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root/jsr166/jsr166/src/jsr166e/ForkJoinTask.java

Comparing jsr166/src/jsr166e/ForkJoinTask.java (file contents):
Revision 1.1 by dl, Mon Aug 13 15:52:33 2012 UTC vs.
Revision 1.18 by jsr166, Tue Oct 13 19:45:34 2015 UTC

#	Line 30 | 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 52 | Line 55 | import java.lang.reflect.Constructor;
55		* minimize other blocking synchronization apart from joining other
56		* tasks or using synchronizers such as Phasers that are advertised to
57		* cooperate with fork/join scheduling. Subdividable tasks should also
58	<	* not perform blocking IO, and should ideally access variables that
58	>	* not perform blocking I/O, and should ideally access variables that
59		* are completely independent of those accessed by other running
60		* tasks. These guidelines are loosely enforced by not permitting
61		* checked exceptions such as {@code IOExceptions} to be
#	Line 70 | Line 73 | import java.lang.reflect.Constructor;
73		* <p>It is possible to define and use ForkJoinTasks that may block,
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
76	>	* that blocks on external synchronization or I/O. Event-style async
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
#	Line 123 | Line 126 | import java.lang.reflect.Constructor;
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. While these methods have
127	<	* {@code public} access (to allow instances of different task
128	<	* subclasses to call each other's methods), some of them may only be
129	<	* called from within other ForkJoinTasks (as may be determined using
130	<	* method {@link #inForkJoinPool}).  Attempts to invoke them in other
131	<	* contexts result in exceptions or errors, possibly including {@code
132	<	* ClassCastException}.
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 134 | import java.lang.reflect.Constructor;
134		* (DAG). Otherwise, executions may encounter a form of deadlock as
135		* tasks cyclically wait for each other.  However, this framework
136		* supports other methods and techniques (for example the use of
137	<	* {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
137	>	* {@link java.util.concurrent.Phaser 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
139	>	* are not statically structured as DAGs. To support such usages, a
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
#	Line 287 | Line 284 | public abstract class ForkJoinTask<V> im
284		* @return status upon completion
285		*/
286		private int externalAwaitDone() {
290	–	boolean interrupted = false;
287		int s;
288	<	while ((s = status) >= 0) {
289	<	if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
290	<	synchronized (this) {
291	<	if (status >= 0) {
292	<	try {
293	<	wait();
294	<	} catch (InterruptedException ie) {
295	<	interrupted = true;
288	>	ForkJoinPool cp = ForkJoinPool.common;
289	>	if ((s = status) >= 0) {
290	>	if (cp != null) {
291	>	if (this instanceof CountedCompleter)
292	>	s = cp.externalHelpComplete((CountedCompleter<?>)this);
293	>	else if (cp.tryExternalUnpush(this))
294	>	s = doExec();
295	>	}
296	>	if (s >= 0 && (s = status) >= 0) {
297	>	boolean interrupted = false;
298	>	do {
299	>	if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
300	>	synchronized (this) {
301	>	if (status >= 0) {
302	>	try {
303	>	wait();
304	>	} catch (InterruptedException ie) {
305	>	interrupted = true;
306	>	}
307	>	}
308	>	else
309	>	notifyAll();
310		}
311		}
312	<	else
313	<	notifyAll();
314	<	}
312	>	} while ((s = status) >= 0);
313	>	if (interrupted)
314	>	Thread.currentThread().interrupt();
315		}
316		}
307	–	if (interrupted)
308	–	Thread.currentThread().interrupt();
317		return s;
318		}
319
#	Line 314 | Line 322 | public abstract class ForkJoinTask<V> im
322		*/
323		private int externalInterruptibleAwaitDone() throws InterruptedException {
324		int s;
325	+	ForkJoinPool cp = ForkJoinPool.common;
326		if (Thread.interrupted())
327		throw new InterruptedException();
328	+	if ((s = status) >= 0 && cp != null) {
329	+	if (this instanceof CountedCompleter)
330	+	cp.externalHelpComplete((CountedCompleter<?>)this);
331	+	else if (cp.tryExternalUnpush(this))
332	+	doExec();
333	+	}
334		while ((s = status) >= 0) {
335		if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
336		synchronized (this) {
#	Line 329 | Line 344 | public abstract class ForkJoinTask<V> im
344		return s;
345		}
346
347	+
348		/**
349		* Implementation for join, get, quietlyJoin. Directly handles
350		* only cases of already-completed, external wait, and
#	Line 338 | Line 354 | public abstract class ForkJoinTask<V> im
354		*/
355		private int doJoin() {
356		int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
357	<	if ((s = status) >= 0) {
358	<	if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
359	<	if (!(w = (wt = (ForkJoinWorkerThread)t).workQueue).
