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Added in API level 21

Summary: Ctors | Methods | Protected Methods | Inherited Methods | [Expand All]

ForkJoinTask

public abstract class ForkJoinTask
extends Object implements Future<V>, Serializable

java.lang.Object
↳	java.util.concurrent.ForkJoinTask<V>

Known Direct Subclasses

CountedCompleter<T>, RecursiveAction, RecursiveTask<V>

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 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. 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 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 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 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 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(V)) 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(short) or compareAndSetForkJoinTaskTag(short, short) 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(V), 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.

Summary

Public constructors
`ForkJoinTask()`

Public methods
`static <T> ForkJoinTask<T>`	`adapt(Runnable runnable, T result)` Returns a new `ForkJoinTask` that performs the `run` method of the given `Runnable` as its action, and returns the given result upon `join()`.
`static ForkJoinTask<?>`	`adapt(Runnable runnable)` Returns a new `ForkJoinTask` that performs the `run` method of the given `Runnable` as its action, and returns a null result upon `join()`.
`static <T> ForkJoinTask<T>`	`adapt(Callable<? extends T> callable)` Returns a new `ForkJoinTask` that performs the `call` method of the given `Callable` as its action, and returns its result upon `join()`, translating any checked exceptions encountered into `RuntimeException`.
`boolean`	`cancel(boolean mayInterruptIfRunning)` Attempts to cancel execution of this task.
`final boolean`	`compareAndSetForkJoinTaskTag(short expect, short update)` Atomically conditionally sets the tag value for this task.
`void`	`complete(V value)` 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.
`void`	`completeExceptionally(Throwable ex)` Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the given exception upon `join` and related operations.
`final ForkJoinTask<V>`	`fork()` Arranges to asynchronously execute this task in the pool the current task is running in, if applicable, or using the `commonPool()` if not `inForkJoinPool()`.
`final V`	`get(long timeout, TimeUnit unit)` Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
`final V`	`get()` Waits if necessary for the computation to complete, and then retrieves its result.
`final Throwable`	`getException()` 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.
`final short`	`getForkJoinTaskTag()` Returns the tag for this task.
`static ForkJoinPool`	`getPool()` Returns the pool hosting the current thread, or `null` if the current thread is executing outside of any ForkJoinPool.
`static int`	`getQueuedTaskCount()` Returns an estimate of the number of tasks that have been forked by the current worker thread but not yet executed.
`abstract V`	`getRawResult()` 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.
`static int`	`getSurplusQueuedTaskCount()` Returns an estimate of how many more locally queued tasks are held by the current worker thread than there are other worker threads that might steal them, or zero if this thread is not operating in a ForkJoinPool.
`static void`	`helpQuiesce()` Possibly executes tasks until the pool hosting the current task is quiescent.
`static boolean`	`inForkJoinPool()` Returns `true` if the current thread is a `ForkJoinWorkerThread` executing as a ForkJoinPool computation.
`final V`	`invoke()` 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.
`static void`	`invokeAll(ForkJoinTask...<?> tasks)` Forks the given tasks, returning when `isDone` holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown.
`static void`	`invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2)` Forks the given tasks, returning when `isDone` holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown.
`static <T extends ForkJoinTask<?>> Collection<T>`	`invokeAll(Collection<T> tasks)` Forks all tasks in the specified collection, returning when `isDone` holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown.
`final boolean`	`isCancelled()` Returns `true` if this task was cancelled before it completed normally.
`final boolean`	`isCompletedAbnormally()` Returns `true` if this task threw an exception or was cancelled.
`final boolean`	`isCompletedNormally()` Returns `true` if this task completed without throwing an exception and was not cancelled.
`final boolean`	`isDone()` Returns `true` if this task completed.
`final V`	`join()` Returns the result of the computation when it `is done`.
`final void`	`quietlyComplete()` Completes this task normally without setting a value.
`final void`	`quietlyInvoke()` Commences performing this task and awaits its completion if necessary, without returning its result or throwing its exception.
`final void`	`quietlyJoin()` Joins this task, without returning its result or throwing its exception.
`void`	`reinitialize()` Resets the internal bookkeeping state of this task, allowing a subsequent `fork`.
`final short`	`setForkJoinTaskTag(short newValue)` Atomically sets the tag value for this task and returns the old value.
`boolean`	`tryUnfork()` Tries to unschedule this task for execution.

Protected methods
`abstract boolean`	`exec()` 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.
`static ForkJoinTask<?>`	`peekNextLocalTask()` Returns, but does not unschedule or execute, a task queued by the current thread but not yet executed, if one is immediately available.
`static ForkJoinTask<?>`	`pollNextLocalTask()` Unschedules and returns, without executing, the next task queued by the current thread but not yet executed, if the current thread is operating in a ForkJoinPool.
`static ForkJoinTask<?>`	`pollTask()` If the current thread is operating in a ForkJoinPool, unschedules and returns, without executing, the next task queued by the current thread but not yet executed, if one is available, or if not available, a task that was forked by some other thread, if available.
`abstract void`	`setRawResult(V value)` Forces the given value to be returned as a result.

Inherited methods

From class


  
    java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long millis, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void`	`wait(long millis)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void`	`wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.

From interface


  
    java.util.concurrent.Future

Parameters
`runnable`	`Runnable`: the runnable action
`result`	`T`: the result upon completion

Parameters
`expect`	`short`: the expected tag value
`update`	`short`: the new tag value

Parameters
`timeout`	`long`: the maximum time to wait
`unit`	`TimeUnit`: the time unit of the timeout argument

Throws
`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

Parameters
`t1`	`ForkJoinTask`: the first task
`t2`	`ForkJoinTask`: the second task

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Results for

ForkJoinTask

Summary

Public constructors

Public methods

Protected methods

Inherited methods

Public constructors

ForkJoinTask

Public methods

adapt

adapt

adapt

cancel

compareAndSetForkJoinTaskTag

complete

completeExceptionally

fork

get

get

getException

getForkJoinTaskTag

getPool

getQueuedTaskCount

getRawResult

getSurplusQueuedTaskCount

helpQuiesce

inForkJoinPool

invoke

invokeAll

invokeAll

invokeAll

isCancelled

isCompletedAbnormally

isCompletedNormally

isDone

join

quietlyComplete

quietlyInvoke

quietlyJoin

reinitialize

setForkJoinTaskTag

tryUnfork

Protected methods

exec

peekNextLocalTask

pollNextLocalTask

pollTask

setRawResult