Packages

  • package root

    This is the documentation for the Scala standard library.

    This is the documentation for the Scala standard library.

    Package structure

    The scala package contains core types like Int, Float, Array or Option which are accessible in all Scala compilation units without explicit qualification or imports.

    Notable packages include:

    Other packages exist. See the complete list on the right.

    Additional parts of the standard library are shipped as separate libraries. These include:

    Automatic imports

    Identifiers in the scala package and the scala.Predef object are always in scope by default.

    Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example, List is an alias for scala.collection.immutable.List.

    Other aliases refer to classes provided by the underlying platform. For example, on the JVM, String is an alias for java.lang.String.

    Definition Classes
    root
  • package scala

    Core Scala types.

    Core Scala types. They are always available without an explicit import.

    Definition Classes
    root
  • package concurrent

    This package object contains primitives for concurrent and parallel programming.

    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 https://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) = …
//Or
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 words = Files.readAllLines("/etc/dictionaries-common/words").asScala
  words.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")
}
    Definition Classes
    scala
  • package duration
    Definition Classes
    concurrent
  • Await
  • Awaitable
  • Batchable
  • BlockContext
  • CanAwait
  • Channel
  • DelayedLazyVal
  • ExecutionContext
  • ExecutionContextExecutor
  • ExecutionContextExecutorService
  • Future
  • JavaConversions
  • OnCompleteRunnable
  • Promise
  • SyncChannel
  • SyncVar

object ExecutionContext

Contains factory methods for creating execution contexts.

Source
ExecutionContext.scala
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Value Members

  1. final val defaultReporter: (Throwable) => Unit

    The default reporter simply prints the stack trace of the Throwable to System.err.

    The default reporter simply prints the stack trace of the Throwable to System.err.

    returns

    the function for error reporting

  2. def fromExecutor(e: Executor): ExecutionContextExecutor

    Creates an ExecutionContext from the given Executor with the default reporter.

    Creates an ExecutionContext from the given Executor with the default reporter.

    e

    the Executor to use. If null, a new Executor is created with default configuration.

    returns

    the ExecutionContext using the given Executor

  3. def fromExecutor(e: Executor, reporter: (Throwable) => Unit): ExecutionContextExecutor

    Creates an ExecutionContext from the given Executor.

    Creates an ExecutionContext from the given Executor.

    e

    the Executor to use. If null, a new Executor is created with default configuration.

    reporter

    a function for error reporting

    returns

    the ExecutionContext using the given Executor

  4. def fromExecutorService(e: ExecutorService): ExecutionContextExecutorService

    Creates an ExecutionContext from the given ExecutorService with the default reporter.

    Creates an ExecutionContext from the given ExecutorService with the default reporter.

    If it is guaranteed that none of the executed tasks are blocking, a single-threaded ExecutorService can be used to create an ExecutionContext as follows:

    import java.util.concurrent.Executors
val ec = ExecutionContext.fromExecutorService(Executors.newSingleThreadExecutor())
    e

    the ExecutorService to use. If null, a new ExecutorService is created with default configuration.

    returns

    the ExecutionContext using the given ExecutorService

  5. def fromExecutorService(e: ExecutorService, reporter: (Throwable) => Unit): ExecutionContextExecutorService

    Creates an ExecutionContext from the given ExecutorService.

    Creates an ExecutionContext from the given ExecutorService.

    e

    the ExecutorService to use. If null, a new ExecutorService is created with default configuration.

    reporter

    a function for error reporting

    returns

    the ExecutionContext using the given ExecutorService

  6. final lazy val global: ExecutionContextExecutor

    The global ExecutionContext.

    The global ExecutionContext. This default ExecutionContext implementation is backed by a work-stealing thread pool. It can be configured via the following system properties:

    • scala.concurrent.context.minThreads = defaults to "1"
    • scala.concurrent.context.numThreads = defaults to "x1" (i.e. the current number of available processors * 1)
    • scala.concurrent.context.maxThreads = defaults to "x1" (i.e. the current number of available processors * 1)
    • scala.concurrent.context.maxExtraThreads = defaults to "256"

    The pool size of threads is then numThreads bounded by minThreads on the lower end and maxThreads on the high end.

    The maxExtraThreads is the maximum number of extra threads to have at any given time to evade deadlock, see scala.concurrent.blocking.

    The global execution context can be used explicitly, by defining an implicit val ec: scala.concurrent.ExecutionContext = scala.concurrent.ExecutionContext.global, or by importing ExecutionContext.Implicits.global.

    Batching short-lived nested tasks

    Asynchronous code with short-lived nested tasks is executed more efficiently when using ExecutionContext.opportunistic (continue reading to learn why it is private[scala] and how to access it).

    ExecutionContext.opportunistic uses the same thread pool as ExecutionContext.global. It attempts to batch nested task and execute them on the same thread as the enclosing task. This is ideally suited to execute short-lived tasks as it reduces the overhead of context switching.

    WARNING: long-running and/or blocking tasks should be demarcated within scala.concurrent.blocking-blocks to ensure that any pending tasks in the current batch can be executed by another thread on global.

    How to use

    This field is private[scala] to maintain binary compatibility. It was added in 2.13.4, code that references it directly fails to run with a 2.13.0-3 Scala library.

    Libraries should not reference this field directly because users of the library might be using an earlier Scala version. In order to use the batching ExecutionContext in a library, the code needs to fall back to global in case the opportunistic field is missing (example below). The resulting ExecutionContext has batching behavior in all Scala 2.13 versions (global is batching in 2.13.0-3).

    implicit val ec: scala.concurrent.ExecutionContext = try {
  scala.concurrent.ExecutionContext.getClass
    .getDeclaredMethod("opportunistic")
    .invoke(scala.concurrent.ExecutionContext)
    .asInstanceOf[scala.concurrent.ExecutionContext]
} catch {
  case _: NoSuchMethodException =>
    scala.concurrent.ExecutionContext.global
}

    Application authors can safely use the field because the Scala version at run time is the same as at compile time. Options to bypass the access restriction include:

    1. Using a structural type (example below). This uses reflection at run time.
    2. Writing a Scala object in the scala package (example below).
    3. Writing a Java source file. This works because private[scala] is emitted as public in Java bytecode.
    // Option 1
implicit val ec: scala.concurrent.ExecutionContext =
  (scala.concurrent.ExecutionContext:
    {def opportunistic: scala.concurrent.ExecutionContextExecutor}
  ).opportunistic

// Option 2
package scala {
  object OpportunisticEC {
    implicit val ec: scala.concurrent.ExecutionContext =
      scala.concurrent.ExecutionContext.opportunistic
  }
}
    returns

    the global ExecutionContext

  7. object Implicits
  8. object parasitic extends ExecutionContextExecutor with BatchingExecutor

    WARNING: Only ever execute logic which will quickly return control to the caller.

    WARNING: Only ever execute logic which will quickly return control to the caller.

    This ExecutionContext steals execution time from other threads by having its Runnables run on the Thread which calls execute and then yielding back control to the caller after *all* its Runnables have been executed. Nested invocations of execute will be trampolined to prevent uncontrolled stack space growth.

    When using parasitic with abstractions such as Future it will in many cases be non-deterministic as to which Thread will be executing the logic, as it depends on when/if that Future is completed.

    Do *not* call any blocking code in the Runnables submitted to this ExecutionContext as it will prevent progress by other enqueued Runnables and the calling Thread.

    Symptoms of misuse of this ExecutionContext include, but are not limited to, deadlocks and severe performance problems.

    Any NonFatal or InterruptedExceptions will be reported to the defaultReporter.