ForkJoinTasks are forms of Futures supporting a - * limited range of use. The "lightness" of ForkJoinTasks is due to 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 supported for ForkJoinTasks are - * fork, that arranges asynchronous execution, and - * join, that doesn't proceed until the task's result has - * been computed. (Cancellation is also supported). The computation - * defined in the compute method should avoid - * synchronized methods or blocks, and should minimize - * 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 external synchronization becomes - * exhausted. This usage restriction is in part enforced by not - * permitting checked exceptions such as IOExceptions to be - * thrown. However, computations may still encounter unchecked - * exceptions, that are rethrown to callers attempting join - * them. These exceptions may additionally include - * RejectedExecutionExceptions stemming from internal resource - * exhaustion such as failure to allocate internal task queues. + *
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. * - *
The ForkJoinTask class is not usually directly - * subclassed. Instead, you subclass one of the abstract classes that - * support different styles of fork/join processing. Normally, a - * concrete ForkJoinTask subclass declares fields comprising its - * parameters, established in a constructor, and then defines a - * compute method that somehow uses the control methods - * supplied by this base class. While these methods have - * public access, some of them may only be called from within - * other ForkJoinTasks. Attempts to invoke them in other contexts - * result in exceptions or errors including ClassCastException. The - * only way to invoke a "main" driver task is to submit it to a - * ForkJoinPool. Once started, this will usually in turn start other - * subtasks. + *
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. * - *
Most base support methods are final because their - * implementations are intrinsically tied to the underlying - * lightweight task scheduling framework, and so cannot be overridden. - * Developers creating new basic styles of fork/join processing should - * minimally implement protected methods exec, - * setRawResult, and getRawResult, while also - * introducing an abstract computational method that can be - * implemented in its subclasses. To support such extensions, - * instances of ForkJoinTasks maintain an atomically updated - * short representing user-defined control state. Control - * state is guaranteed initially to be zero, and to be negative upon - * completion, but may otherwise be used for any other control - * purposes, such as maintaining join counts. The {@link - * ForkJoinWorkerThread} class supports additional inspection and - * tuning methods that can be useful when developing extensions. + *
The primary method for awaiting completion and extracting + * results of a task is {@link #join}, but there are several variants: + * The {@link Future#get} methods support interruptible and/or timed + * waits for completion and report results using {@code Future} + * conventions. Method {@link #invoke} is semantically + * equivalent to {@code fork(); join()} but always attempts to begin + * execution in the current thread. The "quiet" forms of + * these methods do not extract results or report exceptions. These + * may be useful when a set of tasks are being executed, and you need + * to delay processing of results or exceptions until all complete. + * Method {@code invokeAll} (available in multiple versions) + * performs the most common form of parallel invocation: forking a set + * of tasks and joining them all. + * + *
The execution status of tasks may be queried at several levels + * of detail: {@link #isDone} is true if a task completed in any way + * (including the case where a task was cancelled without executing); + * {@link #isCompletedNormally} is true if a task completed without + * cancellation or encountering an exception; {@link #isCancelled} is + * true if the task was cancelled (in which case {@link #getException} + * returns a {@link java.util.concurrent.CancellationException}); and + * {@link #isCompletedAbnormally} is true if a task was either + * cancelled or encountered an exception, in which case {@link + * #getException} will return either the encountered exception or + * {@link java.util.concurrent.CancellationException}. + * + *