360	<	tryUnpush(this) || (s = doExec()) >= 0)
361	<	s = wt.pool.awaitJoin(w, this);
362	<	}
347	<	else
348	<	s = externalAwaitDone();
349	<	}
350	<	return s;
357	>	return (s = status) < 0 ? s :
358	>	((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
359	>	(w = (wt = (ForkJoinWorkerThread)t).workQueue).
360	>	tryUnpush(this) && (s = doExec()) < 0 ? s :
361	>	wt.pool.awaitJoin(w, this) :
362	>	externalAwaitDone();
363		}
364
365		/**
#	Line 357 | Line 369 | public abstract class ForkJoinTask<V> im
369		*/
370		private int doInvoke() {
371		int s; Thread t; ForkJoinWorkerThread wt;
372	<	if ((s = doExec()) >= 0) {
373	<	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
374	<	s = (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue,
375	<	this);
364	<	else
365	<	s = externalAwaitDone();
366	<	}
367	<	return s;
372	>	return (s = doExec()) < 0 ? s :
373	>	((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
374	>	(wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
375	>	externalAwaitDone();
376		}
377
378		// Exception table support
#	Line 403 | Line 411 | public abstract class ForkJoinTask<V> im
411		final Throwable ex;
412		ExceptionNode next;
413		final long thrower;  // use id not ref to avoid weak cycles
414	+	final int hashCode;  // store task hashCode before weak ref disappears
415		ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
416		super(task, exceptionTableRefQueue);
417		this.ex = ex;
418		this.next = next;
419		this.thrower = Thread.currentThread().getId();
420	+	this.hashCode = System.identityHashCode(task);
421		}
422		}
423
#	Line 443 | Line 453 | public abstract class ForkJoinTask<V> im
453		}
454
455		/**
456	<	* Records exception and possibly propagates
456	>	* Records exception and possibly propagates.
457		*
458		* @return status on exit
459		*/
#	Line 476 | Line 486 | public abstract class ForkJoinTask<V> im
486		}
487
488		/**
489	<	* Removes exception node and clears status
489	>	* Removes exception node and clears status.
490		*/
491		private void clearExceptionalCompletion() {
492		int h = System.identityHashCode(this);
#	Line 566 | Line 576 | public abstract class ForkJoinTask<V> im
576		/**
577		* Poll stale refs and remove them. Call only while holding lock.
578		*/
579	+	/**
580	+	* Poll stale refs and remove them. Call only while holding lock.
581	+	*/
582		private static void expungeStaleExceptions() {
583		for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
584		if (x instanceof ExceptionNode) {
585	<	ForkJoinTask<?> key = ((ExceptionNode)x).get();
585	>	int hashCode = ((ExceptionNode)x).hashCode;
586		ExceptionNode[] t = exceptionTable;
587	<	int i = System.identityHashCode(key) & (t.length - 1);
587	>	int i = hashCode & (t.length - 1);
588		ExceptionNode e = t[i];
589		ExceptionNode pred = null;
590		while (e != null) {
#	Line 606 | Line 619 | public abstract class ForkJoinTask<V> im
619		}
620
621		/**
622	+	* A version of "sneaky throw" to relay exceptions
623	+	*/
624	+	static void rethrow(Throwable ex) {
625	+	if (ex != null)
626	+	ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
627	+	}
628	+
629	+	/**
630	+	* The sneaky part of sneaky throw, relying on generics
631	+	* limitations to evade compiler complaints about rethrowing
632	+	* unchecked exceptions
633	+	*/
634	+	@SuppressWarnings("unchecked") static <T extends Throwable>
635	+	void uncheckedThrow(Throwable t) throws T {
636	+	throw (T)t; // rely on vacuous cast
637	+	}
638	+
639	+	/**
640		* Throws exception, if any, associated with the given status.
