Scala example source code file (Future.scala)

This example Scala source code file (Future.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

collection, concurrent, executioncontext, failure, future, generics, mutable, promise, r, reflection, success, t, try, u, unit, utilities

The Future.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.concurrent

import scala.language.higherKinds

import java.util.concurrent.{ ConcurrentLinkedQueue, TimeUnit, Callable }
import java.util.concurrent.TimeUnit.{ NANOSECONDS => NANOS, MILLISECONDS ⇒ MILLIS }
import java.lang.{ Iterable => JIterable }
import java.util.{ LinkedList => JLinkedList }
import java.util.concurrent.atomic.{ AtomicReferenceFieldUpdater, AtomicInteger, AtomicLong, AtomicBoolean }

import scala.util.control.NonFatal
import scala.Option
import scala.util.{Try, Success, Failure}

import scala.annotation.tailrec
import scala.collection.mutable.Builder
import scala.collection.generic.CanBuildFrom
import scala.reflect.ClassTag



/** 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)
 *  }
 *  }}}
 *
 *  @author  Philipp Haller, Heather Miller, Aleksandar Prokopec, Viktor Klang
 *
 *  @define multipleCallbacks
 *  Multiple callbacks may be registered; there is no guarantee that they will be
 *  executed in a particular order.
 *
 *  @define caughtThrowables
 *  The future may contain a throwable object and this means that the future failed.
 *  Futures obtained through combinators have the same exception as the future they were obtained from.
 *  The following throwable objects are not contained in the future:
 *  - `Error` - errors are not contained within futures
 *  - `InterruptedException` - not contained within futures
 *  - all `scala.util.control.ControlThrowable` except `NonLocalReturnControl` - not contained within futures
 *
 *  Instead, the future is completed with a ExecutionException with one of the exceptions above
 *  as the cause.
 *  If a future is failed with a `scala.runtime.NonLocalReturnControl`,
 *  it is completed with a value from that throwable instead.
 *
 *  @define nonDeterministic
 *  Note: using this method yields nondeterministic dataflow programs.
 *
 *  @define forComprehensionExamples
 *  Example:
 *
 *  {{{
 *  val f = Future { 5 }
 *  val g = Future { 3 }
 *  val h = for {
 *    x: Int <- f // returns Future(5)
 *    y: Int <- g // returns Future(3)
 *  } yield x + y
 *  }}}
 *
 *  is translated to:
 *
 *  {{{
 *  f flatMap { (x: Int) => g map { (y: Int) => x + y } }
 *  }}}
 *
 * @define callbackInContext
 * The provided callback always runs in the provided implicit
 *`ExecutionContext`, though there is no guarantee that the
 * `execute()` method on the `ExecutionContext` will be called once
 * per callback or that `execute()` will be called in the current
 * thread. That is, the implementation may run multiple callbacks
 * in a batch within a single `execute()` and it may run
 * `execute()` either immediately or asynchronously.
 */
trait Future[+T] extends Awaitable[T] {

  // The executor within the lexical scope
  // of the Future trait. Note that this will
  // (modulo bugs) _never_ execute a callback
  // other than those below in this same file.
  //
  // See the documentation on `InternalCallbackExecutor` for more details.
  private def internalExecutor = Future.InternalCallbackExecutor

  /* Callbacks */

  /** When this future is completed successfully (i.e. with a value),
   *  apply the provided partial function to the value if the partial function
   *  is defined at that value.
   *
   *  If the future has already been completed with a value,
   *  this will either be applied immediately or be scheduled asynchronously.
   *
   *  $multipleCallbacks
   *  $callbackInContext
   */
  def onSuccess[U](pf: PartialFunction[T, U])(implicit executor: ExecutionContext): Unit = onComplete {
    case Success(v) =>
      pf.applyOrElse[T, Any](v, Predef.conforms[T]) // Exploiting the cached function to avoid MatchError
    case _ =>
  }

  /** When this future is completed with a failure (i.e. with a throwable),
   *  apply the provided callback to the throwable.
   *
   *  $caughtThrowables
   *
   *  If the future has already been completed with a failure,
   *  this will either be applied immediately or be scheduled asynchronously.
   *
   *  Will not be called in case that the future is completed with a value.
   *
   *  $multipleCallbacks
   *  $callbackInContext
   */
  def onFailure[U](@deprecatedName('callback) pf: PartialFunction[Throwable, U])(implicit executor: ExecutionContext): Unit = onComplete {
    case Failure(t) =>
      pf.applyOrElse[Throwable, Any](t, Predef.conforms[Throwable]) // Exploiting the cached function to avoid MatchError
    case _ =>
  }

