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FlexDoc/Javadoc 2.0 Demo Java Doc |
Class ForkJoinPool
java.util.concurrent.ForkJoinPool
All Implemented Interfaces:
public class ForkJoinPool
An
ExecutorService for running
ForkJoinTasks.
A
ForkJoinPool provides the entry point for submissions
from non-
ForkJoinTask clients, as well as management and
monitoring operations.
A ForkJoinPool differs from other kinds of ExecutorService mainly by virtue of employing
work-stealing: all threads in the pool attempt to find and
execute tasks submitted to the pool and/or created by other active
tasks (eventually blocking waiting for work if none exist). This
enables efficient processing when most tasks spawn other subtasks
(as do most ForkJoinTasks), as well as when many small
tasks are submitted to the pool from external clients. Especially
when setting asyncMode to true in constructors,
ForkJoinPools may also be appropriate for use with event-style
tasks that are never joined. All worker threads are initialized
with Thread.isDaemon() set true.
A static commonPool() is available and appropriate for
most applications. The common pool is used by any ForkJoinTask that
is not explicitly submitted to a specified pool. Using the common
pool normally reduces resource usage (its threads are slowly
reclaimed during periods of non-use, and reinstated upon subsequent
use).
For applications that require separate or custom pools, a
ForkJoinPool may be constructed with a given target parallelism
level; by default, equal to the number of available processors.
The pool attempts to maintain enough active (or available) threads
by dynamically adding, suspending, or resuming internal worker
threads, even if some tasks are stalled waiting to join others.
However, no such adjustments are guaranteed in the face of blocked
I/O or other unmanaged synchronization. The nested ForkJoinPool.ManagedBlocker interface enables extension of the kinds of
synchronization accommodated. The default policies may be
overridden using a constructor with parameters corresponding to
those documented in class ThreadPoolExecutor.
In addition to execution and lifecycle control methods, this
class provides status check methods (for example
getStealCount()) that are intended to aid in developing,
tuning, and monitoring fork/join applications. Also, method
toString() returns indications of pool state in a
convenient form for informal monitoring.
As is the case with other ExecutorServices, there are three
main task execution methods summarized in the following table.
These are designed to be used primarily by clients not already
engaged in fork/join computations in the current pool. The main
forms of these methods accept instances of ForkJoinTask,
but overloaded forms also allow mixed execution of plain
Runnable- or Callable- based activities as well. However,
tasks that are already executing in a pool should normally instead
use the within-computation forms listed in the table unless using
async event-style tasks that are not usually joined, in which case
there is little difference among choice of methods.
The parameters used to construct the common pool may be controlled by
setting the following system properties:
- java.util.concurrent.ForkJoinPool.common.parallelism
- the parallelism level, a non-negative integer
- java.util.concurrent.ForkJoinPool.common.threadFactory
- the class name of a ForkJoinPool.ForkJoinWorkerThreadFactory.
The system class loader
is used to load this class.
- java.util.concurrent.ForkJoinPool.common.exceptionHandler
- the class name of a UncaughtExceptionHandler.
The system class loader
is used to load this class.
- java.util.concurrent.ForkJoinPool.common.maximumSpares
- the maximum number of allowed extra threads to maintain target
parallelism (default 256).
If no thread factory is supplied via a system property, then the
common pool uses a factory that uses the system class loader as the
thread context class loader.
In addition, if a
SecurityManager is present, then
the common pool uses a factory supplying threads that have no
Permissions enabled.
Upon any error in establishing these settings, default parameters
are used. It is possible to disable or limit the use of threads in
the common pool by setting the parallelism property to zero, and/or
using a factory that may return
null. However doing so may
cause unjoined tasks to never be executed.
Implementation notes: This implementation restricts the
maximum number of running threads to 32767. Attempts to create
pools with greater than the maximum number result in
IllegalArgumentException.
This implementation rejects submitted tasks (that is, by throwing
RejectedExecutionException) only when the pool is shut down
or internal resources have been exhausted.
