutionException = java.util.concurrent.ExecutionException

trait Future[+T] extends Awaitable[T]

A Future represents a value which may or may not *currently* be available, but will be available at some point, or an exception if that value could not be made available.

Asynchronous computations that yield futures are created with the Future.apply call and are computed using a supplied ExecutionContext, which can be backed by a Thread pool.

import ExecutionContext.Implicits.global
val s = "Hello"
val f: Future[String] = Future {
  s + " future!"
f foreach {
  msg => println(msg)
See also

Futures and Promises

trait OnCompleteRunnable extends AnyRef

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.

trait Promise[T] extends AnyRef

class SyncChannel[A] extends AnyRef

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.



class SyncVar[A] extends AnyRef

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


type of the contained value

type TimeoutException = java.util.concurrent.TimeoutException

class Lock extends AnyRef

This class ...


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

Value Members

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. Properly marking blocking code may improve performance or avoid deadlocks.

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


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

@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

object Await

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 whenever possible— instead favoring combinators and/or callbacks. Await's result and ready methods will block the calling thread's execution until they return, which will cause performance degradation, and possibly, deadlock issues.

object BlockContext

object ExecutionContext

object Future

object JavaConversions

object Promise

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Licensed under the Apache License, Version 2.0.