/*
* 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
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*/
/*
* 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.
*
* <p>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.
*
* <p>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.
*
* <p>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 #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 {@code fork(); join()} but always attempts to begin
* execution in the current thread. The "<em>quiet</em>" 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.
*
* <p>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}.
*
* <p>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
* ClassCastException.
*
* <p>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.
*
* <p>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.
*
* <p>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 in
* {@linkplain ForkJoinPool#setAsyncMode async mode}.
*
* <p>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<V> implements Future<V>, Serializable {
/**
* Run control status bits packed into a single int to minimize
* footprint and to ensure atomicity (via CAS). Status is
* initially zero, and takes on nonnegative values until
* completed, upon which status holds COMPLETED. CANCELLED, or
* EXCEPTIONAL, which use the top 3 bits. Tasks undergoing
* blocking waits by other threads have SIGNAL_MASK bits set --
* bit 15 for external (nonFJ) waits, and the rest a count of
* waiting FJ threads. (This representation relies on
* ForkJoinPool max thread limits). Completion of a stolen task
* with SIGNAL_MASK bits set awakens waiter via notifyAll. Even
* though suboptimal for some purposes, we use basic builtin
* wait/notify to take advantage of "monitor inflation" in JVMs
* that we would otherwise need to emulate to avoid adding further
* per-task bookkeeping overhead. Note that bits 16-28 are
* currently unused. Also value 0x80000000 is available as spare
* completion value.
*/
volatile int status; // accessed directly by pool and workers
static final int COMPLETION_MASK = 0xe0000000;
static final int NORMAL = 0xe0000000; // == mask
static final int CANCELLED = 0xc0000000;
static final int EXCEPTIONAL = 0xa0000000;
static final int SIGNAL_MASK = 0x0000ffff;
static final int INTERNAL_SIGNAL_MASK = 0x00007fff;
static final int EXTERNAL_SIGNAL = 0x00008000; // top bit of low word
/**
* Table of exceptions thrown by tasks, to enable reporting by
* callers. Because exceptions are rare, we don't directly keep
* them with task objects, but instead use a weak ref table. Note
* that cancellation exceptions don't appear in the table, but are
* instead recorded as status values.
* TODO: Use ConcurrentReferenceHashMap
*/
static final Map<ForkJoinTask<?>, Throwable> exceptionMap =
Collections.synchronizedMap
(new WeakHashMap<ForkJoinTask<?>, Throwable>());
// within-package utilities
/**
* Gets current worker thread, or null if not a worker thread.
*/
static ForkJoinWorkerThread getWorker() {
Thread t = Thread.currentThread();
return ((t instanceof ForkJoinWorkerThread) ?
(ForkJoinWorkerThread) t : null);
}
final boolean casStatus(int cmp, int val) {
return UNSAFE.compareAndSwapInt(this, statusOffset, cmp, val);
}
/**
* Workaround for not being able to rethrow unchecked exceptions.
*/
static void rethrowException(Throwable ex) {
if (ex != null)
UNSAFE.throwException(ex);
}
// Setting completion status
/**
* Marks completion and wakes up threads waiting to join this task.
*
* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
*/
final void setCompletion(int completion) {
ForkJoinPool pool = getPool();
if (pool != null) {
int s; // Clear signal bits while setting completion status
do {} while ((s = status) >= 0 && !casStatus(s, completion));
if ((s & SIGNAL_MASK) != 0) {
if ((s &= INTERNAL_SIGNAL_MASK) != 0)
pool.updateRunningCount(s);
synchronized (this) { notifyAll(); }
}
}
else
externallySetCompletion(completion);
}
/**
* Version of setCompletion for non-FJ threads. Leaves signal
* bits for unblocked threads to adjust, and always notifies.
*/
private void externallySetCompletion(int completion) {
int s;
do {} while ((s = status) >= 0 &&
!casStatus(s, (s & SIGNAL_MASK) | completion));
synchronized (this) { notifyAll(); }
}
/**
* Sets status to indicate normal completion.
*/
final void setNormalCompletion() {
// Try typical fast case -- single CAS, no signal, not already done.
// Manually expand casStatus to improve chances of inlining it
if (!UNSAFE.compareAndSwapInt(this, statusOffset, 0, NORMAL))
setCompletion(NORMAL);
}
// internal waiting and notification
/**
* Performs the actual monitor wait for awaitDone.
*/
private void doAwaitDone() {
// Minimize lock bias and in/de-flation effects by maximizing
// chances of waiting inside sync
try {
while (status >= 0)
synchronized (this) { if (status >= 0) wait(); }
} catch (InterruptedException ie) {
onInterruptedWait();
}
}
/**
* Performs the actual timed monitor wait for awaitDone.
