A "main" ForkJoinTask begins execution when submitted to a
- * {@link ForkJoinPool}. Once started, it will usually in turn start
- * other subtasks. As indicated by the name of this class, many
- * programs using ForkJoinTasks 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 {@link Future}. The
- * efficiency of ForkJoinTasks stems from a set of restrictions (that
- * are only partially statically enforceable) reflecting their
- * intended use as computational tasks calculating pure functions or
- * operating on purely isolated objects. The primary coordination
- * mechanisms are {@link #fork}, that arranges asynchronous execution,
- * and {@link #join}, that doesn't proceed until the task's result has
- * been computed. Computations should 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. Tasks should also not perform blocking IO,
- * and should ideally access variables that are completely independent
- * of those accessed by other running tasks. Minor breaches of these
- * restrictions, for example using shared output streams, may be
- * tolerable in practice, but frequent use may result in poor
- * performance, and the potential to indefinitely stall if the number
- * of threads not waiting for IO or other external synchronization
- * becomes exhausted. This usage restriction is in part 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 join
- * them. These exceptions may additionally include
- * RejectedExecutionExceptions stemming from internal resource
- * exhaustion such as failure to allocate internal task queues.
+ *
+ *
A "main" {@code ForkJoinTask} begins execution when submitted + * to a {@link ForkJoinPool}. Once started, it will usually in turn + * start other subtasks. As indicated by the name of this class, + * many programs using {@code ForkJoinTask} employ only methods + * {@link #fork} and {@link #join}, or derivatives such as {@link + * #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 {@code ForkJoinTask} is a lightweight form of {@link Future}. + * The efficiency of {@code ForkJoinTask}s stems from a set of + * restrictions (that are only partially statically enforceable) + * reflecting their intended use as computational tasks calculating + * pure functions or operating on purely isolated objects. The + * primary coordination mechanisms are {@link #fork}, that arranges + * asynchronous execution, and {@link #join}, that doesn't proceed + * until the task's result has been computed. Computations should + * avoid {@code 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. Tasks should also not perform + * blocking IO, and should ideally access variables that are + * completely independent of those accessed by other running + * tasks. Minor breaches of these restrictions, for example using + * shared output streams, may be tolerable in practice, but frequent + * use may result in poor performance, and the potential to + * indefinitely stall if the number of threads not waiting for IO or + * other external synchronization becomes exhausted. This usage + * restriction is in part enforced by not permitting checked + * exceptions such as {@code 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 RejectedExecutionExceptions stemming from + * internal resource exhaustion such as failure to allocate internal + * task queues. * *
The primary method for awaiting completion and extracting
* results of a task is {@link #join}, but there are several variants:
* The {@link Future#get} methods support interruptible and/or timed
- * waits for completion and report results using Future
+ * waits for completion and report results using {@code Future}
* conventions. Method {@link #helpJoin} enables callers to actively
* execute other tasks while awaiting joins, which is sometimes more
* efficient but only applies when all subtasks are known to be
* strictly tree-structured. Method {@link #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
+ * equivalent to {@code 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)
+ * Method {@code invokeAll} (available in multiple versions)
* performs the most common form of parallel invocation: forking a set
* of tasks and joining them all.
*
- *
The ForkJoinTask class is not usually directly subclassed. + *
The execution status of tasks may be queried at several levels + * of detail: {@link #isDone} is true if a task completed in any way + * (including the case where a task was cancelled without executing); + * {@link #isCancelled} is true if completion was due to cancellation; + * {@link #isCompletedNormally} is true if a task completed without + * cancellation or encountering an exception; {@link + * #isCompletedExceptionally} is true if if the task encountered an + * exception (in which case {@link #getException} returns the + * exception); {@link #isCancelled} is true if the task was cancelled + * (in which case {@link #getException} returns a {@link + * java.util.concurrent.CancellationException}; and {@link + * #isCompletedAbnormally} is true if a task was either cancelled or + * encountered an exception. + * + *
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. Normally, a concrete
+ * particular style of fork/join processing, typically {@link
+ * RecursiveAction} for computations that do not return results, or
+ * {@link RecursiveTask} for those that do. Normally, a concrete
* ForkJoinTask subclass declares fields comprising its parameters,
- * established in a constructor, and then defines a compute
+ * established in a constructor, and then defines a {@code compute}
* method that somehow uses the control methods supplied by this base
- * class. While these methods have public access (to allow
- * instances of different task subclasses to call each others
+ * class. While these methods have {@code public} access (to allow
+ * instances of different task subclasses to call each other's
* methods), some of them may only be called from within other
- * ForkJoinTasks. Attempts to invoke them in other contexts result in
- * exceptions or errors including ClassCastException.
