/*
* 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/publicdomain/zero/1.0/
*/
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.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
import java.lang.reflect.Constructor;
/**
* 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(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.
*
* <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. Rethrown exceptions behave in the same way as
* regular exceptions, but, when possible, contain stack traces (as
* displayed for example using {@code ex.printStackTrace()}) of both
* the thread that initiated the computation as well as the thread
* actually encountering the exception; minimally only the latter.
*
* <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 #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
* {@code ClassCastException}.
*
* <p>Method {@link #join} and its variants are appropriate for use
* only when completion dependencies are acyclic; that is, the
* parallel computation can be described as a directed acyclic graph
* (DAG). Otherwise, executions may encounter a form of deadlock as
* tasks cyclically wait for each other. However, this framework
* supports other methods and techniques (for example the use of
* {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
* may be of use in constructing custom subclasses for problems that
* are not statically structured as DAGs.
*
* <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, and should avoid indefinite looping. 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 constructed in <em>asyncMode</em>.
*
* <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 {
/*
* See the internal documentation of class ForkJoinPool for a
* general implementation overview. ForkJoinTasks are mainly
* responsible for maintaining their "status" field amidst relays
* to methods in ForkJoinWorkerThread and ForkJoinPool. The
* methods of this class are more-or-less layered into (1) basic
* status maintenance (2) execution and awaiting completion (3)
* user-level methods that additionally report results. This is
* sometimes hard to see because this file orders exported methods
* in a way that flows well in javadocs.
*/
/*
* The status field holds 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 value
* NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking
* waits by other threads have the SIGNAL bit set. Completion of
* a stolen task with SIGNAL set awakens any waiters 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. We want
* these monitors to be "fat", i.e., not use biasing or thin-lock
* techniques, so use some odd coding idioms that tend to avoid
* them.
*/
/** The run status of this task */
volatile int status; // accessed directly by pool and workers
private static final int NORMAL = -1;
private static final int CANCELLED = -2;
private static final int EXCEPTIONAL = -3;
private static final int SIGNAL = 1;
/**
* Marks completion and wakes up threads waiting to join this task,
* also clearing signal request bits.
*
* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
* @return completion status on exit
*/
private int setCompletion(int completion) {
for (int s;;) {
if ((s = status) < 0)
return s;
if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
if (s != 0)
synchronized (this) { notifyAll(); }
return completion;
}
}
}
/**
* Tries to block a worker thread until completed or timed out.
* Uses Object.wait time argument conventions.
* May fail on contention or interrupt.
*
* @param millis if > 0, wait time.
*/
final void tryAwaitDone(long millis) {
int s;
try {
if (((s = status) > 0 ||
(s == 0 &&
UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&
status > 0) {
synchronized (this) {
if (status > 0)
wait(millis);
}
}
} catch (InterruptedException ie) {
// caller must check termination
}
}
/**
* Blocks a non-worker-thread until completion.
* @return status upon completion
*/
private int externalAwaitDone() {
int s;
if ((s = status) >= 0) {
boolean interrupted = false;
synchronized (this) {
while ((s = status) >= 0) {
if (s == 0)
UNSAFE.compareAndSwapInt(this, statusOffset,
0, SIGNAL);
else {
try {
wait();
} catch (InterruptedException ie) {
interrupted = true;
}
}
}
}
if (interrupted)
Thread.currentThread().interrupt();
}
return s;
}
/**
* Blocks a non-worker-thread until completion or interruption or timeout.
*/
private int externalInterruptibleAwaitDone(long millis)
throws InterruptedException {
int s;
if (Thread.interrupted())
throw new InterruptedException();
if ((s = status) >= 0) {
synchronized (this) {
while ((s = status) >= 0) {
if (s == 0)
UNSAFE.compareAndSwapInt(this, statusOffset,
0, SIGNAL);
else {
wait(millis);
if (millis > 0L)
break;
}
}
}
}
return s;
}
/**
* Primary execution method for stolen tasks. Unless done, calls
* exec and records status if completed, but doesn't wait for
* completion otherwise.
*/
final void doExec() {
if (status >= 0) {
boolean completed;
try {
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
if (completed)
setCompletion(NORMAL); // must be outside try block
}
}
/**
* Primary mechanics for join, get, quietlyJoin.
* @return status upon completion
*/
private int doJoin() {
Thread t; ForkJoinWorkerThread w; int s; boolean completed;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
if ((s = status) < 0)
return s;
if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
try {
completed = exec();
} catch (Throwable rex) {
return setExceptionalCompletion(rex);
}
if (completed)
return setCompletion(NORMAL);
}
return w.joinTask(this);
}
else
return externalAwaitDone();
}
/**
* Primary mechanics for invoke, quietlyInvoke.
* @return status upon completion
*/
private int doInvoke() {
int s; boolean completed;
if ((s = status) < 0)
return s;
try {
completed = exec();
} catch (Throwable rex) {
return setExceptionalCompletion(rex);
}
if (completed)
return setCompletion(NORMAL);
else
return doJoin();
}
// Exception table support
/**
* 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.
