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
  • object ExecutionContext

    Contains factory methods for creating execution contexts.

    Contains factory methods for creating execution contexts.

    Definition Classes
    concurrent
  • Implicits
  • parasitic

object parasitic extends ExecutionContextExecutor with BatchingExecutor

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.

Source
ExecutionContext.scala
Linear Supertypes
Ordering
  1. Alphabetic
  2. By Inheritance
Inherited
  1. parasitic
  2. BatchingExecutor
  3. ExecutionContextExecutor
  4. Executor
  5. ExecutionContext
  6. AnyRef
  7. Any
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Visibility
  1. Public
  2. Protected

Value Members

  1. final def execute(runnable: Runnable): Unit

    Runs a block of code on this execution context.

    Runs a block of code on this execution context.

    runnable

    the task to execute

    Definition Classes
    parasitic → Executor → ExecutionContext
  2. final def reportFailure(t: Throwable): Unit

    Reports that an asynchronous computation failed.

    Reports that an asynchronous computation failed. See ExecutionContext.reportFailure(throwable: Throwable)

    Definition Classes
    parasitic → BatchingExecutor → ExecutionContext
  3. final def submitForExecution(runnable: Runnable): Unit

    MUST throw a NullPointerException when runnable is null When implementing a sync BatchingExecutor, it is RECOMMENDED to implement this method as runnable.run()

    MUST throw a NullPointerException when runnable is null When implementing a sync BatchingExecutor, it is RECOMMENDED to implement this method as runnable.run()

    Definition Classes
    parasitic → BatchingExecutor

Deprecated Value Members

  1. def prepare(): ExecutionContext

    Prepares for the execution of a task.

    Prepares for the execution of a task. Returns the prepared execution context. The recommended implementation of prepare is to return this.

    This method should no longer be overridden or called. It was originally expected that prepare would be called by all libraries that consume ExecutionContexts, in order to capture thread local context. However, this usage has proven difficult to implement in practice and instead it is now better to avoid using prepare entirely.

    Instead, if an ExecutionContext needs to capture thread local context, it should capture that context when it is constructed, so that it doesn't need any additional preparation later.

    Definition Classes
    ExecutionContext
    Annotations
    @deprecated
    Deprecated

    (Since version 2.12.0) preparation of ExecutionContexts will be removed