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root/jsr166/jsr166/src/jsr166y/ForkJoinTask.java

Comparing jsr166/src/jsr166y/ForkJoinTask.java (file contents):
Revision 1.88 by dl, Sun Mar 4 19:47:08 2012 UTC vs.
Revision 1.91 by dl, Sun Oct 28 22:36:01 2012 UTC

#	Line 5 | Line 5
5		*/
6
7		package jsr166y;
8	+
9		import java.io.Serializable;
10		import java.util.Collection;
11		import java.util.List;
#	Line 29 | Line 30 | import java.lang.reflect.Constructor;
30		* subtasks may be hosted by a small number of actual threads in a
31		* ForkJoinPool, at the price of some usage limitations.
32		*
33	<	* <p>A "main" {@code ForkJoinTask} begins execution when submitted
34	<	* to a {@link ForkJoinPool}.  Once started, it will usually in turn
35	<	* start other subtasks.  As indicated by the name of this class,
36	<	* many programs using {@code ForkJoinTask} employ only methods
37	<	* {@link #fork} and {@link #join}, or derivatives such as {@link
33	>	* <p>A "main" {@code ForkJoinTask} begins execution when it is
34	>	* explicitly submitted to a {@link ForkJoinPool}, or, if not already
35	>	* engaged in a ForkJoin computation, commenced in the {@link
36	>	* ForkJoinPool#commonPool} via {@link #fork}, {@link #invoke}, or
37	>	* related methods.  Once started, it will usually in turn start other
38	>	* subtasks.  As indicated by the name of this class, many programs
39	>	* using {@code ForkJoinTask} employ only methods {@link #fork} and
40	>	* {@link #join}, or derivatives such as {@link
41		* #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
42		* provides a number of other methods that can come into play in
43	<	* advanced usages, as well as extension mechanics that allow
44	<	* support of new forms of fork/join processing.
43	>	* advanced usages, as well as extension mechanics that allow support
44	>	* of new forms of fork/join processing.
45		*
46		* <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
47		* The efficiency of {@code ForkJoinTask}s stems from a set of
#	Line 70 | Line 74 | import java.lang.reflect.Constructor;
74		* but doing do requires three further considerations: (1) Completion
75		* of few if any <em>other</em> tasks should be dependent on a task
76		* that blocks on external synchronization or IO. Event-style async
77	<	* tasks that are never joined often fall into this category.  (2) To
78	<	* minimize resource impact, tasks should be small; ideally performing
79	<	* only the (possibly) blocking action. (3) Unless the {@link
77	>	* tasks that are never joined (for example, those subclassing {@link
78	>	* CountedCompleter}) often fall into this category.  (2) To minimize
79	>	* resource impact, tasks should be small; ideally performing only the
80	>	* (possibly) blocking action. (3) Unless the {@link
81		* ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
82		* blocked tasks is known to be less than the pool's {@link
83		* ForkJoinPool#getParallelism} level, the pool cannot guarantee that
#	Line 115 | Line 120 | import java.lang.reflect.Constructor;
120		* <p>The ForkJoinTask class is not usually directly subclassed.
121		* Instead, you subclass one of the abstract classes that support a
122		* particular style of fork/join processing, typically {@link
123	<	* RecursiveAction} for computations that do not return results, or
124	<	* {@link RecursiveTask} for those that do.  Normally, a concrete
125	<	* ForkJoinTask subclass declares fields comprising its parameters,
126	<	* established in a constructor, and then defines a {@code compute}
127	<	* method that somehow uses the control methods supplied by this base
128	<	* class. While these methods have {@code public} access (to allow
129	<	* instances of different task subclasses to call each other's
125	<	* methods), some of them may only be called from within other
126	<	* ForkJoinTasks (as may be determined using method {@link
127	<	* #inForkJoinPool}).  Attempts to invoke them in other contexts
128	<	* result in exceptions or errors, possibly including
129	<	* {@code ClassCastException}.