*/
private void doAwaitDone(long startTime, long nanos) {
synchronized (this) {
try {
while (status >= 0) {
long nt = nanos - (System.nanoTime() - startTime);
if (nt <= 0)
break;
wait(nt / 1000000, (int) (nt % 1000000));
}
} catch (InterruptedException ie) {
onInterruptedWait();
}
}
}
// Awaiting completion
/**
* Sets status to indicate there is joiner, then waits for join,
* surrounded with pool notifications.
*
* @return status upon exit
*/
private int awaitDone(ForkJoinWorkerThread w,
boolean maintainParallelism) {
ForkJoinPool pool = (w == null) ? null : w.pool;
int s;
while ((s = status) >= 0) {
if (casStatus(s, (pool == null) ? s|EXTERNAL_SIGNAL : s+1)) {
if (pool == null || !pool.preJoin(this, maintainParallelism))
doAwaitDone();
if (((s = status) & INTERNAL_SIGNAL_MASK) != 0)
adjustPoolCountsOnUnblock(pool);
break;
}
}
return s;
}
/**
* Timed version of awaitDone
*
* @return status upon exit
*/
private int awaitDone(ForkJoinWorkerThread w, long nanos) {
ForkJoinPool pool = (w == null) ? null : w.pool;
int s;
while ((s = status) >= 0) {
if (casStatus(s, (pool == null) ? s|EXTERNAL_SIGNAL : s+1)) {
long startTime = System.nanoTime();
if (pool == null || !pool.preJoin(this, false))
doAwaitDone(startTime, nanos);
if ((s = status) >= 0) {
adjustPoolCountsOnCancelledWait(pool);
s = status;
}
if (s < 0 && (s & INTERNAL_SIGNAL_MASK) != 0)
adjustPoolCountsOnUnblock(pool);
break;
}
}
return s;
}
/**
* Notifies pool that thread is unblocked. Called by signalled
* threads when woken by non-FJ threads (which is atypical).
*/
private void adjustPoolCountsOnUnblock(ForkJoinPool pool) {
int s;
do {} while ((s = status) < 0 && !casStatus(s, s & COMPLETION_MASK));
if (pool != null && (s &= INTERNAL_SIGNAL_MASK) != 0)
pool.updateRunningCount(s);
}
/**
* Notifies pool to adjust counts on cancelled or timed out wait.
*/
private void adjustPoolCountsOnCancelledWait(ForkJoinPool pool) {
if (pool != null) {
int s;
while ((s = status) >= 0 && (s & INTERNAL_SIGNAL_MASK) != 0) {
if (casStatus(s, s - 1)) {
pool.updateRunningCount(1);
break;
}
}
}
}
/**
* Handles interruptions during waits.
*/
private void onInterruptedWait() {
ForkJoinWorkerThread w = getWorker();
if (w == null)
Thread.currentThread().interrupt(); // re-interrupt
else if (w.isTerminating())
cancelIgnoringExceptions();
// else if FJworker, ignore interrupt
}
// Recording and reporting exceptions
private void setDoneExceptionally(Throwable rex) {
exceptionMap.put(this, rex);
setCompletion(EXCEPTIONAL);
}
/**
* Throws the exception associated with status s.
*
* @throws the exception
*/
private void reportException(int s) {
if ((s &= COMPLETION_MASK) < NORMAL) {
if (s == CANCELLED)
throw new CancellationException();
else
rethrowException(exceptionMap.get(this));
}
}
/**
* Returns result or throws exception using j.u.c.Future conventions.
* Only call when {@code isDone} known to be true or thread known
* to be interrupted.
*/
private V reportFutureResult()
throws InterruptedException, ExecutionException {
if (Thread.interrupted())
throw new InterruptedException();
int s = status & COMPLETION_MASK;
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();
}
/**
* Returns result or throws exception using j.u.c.Future conventions
* with timeouts.
*/
private V reportTimedFutureResult()
throws InterruptedException, ExecutionException, TimeoutException {
if (Thread.interrupted())
throw new InterruptedException();
Throwable ex;
int s = status & COMPLETION_MASK;
if (s == NORMAL)
return getRawResult();
else if (s == CANCELLED)
throw new CancellationException();
else if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
throw new ExecutionException(ex);
else
throw new TimeoutException();
}
// internal execution methods
/**
* Calls exec, recording completion, and rethrowing exception if
* encountered. Caller should normally check status before calling.
