Packages

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akka.dispatch.forkjoin

ForkJoinTask

abstract class ForkJoinTask[V] extends Future[V] with Serializable

Abstract base class for tasks that run within a ForkJoinPool. A ForkJoinTask is a thread-like entity that is much lighter weight than a normal thread. Huge numbers of tasks and subtasks may be hosted by a small number of actual threads in a ForkJoinPool, at the price of some usage limitations.

A "main" ForkJoinTask begins execution when it is explicitly submitted to a ForkJoinPool, or, if not already engaged in a ForkJoin computation, commenced in the ForkJoinPool#commonPool() via #fork, #invoke, or related methods. Once started, it will usually in turn start other subtasks. As indicated by the name of this class, many programs using ForkJoinTask employ only methods #fork and #join, or derivatives such as #invokeAll(ForkJoinTask...) invokeAll. However, this class also provides a number of other methods that can come into play in advanced usages, as well as extension mechanics that allow support of new forms of fork/join processing.

A ForkJoinTask is a lightweight form of Future. The efficiency of ForkJoinTasks stems from a set of restrictions (that are only partially statically enforceable) reflecting their main use as computational tasks calculating pure functions or operating on purely isolated objects. The primary coordination mechanisms are #fork, that arranges asynchronous execution, and #join, that doesn't proceed until the task's result has been computed. Computations should ideally avoid synchronized methods or blocks, and should minimize other blocking synchronization apart from joining other tasks or using synchronizers such as Phasers that are advertised to cooperate with fork/join scheduling. Subdividable tasks should also not perform blocking I/O, and should ideally access variables that are completely independent of those accessed by other running tasks. These guidelines are loosely enforced by not permitting checked exceptions such as IOExceptions to be thrown. However, computations may still encounter unchecked exceptions, that are rethrown to callers attempting to join them. These exceptions may additionally include RejectedExecutionException stemming from internal resource exhaustion, such as failure to allocate internal task queues. Rethrown exceptions behave in the same way as regular exceptions, but, when possible, contain stack traces (as displayed for example using ex.printStackTrace()) of both the thread that initiated the computation as well as the thread actually encountering the exception; minimally only the latter.

It is possible to define and use ForkJoinTasks that may block, but doing do requires three further considerations: (1) Completion of few if any other tasks should be dependent on a task that blocks on external synchronization or I/O. Event-style async tasks that are never joined (for example, those subclassing CountedCompleter) often fall into this category. (2) To minimize resource impact, tasks should be small; ideally performing only the (possibly) blocking action. (3) Unless the ForkJoinPool.ManagedBlocker API is used, or the number of possibly blocked tasks is known to be less than the pool's ForkJoinPool#getParallelism level, the pool cannot guarantee that enough threads will be available to ensure progress or good performance.

The primary method for awaiting completion and extracting results of a task is #join, but there are several variants: The Future#get methods support interruptible and/or timed waits for completion and report results using Future conventions. Method #invoke is semantically equivalent to fork(); join() but always attempts to begin execution in the current thread. The "quiet" forms of these methods do not extract results or report exceptions. These may be useful when a set of tasks are being executed, and you need to delay processing of results or exceptions until all complete. Method invokeAll (available in multiple versions) performs the most common form of parallel invocation: forking a set of tasks and joining them all.

In the most typical usages, a fork-join pair act like a call (fork) and return (join) from a parallel recursive function. As is the case with other forms of recursive calls, returns (joins) should be performed innermost-first. For example, a.fork(); b.fork(); b.join(); a.join(); is likely to be substantially more efficient than joining a before b.

The execution status of tasks may be queried at several levels of detail: #isDone is true if a task completed in any way (including the case where a task was cancelled without executing); #isCompletedNormally is true if a task completed without cancellation or encountering an exception; #isCancelled is true if the task was cancelled (in which case #getException returns a java.util.concurrent.CancellationException); and #isCompletedAbnormally is true if a task was either cancelled or encountered an exception, in which case #getException will return either the encountered exception or java.util.concurrent.CancellationException.

The ForkJoinTask class is not usually directly subclassed. Instead, you subclass one of the abstract classes that support a particular style of fork/join processing, typically RecursiveAction for most computations that do not return results, RecursiveTask for those that do, and CountedCompleter for those in which completed actions trigger other actions. Normally, a concrete ForkJoinTask subclass declares fields comprising its parameters, established in a constructor, and then defines a compute method that somehow uses the control methods supplied by this base class.