Since:
1.7
Author:
Doug Lea
Nested Class Summary |
static interface |
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static interface |
Interface for extending managed parallelism for tasks running
in ForkJoinPools.
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Field Summary |
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Creates a new ForkJoinWorkerThread.
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Constructor Summary |
Creates a ForkJoinPool with parallelism equal to Runtime.availableProcessors(), using defaults for all
other parameters (see ForkJoinPool(int,
ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
int, int, int, Predicate, long, TimeUnit)).
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Creates a ForkJoinPool with the indicated parallelism
level, using defaults for all other parameters (see ForkJoinPool(int, ForkJoinWorkerThreadFactory,
UncaughtExceptionHandler, boolean, int, int, int, Predicate,
long, TimeUnit)).
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Creates a ForkJoinPool with the given parameters.
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Method Summary |
boolean |
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boolean |
Blocks until all tasks have completed execution after a
shutdown request, or the timeout occurs, or the current thread
is interrupted, whichever happens first.
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Returns the common pool instance.
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protected int |
Removes all available unexecuted submitted and forked tasks
from scheduling queues and adds them to the given collection,
without altering their execution status.
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void |
Arranges for (asynchronous) execution of the given task.
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void |
Executes the given command at some time in the future.
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int |
Returns an estimate of the number of threads that are currently
stealing or executing tasks.
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boolean |
Returns true if this pool uses local first-in-first-out
scheduling mode for forked tasks that are never joined.
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static int |
Returns the targeted parallelism level of the common pool.
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Returns the factory used for constructing new workers.
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int |
Returns the targeted parallelism level of this pool.
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int |
Returns the number of worker threads that have started but not
yet terminated.
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int |
Returns an estimate of the number of tasks submitted to this
pool that have not yet begun executing.
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long |
Returns an estimate of the total number of tasks currently held
in queues by worker threads (but not including tasks submitted
to the pool that have not begun executing).
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int |
Returns an estimate of the number of worker threads that are
not blocked waiting to join tasks or for other managed
synchronization.
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long |
Returns an estimate of the total number of completed tasks that
were executed by a thread other than their submitter.
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Returns the handler for internal worker threads that terminate
due to unrecoverable errors encountered while executing tasks.
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boolean |
Returns true if there are any tasks submitted to this
pool that have not yet begun executing.
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Performs the given task, returning its result upon completion.
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Executes the given tasks, returning a list of Futures holding
their status and results when all complete.
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Executes the given tasks, returning a list of Futures holding
their status and results
when all complete or the timeout expires, whichever happens first.
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Executes the given tasks, returning the result
of one that has completed successfully (i.e., without throwing
an exception), if any do.
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Executes the given tasks, returning the result
of one that has completed successfully (i.e., without throwing
an exception), if any do before the given timeout elapses.
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boolean |
Returns true if all worker threads are currently idle.
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boolean |
Returns true if this pool has been shut down.
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boolean |
Returns true if all tasks have completed following shut down.
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boolean |
Returns true if the process of termination has
commenced but not yet completed.
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static void |
Runs the given possibly blocking task.
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Returns a RunnableFuture for the given callable task.
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Returns a RunnableFuture for the given runnable and default
value.
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Removes and returns the next unexecuted submission if one is
available.
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void |
Possibly initiates an orderly shutdown in which previously
submitted tasks are executed, but no new tasks will be
accepted.
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Possibly attempts to cancel and/or stop all tasks, and reject
all subsequently submitted tasks.
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Submits a value-returning task for execution and returns a
Future representing the pending results of the task.
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Submits a ForkJoinTask for execution.
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Submits a Runnable task for execution and returns a Future
representing that task.
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Submits a Runnable task for execution and returns a Future
representing that task.
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Returns a string identifying this pool, as well as its state,
including indications of run state, parallelism level, and
worker and task counts.
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Methods inherited from class java.lang. Object |
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defaultForkJoinWorkerThreadFactory
Creates a new ForkJoinWorkerThread. This factory is used unless
overridden in ForkJoinPool constructors.