  /** When this future is completed, either through an exception, or a value,
   *  apply the provided function.
   *
   *  If the future has already been completed,
   *  this will either be applied immediately or be scheduled asynchronously.
   *
   *  $multipleCallbacks
   *  $callbackInContext
   */
  def onComplete[U](@deprecatedName('func) f: Try[T] => U)(implicit executor: ExecutionContext): Unit


  /* Miscellaneous */

  /** Returns whether the future has already been completed with
   *  a value or an exception.
   *
   *  $nonDeterministic
   *
   *  @return    `true` if the future is already completed, `false` otherwise
   */
  def isCompleted: Boolean

  /** The value of this `Future`.
   *
   *  If the future is not completed the returned value will be `None`.
   *  If the future is completed the value will be `Some(Success(t))`
   *  if it contains a valid result, or `Some(Failure(error))` if it contains
   *  an exception.
   */
  def value: Option[Try[T]]


  /* Projections */

  /** Returns a failed projection of this future.
   *
   *  The failed projection is a future holding a value of type `Throwable`.
   *
   *  It is completed with a value which is the throwable of the original future
   *  in case the original future is failed.
   *
   *  It is failed with a `NoSuchElementException` if the original future is completed successfully.
   *
   *  Blocking on this future returns a value if the original future is completed with an exception
   *  and throws a corresponding exception if the original future fails.
   */
  def failed: Future[Throwable] = {
    implicit val ec = internalExecutor
    val p = Promise[Throwable]()
    onComplete {
      case Failure(t) => p success t
      case Success(v) => p failure (new NoSuchElementException("Future.failed not completed with a throwable."))
    }
    p.future
  }


  /* Monadic operations */

  /** Asynchronously processes the value in the future once the value becomes available.
   *
   *  Will not be called if the future fails.
   */
  def foreach[U](f: T => U)(implicit executor: ExecutionContext): Unit = onComplete { _ foreach f }

  /** Creates a new future by applying the 's' function to the successful result of
   *  this future, or the 'f' function to the failed result. If there is any non-fatal
   *  exception thrown when 's' or 'f' is applied, that exception will be propagated
   *  to the resulting future.
   *
   *  @param  s  function that transforms a successful result of the receiver into a
   *             successful result of the returned future
   *  @param  f  function that transforms a failure of the receiver into a failure of
   *             the returned future
   *  @return    a future that will be completed with the transformed value
   */
  def transform[S](s: T => S, f: Throwable => Throwable)(implicit executor: ExecutionContext): Future[S] = {
    val p = Promise[S]()
    // transform on Try has the wrong shape for us here
    onComplete {
      case Success(r) => p complete Try(s(r))
      case Failure(t) => p complete Try(throw f(t)) // will throw fatal errors!
    }
    p.future
  }

  /** Creates a new future by applying a function to the successful result of
   *  this future. If this future is completed with an exception then the new
   *  future will also contain this exception.
   *
   *  $forComprehensionExamples
   */
  def map[S](f: T => S)(implicit executor: ExecutionContext): Future[S] = { // transform(f, identity)
    val p = Promise[S]()
    onComplete { v => p complete (v map f) }
    p.future
  }

  /** Creates a new future by applying a function to the successful result of
   *  this future, and returns the result of the function as the new future.
   *  If this future is completed with an exception then the new future will
   *  also contain this exception.
   *
   *  $forComprehensionExamples
   */
  def flatMap[S](f: T => Future[S])(implicit executor: ExecutionContext): Future[S] = {
    import impl.Promise.DefaultPromise
    val p = new DefaultPromise[S]()
    onComplete {
      case f: Failure[_] => p complete f.asInstanceOf[Failure[S]]
      case Success(v) => try f(v) match {
        // If possible, link DefaultPromises to avoid space leaks
        case dp: DefaultPromise[_] => dp.asInstanceOf[DefaultPromise[S]].linkRootOf(p)
        case fut => fut.onComplete(p.complete)(internalExecutor)
      } catch { case NonFatal(t) => p failure t }
    }
    p.future
  }