123	>	* RecursiveAction} for most computations that do not return results,
124	>	* {@link RecursiveTask} for those that do, and {@link
125	>	* CountedCompleter} for those in which completed actions trigger
126	>	* other actions.  Normally, a concrete ForkJoinTask subclass declares
127	>	* fields comprising its parameters, established in a constructor, and
128	>	* then defines a {@code compute} method that somehow uses the control
129	>	* methods supplied by this base class.
130		*
131		* <p>Method {@link #join} and its variants are appropriate for use
132		* only when completion dependencies are acyclic; that is, the
#	Line 137 | Line 137 | import java.lang.reflect.Constructor;
137		* {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
138		* may be of use in constructing custom subclasses for problems that
139		* are not statically structured as DAGs. To support such usages a
140	<	* ForkJoinTask may be atomically <em>tagged</em> with a {@code
141	<	* short} value using {@link #setForkJoinTaskTag} or {@link
140	>	* ForkJoinTask may be atomically <em>tagged</em> with a {@code short}
141	>	* value using {@link #setForkJoinTaskTag} or {@link
142		* #compareAndSetForkJoinTaskTag} and checked using {@link
143	<	* #getForkJoinTaskTag}. The ForkJoinTask implementation does not
144	<	* use these {@code protected} methods or tags for any purpose, but
145	<	* they may be of use in the construction of specialized subclasses.
146	<	* For example, parallel graph traversals can use the supplied methods
147	<	* to avoid revisiting nodes/tasks that have already been processed.
148	<	* Also, completion based designs can use them to record that subtasks
149	<	* have completed. (Method names for tagging are bulky in part to
150	<	* encourage definition of methods that reflect their usage patterns.)
143	>	* #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
144	>	* these {@code protected} methods or tags for any purpose, but they
145	>	* may be of use in the construction of specialized subclasses.  For
146	>	* example, parallel graph traversals can use the supplied methods to
147	>	* avoid revisiting nodes/tasks that have already been processed.
148	>	* (Method names for tagging are bulky in part to encourage definition
149	>	* of methods that reflect their usage patterns.)
150		*
151		* <p>Most base support methods are {@code final}, to prevent
152		* overriding of implementations that are intrinsically tied to the
#	Line 269 | Line 268 | public abstract class ForkJoinTask<V> im
268		}
269
270		/**
271	<	* Tries to set SIGNAL status. Used by ForkJoinPool. Other
272	<	* variants are directly incorporated into externalAwaitDone etc.
271	>	* Tries to set SIGNAL status unless already completed. Used by
272	>	* ForkJoinPool. Other variants are directly incorporated into
273	>	* externalAwaitDone etc.
274		*
275		* @return true if successful
276		*/
277		final boolean trySetSignal() {
278	<	int s;
279	<	return U.compareAndSwapInt(this, STATUS, s = status, s | SIGNAL);
278	>	int s = status;
279	>	return s >= 0 && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL);
280		}
281
282		/**
#	Line 284 | Line 284 | public abstract class ForkJoinTask<V> im
284		* @return status upon completion
285		*/
286		private int externalAwaitDone() {
287	–	boolean interrupted = false;
287		int s;
288	<	while ((s = status) >= 0) {
288	>	boolean interrupted = false;
289	>	if ((s = status) >= 0 && ForkJoinPool.tryUnsubmitFromCommonPool(this))
290	>	s = doExec();
291	>	while (s >= 0) {
292		if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
293		synchronized (this) {
294		if (status >= 0) {
#	Line 300 | Line 302 | public abstract class ForkJoinTask<V> im
302		notifyAll();
303		}
304		}
305	+	s = status;
306		}
307		if (interrupted)
308		Thread.currentThread().interrupt();
#	Line 310 | Line 313 | public abstract class ForkJoinTask<V> im
313		* Blocks a non-worker-thread until completion or interruption.
