/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/licenses/publicdomain */ package java.util.concurrent; import java.io.Serializable; import java.util.Collection; import java.util.Collections; import java.util.List; import java.util.RandomAccess; import java.util.Map; import java.util.WeakHashMap; /** * Abstract base class for tasks that run within a {@link ForkJoinPool}. * A {@code ForkJoinTask} is a thread-like entity that is much * lighter weight than a normal thread. Huge numbers of tasks and * subtasks may be hosted by a small number of actual threads in a * ForkJoinPool, at the price of some usage limitations. * *
A "main" {@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(ForkJoinTask...) 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 {@link RejectedExecutionException} 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 {@code Future} * conventions. Method {@link #invoke} is semantically * 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 {@code invokeAll} (available in multiple versions) * performs the most common form of parallel invocation: forking a set * of tasks and joining them all. * *
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 #isCompletedNormally} is true if a task completed without * cancellation or encountering an 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, in which case {@link * #getException} will return either the encountered exception or * {@link java.util.concurrent.CancellationException}. * *
The ForkJoinTask class is not usually directly subclassed. * Instead, you subclass one of the abstract classes that support a * particular style of fork/join processing, typically {@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 {@code compute} * method that somehow uses the control methods supplied by this base * 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 (as may be determined using method {@link * #inForkJoinPool}). Attempts to invoke them in other contexts * result in exceptions or errors, possibly including * {@code ClassCastException}. * *
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 * 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 Runnable} * and {@link Callable}, that may be of use when mixing execution of * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are * of this form, consider using a pool constructed in asyncMode. * *
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.
*
* @since 1.7
* @author Doug Lea
*/
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 {@code this}, to simplify usage
*/
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(ForkJoinTask> t1, ForkJoinTask> t2) {
t2.fork();
t1.invoke();
t2.join();
}
/**
* 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. If any task encounters an exception, others
* may be cancelled. However, the execution status of individual
* tasks is not guaranteed upon exceptional return. The status of
* each task may be obtained using {@link #getException()} and
* related methods to check if they have been cancelled, completed
* normally or exceptionally, or left unprocessed.
*
* 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;
int last = tasks.length - 1;
for (int i = last; i >= 0; --i) {
ForkJoinTask> t = tasks[i];
if (t == null) {
if (ex == null)
ex = new NullPointerException();
}
else if (i != 0)
t.fork();
else {
t.quietlyInvoke();
if (ex == null && t.status < NORMAL)
ex = t.getException();
}
}
for (int i = 1; i <= last; ++i) {
ForkJoinTask> t = tasks[i];
if (t != null) {
if (ex != null)
t.cancel(false);
else {
t.quietlyJoin();
if (ex == null && t.status < NORMAL)
ex = t.getException();
}
}
}
if (ex != null)
UNSAFE.throwException(ex);
}
/**
* Forks all tasks in the specified collection, 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. If any task encounters an
* exception, others may be cancelled. However, the execution
* status of individual tasks is not guaranteed upon exceptional
* return. The status of each task may be obtained using {@link
* #getException()} and related methods to check if they have been
* cancelled, completed normally or exceptionally, or left
* unprocessed.
*
* 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
* @return the tasks argument, to simplify usage
* @throws NullPointerException if tasks or any element are null
*/
public static This method may be overridden in subclasses, but if so, must
* still ensure that these minimal properties hold. In particular,
* the {@code cancel} method itself must not throw exceptions.
*
* This method is designed to be invoked by other
* tasks. To terminate the current task, you can just return or
* throw an unchecked exception from its computation method, or
* invoke {@link #completeExceptionally}.
*
* @param mayInterruptIfRunning this value is ignored in the
* default implementation because tasks are not
* cancelled via interruption
*
* @return {@code true} if this task is now cancelled
*/
public boolean cancel(boolean mayInterruptIfRunning) {
setCompletion(CANCELLED);
return status == CANCELLED;
}
/**
* Cancels, ignoring any exceptions thrown by cancel. Used during
* worker and pool shutdown. Cancel is spec'ed not to throw any
* exceptions, but if it does anyway, we have no recourse during
* shutdown, so guard against this case.