  /** Creates a new future by filtering the value of the current future with a predicate.
   *
   *  If the current future contains a value which satisfies the predicate, the new future will also hold that value.
   *  Otherwise, the resulting future will fail with a `NoSuchElementException`.
   *
   *  If the current future fails, then the resulting future also fails.
   *
   *  Example:
   *  {{{
   *  val f = Future { 5 }
   *  val g = f filter { _ % 2 == 1 }
   *  val h = f filter { _ % 2 == 0 }
   *  Await.result(g, Duration.Zero) // evaluates to 5
   *  Await.result(h, Duration.Zero) // throw a NoSuchElementException
   *  }}}
   */
  def filter(@deprecatedName('pred) p: T => Boolean)(implicit executor: ExecutionContext): Future[T] =
    map {
      r => if (p(r)) r else throw new NoSuchElementException("Future.filter predicate is not satisfied")
    }

  /** Used by for-comprehensions.
   */
  final def withFilter(p: T => Boolean)(implicit executor: ExecutionContext): Future[T] = filter(p)(executor)

  /** Creates a new future by mapping the value of the current future, if the given partial function is defined at that value.
   *
   *  If the current future contains a value for which the partial function is defined, the new future will also hold that value.
   *  Otherwise, the resulting future will fail with a `NoSuchElementException`.
   *
   *  If the current future fails, then the resulting future also fails.
   *
   *  Example:
   *  {{{
   *  val f = Future { -5 }
   *  val g = f collect {
   *    case x if x < 0 => -x
   *  }
   *  val h = f collect {
   *    case x if x > 0 => x * 2
   *  }
   *  Await.result(g, Duration.Zero) // evaluates to 5
   *  Await.result(h, Duration.Zero) // throw a NoSuchElementException
   *  }}}
   */
  def collect[S](pf: PartialFunction[T, S])(implicit executor: ExecutionContext): Future[S] =
    map {
      r => pf.applyOrElse(r, (t: T) => throw new NoSuchElementException("Future.collect partial function is not defined at: " + t))
    }

  /** Creates a new future that will handle any matching throwable that this
   *  future might contain. If there is no match, or if this future contains
   *  a valid result then the new future will contain the same.
   *
   *  Example:
   *
   *  {{{
   *  Future (6 / 0) recover { case e: ArithmeticException => 0 } // result: 0
   *  Future (6 / 0) recover { case e: NotFoundException   => 0 } // result: exception
   *  Future (6 / 2) recover { case e: ArithmeticException => 0 } // result: 3
   *  }}}
   */
  def recover[U >: T](pf: PartialFunction[Throwable, U])(implicit executor: ExecutionContext): Future[U] = {
    val p = Promise[U]()
    onComplete { v => p complete (v recover pf) }
    p.future
  }

  /** Creates a new future that will handle any matching throwable that this
   *  future might contain by assigning it a value of another future.
   *
   *  If there is no match, or if this future contains
   *  a valid result then the new future will contain the same result.
   *
   *  Example:
   *
   *  {{{
   *  val f = Future { Int.MaxValue }
   *  Future (6 / 0) recoverWith { case e: ArithmeticException => f } // result: Int.MaxValue
   *  }}}
   */
  def recoverWith[U >: T](pf: PartialFunction[Throwable, Future[U]])(implicit executor: ExecutionContext): Future[U] = {
    val p = Promise[U]()
    onComplete {
      case Failure(t) => try pf.applyOrElse(t, (_: Throwable) => this).onComplete(p.complete)(internalExecutor) catch { case NonFatal(t) => p failure t }
      case other => p complete other
    }
    p.future
  }

  /** Zips the values of `this` and `that` future, and creates
   *  a new future holding the tuple of their results.
   *
   *  If `this` future fails, the resulting future is failed
   *  with the throwable stored in `this`.
   *  Otherwise, if `that` future fails, the resulting future is failed
   *  with the throwable stored in `that`.
   */
  def zip[U](that: Future[U]): Future[(T, U)] = {
    implicit val ec = internalExecutor
    val p = Promise[(T, U)]()
    onComplete {
      case f: Failure[_] => p complete f.asInstanceOf[Failure[(T, U)]]
      case Success(s) => that onComplete { c => p.complete(c map { s2 => (s, s2) }) }
    }
    p.future
  }

  /** Creates a new future which holds the result of this future if it was completed successfully, or, if not,
   *  the result of the `that` future if `that` is completed successfully.
   *  If both futures are failed, the resulting future holds the throwable object of the first future.
   *
   *  Using this method will not cause concurrent programs to become nondeterministic.
   *
   *  Example:
   *  {{{
   *  val f = Future { sys.error("failed") }
   *  val g = Future { 5 }
   *  val h = f fallbackTo g
   *  Await.result(h, Duration.Zero) // evaluates to 5
   *  }}}
   */
  def fallbackTo[U >: T](that: Future[U]): Future[U] = {
    implicit val ec = internalExecutor
    val p = Promise[U]()
    onComplete {
      case s @ Success(_) => p complete s
      case f @ Failure(_) => that onComplete {
        case s2 @ Success(_) => p complete s2
        case _ => p complete f // Use the first failure as the failure
      }
    }
    p.future
  }