Method #join and its variants are appropriate for use only when completion dependencies are acyclic; that is, the parallel computation can be described as a directed acyclic graph (DAG). Otherwise, executions may encounter a form of deadlock as tasks cyclically wait for each other. However, this framework supports other methods and techniques (for example the use of Phaser, #helpQuiesce, and #complete) that may be of use in constructing custom subclasses for problems that are not statically structured as DAGs. To support such usages a ForkJoinTask may be atomically tagged with a short value using #setForkJoinTaskTag or #compareAndSetForkJoinTaskTag and checked using #getForkJoinTaskTag. The ForkJoinTask implementation does not use these protected methods or tags for any purpose, but they may be of use in the construction of specialized subclasses. For example, parallel graph traversals can use the supplied methods to avoid revisiting nodes/tasks that have already been processed. (Method names for tagging are bulky in part to encourage definition of methods that reflect their usage patterns.)

Most base support methods are final, to prevent overriding of implementations that are intrinsically tied to the underlying lightweight task scheduling framework. Developers creating new basic styles of fork/join processing should minimally implement protected methods #exec, #setRawResult, and #getRawResult, while also introducing an abstract computational method that can be implemented in its subclasses, possibly relying on other protected methods provided by this class.

ForkJoinTasks should perform relatively small amounts of computation. Large tasks should be split into smaller subtasks, usually via recursive decomposition. As a very rough rule of thumb, a task should perform more than 100 and less than 10000 basic computational steps, and should avoid indefinite looping. If tasks are too big, then parallelism cannot improve throughput. If too small, then memory and internal task maintenance overhead may overwhelm processing.

This class provides adapt methods for Runnable and Callable, that may be of use when mixing execution of ForkJoinTasks with other kinds of tasks. When all tasks are of this form, consider using a pool constructed in asyncMode.

ForkJoinTasks are Serializable, which enables them to be used in extensions such as remote execution frameworks. It is sensible to serialize tasks only before or after, but not during, execution. Serialization is not relied on during execution itself.

Source
ForkJoinTask.java
Since

1.7

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  1. ForkJoinTask
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  1. by any2stringadd
  2. by StringFormat
  3. by Ensuring
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Instance Constructors

  1. new ForkJoinTask()

Abstract Value Members

  1. abstract def exec(): Boolean

    Immediately performs the base action of this task and returns true if, upon return from this method, this task is guaranteed to have completed normally.

    Immediately performs the base action of this task and returns true if, upon return from this method, this task is guaranteed to have completed normally. This method may return false otherwise, to indicate that this task is not necessarily complete (or is not known to be complete), for example in asynchronous actions that require explicit invocations of completion methods. This method may also throw an (unchecked) exception to indicate abnormal exit. This method is designed to support extensions, and should not in general be called otherwise.

    returns

    true if this task is known to have completed normally

    Attributes
    protected[akka.dispatch.forkjoin]
  2. abstract def getRawResult(): V

    Returns the result that would be returned by #join, even if this task completed abnormally, or null if this task is not known to have been completed.

    Returns the result that would be returned by #join, even if this task completed abnormally, or null if this task is not known to have been completed. This method is designed to aid debugging, as well as to support extensions. Its use in any other context is discouraged.

    returns

    the result, or null if not completed

  3. abstract def setRawResult(value: V): Unit

    Forces the given value to be returned as a result.

    Forces the given value to be returned as a result. This method is designed to support extensions, and should not in general be called otherwise.

    value

    the value

    Attributes
    protected[akka.dispatch.forkjoin]

Concrete Value Members

  1. final def !=(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  2. final def ##(): Int
    Definition Classes
    AnyRef → Any
  3. def +(other: String): String
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to any2stringadd[ForkJoinTask[V]] performed by method any2stringadd in scala.Predef.
    Definition Classes
    any2stringadd
  4. def ->[B](y: B): (ForkJoinTask[V], B)
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to ArrowAssoc[ForkJoinTask[V]] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  5. final def ==(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  6. final def asInstanceOf[T0]: T0
    Definition Classes
    Any
  7. def cancel(mayInterruptIfRunning: Boolean): Boolean

    Attempts to cancel execution of this task.

    Attempts to cancel execution of this task. This attempt will fail if the task has already completed or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, execution of this task is suppressed. After this method returns successfully, unless there is an intervening call to #reinitialize, subsequent calls to #isCancelled, #isDone, and cancel will return true and calls to #join and related methods will result in CancellationException.

    This method may be overridden in subclasses, but if so, must still ensure that these properties hold. In particular, the cancel method itself must not throw exceptions.