Creates a
ForkJoinPool with parallelism equal to
Runtime.availableProcessors(), using defaults for all
other parameters (see
ForkJoinPool(int,
ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
int, int, int, Predicate, long, TimeUnit)).
Throws:
public ForkJoinPool |
(int parallelism) |
Creates a
ForkJoinPool with the indicated parallelism
level, using defaults for all other parameters (see
ForkJoinPool(int, ForkJoinWorkerThreadFactory,
UncaughtExceptionHandler, boolean, int, int, int, Predicate,
long, TimeUnit)).
Parameters:
parallelism - the parallelism level
Throws:
Creates a
ForkJoinPool with the given parameters (using
defaults for others -- see
ForkJoinPool(int,
ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
int, int, int, Predicate, long, TimeUnit)).
Parameters:
parallelism - the parallelism level. For default value,
use
Runtime.availableProcessors().
handler - the handler for internal worker threads that
terminate due to unrecoverable errors encountered while executing
tasks. For default value, use null.
asyncMode - if true,
establishes local first-in-first-out scheduling mode for forked
tasks that are never joined. This mode may be more appropriate
than default locally stack-based mode in applications in which
worker threads only process event-style asynchronous tasks.
For default value, use false.
Throws:
Creates a
ForkJoinPool with the given parameters.
Parameters:
parallelism - the parallelism level. For default value,
use
Runtime.availableProcessors().
handler - the handler for internal worker threads that
terminate due to unrecoverable errors encountered while
executing tasks. For default value, use null.
asyncMode - if true, establishes local first-in-first-out
scheduling mode for forked tasks that are never joined. This
mode may be more appropriate than default locally stack-based
mode in applications in which worker threads only process
event-style asynchronous tasks. For default value, use
false.
corePoolSize - the number of threads to keep in the pool
(unless timed out after an elapsed keep-alive). Normally (and
by default) this is the same value as the parallelism level,
but may be set to a larger value to reduce dynamic overhead if
tasks regularly block. Using a smaller value (for example
0) has the same effect as the default.
maximumPoolSize - the maximum number of threads allowed.
When the maximum is reached, attempts to replace blocked
threads fail. (However, because creation and termination of
different threads may overlap, and may be managed by the given
thread factory, this value may be transiently exceeded.) To
arrange the same value as is used by default for the common
pool, use 256 plus the parallelism level. (By
default, the common pool allows a maximum of 256 spare
threads.) Using a value (for example
Integer.MAX_VALUE) larger than the implementation's total
thread limit has the same effect as using this limit (which is
the default).
minimumRunnable - the minimum allowed number of core
threads not blocked by a join or
ForkJoinPool.ManagedBlocker. To
ensure progress, when too few unblocked threads exist and
unexecuted tasks may exist, new threads are constructed, up to
the given maximumPoolSize. For the default value, use
1, that ensures liveness. A larger value might improve
throughput in the presence of blocked activities, but might
not, due to increased overhead. A value of zero may be
acceptable when submitted tasks cannot have dependencies
requiring additional threads.
saturate - if non-null, a predicate invoked upon attempts
to create more than the maximum total allowed threads. By
default, when a thread is about to block on a join or
ForkJoinPool.ManagedBlocker, but cannot be replaced because the
maximumPoolSize would be exceeded, a
RejectedExecutionException is thrown. But if this predicate
returns
true, then no exception is thrown, so the pool
continues to operate with fewer than the target number of
runnable threads, which might not ensure progress.
keepAliveTime - the elapsed time since last use before
a thread is terminated (and then later replaced if needed).
For the default value, use 60, TimeUnit.SECONDS.
unit - the time unit for the keepAliveTime argument
Throws:
IllegalArgumentException - if parallelism is less than or
equal to zero, or is greater than implementation limit,
or if maximumPoolSize is less than parallelism,
of if the keepAliveTime is less than or equal to zero.