  /** Creates a new `Future[S]` which is completed with this `Future`'s result if
   *  that conforms to `S`'s erased type or a `ClassCastException` otherwise.
   */
  def mapTo[S](implicit tag: ClassTag[S]): Future[S] = {
    implicit val ec = internalExecutor
    val boxedClass = {
      val c = tag.runtimeClass
      if (c.isPrimitive) Future.toBoxed(c) else c
    }
    require(boxedClass ne null)
    map(s => boxedClass.cast(s).asInstanceOf[S])
  }

  /** Applies the side-effecting function to the result of this future, and returns
   *  a new future with the result of this future.
   *
   *  This method allows one to enforce that the callbacks are executed in a
   *  specified order.
   *
   *  Note that if one of the chained `andThen` callbacks throws
   *  an exception, that exception is not propagated to the subsequent `andThen`
   *  callbacks. Instead, the subsequent `andThen` callbacks are given the original
   *  value of this future.
   *
   *  The following example prints out `5`:
   *
   *  {{{
   *  val f = Future { 5 }
   *  f andThen {
   *    case r => sys.error("runtime exception")
   *  } andThen {
   *    case Failure(t) => println(t)
   *    case Success(v) => println(v)
   *  }
   *  }}}
   */
  def andThen[U](pf: PartialFunction[Try[T], U])(implicit executor: ExecutionContext): Future[T] = {
    val p = Promise[T]()
    onComplete {
      case r => try pf.applyOrElse[Try[T], Any](r, Predef.conforms[Try[T]]) finally p complete r
    }
    p.future
  }

}



/** Future companion object.
 *
 *  @define nonDeterministic
 *  Note: using this method yields nondeterministic dataflow programs.
 */
object Future {

  private[concurrent] val toBoxed = Map[Class[_], Class[_]](
    classOf[Boolean] -> classOf[java.lang.Boolean],
    classOf[Byte]    -> classOf[java.lang.Byte],
    classOf[Char]    -> classOf[java.lang.Character],
    classOf[Short]   -> classOf[java.lang.Short],
    classOf[Int]     -> classOf[java.lang.Integer],
    classOf[Long]    -> classOf[java.lang.Long],
    classOf[Float]   -> classOf[java.lang.Float],
    classOf[Double]  -> classOf[java.lang.Double],
    classOf[Unit]    -> classOf[scala.runtime.BoxedUnit]
  )

  /** Creates an already completed Future with the specified exception.
   *
   *  @tparam T       the type of the value in the future
   *  @return         the newly created `Future` object
   */
  def failed[T](exception: Throwable): Future[T] = Promise.failed(exception).future

  /** Creates an already completed Future with the specified result.
   *
   *  @tparam T       the type of the value in the future
   *  @return         the newly created `Future` object
   */
  def successful[T](result: T): Future[T] = Promise.successful(result).future

  /** Creates an already completed Future with the specified result or exception.
   *
   *  @tparam T       the type of the value in the promise
   *  @return         the newly created `Future` object
   */
  def fromTry[T](result: Try[T]): Future[T] = Promise.fromTry(result).future

  /** 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.
  *
  *  @tparam T       the type of the result
  *  @param body     the asychronous computation
  *  @param executor  the execution context on which the future is run
  *  @return         the `Future` holding the result of the computation
  */
  def apply[T](body: =>T)(implicit @deprecatedName('execctx) executor: ExecutionContext): Future[T] = impl.Future(body)

  /** Simple version of `Future.traverse`. Transforms a `TraversableOnce[Future[A]]` into a `Future[TraversableOnce[A]]`.
   *  Useful for reducing many `Future`s into a single `Future`.
   */
  def sequence[A, M[X] <: TraversableOnce[X]](in: M[Future[A]])(implicit cbf: CanBuildFrom[M[Future[A]], A, M[A]], executor: ExecutionContext): Future[M[A]] = {
    in.foldLeft(successful(cbf(in))) {
      (fr, fa) => for (r <- fr; a <- fa) yield (r += a)
    } map (_.result())
  }