*
* @return true if completed normally
*/
private boolean tryExec() {
try { // try block must contain only call to exec
if (!exec())
return false;
} catch (Throwable rex) {
setDoneExceptionally(rex);
rethrowException(rex);
return false; // not reached
}
setNormalCompletion();
return true;
}
/**
* Main execution method used by worker threads. Invokes
* base computation unless already complete.
*/
final void quietlyExec() {
if (status >= 0) {
try {
if (!exec())
return;
} catch (Throwable rex) {
setDoneExceptionally(rex);
return;
}
setNormalCompletion();
}
}
/**
* Calls exec(), recording but not rethrowing exception.
* Caller should normally check status before calling.
*
* @return true if completed normally
*/
private boolean tryQuietlyInvoke() {
try {
if (!exec())
return false;
} catch (Throwable rex) {
setDoneExceptionally(rex);
return false;
}
setNormalCompletion();
return true;
}
/**
* Cancels, ignoring any exceptions it throws.
*/
final void cancelIgnoringExceptions() {
try {
cancel(false);
} catch (Throwable ignore) {
}
}
/**
* Main implementation of helpJoin
*/
private int busyJoin(ForkJoinWorkerThread w) {
int s;
ForkJoinTask<?> t;
while ((s = status) >= 0 && (t = w.scanWhileJoining(this)) != null)
t.quietlyExec();
return (s >= 0) ? awaitDone(w, false) : s; // block if no work
}
// public methods
/**
* Arranges to asynchronously execute this task. While it is not
* necessarily enforced, it is a usage error to fork a task more
* than once unless it has completed and been reinitialized.
* Subsequent modifications to the state of this task or any data
* it operates on are not necessarily consistently observable by
* any thread other than the one executing it unless preceded by a
* call to {@link #join} or related methods, or a call to {@link
* #isDone} returning {@code true}.
*
* <p>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<V> fork() {
((ForkJoinWorkerThread) Thread.currentThread())
.pushTask(this);
return this;
}
/**
* Returns the result of the computation when it {@link #isDone is done}.
* This method differs from {@link #get()} in that
* abnormal completion results in {@code RuntimeException} or
* {@code Error}, not {@code ExecutionException}.
*
* @return the computed result
*/
public final V join() {
ForkJoinWorkerThread w = getWorker();
if (w == null || status < 0 || !w.unpushTask(this) || !tryExec())
reportException(awaitDone(w, true));
return getRawResult();
}
/**
* Commences performing this task, awaits its completion if
* necessary, and return its result, or throws an (unchecked)
* exception if the underlying computation did so.
*
* @return the computed result
*/
public final V invoke() {
if (status >= 0 && tryExec())
return getRawResult();
else
return 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 either task encounters an
* exception, the other one may be, but is not guaranteed to be,
* cancelled. If both tasks throw an exception, then this method
* throws one of them. The individual status of each task may be
* checked using {@link #getException()} and related methods.
*
* <p>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 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.
*
* <p>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)
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)
ex = t.getException();
}
}
}
if (ex != null)
rethrowException(ex);
}
/**
* 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.
*
* <p>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 <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()]));
return tasks;
}
@SuppressWarnings("unchecked")
List<? extends ForkJoinTask<?>> ts =
(List<? extends ForkJoinTask<?>>) tasks;
Throwable ex = null;
int last = ts.size() - 1;
for (int i = last; i >= 0; --i) {
ForkJoinTask<?> t = ts.get(i);
if (t == null) {
if (ex == null)
ex = new NullPointerException();
}
else if (i != 0)
t.fork();
else {
t.quietlyInvoke();
if (ex == null)
ex = t.getException();
}
}
for (int i = 1; i <= last; ++i) {
ForkJoinTask<?> t = ts.get(i);
if (t != null) {
if (ex != null)
t.cancel(false);
else {
t.quietlyJoin();
if (ex == null)
ex = t.getException();
}
}
}
if (ex != null)
rethrowException(ex);
return tasks;
}
/**
* 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.
*
* <p>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.
*
* <p>This method is designed to be invoked by <em>other</em>
* 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 & COMPLETION_MASK) == CANCELLED;
}
public final boolean isDone() {
return status < 0;
}
public final boolean isCancelled() {
return (status & COMPLETION_MASK) == 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 & COMPLETION_MASK) < 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 & COMPLETION_MASK) == 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 & COMPLETION_MASK;
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) {
setDoneExceptionally((ex instanceof RuntimeException) ||
(ex instanceof Error) ? ex :
new RuntimeException(ex));
}
/**
* Completes this task, and if not already aborted or cancelled,
* 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 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) {
setDoneExceptionally(rex);
return;
}
setNormalCompletion();
}
public final V get() throws InterruptedException, ExecutionException {
ForkJoinWorkerThread w = getWorker();
if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke())
awaitDone(w, true);
return reportFutureResult();
}
public final V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
long nanos = unit.toNanos(timeout);
ForkJoinWorkerThread w = getWorker();
if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke())
awaitDone(w, nanos);
return reportTimedFutureResult();
}
/**
* Possibly executes other tasks until this task {@link #isDone is
* done}, then returns the result of the computation. This method
* may be more 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).