+ * ForkJoinTasks (as may be determined using method {@link
+ * #inForkJoinPool}). Attempts to invoke them in other contexts
+ * result in exceptions or errors, possibly including
+ * ClassCastException.
*
- *
Most base support methods are final because their
- * implementations are intrinsically tied to the underlying
- * lightweight task scheduling framework, and so cannot be overridden.
- * Developers creating new basic styles of fork/join processing should
- * minimally implement protected methods
- * exec, setRawResult, and
- * getRawResult, while also introducing an abstract
- * computational method that can be implemented in its subclasses,
- * possibly relying on other protected methods provided
- * by this class.
+ *
Most base support methods are {@code 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 {@code protected} methods {@link #exec}, {@link + * #setRawResult}, and {@link #getRawResult}, while also introducing + * an abstract computational method that can be implemented in its + * subclasses, possibly relying on other {@code protected} methods + * provided by this class. * *
ForkJoinTasks should perform relatively small amounts of - * computations, othewise splitting into smaller tasks. As a very - * rough rule of thumb, a task should perform more than 100 and less - * than 10000 basic computational steps. If tasks are too big, then - * parellelism cannot improve throughput. If too small, then memory - * and internal task maintenance overhead may overwhelm processing. + * 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. 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 {@code adapt} methods for {@link + * java.lang.Runnable} and {@link java.util.concurrent.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 in {@link ForkJoinPool#setAsyncMode async mode}. + * + *
ForkJoinTasks are {@code 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. * - *
ForkJoinTasks are This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return {@code this}, to simplify usage
*/
- public final void fork() {
- ((ForkJoinWorkerThread)(Thread.currentThread())).pushTask(this);
+ public final ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @param t1 the first task
+ * @param t2 the second task
+ * @throws NullPointerException if any task is null
*/
- public static void invokeAll(ForkJoinTaskt1, ForkJoinTask t2) {
+ public static void invokeAll(ForkJoinTask t1, ForkJoinTask t2) {
t2.fork();
t1.invoke();
t2.join();
}
/**
- * Forks the given tasks, returning when This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @param tasks the tasks
+ * @throws NullPointerException if any task is null
*/
public static void invokeAll(ForkJoinTask... tasks) {
Throwable ex = null;
@@ -571,23 +632,35 @@ public abstract class ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
* @param tasks the collection of tasks
- * @throws NullPointerException if tasks or any element are null.
- * @throws RuntimeException or Error if any task did so.
+ * @return the tasks argument, to simplify usage
+ * @throws NullPointerException if tasks or any element are null
*/
- public static void invokeAll(Collection> tasks) {
- if (!(tasks instanceof List)) {
- invokeAll(tasks.toArray(new ForkJoinTask[tasks.size()]));
- return;
+ public static This method may be overridden in subclasses, but if so, must
* still ensure that these minimal properties hold. In particular,
- * the cancel method itself must not throw exceptions.
+ * the {@code cancel} method itself must not throw exceptions.
*
- * This method is designed to be invoked by other
+ * 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 This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
* @return the computed result
*/
public final V helpJoin() {
- ForkJoinWorkerThread w = (ForkJoinWorkerThread)(Thread.currentThread());
+ ForkJoinWorkerThread w = (ForkJoinWorkerThread) Thread.currentThread();
if (status < 0 || !w.unpushTask(this) || !tryExec())
reportException(busyJoin(w));
return getRawResult();
@@ -770,14 +871,19 @@ public abstract class ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
*/
public final void quietlyHelpJoin() {
if (status >= 0) {
ForkJoinWorkerThread w =
- (ForkJoinWorkerThread)(Thread.currentThread());
+ (ForkJoinWorkerThread) Thread.currentThread();
if (!w.unpushTask(this) || !tryQuietlyInvoke())
busyJoin(w);
}
@@ -811,23 +917,30 @@ public abstract class ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
*/
public static void helpQuiesce() {
- ((ForkJoinWorkerThread)(Thread.currentThread())).
- helpQuiescePool();
+ ((ForkJoinWorkerThread) Thread.currentThread())
+ .helpQuiescePool();
}
/**
* Resets the internal bookkeeping state of this task, allowing a
- * subsequent This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return {@code true} if unforked
*/
public boolean tryUnfork() {
- return ((ForkJoinWorkerThread)(Thread.currentThread())).unpushTask(this);
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .unpushTask(this);
}
/**
@@ -867,15 +999,22 @@ public abstract class ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
* @return the number of tasks
*/
public static int getQueuedTaskCount() {
- return ((ForkJoinWorkerThread)(Thread.currentThread())).