Since:
9
Returns the common pool instance. This pool is statically
constructed; its run state is unaffected by attempts to
shutdown() or
shutdownNow(). However this pool and any
ongoing processing are automatically terminated upon program
System.exit(int). Any program that relies on asynchronous
task processing to complete before program termination should
invoke
commonPool().awaitQuiescence,
before exit.
Returns:
the common pool instance
Since:
1.8
Performs the given task, returning its result upon completion.
If the computation encounters an unchecked Exception or Error,
it is rethrown as the outcome of this invocation. 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 current thread
as well as the thread actually encountering the exception;
minimally only the latter.
Type Parameters:
T - the type of the task's result
Parameters:
task - the task
Returns:
the task's result
Throws:
Arranges for (asynchronous) execution of the given task.
Parameters:
task - the task
Throws:
Description copied from interface:
Executor
Executes the given command at some time in the future. The command
may execute in a new thread, in a pooled thread, or in the calling
thread, at the discretion of the Executor implementation.
Specified by:
Parameters:
task - the runnable task
Throws:
Submits a ForkJoinTask for execution.
Type Parameters:
T - the type of the task's result
Parameters:
task - the task to submit
Returns:
the task
Throws:
Submits a value-returning task for execution and returns a
Future representing the pending results of the task. The
Future's
get method will return the task's result upon
successful completion.
If you would like to immediately block waiting
for a task, you can use constructions of the form
result = exec.submit(aCallable).get();
Note: The Executors class includes a set of methods
that can convert some other common closure-like objects,
for example, PrivilegedAction to
Callable form so they can be submitted.
Specified by:
Overrides:
Type Parameters:
T - the type of the task's result
Parameters:
task - the task to submit
Returns:
a Future representing pending completion of the task
Throws:
Submits a Runnable task for execution and returns a Future
representing that task. The Future's get method will
return the given result upon successful completion.
Specified by:
Overrides:
Type Parameters:
T - the type of the result
Parameters:
task - the task to submit
result - the result to return
Returns:
a Future representing pending completion of the task
Throws:
Submits a Runnable task for execution and returns a Future
representing that task. The Future's get method will
return null upon successful completion.
Specified by:
Overrides:
Parameters:
task - the task to submit
Returns:
a Future representing pending completion of the task
Throws:
Executes the given tasks, returning a list of Futures holding
their status and results when all complete.
Future.isDone() is
true for each
element of the returned list.
Note that a
completed task could have
terminated either normally or by throwing an exception.
The results of this method are undefined if the given
collection is modified while this operation is in progress.
Specified by:
Overrides:
Type Parameters:
T - the type of the values returned from the tasks
Parameters:
tasks - the collection of tasks
Returns:
a list of Futures representing the tasks, in the same
sequential order as produced by the iterator for the
given task list, each of which has completed
Throws:
Executes the given tasks, returning a list of Futures holding
their status and results
when all complete or the timeout expires, whichever happens first.
Future.isDone() is
true for each
element of the returned list.
Upon return, tasks that have not completed are cancelled.
Note that a
completed task could have
terminated either normally or by throwing an exception.
The results of this method are undefined if the given
collection is modified while this operation is in progress.
Specified by:
Overrides:
Type Parameters:
T - the type of the values returned from the tasks
Parameters:
tasks - the collection of tasks
timeout - the maximum time to wait
unit - the time unit of the timeout argument
Returns:
a list of Futures representing the tasks, in the same
sequential order as produced by the iterator for the
given task list. If the operation did not time out,
each task will have completed. If it did time out, some
of these tasks will not have completed.
Throws:
Executes the given tasks, returning the result
of one that has completed successfully (i.e., without throwing
an exception), if any do. Upon normal or exceptional return,
tasks that have not completed are cancelled.
The results of this method are undefined if the given
collection is modified while this operation is in progress.
Specified by:
Overrides:
Type Parameters:
T - the type of the values returned from the tasks
Parameters:
tasks - the collection of tasks
Returns:
the result returned by one of the tasks
Throws:
Executes the given tasks, returning the result
of one that has completed successfully (i.e., without throwing
an exception), if any do before the given timeout elapses.
Upon normal or exceptional return, tasks that have not
completed are cancelled.