*
* <p>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();
if (status < 0 || !w.unpushTask(this) || !tryExec())
reportException(busyJoin(w));
return getRawResult();
}
/**
* Possibly executes other tasks until this task {@link #isDone is
* done}. This method may be useful when processing collections
* of tasks when some have been cancelled or otherwise known to
* have aborted.
*
* <p>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();
if (!w.unpushTask(this) || !tryQuietlyInvoke())
busyJoin(w);
}
}
/**
* Joins this task, without returning its result or throwing an
* 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() {
if (status >= 0) {
ForkJoinWorkerThread w = getWorker();
if (w == null || !w.unpushTask(this) || !tryQuietlyInvoke())
awaitDone(w, true);
}
}
/**
* Commences performing this task and awaits its completion if
* necessary, without returning its result or throwing an
* 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 quietlyInvoke() {
if (status >= 0 && !tryQuietlyInvoke())
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.
*
* <p>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 & COMPLETION_MASK) == 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.
*
* <p>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.
*
* <p>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.
*
* <p>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.
*
* <p>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.
*
* <p>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.
*
* <p>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<T> extends ForkJoinTask<T>
implements RunnableFuture<T> {
final Runnable runnable;
final T resultOnCompletion;
T result;
AdaptedRunnable(Runnable runnable, T result) {
if (runnable == null) throw new NullPointerException();
this.runnable = runnable;
this.resultOnCompletion = result;
}
public T getRawResult() { return result; }
public void setRawResult(T v) { result = v; }
public boolean exec() {
runnable.run();
result = resultOnCompletion;
return true;
}
public void run() { invoke(); }
private static final long serialVersionUID = 5232453952276885070L;
}
/**
* Adaptor for Callables
*/
static final class AdaptedCallable<T> extends ForkJoinTask<T>
implements RunnableFuture<T> {
final Callable<? extends T> callable;
T result;
AdaptedCallable(Callable<? extends T> callable) {
if (callable == null) throw new NullPointerException();
this.callable = callable;
}
public T getRawResult() { return result; }
public void setRawResult(T v) { result = v; }
public boolean exec() {
try {
result = callable.call();
return true;
} catch (Error err) {
throw err;
} catch (RuntimeException rex) {
throw rex;
} catch (Exception ex) {
throw new RuntimeException(ex);
}
}
public void run() { invoke(); }
private static final long serialVersionUID = 2838392045355241008L;
}
/**
* Returns a new {@code ForkJoinTask} that performs the {@code run}
* method of the given {@code Runnable} as its action, and returns
* a null result upon {@link #join}.
*
* @param runnable the runnable action
* @return the task
*/
public static ForkJoinTask<?> adapt(Runnable runnable) {
return new AdaptedRunnable<Void>(runnable, null);
}
/**
* Returns a new {@code ForkJoinTask} that performs the {@code run}
* method of the given {@code Runnable} as its action, and returns
* the given result upon {@link #join}.
*
* @param runnable the runnable action
* @param result the result upon completion
* @return the task
*/
public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
return new AdaptedRunnable<T>(runnable, result);
}
/**
* Returns a new {@code ForkJoinTask} that performs the {@code call}
* method of the given {@code Callable} as its action, and returns
* its result upon {@link #join}, translating any checked exceptions
* encountered into {@code RuntimeException}.
*
* @param callable the callable action
* @return the task
*/
public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
return new AdaptedCallable<T>(callable);
}
// Serialization support
private static final long serialVersionUID = -7721805057305804111L;
/**
* Saves the state to a stream.
*
* @serialData the current run status and the exception thrown
* during execution, or {@code null} if none
* @param s the stream
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
s.defaultWriteObject();
s.writeObject(getException());
}
/**
* Reconstitutes the instance from a stream.
*
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
status &= ~INTERNAL_SIGNAL_MASK; // clear internal signal counts
status |= EXTERNAL_SIGNAL; // conservatively set external signal
Object ex = s.readObject();
if (ex != null)
setDoneExceptionally((Throwable) ex);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long statusOffset =
objectFieldOffset("status", ForkJoinTask.class);
private static long objectFieldOffset(String field, Class<?> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
}