641		*/
642		private void reportException(int s) {
643	<	Throwable ex = ((s == CANCELLED) ?  new CancellationException() :
644	<	(s == EXCEPTIONAL) ? getThrowableException() :
645	<	null);
646	<	if (ex != null)
616	<	U.throwException(ex);
643	>	if (s == CANCELLED)
644	>	throw new CancellationException();
645	>	if (s == EXCEPTIONAL)
646	>	rethrow(getThrowableException());
647		}
648
649		// public methods
650
651		/**
652	<	* Arranges to asynchronously execute this task.  While it is not
653	<	* necessarily enforced, it is a usage error to fork a task more
654	<	* than once unless it has completed and been reinitialized.
655	<	* Subsequent modifications to the state of this task or any data
656	<	* it operates on are not necessarily consistently observable by
657	<	* any thread other than the one executing it unless preceded by a
658	<	* call to {@link #join} or related methods, or a call to {@link
659	<	* #isDone} returning {@code true}.
660	<	*
661	<	* <p>This method may be invoked only from within {@code
662	<	* ForkJoinPool} computations (as may be determined using method
633	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
634	<	* result in exceptions or errors, possibly including {@code
635	<	* ClassCastException}.
652	>	* Arranges to asynchronously execute this task in the pool the
653	>	* current task is running in, if applicable, or using the {@link
654	>	* ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.  While
655	>	* it is not necessarily enforced, it is a usage error to fork a
656	>	* task more than once unless it has completed and been
657	>	* reinitialized.  Subsequent modifications to the state of this
658	>	* task or any data it operates on are not necessarily
659	>	* consistently observable by any thread other than the one
660	>	* executing it unless preceded by a call to {@link #join} or
661	>	* related methods, or a call to {@link #isDone} returning {@code
662	>	* true}.
663		*
664		* @return {@code this}, to simplify usage
665		*/
666		public final ForkJoinTask<V> fork() {
667	<	((ForkJoinWorkerThread)Thread.currentThread()).workQueue.push(this);
667	>	Thread t;
668	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
669	>	((ForkJoinWorkerThread)t).workQueue.push(this);
670	>	else
671	>	ForkJoinPool.common.externalPush(this);
672		return this;
673		}
674
#	Line 687 | Line 718 | public abstract class ForkJoinTask<V> im
718		* cancelled, completed normally or exceptionally, or left
719		* unprocessed.
720		*
690	–	* <p>This method may be invoked only from within {@code
691	–	* ForkJoinPool} computations (as may be determined using method
692	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
693	–	* result in exceptions or errors, possibly including {@code
694	–	* ClassCastException}.
695	–	*
721		* @param t1 the first task
722		* @param t2 the second task
723		* @throws NullPointerException if any task is null
#	Line 718 | Line 743 | public abstract class ForkJoinTask<V> im
743		* related methods to check if they have been cancelled, completed
744		* normally or exceptionally, or left unprocessed.
745		*
721	–	* <p>This method may be invoked only from within {@code
722	–	* ForkJoinPool} computations (as may be determined using method
723	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
724	–	* result in exceptions or errors, possibly including {@code
725	–	* ClassCastException}.
726	–	*
746		* @param tasks the tasks
747		* @throws NullPointerException if any task is null
748		*/
#	Line 751 | Line 770 | public abstract class ForkJoinTask<V> im
770		}
771		}
772		if (ex != null)
773	<	U.throwException(ex);
773	>	rethrow(ex);
774		}
775
776		/**
#	Line 767 | Line 786 | public abstract class ForkJoinTask<V> im
786		* cancelled, completed normally or exceptionally, or left
787		* unprocessed.