The results of this method are undefined if the given
collection is modified while this operation is in progress.
Specified by:
Overrides:
Type Parameters:
T - the type of the values returned from the tasks
Parameters:
tasks - the collection of tasks
timeout - the maximum time to wait
unit - the time unit of the timeout argument
Returns:
the result returned by one of the tasks
Throws:
Returns the factory used for constructing new workers.
Returns:
the factory used for constructing new workers
getUncaughtExceptionHandler
Returns the handler for internal worker threads that terminate
due to unrecoverable errors encountered while executing tasks.
Returns:
the handler, or null if none
public int getParallelism |
() |
Returns the targeted parallelism level of this pool.
Returns:
the targeted parallelism level of this pool
public static int getCommonPoolParallelism |
() |
Returns the targeted parallelism level of the common pool.
Returns:
the targeted parallelism level of the common pool
Since:
1.8
public int getPoolSize |
() |
Returns the number of worker threads that have started but not
yet terminated. The result returned by this method may differ
from
getParallelism() when threads are created to
maintain parallelism when others are cooperatively blocked.
Returns:
the number of worker threads
public boolean getAsyncMode |
() |
Returns
true if this pool uses local first-in-first-out
scheduling mode for forked tasks that are never joined.
Returns:
true if this pool uses async mode
public int getRunningThreadCount |
() |
Returns an estimate of the number of worker threads that are
not blocked waiting to join tasks or for other managed
synchronization. This method may overestimate the
number of running threads.
Returns:
the number of worker threads
public int getActiveThreadCount |
() |
Returns an estimate of the number of threads that are currently
stealing or executing tasks. This method may overestimate the
number of active threads.
Returns:
the number of active threads
public boolean isQuiescent |
() |
Returns
true if all worker threads are currently idle.
An idle worker is one that cannot obtain a task to execute
because none are available to steal from other threads, and
there are no pending submissions to the pool. This method is
conservative; it might not return
true immediately upon
idleness of all threads, but will eventually become true if
threads remain inactive.
Returns:
true if all threads are currently idle
public long getStealCount |
() |
Returns an estimate of the total number of completed tasks that
were executed by a thread other than their submitter. The
reported value underestimates the actual total number of steals
when the pool is not quiescent. This value may be useful for
monitoring and tuning fork/join programs: in general, steal
counts should be high enough to keep threads busy, but low
enough to avoid overhead and contention across threads.
Returns:
the number of steals
public long getQueuedTaskCount |
() |
Returns an estimate of the total number of tasks currently held
in queues by worker threads (but not including tasks submitted
to the pool that have not begun executing). This value is only
an approximation, obtained by iterating across all threads in
the pool. This method may be useful for tuning task
granularities.
Returns:
the number of queued tasks
public int getQueuedSubmissionCount |
() |
Returns an estimate of the number of tasks submitted to this
pool that have not yet begun executing. This method may take
time proportional to the number of submissions.
Returns:
the number of queued submissions
public boolean hasQueuedSubmissions |
() |
Returns
true if there are any tasks submitted to this
pool that have not yet begun executing.
Returns:
true if there are any queued submissions
Removes and returns the next unexecuted submission if one is
available. This method may be useful in extensions to this
class that re-assign work in systems with multiple pools.
Returns:
the next submission, or null if none
protected int drainTasksTo |
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Removes all available unexecuted submitted and forked tasks
from scheduling queues and adds them to the given collection,
without altering their execution status. These may include
artificially generated or wrapped tasks. This method is
designed to be invoked only when the pool is known to be
quiescent. Invocations at other times may not remove all
tasks. A failure encountered while attempting to add elements
to collection
c may result in elements being in
neither, either or both collections when the associated
exception is thrown. The behavior of this operation is
undefined if the specified collection is modified while the
operation is in progress.
Parameters:
c - the collection to transfer elements into
Returns:
the number of elements transferred
Returns a string identifying this pool, as well as its state,
including indications of run state, parallelism level, and
worker and task counts.
