/*

* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

*

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/*

*

*

*

*

*

* 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