788		*
770	–	* <p>This method may be invoked only from within {@code
771	–	* ForkJoinPool} computations (as may be determined using method
772	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
773	–	* result in exceptions or errors, possibly including {@code
774	–	* ClassCastException}.
775	–	*
789		* @param tasks the collection of tasks
790	+	* @param <T> the type of the values returned from the tasks
791		* @return the tasks argument, to simplify usage
792		* @throws NullPointerException if tasks or any element are null
793		*/
#	Line 808 | Line 822 | public abstract class ForkJoinTask<V> im
822		}
823		}
824		if (ex != null)
825	<	U.throwException(ex);
825	>	rethrow(ex);
826		return tasks;
827		}
828
#	Line 831 | Line 845 | public abstract class ForkJoinTask<V> im
845		* <p>This method is designed to be invoked by <em>other</em>
846		* tasks. To terminate the current task, you can just return or
847		* throw an unchecked exception from its computation method, or
848	<	* invoke {@link #completeExceptionally}.
848	>	* invoke {@link #completeExceptionally(Throwable)}.
849		*
850		* @param mayInterruptIfRunning this value has no effect in the
851		* default implementation because interrupts are not used to
#	Line 981 | Line 995 | public abstract class ForkJoinTask<V> im
995		if (Thread.interrupted())
996		throw new InterruptedException();
997		// Messy in part because we measure in nanosecs, but wait in millisecs
998	<	int s; long ns, ms;
999	<	if ((s = status) >= 0 && (ns = unit.toNanos(timeout)) > 0L) {
998	>	int s; long ms;
999	>	long ns = unit.toNanos(timeout);
1000	>	ForkJoinPool cp;
1001	>	if ((s = status) >= 0 && ns > 0L) {
1002		long deadline = System.nanoTime() + ns;
1003		ForkJoinPool p = null;
1004		ForkJoinPool.WorkQueue w = null;
#	Line 991 | Line 1007 | public abstract class ForkJoinTask<V> im
1007		ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
1008		p = wt.pool;
1009		w = wt.workQueue;
1010	<	s = p.helpJoinOnce(w, this); // no retries on failure
1010	>	p.helpJoinOnce(w, this); // no retries on failure
1011	>	}
1012	>	else if ((cp = ForkJoinPool.common) != null) {
1013	>	if (this instanceof CountedCompleter)
1014	>	cp.externalHelpComplete((CountedCompleter<?>)this);
1015	>	else if (cp.tryExternalUnpush(this))
1016	>	doExec();
1017		}
1018		boolean canBlock = false;
1019		boolean interrupted = false;
1020		try {
1021		while ((s = status) >= 0) {
1022	<	if (w != null && w.runState < 0)
1022	>	if (w != null && w.qlock < 0)
1023		cancelIgnoringExceptions(this);
1024		else if (!canBlock) {
1025	<	if (p == null || p.tryCompensate(this, null))
1025	>	if (p == null || p.tryCompensate(p.ctl))
1026		canBlock = true;
1027		}
1028		else {
#	Line 1064 | Line 1086 | public abstract class ForkJoinTask<V> im
1086
1087		/**
1088		* Possibly executes tasks until the pool hosting the current task
1089	<	* {@link ForkJoinPool#isQuiescent is quiescent}. This method may
1090	<	* be of use in designs in which many tasks are forked, but none
1091	<	* are explicitly joined, instead executing them until all are
1092	<	* processed.
1071	<	*
1072	<	* <p>This method may be invoked only from within {@code
1073	<	* ForkJoinPool} computations (as may be determined using method
1074	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1075	<	* result in exceptions or errors, possibly including {@code
1076	<	* ClassCastException}.
1089	>	* {@linkplain ForkJoinPool#isQuiescent is quiescent}.  This
1090	>	* method may be of use in designs in which many tasks are forked,
1091	>	* but none are explicitly joined, instead executing them until
1092	>	* all are processed.