314		*/
315		private int externalInterruptibleAwaitDone() throws InterruptedException {
313	–	int s;
316		if (Thread.interrupted())
317		throw new InterruptedException();
318	<	while ((s = status) >= 0) {
318	>	int s;
319	>	if ((s = status) >= 0 && ForkJoinPool.tryUnsubmitFromCommonPool(this))
320	>	s = doExec();
321	>	while (s >= 0) {
322		if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
323		synchronized (this) {
324		if (status >= 0)
#	Line 322 | Line 327 | public abstract class ForkJoinTask<V> im
327		notifyAll();
328		}
329		}
330	+	s = status;
331		}
332		return s;
333		}
#	Line 335 | Line 341 | public abstract class ForkJoinTask<V> im
341		*/
342		private int doJoin() {
343		int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
344	<	if ((s = status) >= 0) {
345	<	if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
346	<	if (!(w = (wt = (ForkJoinWorkerThread)t).workQueue).
347	<	tryUnpush(this) || (s = doExec()) >= 0)
348	<	s = wt.pool.awaitJoin(w, this);
349	<	}
344	<	else
345	<	s = externalAwaitDone();
346	<	}
347	<	return s;
344	>	return (s = status) < 0 ? s :
345	>	((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
346	>	(w = (wt = (ForkJoinWorkerThread)t).workQueue).
347	>	tryUnpush(this) && (s = doExec()) < 0 ? s :
348	>	wt.pool.awaitJoin(w, this) :
349	>	externalAwaitDone();
350		}
351
352		/**
#	Line 354 | Line 356 | public abstract class ForkJoinTask<V> im
356		*/
357		private int doInvoke() {
358		int s; Thread t; ForkJoinWorkerThread wt;
359	<	if ((s = doExec()) >= 0) {
360	<	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
361	<	s = (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue,
362	<	this);
361	<	else
362	<	s = externalAwaitDone();
363	<	}
364	<	return s;
359	>	return (s = doExec()) < 0 ? s :
360	>	((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
361	>	(wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
362	>	externalAwaitDone();
363		}
364
365		// Exception table support
#	Line 409 | Line 407 | public abstract class ForkJoinTask<V> im
407		}
408
409		/**
410	<	* Records exception and sets exceptional completion.
410	>	* Records exception and sets status.
411		*
412		* @return status on exit
413		*/
414	<	private int setExceptionalCompletion(Throwable ex) {
415	<	int h = System.identityHashCode(this);
416	<	final ReentrantLock lock = exceptionTableLock;
417	<	lock.lock();
418	<	try {
419	<	expungeStaleExceptions();
420	<	ExceptionNode[] t = exceptionTable;
421	<	int i = h & (t.length - 1);
422	<	for (ExceptionNode e = t[i]; ; e = e.next) {
423	<	if (e == null) {
424	<	t[i] = new ExceptionNode(this, ex, t[i]);
425	<	break;
414	>	final int recordExceptionalCompletion(Throwable ex) {
415	>	int s;
416	>	if ((s = status) >= 0) {
417	>	int h = System.identityHashCode(this);
418	>	final ReentrantLock lock = exceptionTableLock;
419	>	lock.lock();
420	>	try {
421	>	expungeStaleExceptions();
422	>	ExceptionNode[] t = exceptionTable;
423	>	int i = h & (t.length - 1);
424	>	for (ExceptionNode e = t[i]; ; e = e.next) {
425	>	if (e == null) {
426	>	t[i] = new ExceptionNode(this, ex, t[i]);
427	>	break;
428	>	}
429	>	if (e.get() == this) // already present
430	>	break;
431		}
432	<	if (e.get() == this) // already present
433	<	break;
432	>	} finally {
433	>	lock.unlock();
434		}
435	<	} finally {
433	<	lock.unlock();
435	>	s = setCompletion(EXCEPTIONAL);
436		}
437	<	return setCompletion(EXCEPTIONAL);
437	>	return s;
438	>	}
439	>
440	>	/**
441	>	* Records exception and possibly propagates
442	>	*
443	>	* @return status on exit
444	>	*/
445	>	private int setExceptionalCompletion(Throwable ex) {
446	>	int s = recordExceptionalCompletion(ex);
447	>	if ((s & DONE_MASK) == EXCEPTIONAL)
448	>	internalPropagateException(ex);
449	>	return s;
450	>	}
451	>
452	>	/**
453	>	* Hook for exception propagation support for tasks with completers.