- getQueueSize();
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .getQueueSize();
}
/**
- * Returns a estimate of how many more locally queued tasks are
+ * 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. This value may be useful for
* heuristic decisions about whether to fork other tasks. In many
@@ -883,23 +1022,30 @@ public abstract class ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
* @return the surplus number of tasks, which may be negative
*/
public static int getSurplusQueuedTaskCount() {
- return ((ForkJoinWorkerThread)(Thread.currentThread()))
+ return ((ForkJoinWorkerThread) Thread.currentThread())
.getEstimatedSurplusTaskCount();
}
// Extension methods
/**
- * Returns the result that would be returned by This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
*
- * @return the next task, or null if none are available
+ * @return the next task, or {@code null} if none are available
*/
protected static ForkJoinTask peekNextLocalTask() {
- return ((ForkJoinWorkerThread)(Thread.currentThread())).peekTask();
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .peekTask();
}
/**
- * Unschedules and returns, without executing, the task most
- * recently forked by the current thread but not yet executed.
- * This method is designed primarily to support extensions,
- * and is unlikely to be useful otherwise.
+ * Unschedules and returns, without executing, the next task
+ * queued by the current thread but not yet executed. This method
+ * is designed primarily to support extensions, and is unlikely to
+ * be useful otherwise.
*
- * @return the next task, or null if none are available
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return the next task, or {@code null} if none are available
*/
protected static ForkJoinTask pollNextLocalTask() {
- return ((ForkJoinWorkerThread)(Thread.currentThread())).popTask();
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .pollLocalTask();
}
/**
- * Unschedules and returns, without executing, the task most
- * recently forked 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 This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return a task, or {@code null} if none are available
*/
protected static ForkJoinTask pollTask() {
- return ((ForkJoinWorkerThread)(Thread.currentThread())).
- pollLocalOrStolenTask();
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .pollTask();
+ }
+
+ /**
+ * Adaptor for Runnables. This implements RunnableFuture
+ * to be compliant with AbstractExecutorService constraints
+ * when used in ForkJoinPool.
+ */
+ static final class AdaptedRunnableSerializable, which enables them
- * to be used in extensions such as remote execution frameworks. It is
- * in general sensible to serialize tasks only before or after, but
- * not during execution. Serialization is not relied on during
- * execution itself.
+ * @since 1.7
+ * @author Doug Lea
*/
public abstract class ForkJoinTaskget in that abnormal
- * completion results in RuntimeExceptions or Errors, not
- * ExecutionExceptions.
+ * This method differs from {@link #get()} in that
+ * abnormal completion results in {@code RuntimeException} or
+ * {@code Error}, not {@code ExecutionException}.
*
* @return the computed result
*/
@@ -498,9 +543,9 @@ public abstract class ForkJoinTaskisDone holds for
- * both of them or an exception is encountered. This method may be
- * invoked only from within ForkJoinTask computations. Attempts to
- * invoke in other contexts result in exceptions or errors
- * including ClassCastException.
- * @param t1 one task
- * @param t2 the other task
- * @throws NullPointerException if t1 or t2 are null
- * @throws RuntimeException or Error if either task did so.
+ * Forks the given tasks, returning when {@code 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. The individual status of each task may be
+ * checked using {@link #getException()} and related methods.
+ *
+ * isDone holds
- * for all of them. If any task encounters an exception, others
- * may be cancelled. This method may be invoked only from within
- * ForkJoinTask computations. Attempts to invoke in other contexts
- * result in exceptions or errors including ClassCastException.
- * @param tasks the array of tasks
- * @throws NullPointerException if tasks or any element are null.
- * @throws RuntimeException or Error if any task did so.
+ * Forks the given tasks, returning when {@code isDone} holds for
+ * each task or an (unchecked) exception is encountered, in which
+ * case the exception is rethrown. If any task encounters an
+ * exception, others may be, but are not guaranteed to be,
+ * cancelled. If more than one task encounters an exception, then
+ * this method throws any one of these exceptions. The individual
+ * status of each task may be checked using {@link #getException()}
+ * and related methods.
+ *
+ * isDone holds for all of them. If any task
- * encounters an exception, others may be cancelled. This method
- * may be invoked only from within ForkJoinTask
- * computations. Attempts to invoke in other contexts resul!t in
- * exceptions or errors including ClassCastException.