1093		*/
1094		public static void helpQuiesce() {
1095	<	ForkJoinWorkerThread wt =
1096	<	(ForkJoinWorkerThread)Thread.currentThread();
1097	<	wt.pool.helpQuiescePool(wt.workQueue);
1095	>	Thread t;
1096	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
1097	>	ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
1098	>	wt.pool.helpQuiescePool(wt.workQueue);
1099	>	}
1100	>	else
1101	>	ForkJoinPool.quiesceCommonPool();
1102		}
1103
1104		/**
#	Line 1131 | Line 1151 | public abstract class ForkJoinTask<V> im
1151
1152		/**
1153		* Tries to unschedule this task for execution. This method will
1154	<	* typically succeed if this task is the most recently forked task
1155	<	* by the current thread, and has not commenced executing in
1156	<	* another thread.  This method may be useful when arranging
1157	<	* alternative local processing of tasks that could have been, but
1158	<	* were not, stolen.
1139	<	*
1140	<	* <p>This method may be invoked only from within {@code
1141	<	* ForkJoinPool} computations (as may be determined using method
1142	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1143	<	* result in exceptions or errors, possibly including {@code
1144	<	* ClassCastException}.
1154	>	* typically (but is not guaranteed to) succeed if this task is
1155	>	* the most recently forked task by the current thread, and has
1156	>	* not commenced executing in another thread.  This method may be
1157	>	* useful when arranging alternative local processing of tasks
1158	>	* that could have been, but were not, stolen.
1159		*
1160		* @return {@code true} if unforked
1161		*/
1162		public boolean tryUnfork() {
1163	<	return ((ForkJoinWorkerThread)Thread.currentThread())
1164	<	.workQueue.tryUnpush(this);
1163	>	Thread t;
1164	>	return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1165	>	((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
1166	>	ForkJoinPool.common.tryExternalUnpush(this));
1167		}
1168
1169		/**
#	Line 1156 | Line 1172 | public abstract class ForkJoinTask<V> im
1172		* value may be useful for heuristic decisions about whether to
1173		* fork other tasks.
1174		*
1159	–	* <p>This method may be invoked only from within {@code
1160	–	* ForkJoinPool} computations (as may be determined using method
1161	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1162	–	* result in exceptions or errors, possibly including {@code
1163	–	* ClassCastException}.
1164	–	*
1175		* @return the number of tasks
1176		*/
1177		public static int getQueuedTaskCount() {
1178	<	return ((ForkJoinWorkerThread) Thread.currentThread())
1179	<	.workQueue.queueSize();
1178	>	Thread t; ForkJoinPool.WorkQueue q;
1179	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
1180	>	q = ((ForkJoinWorkerThread)t).workQueue;
1181	>	else
1182	>	q = ForkJoinPool.commonSubmitterQueue();
1183	>	return (q == null) ? 0 : q.queueSize();
1184		}
1185
1186		/**
1187		* Returns an estimate of how many more locally queued tasks are
1188		* held by the current worker thread than there are other worker
1189	<	* threads that might steal them.  This value may be useful for
1189	>	* threads that might steal them, or zero if this thread is not
1190	>	* operating in a ForkJoinPool. This value may be useful for
1191		* heuristic decisions about whether to fork other tasks. In many
1192		* usages of ForkJoinTasks, at steady state, each worker should
1193		* aim to maintain a small constant surplus (for example, 3) of
1194		* tasks, and to process computations locally if this threshold is
1195		* exceeded.
1196		*
1182	–	* <p>This method may be invoked only from within {@code
1183	–	* ForkJoinPool} computations (as may be determined using method
1184	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1185	–	* result in exceptions or errors, possibly including {@code
1186	–	* ClassCastException}.
1187	–	*
1197		* @return the surplus number of tasks, which may be negative
1198		*/
1199		public static int getSurplusQueuedTaskCount() {
1200	<	/*
1192	<	* The aim of this method is to return a cheap heuristic guide
1193	<	* for task partitioning when programmers, frameworks, tools,
1194	<	* or languages have little or no idea about task granularity.