    This method is designed to be invoked by other tasks. To terminate the current task, you can just return or throw an unchecked exception from its computation method, or invoke #completeExceptionally.

    mayInterruptIfRunning

    this value has no effect in the default implementation because interrupts are not used to control cancellation.

    returns

    true if this task is now cancelled

    Definition Classes
    ForkJoinTask → Future
  8. def clone(): AnyRef
    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @native() @throws( ... )
  9. final def compareAndSetForkJoinTaskTag(e: Short, tag: Short): Boolean

    Atomically conditionally sets the tag value for this task.

    Atomically conditionally sets the tag value for this task. Among other applications, tags can be used as visit markers in tasks operating on graphs, as in methods that check: if (task.compareAndSetForkJoinTaskTag((short)0, (short)1)) before processing, otherwise exiting because the node has already been visited.

    e

    the expected tag value

    tag

    the new tag value

    returns

    true if successful; i.e., the current value was equal to e and is now tag.

    Since

    1.8

  10. def complete(value: V): Unit

    Completes this task, and if not already aborted or cancelled, returning the given value as the result of subsequent invocations of join and related operations.

    Completes this task, and if not already aborted or cancelled, returning the given value as the result of subsequent invocations of join and related operations. This method may be used to provide results for asynchronous tasks, or to provide alternative handling for tasks that would not otherwise complete normally. Its use in other situations is discouraged. This method is overridable, but overridden versions must invoke super implementation to maintain guarantees.

    value

    the result value for this task

  11. def completeExceptionally(ex: Throwable): Unit

    Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the given exception upon join and related operations.

    Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the given exception upon join and related operations. This method may be used to induce exceptions in asynchronous tasks, or to force completion of tasks that would not otherwise complete. Its use in other situations is discouraged. This method is overridable, but overridden versions must invoke super implementation to maintain guarantees.

    ex

    the exception to throw. If this exception is not a RuntimeException or Error, the actual exception thrown will be a RuntimeException with cause ex.

  12. def ensuring(cond: (ForkJoinTask[V]) ⇒ Boolean, msg: ⇒ Any): ForkJoinTask[V]
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to Ensuring[ForkJoinTask[V]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  13. def ensuring(cond: (ForkJoinTask[V]) ⇒ Boolean): ForkJoinTask[V]
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to Ensuring[ForkJoinTask[V]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  14. def ensuring(cond: Boolean, msg: ⇒ Any): ForkJoinTask[V]
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to Ensuring[ForkJoinTask[V]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  15. def ensuring(cond: Boolean): ForkJoinTask[V]
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to Ensuring[ForkJoinTask[V]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  16. final def eq(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  17. def equals(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  18. def finalize(): Unit
    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] )
  19. final def fork(): ForkJoinTask[V]

    Arranges to asynchronously execute this task in the pool the current task is running in, if applicable, or using the ForkJoinPool#commonPool() if not #inForkJoinPool.

    Arranges to asynchronously execute this task in the pool the current task is running in, if applicable, or using the ForkJoinPool#commonPool() if not #inForkJoinPool. While it is not necessarily enforced, it is a usage error to fork a task more than once unless it has completed and been reinitialized. Subsequent modifications to the state of this task or any data it operates on are not necessarily consistently observable by any thread other than the one executing it unless preceded by a call to #join or related methods, or a call to #isDone returning true.

    returns

    this, to simplify usage

  20. def formatted(fmtstr: String): String
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to StringFormat[ForkJoinTask[V]] performed by method StringFormat in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @inline()
  21. final def get(timeout: Long, unit: TimeUnit): V

    Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

    Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

    timeout

    the maximum time to wait

    unit

    the time unit of the timeout argument

    returns

    the computed result

    Definition Classes
    ForkJoinTask → Future
    Exceptions thrown

    CancellationException if the computation was cancelled

    ExecutionException if the computation threw an exception

    InterruptedException if the current thread is not a member of a ForkJoinPool and was interrupted while waiting

    TimeoutException if the wait timed out

  22. final def get(): V

    Waits if necessary for the computation to complete, and then retrieves its result.

    Waits if necessary for the computation to complete, and then retrieves its result.

    returns

    the computed result

    Definition Classes
    ForkJoinTask → Future
    Exceptions thrown

    CancellationException if the computation was cancelled

    ExecutionException if the computation threw an exception

    InterruptedException if the current thread is not a member of a ForkJoinPool and was interrupted while waiting

  23. final def getClass(): Class[_]
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  24. final def getException(): Throwable

    Returns the exception thrown by the base computation, or a CancellationException if cancelled, or null if none or if the method has not yet completed.

