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 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) = …
//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 def !=(arg0: Any): Boolean

    Test two objects for inequality.

    Test two objects for inequality.

    returns

    true if !(this == that), false otherwise.

    Definition Classes
    AnyRef → Any
  2. final def ##: Int

    Equivalent to x.hashCode except for boxed numeric types and null.

    Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  3. final def ==(arg0: Any): Boolean

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  4. final def asInstanceOf[T0]: T0

    Cast the receiver object to be of type T0.

    Cast the receiver object to be of type T0.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

    returns

    the receiver object.

    Definition Classes
    Any
    Exceptions thrown

    ClassCastException if the receiver object is not an instance of the erasure of type T0.

  5. def clone(): AnyRef

    Create a copy of the receiver object.

    Create a copy of the receiver object.

    The default implementation of the clone method is platform dependent.

    returns

    a copy of the receiver object.

    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.CloneNotSupportedException]) @native()
    Note

    not specified by SLS as a member of AnyRef

  6. 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

  7. final def eq(arg0: AnyRef): Boolean

    Tests whether the argument (that) is a reference to the receiver object (this).

    Tests whether the argument (that) is a reference to the receiver object (this).

    The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

    • It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
    • For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
    • null.eq(null) returns true.

    When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

    returns

    true if the argument is a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  8. def equals(arg0: AnyRef): Boolean

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to eq.

    See also equals in scala.Any.

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  9. def finalize(): Unit

    Called by the garbage collector on the receiver object when there are no more references to the object.

    Called by the garbage collector on the receiver object when there are no more references to the object.

    The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.Throwable])
    Note

    not specified by SLS as a member of AnyRef

  10. 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

  11. 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

  12. 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

  13. 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

  14. final def getClass(): Class[_ <: AnyRef]

    Returns the runtime class representation of the object.

    Returns the runtime class representation of the object.

    returns

    a class object corresponding to the runtime type of the receiver.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  15. 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

  16. def hashCode(): Int

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  17. final def isInstanceOf[T0]: Boolean

    Test whether the dynamic type of the receiver object is T0.

    Test whether the dynamic type of the receiver object is T0.

    Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

    returns

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
  18. final def ne(arg0: AnyRef): Boolean

    Equivalent to !(this eq that).

    Equivalent to !(this eq that).

    returns

    true if the argument is not a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  19. final def notify(): Unit

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  20. final def notifyAll(): Unit

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  21. final def synchronized[T0](arg0: => T0): T0

    Executes the code in body with an exclusive lock on this.

    Executes the code in body with an exclusive lock on this.

    returns

    the result of body

    Definition Classes
    AnyRef
  22. def toString(): String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    AnyRef → Any
  23. final def wait(): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.

    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
    Note

    not specified by SLS as a member of AnyRef

  24. final def wait(arg0: Long, arg1: Int): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-

    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
    Note

    not specified by SLS as a member of AnyRef

  25. final def wait(arg0: Long): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.

    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException]) @native()
    Note

    not specified by SLS as a member of AnyRef

  26. object Implicits
  27. 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.

Inherited from AnyRef

Inherited from Any

Ungrouped