454	>	*/
455	>	void internalPropagateException(Throwable ex) {
456		}
457
458		/**
#	Line 514 | Line 534 | public abstract class ForkJoinTask<V> im
534		Throwable ex;
535		if (e == null || (ex = e.ex) == null)
536		return null;
537	<	if (e.thrower != Thread.currentThread().getId()) {
537	>	if (false && e.thrower != Thread.currentThread().getId()) {
538		Class<? extends Throwable> ec = ex.getClass();
539		try {
540		Constructor<?> noArgCtor = null;
#	Line 594 | Line 614 | public abstract class ForkJoinTask<V> im
614		// public methods
615
616		/**
617	<	* Arranges to asynchronously execute this task.  While it is not
618	<	* necessarily enforced, it is a usage error to fork a task more
619	<	* than once unless it has completed and been reinitialized.
620	<	* Subsequent modifications to the state of this task or any data
621	<	* it operates on are not necessarily consistently observable by
622	<	* any thread other than the one executing it unless preceded by a
623	<	* call to {@link #join} or related methods, or a call to {@link
624	<	* #isDone} returning {@code true}.
625	<	*
626	<	* <p>This method may be invoked only from within {@code
627	<	* ForkJoinPool} computations (as may be determined using method
608	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
609	<	* result in exceptions or errors, possibly including {@code
610	<	* ClassCastException}.
617	>	* Arranges to asynchronously execute this task in the pool the
618	>	* current task is running in, if applicable, or using the {@link
619	>	* ForkJoinPool#commonPool} if not {@link #inForkJoinPool}.  While
620	>	* it is not necessarily enforced, it is a usage error to fork a
621	>	* task more than once unless it has completed and been
622	>	* reinitialized.  Subsequent modifications to the state of this
623	>	* task or any data it operates on are not necessarily
624	>	* consistently observable by any thread other than the one
625	>	* executing it unless preceded by a call to {@link #join} or
626	>	* related methods, or a call to {@link #isDone} returning {@code
627	>	* true}.
628		*
629		* @return {@code this}, to simplify usage
630		*/
631		public final ForkJoinTask<V> fork() {
632	<	((ForkJoinWorkerThread)Thread.currentThread()).workQueue.push(this);
632	>	Thread t;
633	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
634	>	((ForkJoinWorkerThread)t).workQueue.push(this);
635	>	else
636	>	ForkJoinPool.submitToCommonPool(this);
637		return this;
638		}
639
#	Line 662 | Line 683 | public abstract class ForkJoinTask<V> im
683		* cancelled, completed normally or exceptionally, or left
684		* unprocessed.
685		*
665	–	* <p>This method may be invoked only from within {@code
666	–	* ForkJoinPool} computations (as may be determined using method
667	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
668	–	* result in exceptions or errors, possibly including {@code
669	–	* ClassCastException}.
670	–	*
686		* @param t1 the first task
687		* @param t2 the second task
688		* @throws NullPointerException if any task is null
#	Line 693 | Line 708 | public abstract class ForkJoinTask<V> im
708		* related methods to check if they have been cancelled, completed
709		* normally or exceptionally, or left unprocessed.
710		*
696	–	* <p>This method may be invoked only from within {@code
697	–	* ForkJoinPool} computations (as may be determined using method
698	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
699	–	* result in exceptions or errors, possibly including {@code
700	–	* ClassCastException}.