*/
final void cancelIgnoringExceptions() {
try {
cancel(false);
} catch (Throwable ignore) {
}
}
/**
* Cancels if current thread is a terminating worker thread,
* ignoring any exceptions thrown by cancel.
*/
final void cancelIfTerminating() {
Thread t = Thread.currentThread();
if ((t instanceof ForkJoinWorkerThread) &&
((ForkJoinWorkerThread) t).isTerminating()) {
try {
cancel(false);
} catch (Throwable ignore) {
}
}
}
public final boolean isDone() {
return status < 0;
}
public final boolean isCancelled() {
return status == CANCELLED;
}
/**
* Returns {@code true} if this task threw an exception or was cancelled.
*
* @return {@code true} if this task threw an exception or was cancelled
*/
public final boolean isCompletedAbnormally() {
return status < NORMAL;
}
/**
* Returns {@code true} if this task completed without throwing an
* exception and was not cancelled.
*
* @return {@code true} if this task completed without throwing an
* exception and was not cancelled
*/
public final boolean isCompletedNormally() {
return status == NORMAL;
}
/**
* Returns the exception thrown by the base computation, or a
* {@code CancellationException} if cancelled, or {@code null} if
* none or if the method has not yet completed.
*
* @return the exception, or {@code null} if none
*/
public final Throwable getException() {
int s = status;
return ((s >= NORMAL) ? null :
(s == CANCELLED) ? new CancellationException() :
exceptionMap.get(this));
}
/**
* Completes this task abnormally, and if not already aborted or
* cancelled, causes it to throw the given exception upon
* {@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 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 not a
* {@code RuntimeException} or {@code Error}, the actual exception
* thrown will be a {@code RuntimeException} with cause {@code ex}.
*/
public void completeExceptionally(Throwable ex) {
setExceptionalCompletion((ex instanceof RuntimeException) ||
(ex instanceof Error) ? ex :
new RuntimeException(ex));
}
/**
* Completes this task, and if not already aborted or cancelled,
* returning the given value as the result of subsequent
* invocations of {@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
* discouraged. This method is overridable, but overridden
* versions must invoke {@code super} implementation to maintain
* guarantees.
*
* @param value the result value for this task
*/
public void complete(V value) {
try {
setRawResult(value);
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
setCompletion(NORMAL);
}
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread is not a
* member of a ForkJoinPool and was interrupted while waiting
*/
public final V get() throws InterruptedException, ExecutionException {
int s;
if (Thread.currentThread() instanceof ForkJoinWorkerThread) {
quietlyJoin();
s = status;
}
else {
while ((s = status) >= 0) {
synchronized (this) { // interruptible form of awaitDone
if (UNSAFE.compareAndSwapInt(this, statusOffset,
s, SIGNAL)) {
while (status >= 0)
wait();
}
}
}
}
if (s < NORMAL) {
Throwable ex;
if (s == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
throw new ExecutionException(ex);
}
return getRawResult();
}
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread is not a
* member of a ForkJoinPool and was interrupted while waiting
* @throws TimeoutException if the wait timed out
*/
public final V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
Thread t = Thread.currentThread();
ForkJoinPool pool;
if (t instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
if (status >= 0 && w.unpushTask(this))
quietlyExec();
pool = w.pool;
}
else
pool = null;
/*
* Timed wait loop intermixes cases for FJ (pool != null) and
* non FJ threads. For FJ, decrement pool count but don't try
* for replacement; increment count on completion. For non-FJ,
* deal with interrupts. This is messy, but a little less so
* than is splitting the FJ and nonFJ cases.
*/
boolean interrupted = false;
boolean dec = false; // true if pool count decremented
long nanos = unit.toNanos(timeout);
for (;;) {
if (pool == null && Thread.interrupted()) {
interrupted = true;
break;
}
int s = status;
if (s < 0)
break;
if (UNSAFE.compareAndSwapInt(this, statusOffset, s, SIGNAL)) {
long startTime = System.nanoTime();
long nt; // wait time
while (status >= 0 &&
(nt = nanos - (System.nanoTime() - startTime)) > 0) {
if (pool != null && !dec)
dec = pool.tryDecrementRunningCount();
else {
long ms = nt / 1000000;
int ns = (int) (nt % 1000000);
try {
synchronized (this) {
if (status >= 0)
wait(ms, ns);
}
} catch (InterruptedException ie) {
if (pool != null)
cancelIfTerminating();
else {
interrupted = true;
break;
}
}
}
}
break;
}
}
if (pool != null && dec)
pool.incrementRunningCount();
if (interrupted)
throw new InterruptedException();
int es = status;
if (es != NORMAL) {
Throwable ex;
if (es == CANCELLED)
throw new CancellationException();
if (es == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
throw new ExecutionException(ex);
throw new TimeoutException();
}
return getRawResult();
}
/**
* Joins this task, without returning its result or throwing its
* exception. This method may be useful when processing
* collections of tasks when some have been cancelled or otherwise
* known to have aborted.