Overrides:
Returns:
a string identifying this pool, as well as its state
Possibly initiates an orderly shutdown in which previously
submitted tasks are executed, but no new tasks will be
accepted. Invocation has no effect on execution state if this
is the
commonPool(), and no additional effect if
already shut down. Tasks that are in the process of being
submitted concurrently during the course of this method may or
may not be rejected.
Specified by:
Throws:
Possibly attempts to cancel and/or stop all tasks, and reject
all subsequently submitted tasks. Invocation has no effect on
execution state if this is the
commonPool(), and no
additional effect if already shut down. Otherwise, tasks that
are in the process of being submitted or executed concurrently
during the course of this method may or may not be
rejected. This method cancels both existing and unexecuted
tasks, in order to permit termination in the presence of task
dependencies. So the method always returns an empty list
(unlike the case for some other Executors).
Specified by:
Returns:
an empty list
Throws:
public boolean isTerminated |
() |
Returns
true if all tasks have completed following shut down.
Specified by:
Returns:
true if all tasks have completed following shut down
public boolean isTerminating |
() |
Returns
true if the process of termination has
commenced but not yet completed. This method may be useful for
debugging. A return of
true reported a sufficient
period after shutdown may indicate that submitted tasks have
ignored or suppressed interruption, or are waiting for I/O,
causing this executor not to properly terminate. (See the
advisory notes for class
ForkJoinTask stating that
tasks should not normally entail blocking operations. But if
they do, they must abort them on interrupt.)
Returns:
true if terminating but not yet terminated
public boolean isShutdown |
() |
Returns
true if this pool has been shut down.
Specified by:
Returns:
true if this pool has been shut down
public boolean awaitTermination |
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throws |
|
Blocks until all tasks have completed execution after a
shutdown request, or the timeout occurs, or the current thread
is interrupted, whichever happens first. Because the
commonPool() never terminates until program shutdown, when
applied to the common pool, this method is equivalent to
awaitQuiescence(long, TimeUnit) but always returns
false.
Specified by:
Parameters:
timeout - the maximum time to wait
unit - the time unit of the timeout argument
Returns:
true if this executor terminated and
false if the timeout elapsed before termination
Throws:
public boolean awaitQuiescence |
|
If called by a ForkJoinTask operating in this pool, equivalent
in effect to
ForkJoinTask.helpQuiesce(). Otherwise,
waits and/or attempts to assist performing tasks until this
pool
isQuiescent() or the indicated timeout elapses.
Parameters:
timeout - the maximum time to wait
unit - the time unit of the timeout argument
Returns:
true if quiescent; false if the
timeout elapsed.
public static void managedBlock |
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throws |
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Runs the given possibly blocking task. When
running in a ForkJoinPool, this
method possibly arranges for a spare thread to be activated if
necessary to ensure sufficient parallelism while the current
thread is blocked in
blocker.block().
This method repeatedly calls blocker.isReleasable() and
blocker.block() until either method returns true.
Every call to blocker.block() is preceded by a call to
blocker.isReleasable() that returned false.
If not running in a ForkJoinPool, this method is
behaviorally equivalent to
while (!blocker.isReleasable())
if (blocker.block())
break;
If running in a ForkJoinPool, the pool may first be expanded to
ensure sufficient parallelism available during the call to
blocker.block().
Parameters:
blocker - the blocker task
Throws:
Returns a RunnableFuture for the given runnable and default
value.
Overrides:
Type Parameters:
T - the type of the given value
Parameters:
runnable - the runnable task being wrapped
value - the default value for the returned future
Returns:
a RunnableFuture which, when run, will run the
underlying runnable and which, as a Future, will yield
the given value as its result and provide for cancellation of
the underlying task
Returns a RunnableFuture for the given callable task.
Overrides:
Type Parameters:
T - the type of the callable's result
Parameters:
callable - the callable task being wrapped
Returns:
a RunnableFuture which, when run, will call the
underlying callable and which, as a Future, will yield
the callable's result as its result and provide for
cancellation of the underlying task
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