1195	<	* In essence by offering this method, we ask users only about
1196	<	* tradeoffs in overhead vs expected throughput and its
1197	<	* variance, rather than how finely to partition tasks.
1198	<	*
1199	<	* In a steady state strict (tree-structured) computation,
1200	<	* each thread makes available for stealing enough tasks for
1201	<	* other threads to remain active. Inductively, if all threads
1202	<	* play by the same rules, each thread should make available
1203	<	* only a constant number of tasks.
1204	<	*
1205	<	* The minimum useful constant is just 1. But using a value of
1206	<	* 1 would require immediate replenishment upon each steal to
1207	<	* maintain enough tasks, which is infeasible.  Further,
1208	<	* partitionings/granularities of offered tasks should
1209	<	* minimize steal rates, which in general means that threads
1210	<	* nearer the top of computation tree should generate more
1211	<	* than those nearer the bottom. In perfect steady state, each
1212	<	* thread is at approximately the same level of computation
1213	<	* tree. However, producing extra tasks amortizes the
1214	<	* uncertainty of progress and diffusion assumptions.
1215	<	*
1216	<	* So, users will want to use values larger, but not much
1217	<	* larger than 1 to both smooth over transient shortages and
1218	<	* hedge against uneven progress; as traded off against the
1219	<	* cost of extra task overhead. We leave the user to pick a
1220	<	* threshold value to compare with the results of this call to
1221	<	* guide decisions, but recommend values such as 3.
1222	<	*
1223	<	* When all threads are active, it is on average OK to
1224	<	* estimate surplus strictly locally. In steady-state, if one
1225	<	* thread is maintaining say 2 surplus tasks, then so are
1226	<	* others. So we can just use estimated queue length.
1227	<	* However, this strategy alone leads to serious mis-estimates
1228	<	* in some non-steady-state conditions (ramp-up, ramp-down,
1229	<	* other stalls). We can detect many of these by further
1230	<	* considering the number of "idle" threads, that are known to
1231	<	* have zero queued tasks, so compensate by a factor of
1232	<	* (#idle/#active) threads.
1233	<	*/
1234	<	ForkJoinWorkerThread wt =
1235	<	(ForkJoinWorkerThread)Thread.currentThread();
1236	<	return wt.workQueue.queueSize() - wt.pool.idlePerActive();
1200	>	return ForkJoinPool.getSurplusQueuedTaskCount();
1201		}
1202
1203		// Extension methods
#	Line 1284 | Line 1248 | public abstract class ForkJoinTask<V> im
1248		* primarily to support extensions, and is unlikely to be useful
1249		* otherwise.
1250		*
1287	–	* <p>This method may be invoked only from within {@code
1288	–	* ForkJoinPool} computations (as may be determined using method
1289	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1290	–	* result in exceptions or errors, possibly including {@code
1291	–	* ClassCastException}.
1292	–	*
1251		* @return the next task, or {@code null} if none are available
1252		*/
1253		protected static ForkJoinTask<?> peekNextLocalTask() {
1254	<	return ((ForkJoinWorkerThread) Thread.currentThread()).workQueue.peek();
1254	>	Thread t; ForkJoinPool.WorkQueue q;
1255	>	if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
1256	>	q = ((ForkJoinWorkerThread)t).workQueue;
1257	>	else
1258	>	q = ForkJoinPool.commonSubmitterQueue();
1259	>	return (q == null) ? null : q.peek();
1260		}
1261
1262		/**
1263		* Unschedules and returns, without executing, the next task
1264	<	* queued by the current thread but not yet executed.  This method
1265	<	* is designed primarily to support extensions, and is unlikely to
1266	<	* be useful otherwise.
1267	<	*
1305	<	* <p>This method may be invoked only from within {@code
1306	<	* ForkJoinPool} computations (as may be determined using method
1307	<	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1308	<	* result in exceptions or errors, possibly including {@code
1309	<	* ClassCastException}.
1264	>	* queued by the current thread but not yet executed, if the
1265	>	* current thread is operating in a ForkJoinPool.  This method is
1266	>	* designed primarily to support extensions, and is unlikely to be
1267	>	* useful otherwise.