*
* Note: These statics are initialized below in static block.
*/
private static final ExceptionNode[] exceptionTable;
private static final ReentrantLock exceptionTableLock;
private static final ReferenceQueue<Object> exceptionTableRefQueue;
/**
* Fixed capacity for exceptionTable.
*/
private static final int EXCEPTION_MAP_CAPACITY = 32;
/**
* Key-value nodes for exception table. The chained hash table
* uses identity comparisons, full locking, and weak references
* for keys. The table has a fixed capacity because it only
* maintains task exceptions long enough for joiners to access
* them, so should never become very large for sustained
* periods. However, since we do not know when the last joiner
* completes, we must use weak references and expunge them. We do
* so on each operation (hence full locking). Also, some thread in
* any ForkJoinPool will call helpExpungeStaleExceptions when its
* pool becomes isQuiescent.
*/
static final class ExceptionNode extends WeakReference<ForkJoinTask<?>>{
final Throwable ex;
ExceptionNode next;
final long thrower; // use id not ref to avoid weak cycles
ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
super(task, exceptionTableRefQueue);
this.ex = ex;
this.next = next;
this.thrower = Thread.currentThread().getId();
}
}
/**
* Records exception and sets exceptional completion.
*
* @return status on exit
*/
private int setExceptionalCompletion(Throwable ex) {
int h = System.identityHashCode(this);
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
expungeStaleExceptions();
ExceptionNode[] t = exceptionTable;
int i = h & (t.length - 1);
for (ExceptionNode e = t[i]; ; e = e.next) {
if (e == null) {
t[i] = new ExceptionNode(this, ex, t[i]);
break;
}
if (e.get() == this) // already present
break;
}
} finally {
lock.unlock();
}
return setCompletion(EXCEPTIONAL);
}
/**
* Removes exception node and clears status
*/
private void clearExceptionalCompletion() {
int h = System.identityHashCode(this);
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
ExceptionNode[] t = exceptionTable;
int i = h & (t.length - 1);
ExceptionNode e = t[i];
ExceptionNode pred = null;
while (e != null) {
ExceptionNode next = e.next;
if (e.get() == this) {
if (pred == null)
t[i] = next;
else
pred.next = next;
break;
}
pred = e;
e = next;
}
expungeStaleExceptions();
status = 0;
} finally {
lock.unlock();
}
}
/**
* Returns a rethrowable exception for the given task, if
* available. To provide accurate stack traces, if the exception
* was not thrown by the current thread, we try to create a new
* exception of the same type as the one thrown, but with the
* recorded exception as its cause. If there is no such
* constructor, we instead try to use a no-arg constructor,
* followed by initCause, to the same effect. If none of these
* apply, or any fail due to other exceptions, we return the
* recorded exception, which is still correct, although it may
* contain a misleading stack trace.
*
* @return the exception, or null if none
*/
private Throwable getThrowableException() {
if (status != EXCEPTIONAL)
return null;
int h = System.identityHashCode(this);
ExceptionNode e;
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
expungeStaleExceptions();
ExceptionNode[] t = exceptionTable;
e = t[h & (t.length - 1)];
while (e != null && e.get() != this)
e = e.next;
} finally {
lock.unlock();
}
Throwable ex;
if (e == null || (ex = e.ex) == null)
return null;
if (e.thrower != Thread.currentThread().getId()) {
Class ec = ex.getClass();
try {
Constructor<?> noArgCtor = null;
Constructor<?>[] cs = ec.getConstructors();// public ctors only
for (int i = 0; i < cs.length; ++i) {
Constructor<?> c = cs[i];
Class<?>[] ps = c.getParameterTypes();
if (ps.length == 0)
noArgCtor = c;
else if (ps.length == 1 && ps[0] == Throwable.class)
return (Throwable)(c.newInstance(ex));
}
if (noArgCtor != null) {
Throwable wx = (Throwable)(noArgCtor.newInstance());
wx.initCause(ex);
return wx;
}
} catch (Exception ignore) {
}
}
return ex;
}
/**
* Poll stale refs and remove them. Call only while holding lock.
*/
private static void expungeStaleExceptions() {
for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
if (x instanceof ExceptionNode) {
ForkJoinTask<?> key = ((ExceptionNode)x).get();
ExceptionNode[] t = exceptionTable;
int i = System.identityHashCode(key) & (t.length - 1);
ExceptionNode e = t[i];
ExceptionNode pred = null;
while (e != null) {
ExceptionNode next = e.next;
if (e == x) {
if (pred == null)
t[i] = next;
else
pred.next = next;
break;
}
pred = e;
e = next;
}
}
}
}
/**
* If lock is available, poll stale refs and remove them.
* Called from ForkJoinPool when pools become quiescent.