701	–	*
711		* @param tasks the tasks
712		* @throws NullPointerException if any task is null
713		*/
#	Line 742 | Line 751 | public abstract class ForkJoinTask<V> im
751		* cancelled, completed normally or exceptionally, or left
752		* unprocessed.
753		*
745	–	* <p>This method may be invoked only from within {@code
746	–	* ForkJoinPool} computations (as may be determined using method
747	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
748	–	* result in exceptions or errors, possibly including {@code
749	–	* ClassCastException}.
750	–	*
754		* @param tasks the collection of tasks
755		* @return the tasks argument, to simplify usage
756		* @throws NullPointerException if tasks or any element are null
#	Line 904 | Line 907 | public abstract class ForkJoinTask<V> im
907		}
908
909		/**
910	<	* Completes this task. The most recent value established by
911	<	* {@link #setRawResult} (or {@code null}) will be returned as the
912	<	* result of subsequent invocations of {@code join} and related
913	<	* operations. This method may be useful when processing sets of
914	<	* tasks when some do not otherwise complete normally. Its use in
915	<	* other situations is discouraged.
910	>	* Completes this task normally without setting a value. The most
911	>	* recent value established by {@link #setRawResult} (or {@code
912	>	* null} by default) will be returned as the result of subsequent
913	>	* invocations of {@code join} and related operations.
914	>	*
915	>	* @since 1.8
916		*/
917		public final void quietlyComplete() {
918		setCompletion(NORMAL);
#	Line 1043 | Line 1046 | public abstract class ForkJoinTask<V> im
1046		* be of use in designs in which many tasks are forked, but none
1047		* are explicitly joined, instead executing them until all are
1048		* processed.
1046	–	*
1047	–	* <p>This method may be invoked only from within {@code
1048	–	* ForkJoinPool} computations (as may be determined using method
1049	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1050	–	* result in exceptions or errors, possibly including {@code
1051	–	* ClassCastException}.
1049		*/
1050		public static void helpQuiesce() {
1051	<	ForkJoinWorkerThread wt =
1052	<	(ForkJoinWorkerThread)Thread.currentThread();
1053	<	wt.pool.helpQuiescePool(wt.workQueue);
1051	>	Thread t;
1052	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
1053	>	ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
1054	>	wt.pool.helpQuiescePool(wt.workQueue);
1055	>	}
1056	>	else
1057	>	ForkJoinPool.externalHelpQuiescePool();
1058		}
1059
1060		/**
#	Line 1106 | Line 1107 | public abstract class ForkJoinTask<V> im
1107
1108		/**
1109		* Tries to unschedule this task for execution. This method will
1110	<	* typically succeed if this task is the most recently forked task
1111	<	* by the current thread, and has not commenced executing in
1112	<	* another thread.  This method may be useful when arranging
1113	<	* alternative local processing of tasks that could have been, but
1114	<	* were not, stolen.
1114	<	*
1115	<	* <p>This method may be invoked only from within {@code
1116	<	* ForkJoinPool} computations (as may be determined using method
1117	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1118	<	* result in exceptions or errors, possibly including {@code
1119	<	* ClassCastException}.
1110	>	* typically (but is not guaranteed to) succeed if this task is
1111	>	* the most recently forked task by the current thread, and has
1112	>	* not commenced executing in another thread.  This method may be
1113	>	* useful when arranging alternative local processing of tasks
1114	>	* that could have been, but were not, stolen.
1115		*
1116		* @return {@code true} if unforked
1117		*/
1118		public boolean tryUnfork() {
1119	<	return ((ForkJoinWorkerThread)Thread.currentThread())
1120	<	.workQueue.tryUnpush(this);
1119	>	Thread t;
1120	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1121	>	((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
1122	>	ForkJoinPool.tryUnsubmitFromCommonPool(this);
1123		}
1124
1125		/**
#	Line 1131 | Line 1128 | public abstract class ForkJoinTask<V> im
1128		* value may be useful for heuristic decisions about whether to
1129		* fork other tasks.