1268		*
1269		* @return the next task, or {@code null} if none are available
1270		*/
1271		protected static ForkJoinTask<?> pollNextLocalTask() {
1272	<	return ((ForkJoinWorkerThread) Thread.currentThread())
1273	<	.workQueue.nextLocalTask();
1272	>	Thread t;
1273	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1274	>	((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
1275	>	null;
1276		}
1277
1278		/**
1279	<	* Unschedules and returns, without executing, the next task
1279	>	* If the current thread is operating in a ForkJoinPool,
1280	>	* unschedules and returns, without executing, the next task
1281		* queued by the current thread but not yet executed, if one is
1282		* available, or if not available, a task that was forked by some
1283		* other thread, if available. Availability may be transient, so a
1284	<	* {@code null} result does not necessarily imply quiescence
1285	<	* of the pool this task is operating in.  This method is designed
1284	>	* {@code null} result does not necessarily imply quiescence of
1285	>	* the pool this task is operating in.  This method is designed
1286		* primarily to support extensions, and is unlikely to be useful
1287		* otherwise.
1288		*
1328	–	* <p>This method may be invoked only from within {@code
1329	–	* ForkJoinPool} computations (as may be determined using method
1330	–	* {@link #inForkJoinPool}).  Attempts to invoke in other contexts
1331	–	* result in exceptions or errors, possibly including {@code
1332	–	* ClassCastException}.
1333	–	*
1289		* @return a task, or {@code null} if none are available
1290		*/
1291		protected static ForkJoinTask<?> pollTask() {
1292	<	ForkJoinWorkerThread wt =
1293	<	(ForkJoinWorkerThread)Thread.currentThread();
1294	<	return wt.pool.nextTaskFor(wt.workQueue);
1292	>	Thread t; ForkJoinWorkerThread wt;
1293	>	return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
1294	>	(wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
1295	>	null;
1296		}
1297
1298		// tag operations
#	Line 1376 | Line 1332 | public abstract class ForkJoinTask<V> im
1332		*
1333		* @param e the expected tag value
1334		* @param tag the new tag value
1335	<	* @return true if successful; i.e., the current value was
1335	>	* @return {@code true} if successful; i.e., the current value was
1336		* equal to e and is now tag.
1337		* @since 1.8
1338		*/
#	Line 1391 | Line 1347 | public abstract class ForkJoinTask<V> im
1347		}
1348
1349		/**
1350	<	* Adaptor for Runnables. This implements RunnableFuture
1350	>	* Adapter for Runnables. This implements RunnableFuture
1351		* to be compliant with AbstractExecutorService constraints
1352		* when used in ForkJoinPool.
1353		*/
#	Line 1412 | Line 1368 | public abstract class ForkJoinTask<V> im
1368		}
1369
1370		/**
1371	<	* Adaptor for Runnables without results
1371	>	* Adapter for Runnables without results
1372		*/
1373		static final class AdaptedRunnableAction extends ForkJoinTask<Void>
1374		implements RunnableFuture<Void> {
#	Line 1429 | Line 1385 | public abstract class ForkJoinTask<V> im
1385		}
1386
1387		/**
1388	<	* Adaptor for Callables
1388	>	* Adapter for Runnables in which failure forces worker exception
1389	>	*/
1390	>	static final class RunnableExecuteAction extends ForkJoinTask<Void> {
1391	>	final Runnable runnable;
1392	>	RunnableExecuteAction(Runnable runnable) {
1393	>	if (runnable == null) throw new NullPointerException();
1394	>	this.runnable = runnable;
1395	>	}
1396	>	public final Void getRawResult() { return null; }
1397	>	public final void setRawResult(Void v) { }
1398	>	public final boolean exec() { runnable.run(); return true; }
1399	>	void internalPropagateException(Throwable ex) {
1400	>	rethrow(ex); // rethrow outside exec() catches.