*/
static final void helpExpungeStaleExceptions() {
final ReentrantLock lock = exceptionTableLock;
if (lock.tryLock()) {
try {
expungeStaleExceptions();
} finally {
lock.unlock();
}
}
}
/**
* Report the result of invoke or join; called only upon
* non-normal return of internal versions.
*/
private V reportResult() {
int s; Throwable ex;
if ((s = status) == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
UNSAFE.throwException(ex);
return getRawResult();
}
// 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
* ForkJoinPool} 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}, and that
* interrupts of the calling thread do <em>not</em> cause the
* method to abruptly return by throwing {@code
* InterruptedException}.
*
* @return the computed result
*/
public final V join() {
if (doJoin() != NORMAL)
return reportResult();
else
return getRawResult();
}
/**
* Commences performing this task, awaits its completion if
* necessary, and returns its result, or throws an (unchecked)
* {@code RuntimeException} or {@code Error} if the underlying
* computation did so.
*
* @return the computed result
*/
public final V invoke() {
if (doInvoke() != NORMAL)
return reportResult();
else
return getRawResult();
}
/**
* 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, the
* other 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.
*
* <p>This method may be invoked only from within {@code
* ForkJoinPool} 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.
*
* <p>This method may be invoked only from within {@code
* ForkJoinPool} 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 if (t.doInvoke() < NORMAL && 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 if (t.doJoin() < NORMAL && ex == null)
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.
*
* <p>This method may be invoked only from within {@code
* ForkJoinPool} 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 if (t.doInvoke() < NORMAL && 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 if (t.doJoin() < NORMAL && ex == null)
ex = t.getException();
}
}
if (ex != null)
UNSAFE.throwException(ex);
return tasks;
}
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed or could not be
* cancelled for some other reason. If successful, and this task
* has not started when {@code cancel} is called, execution of
* this task is suppressed. After this method returns
* successfully, unless there is an intervening call to {@link
* #reinitialize}, subsequent calls to {@link #isCancelled},
* {@link #isDone}, and {@code cancel} will return {@code true}
* and calls to {@link #join} and related methods will result in
* {@code CancellationException}.
*
* <p>This method may be overridden in subclasses, but if so, must
* still ensure that these 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 has no effect in the
* default implementation because interrupts are not used to
* control cancellation.
*
* @return {@code true} if this task is now cancelled
*/
public boolean cancel(boolean mayInterruptIfRunning) {
return setCompletion(CANCELLED) == 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) {
}
}
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() :
getThrowableException());
}
/**
* 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 = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
doJoin() : externalInterruptibleAwaitDone(0L);
Throwable ex;
if (s == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL && (ex = getThrowableException()) != 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();
if (t instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
long nanos = unit.toNanos(timeout);
if (status >= 0) {
boolean completed = false;
if (w.unpushTask(this)) {
try {
completed = exec();
} catch (Throwable rex) {
setExceptionalCompletion(rex);
}
}
if (completed)
setCompletion(NORMAL);
else if (status >= 0 && nanos > 0)
w.pool.timedAwaitJoin(this, nanos);
}
}
else {
long millis = unit.toMillis(timeout);
if (millis > 0)
externalInterruptibleAwaitDone(millis);
}
int s = status;
if (s != NORMAL) {
Throwable ex;
if (s == CANCELLED)
throw new CancellationException();
if (s != EXCEPTIONAL)
throw new TimeoutException();
if ((ex = getThrowableException()) != null)
throw new ExecutionException(ex);
}
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() {
doJoin();
}
/**
* Commences performing this task and awaits its completion if
* necessary, without returning its result or throwing its
* exception.
*/
public final void quietlyInvoke() {
doInvoke();
}
/**
* 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
* ForkJoinPool} 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.
*
* <p>Upon completion of this method, {@code isDone()} reports
* {@code false}, and {@code getException()} reports {@code
* null}. However, the value returned by {@code getRawResult} is
* unaffected. To clear this value, you can invoke {@code
* setRawResult(null)}.
*/
public void reinitialize() {
if (status == EXCEPTIONAL)
clearExceptionalCompletion();
else
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 a {@link
* ForkJoinWorkerThread} executing as a ForkJoinPool computation.
*
* @return {@code true} if the current thread is a {@link
* ForkJoinWorkerThread} executing as a ForkJoinPool computation,
* or {@code 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
* ForkJoinPool} 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
* ForkJoinPool} 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
* ForkJoinPool} 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
* ForkJoinPool} 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
* ForkJoinPool} 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
* ForkJoinPool} 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 (that is, serializes it).
*
* @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 (that is, deserializes it).
*
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
Object ex = s.readObject();
if (ex != null)
setExceptionalCompletion((Throwable)ex);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long statusOffset;
static {
exceptionTableLock = new ReentrantLock();
exceptionTableRefQueue = new ReferenceQueue<Object>();
exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
statusOffset = UNSAFE.objectFieldOffset
(ForkJoinTask.class.getDeclaredField("status"));
} catch (Exception e) {
throw new Error(e);
}
}
}