1130		*
1134	–	* <p>This method may be invoked only from within {@code
1135	–	* ForkJoinPool} computations (as may be determined using method
1136	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1137	–	* result in exceptions or errors, possibly including {@code
1138	–	* ClassCastException}.
1139	–	*
1131		* @return the number of tasks
1132		*/
1133		public static int getQueuedTaskCount() {
1134	<	return ((ForkJoinWorkerThread) Thread.currentThread())
1135	<	.workQueue.queueSize();
1134	>	Thread t;
1135	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1136	>	((ForkJoinWorkerThread)t).workQueue.queueSize() :
1137	>	ForkJoinPool.getEstimatedSubmitterQueueLength();
1138		}
1139
1140		/**
1141		* Returns an estimate of how many more locally queued tasks are
1142		* held by the current worker thread than there are other worker
1143	<	* threads that might steal them.  This value may be useful for
1143	>	* threads that might steal them, or zero if this thread is not
1144	>	* operating in a ForkJoinPool. This value may be useful for
1145		* heuristic decisions about whether to fork other tasks. In many
1146		* usages of ForkJoinTasks, at steady state, each worker should
1147		* aim to maintain a small constant surplus (for example, 3) of
1148		* tasks, and to process computations locally if this threshold is
1149		* exceeded.
1150		*
1157	–	* <p>This method may be invoked only from within {@code
1158	–	* ForkJoinPool} computations (as may be determined using method
1159	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1160	–	* result in exceptions or errors, possibly including {@code
1161	–	* ClassCastException}.
1162	–	*
1151		* @return the surplus number of tasks, which may be negative
1152		*/
1153		public static int getSurplusQueuedTaskCount() {
#	Line 1206 | Line 1194 | public abstract class ForkJoinTask<V> im
1194		* have zero queued tasks, so compensate by a factor of
1195		* (#idle/#active) threads.
1196		*/
1197	<	ForkJoinWorkerThread wt =
1198	<	(ForkJoinWorkerThread)Thread.currentThread();
1199	<	return wt.workQueue.queueSize() - wt.pool.idlePerActive();
1197	>	Thread t; ForkJoinWorkerThread wt;
1198	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1199	>	(wt = (ForkJoinWorkerThread)t).workQueue.queueSize() - wt.pool.idlePerActive() :
1200	>	0;
1201		}
1202
1203		// Extension methods
#	Line 1234 | Line 1223 | public abstract class ForkJoinTask<V> im
1223		protected abstract void setRawResult(V value);
1224
1225		/**
1226	<	* Immediately performs the base action of this task.  This method
1227	<	* is designed to support extensions, and should not in general be
1228	<	* called otherwise. The return value controls whether this task
1229	<	* is considered to be done normally. It may return false in
1226	>	* Immediately performs the base action of this task and returns
1227	>	* true if, upon return from this method, this task is guaranteed
1228	>	* to have completed normally. This method may return false
1229	>	* otherwise, to indicate that this task is not necessarily
1230	>	* complete (or is not known to be complete), for example in
1231		* asynchronous actions that require explicit invocations of
1232	<	* {@link #complete} to become joinable. It may also throw an
1233	<	* (unchecked) exception to indicate abnormal exit.
1232	>	* completion methods. This method may also throw an (unchecked)
1233	>	* exception to indicate abnormal exit. This method is designed to
1234	>	* support extensions, and should not in general be called
1235	>	* otherwise.