+ * Forks all tasks in the specified collection, returning when
+ * {@code isDone} holds for each task or an (unchecked) exception
+ * is encountered. If any task encounters an exception, others
+ * may be, but are not guaranteed to be, cancelled. If more than
+ * one task encounters an exception, then this method throws any
+ * one of these exceptions. The individual status of each task
+ * may be checked using {@link #getException()} and related
+ * methods. The behavior of this operation is undefined if the
+ * specified collection is modified while the operation is in
+ * progress.
+ *
+ * isCancelled
- * will report true, and join will result in a
- * CancellationException being thrown. Otherwise, when
- * cancellation races with completion, there are no guarantees
- * about whether isCancelled will report true, whether
- * join will return normally or via an exception, or
- * whether these behaviors will remain consistent upon repeated
- * invocation.
+ * Attempts to cancel execution of this task. This attempt will
+ * fail if the task has already completed, has already been
+ * cancelled, or could not be cancelled for some other reason. If
+ * successful, and this task has not started when cancel is
+ * called, execution of this task is suppressed, {@link
+ * #isCancelled} will report true, and {@link #join} will result
+ * in a {@code CancellationException} being thrown.
*
* completeExceptionally.
+ * invoke {@link #completeExceptionally}.
*
* @param mayInterruptIfRunning this value is ignored in the
- * default implementation because tasks are not in general
- * cancelled via interruption.
+ * default implementation because tasks are not
+ * cancelled via interruption
*
- * @return true if this task is now cancelled
+ * @return {@code true} if this task is now cancelled
*/
public boolean cancel(boolean mayInterruptIfRunning) {
setCompletion(CANCELLED);
@@ -670,18 +724,59 @@ public abstract class ForkJoinTaskjoin and related operations. This method may be used
+ * {@code 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 likely to be wrong. This method is
- * overridable, but overridden versions must invoke super
+ * in other situations is discouraged. This method is
+ * overridable, but overridden versions must invoke {@code super}
* implementation to maintain guarantees.
*
* @param ex the exception to throw. If this exception is
@@ -708,26 +803,26 @@ public abstract class ForkJoinTasknull result upon join and related
+ * returning a {@code null} result upon {@code 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 likely to be wrong. This method is
- * overridable, but overridden versions must invoke super
+ * other situations is discouraged. This method is
+ * overridable, but overridden versions must invoke {@code super}
* implementation to maintain guarantees.
*
- * @param value the result value for this task.
+ * @param value the result value for this task
*/
public void complete(V value) {
try {
setRawResult(value);
- } catch(Throwable rex) {
+ } catch (Throwable rex) {
setDoneExceptionally(rex);
return;
}
@@ -743,26 +838,32 @@ public abstract class ForkJoinTaskjoin, but is only applicable when
- * there are no potemtial dependencies between continuation of the
+ * efficient than {@code join}, but is only applicable when
+ * there are no potential dependencies between continuation of the
* current task and that of any other task that might be executed
* while helping. (This usually holds for pure divide-and-conquer
- * tasks). This method may be invoked only from within
- * ForkJoinTask computations. Attempts to invoke in other contexts
- * resul!t in exceptions or errors including ClassCastException.
+ * tasks).
+ *
+ * fork. This method allows repeated reuse of
+ * subsequent {@code 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, and are
- * almost surely wrong. This method may be useful when executing
+ * under any other usage conditions are not guaranteed.
+ * This method may be useful when executing
* pre-constructed trees of subtasks in loops.
*/
public void reinitialize() {
@@ -838,13 +951,26 @@ public abstract class ForkJoinTaskjoin,
- * 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 that would be returned by {@link #join}, even
+ * if this task completed abnormally, or {@code 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.
*
- * @return the result, or null if not completed.
+ * @return the result, or {@code null} if not completed
*/
public abstract V getRawResult();
@@ -918,54 +1064,169 @@ public abstract class ForkJoinTaskcomplete to become joinable. It may throw exceptions
- * to indicate abnormal exit.
- * @return true if completed normally
- * @throws Error or RuntimeException if encountered during computation
+ * {@link #complete} to become joinable. It may also throw an
+ * (unchecked) exception to indicate abnormal exit.
+ *
+ * @return {@code true} if completed normally
*/
protected abstract boolean exec();
/**
- * Returns, but does not unschedule or execute, the task most
- * recently forked by the current thread but not yet executed, if
- * one is available. There is no guarantee that this task will
- * actually be polled or executed next.
- * This method is designed primarily to support extensions,
- * and is unlikely to be useful otherwise.
+ * 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.
+ *
+ * null result does not necessarily
- * imply quiecence of the pool this task is operating in.
- * This method is designed primarily to support extensions,
- * and is unlikely to be useful otherwise.
- *
- * @return a task, or null if none are available
+ * 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
+ * {@code 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.
+ *
+ *