Class ForkJoinTask<V>

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

Constructors

ForkJoinTask

public ForkJoinTask()

Methods

fork

public final ForkJoinTask<V> fork()

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

join

public final V join()

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

invoke

public 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.

Returns:

the computed result

invokeAll

public 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. If more than one task encounters an exception, then this method throws any one of these exceptions. If any task encounters an exception, the other may be cancelled. However, the execution status of individual tasks is not guaranteed upon exceptional return. The status of each task may be obtained using getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed.

Parameters:

t1 - the first task

t2 - the second task

Throws:

NullPointerException - if any task is null

invokeAll

public 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. If more than one task encounters an exception, then this method throws any one of these exceptions. If any task encounters an exception, others may be cancelled. However, the execution status of individual tasks is not guaranteed upon exceptional return. The status of each task may be obtained using getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed.

Parameters:

tasks - the tasks

Throws:

NullPointerException - if any task is null

invokeAll

public 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. If more than one task encounters an exception, then this method throws any one of these exceptions. If any task encounters an exception, others may be cancelled. However, the execution status of individual tasks is not guaranteed upon exceptional return. The status of each task may be obtained using getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed.

Type Parameters:

T - the type of the values returned from the tasks

Parameters:

tasks - the collection of tasks

Returns:

the tasks argument, to simplify usage

Throws:

NullPointerException - if tasks or any element are null

cancel

public boolean cancel(boolean mayInterruptIfRunning)

Specified by:

cancel in interface Future<V>

Parameters:

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

isDone

public final boolean isDone()

Description copied from interface: Future

Returns true if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return true.

Specified by:

isDone in interface Future<V>

Returns:

true if this task completed

isCancelled

public final boolean isCancelled()

Description copied from interface: Future

Returns true if this task was cancelled before it completed normally.

Specified by:

isCancelled in interface Future<V>

Returns:

true if this task was cancelled before it completed

isCompletedAbnormally

public final boolean isCompletedAbnormally()

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

Returns:

true if this task threw an exception or was cancelled

isCompletedNormally

public final boolean isCompletedNormally()

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

getException

public 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.

Returns:

the exception, or null if none

completeExceptionally

public 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. 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.

Parameters:

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.

complete

public 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. 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.

Parameters:

value - the result value for this task

quietlyComplete

public final void quietlyComplete()

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

Since:

1.8

get

public final V get()
            throws InterruptedException,
                   ExecutionException

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

Specified by:

get in interface Future<V>

Returns:

the computed result

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

get

public final V get(long timeout,
                   TimeUnit unit)
            throws InterruptedException,
                   ExecutionException,
                   TimeoutException

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

Specified by:

get in interface Future<V>

Parameters:

timeout - the maximum time to wait

unit - the time unit of the timeout argument

Returns:

the computed result

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

quietlyJoin

public final void quietlyJoin()

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.

quietlyInvoke

public final void quietlyInvoke()

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

helpQuiesce

public static void helpQuiesce()

Possibly executes tasks until the pool hosting the current task is quiescent. This method may be of use in designs in which many tasks are forked, but none are explicitly joined, instead executing them until all are processed.

reinitialize

public void reinitialize()

getPool

public static ForkJoinPool getPool()

Returns the pool hosting the current task execution, or null if this task is executing outside of any ForkJoinPool.

Returns:

the pool, or null if none

See Also:

inForkJoinPool()

inForkJoinPool

public static boolean inForkJoinPool()

Returns true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation.

Returns:

true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation, or false otherwise

tryUnfork

public boolean tryUnfork()

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

getQueuedTaskCount

public static int getQueuedTaskCount()

Returns an estimate of the number of tasks that have been forked by the current worker thread but not yet executed. This value may be useful for heuristic decisions about whether to fork other tasks.

Returns:

the number of tasks

getSurplusQueuedTaskCount

public 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. This value may be useful for heuristic decisions about whether to fork other tasks. In many usages of ForkJoinTasks, at steady state, each worker should aim to maintain a small constant surplus (for example, 3) of tasks, and to process computations locally if this threshold is exceeded.

Returns:

the surplus number of tasks, which may be negative

getRawResult

public 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. 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

setRawResult

protected abstract void setRawResult(V value)

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.

Parameters:

value - the value

exec

protected 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. 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

peekNextLocalTask

protected 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. There is no guarantee that this task will actually be polled or executed next. Conversely, this method may return null even if a task exists but cannot be accessed without contention with other threads. This method is designed primarily to support extensions, and is unlikely to be useful otherwise.

Returns:

the next task, or null if none are available

pollNextLocalTask

protected 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. This method is designed primarily to support extensions, and is unlikely to be useful otherwise.

Returns:

the next task, or null if none are available

pollTask

protected 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. Availability may be transient, so a null result does not necessarily imply quiescence of the pool this task is operating in. This method is designed primarily to support extensions, and is unlikely to be useful otherwise.

Returns:

a task, or null if none are available

getForkJoinTaskTag

public final short getForkJoinTaskTag()

Returns the tag for this task.

Returns:

the tag for this task

Since:

1.8

setForkJoinTaskTag

public final short setForkJoinTaskTag(short tag)

Atomically sets the tag value for this task.

Parameters:

tag - the tag value

Returns:

the previous value of the tag

Since:

1.8

compareAndSetForkJoinTaskTag

public final boolean compareAndSetForkJoinTaskTag(short e,
                                                  short tag)

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.

Parameters:

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

adapt

public 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().

Parameters:

runnable - the runnable action

Returns:

the task

adapt

public 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().

Type Parameters:

T - the type of the result

Parameters:

runnable - the runnable action

result - the result upon completion

Returns:

the task

adapt

public 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.

Type Parameters:

T - the type of the callable's result

Parameters:

callable - the callable action

Returns:

the task