    Returns the exception thrown by the base computation, or a CancellationException if cancelled, or null if none or if the method has not yet completed.

    returns

    the exception, or null if none

  25. final def getForkJoinTaskTag(): Short

    Returns the tag for this task.

    Returns the tag for this task.

    returns

    the tag for this task

    Since

    1.8

  26. def hashCode(): Int
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  27. final def invoke(): V

    Commences performing this task, awaits its completion if necessary, and returns its result, or throws an (unchecked) RuntimeException or Error if the underlying computation did so.

    Commences performing this task, awaits its completion if necessary, and returns its result, or throws an (unchecked) RuntimeException or Error if the underlying computation did so.

    returns

    the computed result

  28. final def isCancelled(): Boolean
    Definition Classes
    ForkJoinTask → Future
  29. final def isCompletedAbnormally(): Boolean

    Returns true if this task threw an exception or was cancelled.

    Returns true if this task threw an exception or was cancelled.

    returns

    true if this task threw an exception or was cancelled

  30. final def isCompletedNormally(): Boolean

    Returns true if this task completed without throwing an exception and was not cancelled.

    Returns true if this task completed without throwing an exception and was not cancelled.

    returns

    true if this task completed without throwing an exception and was not cancelled

  31. final def isDone(): Boolean
    Definition Classes
    ForkJoinTask → Future
  32. final def isInstanceOf[T0]: Boolean
    Definition Classes
    Any
  33. final def join(): V

    Returns the result of the computation when it is done.

    Returns the result of the computation when it is done. This method differs from #get() in that abnormal completion results in RuntimeException or Error, not ExecutionException, and that interrupts of the calling thread do not cause the method to abruptly return by throwing InterruptedException.

    returns

    the computed result

  34. final def ne(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  35. final def notify(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  36. final def notifyAll(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  37. final def quietlyComplete(): Unit

    Completes this task normally without setting a value.

    Completes this task normally without setting a value. The most recent value established by #setRawResult (or null by default) will be returned as the result of subsequent invocations of join and related operations.

    Since

    1.8

  38. final def quietlyInvoke(): Unit

    Commences performing this task and awaits its completion if necessary, without returning its result or throwing its exception.

  39. final def quietlyJoin(): Unit

    Joins this task, without returning its result or throwing its exception.

    Joins this task, without returning its result or throwing its exception. This method may be useful when processing collections of tasks when some have been cancelled or otherwise known to have aborted.

  40. def reinitialize(): Unit

    Resets the internal bookkeeping state of this task, allowing a subsequent fork.

    Resets the internal bookkeeping state of this task, allowing a subsequent fork. This method allows repeated reuse of this task, but only if reuse occurs when this task has either never been forked, or has been forked, then completed and all outstanding joins of this task have also completed. Effects under any other usage conditions are not guaranteed. This method may be useful when executing pre-constructed trees of subtasks in loops.

    Upon completion of this method, isDone() reports false, and getException() reports null. However, the value returned by getRawResult is unaffected. To clear this value, you can invoke setRawResult(null).

  41. final def setForkJoinTaskTag(tag: Short): Short

    Atomically sets the tag value for this task.

    Atomically sets the tag value for this task.

    tag

    the tag value

    returns

    the previous value of the tag

    Since

    1.8

  42. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  43. def toString(): String
    Definition Classes
    AnyRef → Any
  44. def tryUnfork(): Boolean

    Tries to unschedule this task for execution.

    Tries to unschedule this task for execution. This method will typically (but is not guaranteed to) succeed if this task is the most recently forked task by the current thread, and has not commenced executing in another thread. This method may be useful when arranging alternative local processing of tasks that could have been, but were not, stolen.

    returns

    true if unforked

  45. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  46. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  47. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @native() @throws( ... )
  48. def [B](y: B): (ForkJoinTask[V], B)
    Implicit
    This member is added by an implicit conversion from ForkJoinTask[V] to ArrowAssoc[ForkJoinTask[V]] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc

Inherited from Serializable

Inherited from Future[V]

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from ForkJoinTask[V] to any2stringadd[ForkJoinTask[V]]

Inherited by implicit conversion StringFormat from ForkJoinTask[V] to StringFormat[ForkJoinTask[V]]

Inherited by implicit conversion Ensuring from ForkJoinTask[V] to Ensuring[ForkJoinTask[V]]

Inherited by implicit conversion ArrowAssoc from ForkJoinTask[V] to ArrowAssoc[ForkJoinTask[V]]

Ungrouped