public abstract class ForkJoinTask<V> extends Object implements Future<V>, Serializable
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 ForkJoinTask
s 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.
Constructor and Description |
---|
ForkJoinTask() |
Modifier and Type | Method and Description |
---|---|
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 . |
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(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() . |
boolean |
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
|
boolean |
compareAndSetForkJoinTaskTag(short e,
short tag)
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. |
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.
|
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() . |
V |
get()
Waits if necessary for the computation to complete, and then
retrieves its result.
|
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.
|
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. |
short |
getForkJoinTaskTag()
Returns the tag for this task.
|
static ForkJoinPool |
getPool()
Returns the pool hosting the current task execution, or null
if this task 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. |
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 <T extends ForkJoinTask<?>> |
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. |
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. |
boolean |
isCancelled()
Returns
true if this task was cancelled before it completed
normally. |
boolean |
isCompletedAbnormally()
Returns
true if this task threw an exception or was cancelled. |
boolean |
isCompletedNormally()
Returns
true if this task completed without throwing an
exception and was not cancelled. |
boolean |
isDone()
Returns
true if this task completed. |
V |
join()
Returns the result of the computation when it
is
done . |
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.
|
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.
|
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.
|
void |
quietlyComplete()
Completes this task normally without setting a value.
|
void |
quietlyInvoke()
Commences performing this task and awaits its completion if
necessary, without returning its result or throwing its
exception.
|
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 . |
short |
setForkJoinTaskTag(short tag)
Atomically sets the tag value for this task.
|
protected abstract void |
setRawResult(V value)
Forces the given value to be returned as a result.
|
boolean |
tryUnfork()
Tries to unschedule this task for execution.
|
public final ForkJoinTask<V> fork()
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
.this
, to simplify usagepublic final V join()
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
.public final V invoke()
RuntimeException
or Error
if the underlying
computation did so.public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2)
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.t1
- the first taskt2
- the second taskNullPointerException
- if any task is nullpublic static void invokeAll(ForkJoinTask<?>... tasks)
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.tasks
- the tasksNullPointerException
- if any task is nullpublic static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks)
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.T
- the type of the values returned from the taskstasks
- the collection of tasksNullPointerException
- if tasks or any element are nullpublic boolean cancel(boolean mayInterruptIfRunning)
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(Throwable)
.
public final boolean isDone()
Future
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
.public final boolean isCancelled()
Future
true
if this task was cancelled before it completed
normally.isCancelled
in interface Future<V>
true
if this task was cancelled before it completedpublic final boolean isCompletedAbnormally()
true
if this task threw an exception or was cancelled.true
if this task threw an exception or was cancelledpublic final boolean isCompletedNormally()
true
if this task completed without throwing an
exception and was not cancelled.true
if this task completed without throwing an
exception and was not cancelledpublic final Throwable getException()
CancellationException
if cancelled, or null
if
none or if the method has not yet completed.null
if nonepublic void completeExceptionally(Throwable ex)
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
.public void complete(V value)
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 taskpublic final void quietlyComplete()
setRawResult(V)
(or null
by default) will be returned as the result of subsequent
invocations of join
and related operations.public final V get() throws InterruptedException, ExecutionException
get
in interface Future<V>
CancellationException
- if the computation was cancelledExecutionException
- if the computation threw an
exceptionInterruptedException
- if the current thread is not a
member of a ForkJoinPool and was interrupted while waitingpublic final V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException
get
in interface Future<V>
timeout
- the maximum time to waitunit
- the time unit of the timeout argumentCancellationException
- if the computation was cancelledExecutionException
- if the computation threw an
exceptionInterruptedException
- if the current thread is not a
member of a ForkJoinPool and was interrupted while waitingTimeoutException
- if the wait timed outpublic final void quietlyJoin()
public final void quietlyInvoke()
public static void helpQuiesce()
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.public void reinitialize()
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)
.
public static ForkJoinPool getPool()
null
if noneinForkJoinPool()
public static boolean inForkJoinPool()
true
if the current thread is a ForkJoinWorkerThread
executing as a ForkJoinPool computation.true
if the current thread is a ForkJoinWorkerThread
executing as a ForkJoinPool computation,
or false
otherwisepublic boolean tryUnfork()
true
if unforkedpublic static int getQueuedTaskCount()
public static int getSurplusQueuedTaskCount()
public abstract V getRawResult()
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.null
if not completedprotected abstract void setRawResult(V value)
value
- the valueprotected abstract boolean exec()
true
if this task is known to have completed normallyprotected static ForkJoinTask<?> peekNextLocalTask()
null
if none are availableprotected static ForkJoinTask<?> pollNextLocalTask()
null
if none are availableprotected static ForkJoinTask<?> pollTask()
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.null
if none are availablepublic final short getForkJoinTaskTag()
public final short setForkJoinTaskTag(short tag)
tag
- the tag valuepublic final boolean compareAndSetForkJoinTaskTag(short e, short tag)
if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))
before processing, otherwise exiting because the node has
already been visited.e
- the expected tag valuetag
- the new tag valuetrue
if successful; i.e., the current value was
equal to e and is now tag.public static ForkJoinTask<?> adapt(Runnable runnable)
ForkJoinTask
that performs the run
method of the given Runnable
as its action, and returns
a null result upon join()
.runnable
- the runnable actionpublic static <T> ForkJoinTask<T> adapt(Runnable runnable, T result)
ForkJoinTask
that performs the run
method of the given Runnable
as its action, and returns
the given result upon join()
.T
- the type of the resultrunnable
- the runnable actionresult
- the result upon completionpublic static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable)
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
.T
- the type of the callable's resultcallable
- the callable action Submit a bug or feature
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