1236		*
1237	<	* @return {@code true} if completed normally
1237	>	* @return {@code true} if this task is known to have completed normally
1238		*/
1239		protected abstract boolean exec();
1240
1241		/**
1242		* Returns, but does not unschedule or execute, a task queued by
1243		* the current thread but not yet executed, if one is immediately
1244	<	* available. There is no guarantee that this task will actually
1245	<	* be polled or executed next. Conversely, this method may return
1246	<	* null even if a task exists but cannot be accessed without
1247	<	* contention with other threads.  This method is designed
1244	>	* available and the current thread is operating in a
1245	>	* ForkJoinPool. There is no guarantee that this task will
1246	>	* actually be polled or executed next. Conversely, this method
1247	>	* may return null even if a task exists but cannot be accessed
1248	>	* without contention with other threads.  This method is designed
1249		* primarily to support extensions, and is unlikely to be useful
1250		* otherwise.
1251		*
1259	–	* <p>This method may be invoked only from within {@code
1260	–	* ForkJoinPool} computations (as may be determined using method
1261	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1262	–	* result in exceptions or errors, possibly including {@code
1263	–	* ClassCastException}.
1264	–	*
1252		* @return the next task, or {@code null} if none are available
1253		*/
1254		protected static ForkJoinTask<?> peekNextLocalTask() {
1255	<	return ((ForkJoinWorkerThread) Thread.currentThread()).workQueue.peek();
1255	>	Thread t;
1256	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1257	>	((ForkJoinWorkerThread)t).workQueue.peek() :
1258	>	null;
1259		}
1260
1261		/**
1262		* Unschedules and returns, without executing, the next task
1263	<	* queued by the current thread but not yet executed.  This method
1264	<	* is designed primarily to support extensions, and is unlikely to
1265	<	* be useful otherwise.
1266	<	*
1277	<	* <p>This method may be invoked only from within {@code
1278	<	* ForkJoinPool} computations (as may be determined using method
1279	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1280	<	* result in exceptions or errors, possibly including {@code
1281	<	* ClassCastException}.
1263	>	* queued by the current thread but not yet executed, if the
1264	>	* current thread is operating in a ForkJoinPool.  This method is
1265	>	* designed primarily to support extensions, and is unlikely to be
1266	>	* useful otherwise.
1267		*
1268		* @return the next task, or {@code null} if none are available
1269		*/
1270		protected static ForkJoinTask<?> pollNextLocalTask() {
1271	<	return ((ForkJoinWorkerThread) Thread.currentThread())
1272	<	.workQueue.nextLocalTask();
1271	>	Thread t;
1272	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1273	>	((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
1274	>	null;
1275		}
1276
1277		/**
1278	<	* Unschedules and returns, without executing, the next task
1278	>	* If the current thread is operating in a ForkJoinPool,
1279	>	* unschedules and returns, without executing, the next task
1280		* queued by the current thread but not yet executed, if one is
1281		* available, or if not available, a task that was forked by some
1282		* other thread, if available. Availability may be transient, so a
1283	<	* {@code null} result does not necessarily imply quiescence
1284	<	* of the pool this task is operating in.  This method is designed
1283	>	* {@code null} result does not necessarily imply quiescence of
1284	>	* the pool this task is operating in.  This method is designed
1285		* primarily to support extensions, and is unlikely to be useful
1286		* otherwise.
1287		*
1300	–	* <p>This method may be invoked only from within {@code
1301	–	* ForkJoinPool} computations (as may be determined using method
1302	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1303	–	* result in exceptions or errors, possibly including {@code
1304	–	* ClassCastException}.
1305	–	*
1288		* @return a task, or {@code null} if none are available
1289		*/
1290		protected static ForkJoinTask<?> pollTask() {
1291	<	ForkJoinWorkerThread wt =
1292	<	(ForkJoinWorkerThread)Thread.currentThread();
1293	<	return wt.pool.nextTaskFor(wt.workQueue);
1291	>	Thread t; ForkJoinWorkerThread wt;
1292	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1293	>	(wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
1294	>	null;
1295		}
1296
1297		// tag operations

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