*/
public final void quietlyJoin() {
Thread t;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
if (status >= 0) {
if (w.unpushTask(this)) {
boolean completed;
try {
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
if (completed) {
setCompletion(NORMAL);
return;
}
}
w.joinTask(this);
}
}
else
externalAwaitDone();
}
/**
* Commences performing this task and awaits its completion if
* necessary, without returning its result or throwing its
* exception.
*/
public final void quietlyInvoke() {
if (status >= 0) {
boolean completed;
try {
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
if (completed)
setCompletion(NORMAL);
else
quietlyJoin();
}
}
/**
* Possibly executes tasks until the pool hosting the current task
* {@link ForkJoinPool#isQuiescent is quiescent}. This method may
* be of use in designs in which many tasks are forked, but none
* are explicitly joined, instead executing them until all are
* processed.
*
* 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();
}
/**
* Resets the internal bookkeeping state of this task, allowing a
* 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.
* This method may be useful when executing
* pre-constructed trees of subtasks in loops.
*/
public void reinitialize() {
if (status == EXCEPTIONAL)
exceptionMap.remove(this);
status = 0;
}
/**
* Returns the pool hosting the current task execution, or null
* if this task is executing outside of any ForkJoinPool.
*
* @see #inForkJoinPool
* @return the pool, or {@code null} if none
*/
public static ForkJoinPool getPool() {
Thread t = Thread.currentThread();
return (t instanceof ForkJoinWorkerThread) ?
((ForkJoinWorkerThread) t).pool : null;
}
/**
* Returns {@code true} if the current thread is executing as a
* ForkJoinPool computation.
*
* @return {@code true} if the current thread is executing as a
* ForkJoinPool computation, or false otherwise
*/
public static boolean inForkJoinPool() {
return Thread.currentThread() instanceof ForkJoinWorkerThread;
}
/**
* Tries to unschedule this task for execution. This method will
* typically succeed if this task is the most recently forked task
* by the current thread, and has not commenced executing in
* another thread. This method may be useful when arranging
* alternative local processing of tasks that could have been, but
* were not, stolen.
*
* 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);
}
/**
* Returns an estimate of the number of tasks that have been
* forked by the current worker thread but not yet executed. This
* value may be useful for heuristic decisions about whether to
* fork other tasks.
*
* 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();
}
/**
* 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
* usages of ForkJoinTasks, at steady state, each worker should
* aim to maintain a small constant surplus (for example, 3) of
* tasks, and to process computations locally if this threshold is
* exceeded.
*
* 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())
.getEstimatedSurplusTaskCount();
}
// Extension methods
/**
* 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 {@code null} if not completed
*/
public abstract V getRawResult();
/**
* Forces the given value to be returned as a result. This method
* is designed to support extensions, and should not in general be
* called otherwise.
*
* @param value the value
*/
protected abstract void setRawResult(V value);
/**
* Immediately performs the base action of this task. This method
* is designed to support extensions, and should not in general be
* called otherwise. The return value controls whether this task
* is considered to be done normally. It may return false in
* asynchronous actions that require explicit invocations of
* {@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, 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.
*
* 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> peekNextLocalTask() {
return ((ForkJoinWorkerThread) Thread.currentThread())
.peekTask();
}
/**
* 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.
*
* 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())
.pollLocalTask();
}
/**
* 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.
*
* 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())
.pollTask();
}
/**
* Adaptor for Runnables. This implements RunnableFuture
* to be compliant with AbstractExecutorService constraints
* when used in ForkJoinPool.
*/
static final class AdaptedRunnable