  /** Returns a new `Future` to the result of the first future in the list that is completed.
   */
  def firstCompletedOf[T](futures: TraversableOnce[Future[T]])(implicit executor: ExecutionContext): Future[T] = {
    val p = Promise[T]()
    val completeFirst: Try[T] => Unit = p tryComplete _
    futures foreach { _ onComplete completeFirst }
    p.future
  }

  /** Returns a `Future` that will hold the optional result of the first `Future` with a result that matches the predicate.
   */
  def find[T](@deprecatedName('futurestravonce) futures: TraversableOnce[Future[T]])(@deprecatedName('predicate) p: T => Boolean)(implicit executor: ExecutionContext): Future[Option[T]] = {
    val futuresBuffer = futures.toBuffer
    if (futuresBuffer.isEmpty) successful[Option[T]](None)
    else {
      val result = Promise[Option[T]]()
      val ref = new AtomicInteger(futuresBuffer.size)
      val search: Try[T] => Unit = v => try {
        v match {
          case Success(r) if p(r) => result tryComplete Success(Some(r))
          case _ =>
        }
      } finally {
        if (ref.decrementAndGet == 0) {
          result tryComplete Success(None)
        }
      }

      futuresBuffer.foreach(_ onComplete search)

      result.future
    }
  }

  /** A non-blocking fold over the specified futures, with the start value of the given zero.
   *  The fold is performed on the thread where the last future is completed,
   *  the result will be the first failure of any of the futures, or any failure in the actual fold,
   *  or the result of the fold.
   *
   *  Example:
   *  {{{
   *    val result = Await.result(Future.fold(futures)(0)(_ + _), 5 seconds)
   *  }}}
   */
  def fold[T, R](futures: TraversableOnce[Future[T]])(zero: R)(@deprecatedName('foldFun) op: (R, T) => R)(implicit executor: ExecutionContext): Future[R] = {
    if (futures.isEmpty) successful(zero)
    else sequence(futures).map(_.foldLeft(zero)(op))
  }

  /** Initiates a fold over the supplied futures where the fold-zero is the result value of the `Future` that's completed first.
   *
   *  Example:
   *  {{{
   *    val result = Await.result(Future.reduce(futures)(_ + _), 5 seconds)
   *  }}}
   */
  def reduce[T, R >: T](futures: TraversableOnce[Future[T]])(op: (R, T) => R)(implicit executor: ExecutionContext): Future[R] = {
    if (futures.isEmpty) failed(new NoSuchElementException("reduce attempted on empty collection"))
    else sequence(futures).map(_ reduceLeft op)
  }

  /** Transforms a `TraversableOnce[A]` into a `Future[TraversableOnce[B]]` using the provided function `A => Future[B]`.
   *  This is useful for performing a parallel map. For example, to apply a function to all items of a list
   *  in parallel:
   *
   *  {{{
   *    val myFutureList = Future.traverse(myList)(x => Future(myFunc(x)))
   *  }}}
   */
  def traverse[A, B, M[X] <: TraversableOnce[X]](in: M[A])(fn: A => Future[B])(implicit cbf: CanBuildFrom[M[A], B, M[B]], executor: ExecutionContext): Future[M[B]] =
    in.foldLeft(successful(cbf(in))) { (fr, a) =>
      val fb = fn(a)
      for (r <- fr; b <- fb) yield (r += b)
    }.map(_.result())

  // This is used to run callbacks which are internal
  // to scala.concurrent; our own callbacks are only
  // ever used to eventually run another callback,
  // and that other callback will have its own
  // executor because all callbacks come with
  // an executor. Our own callbacks never block
  // and have no "expected" exceptions.
  // As a result, this executor can do nothing;
  // some other executor will always come after
  // it (and sometimes one will be before it),
  // and those will be performing the "real"
  // dispatch to code outside scala.concurrent.
  // Because this exists, ExecutionContext.defaultExecutionContext
  // isn't instantiated by Future internals, so
  // if some code for some reason wants to avoid
  // ever starting up the default context, it can do so
  // by just not ever using it itself. scala.concurrent
  // doesn't need to create defaultExecutionContext as
  // a side effect.
  private[concurrent] object InternalCallbackExecutor extends ExecutionContext with BatchingExecutor {
    override protected def unbatchedExecute(r: Runnable): Unit =
      r.run()
    override def reportFailure(t: Throwable): Unit =
      throw new IllegalStateException("problem in scala.concurrent internal callback", t)
  }
}

/** 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 OnCompleteRunnable {
  self: Runnable =>
}