1401	>	}
1402	>	private static final long serialVersionUID = 5232453952276885070L;
1403	>	}
1404	>
1405	>	/**
1406	>	* Adapter for Callables
1407		*/
1408		static final class AdaptedCallable<T> extends ForkJoinTask<T>
1409		implements RunnableFuture<T> {
#	Line 1476 | Line 1450 | public abstract class ForkJoinTask<V> im
1450		*
1451		* @param runnable the runnable action
1452		* @param result the result upon completion
1453	+	* @param <T> the type of the result
1454		* @return the task
1455		*/
1456		public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
#	Line 1489 | Line 1464 | public abstract class ForkJoinTask<V> im
1464		* encountered into {@code RuntimeException}.
1465		*
1466		* @param callable the callable action
1467	+	* @param <T> the type of the callable's result
1468		* @return the task
1469		*/
1470		public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
#	Line 1502 | Line 1478 | public abstract class ForkJoinTask<V> im
1478		/**
1479		* Saves this task to a stream (that is, serializes it).
1480		*
1481	+	* @param s the stream
1482	+	* @throws java.io.IOException if an I/O error occurs
1483		* @serialData the current run status and the exception thrown
1484		* during execution, or {@code null} if none
1485		*/
#	Line 1513 | Line 1491 | public abstract class ForkJoinTask<V> im
1491
1492		/**
1493		* Reconstitutes this task from a stream (that is, deserializes it).
1494	+	* @param s the stream
1495	+	* @throws ClassNotFoundException if the class of a serialized object
1496	+	*         could not be found
1497	+	* @throws java.io.IOException if an I/O error occurs
1498		*/
1499		private void readObject(java.io.ObjectInputStream s)
1500		throws java.io.IOException, ClassNotFoundException {
#	Line 1525 | Line 1507 | public abstract class ForkJoinTask<V> im
1507		// Unsafe mechanics
1508		private static final sun.misc.Unsafe U;
1509		private static final long STATUS;
1510	+
1511		static {
1512		exceptionTableLock = new ReentrantLock();
1513		exceptionTableRefQueue = new ReferenceQueue<Object>();
1514		exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
1515		try {
1516		U = getUnsafe();
1517	+	Class<?> k = ForkJoinTask.class;
1518		STATUS = U.objectFieldOffset
1519	<	(ForkJoinTask.class.getDeclaredField("status"));
1519	>	(k.getDeclaredField("status"));
1520		} catch (Exception e) {
1521		throw new Error(e);
1522		}
#	Line 1548 | Line 1532 | public abstract class ForkJoinTask<V> im
1532		private static sun.misc.Unsafe getUnsafe() {
1533		try {
1534		return sun.misc.Unsafe.getUnsafe();
1535	<	} catch (SecurityException se) {
1536	<	try {
1537	<	return java.security.AccessController.doPrivileged
1538	<	(new java.security
1539	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1540	<	public sun.misc.Unsafe run() throws Exception {
1541	<	java.lang.reflect.Field f = sun.misc
1542	<	.Unsafe.class.getDeclaredField("theUnsafe");
1543	<	f.setAccessible(true);
1544	<	return (sun.misc.Unsafe) f.get(null);
1545	<	}});
1546	<	} catch (java.security.PrivilegedActionException e) {
1547	<	throw new RuntimeException("Could not initialize intrinsics",
1548	<	e.getCause());
1549	<	}
1535	>	} catch (SecurityException tryReflectionInstead) {}
1536	>	try {
1537	>	return java.security.AccessController.doPrivileged
1538	>	(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1539	>	public sun.misc.Unsafe run() throws Exception {
1540	>	Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
1541	>	for (java.lang.reflect.Field f : k.getDeclaredFields()) {
1542	>	f.setAccessible(true);
1543	>	Object x = f.get(null);
1544	>	if (k.isInstance(x))
1545	>	return k.cast(x);
1546	>	}
1547	>	throw new NoSuchFieldError("the Unsafe");
1548	>	}});
1549	>	} catch (java.security.PrivilegedActionException e) {
1550	>	throw new RuntimeException("Could not initialize intrinsics",
1551	>	e.getCause());
1552		}
1553		}
1554		}

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