The ForkJoinTask class is not usually directly subclassed. + * Instead, you subclass one of the abstract classes that support a + * particular style of fork/join processing, typically {@link + * RecursiveAction} for computations that do not return results, or + * {@link RecursiveTask} for those that do. Normally, a concrete + * ForkJoinTask subclass declares fields comprising its parameters, + * established in a constructor, and then defines a {@code compute} + * method that somehow uses the control methods supplied by this base + * class. While these methods have {@code public} access (to allow + * instances of different task subclasses to call each other's + * methods), some of them may only be called from within other + * ForkJoinTasks (as may be determined using method {@link + * #inForkJoinPool}). Attempts to invoke them in other contexts + * result in exceptions or errors, possibly including + * {@code ClassCastException}. + * + *
Most base support methods are {@code final}, to prevent + * overriding of implementations that are intrinsically tied to the + * underlying lightweight task scheduling framework. Developers + * creating new basic styles of fork/join processing should minimally + * implement {@code protected} methods {@link #exec}, {@link + * #setRawResult}, and {@link #getRawResult}, while also introducing + * an abstract computational method that can be implemented in its + * subclasses, possibly relying on other {@code protected} methods + * provided by this class. * *
ForkJoinTasks should perform relatively small amounts of - * computations, othewise splitting into smaller tasks. 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 - * parellelism cannot improve throughput. If too small, then memory - * and internal task maintenance overhead may overwhelm processing. + * computation. Large tasks should be split into smaller subtasks, + * usually via recursive decomposition. As a very rough rule of thumb, + * a task should perform more than 100 and less than 10000 basic + * computational steps. If tasks are too big, then parallelism cannot + * improve throughput. If too small, then memory and internal task + * maintenance overhead may overwhelm processing. + * + *
This class provides {@code adapt} methods for {@link Runnable} + * and {@link Callable}, that may be of use when mixing execution of + * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are + * of this form, consider using a pool constructed in asyncMode. * - *
ForkJoinTasks are Serializable, which enables them to - * be used in extensions such as remote execution frameworks. However, - * it is in general safe to serialize tasks only before or after, but - * not during execution. Serialization is not relied on during - * execution itself. + *
ForkJoinTasks are {@code Serializable}, which enables them to be
+ * used in extensions such as remote execution frameworks. It is
+ * sensible to serialize tasks only before or after, but not during,
+ * execution. Serialization is not relied on during execution itself.
+ *
+ * @since 1.7
+ * @author Doug Lea
*/
public abstract class ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return {@code this}, to simplify usage
*/
- private void reportException(int s) {
- if ((s &= COMPLETION_MASK) < NORMAL) {
- if (s == CANCELLED)
- throw new CancellationException();
- else
- rethrowException(exceptionMap.get(this));
- }
+ public final ForkJoinTask This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @param t1 the first task
+ * @param t2 the second task
+ * @throws NullPointerException if any task is null
+ */
+ public static void invokeAll(ForkJoinTask t1, ForkJoinTask t2) {
+ t2.fork();
+ t1.invoke();
+ t2.join();
+ }
/**
- * Calls exec, recording completion, and rethrowing exception if
- * encountered. Caller should normally check status before calling
- * @return true if completed normally
+ * Forks the given tasks, returning when {@code isDone} holds for
+ * each task or an (unchecked) exception is encountered, in which
+ * case the exception is rethrown. If more than one task
+ * encounters an exception, then this method throws any one of
+ * these exceptions. If any task encounters an exception, others
+ * may be cancelled. However, the execution status of individual
+ * tasks is not guaranteed upon exceptional return. The status of
+ * each task may be obtained using {@link #getException()} and
+ * related methods to check if they have been cancelled, completed
+ * normally or exceptionally, or left unprocessed.
+ *
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @param tasks the tasks
+ * @throws NullPointerException if any task is null
*/
- 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
+ public static void invokeAll(ForkJoinTask... tasks) {
+ Throwable ex = null;
+ int last = tasks.length - 1;
+ for (int i = last; i >= 0; --i) {
+ ForkJoinTask t = tasks[i];
+ if (t == null) {
+ if (ex == null)
+ ex = new NullPointerException();
+ }
+ else if (i != 0)
+ t.fork();
+ else {
+ t.quietlyInvoke();
+ if (ex == null && t.status < NORMAL)
+ ex = t.getException();
+ }
}
- setNormalCompletion();
- return true;
+ for (int i = 1; i <= last; ++i) {
+ ForkJoinTask t = tasks[i];
+ if (t != null) {
+ if (ex != null)
+ t.cancel(false);
+ else {
+ t.quietlyJoin();
+ if (ex == null && t.status < NORMAL)
+ ex = t.getException();
+ }
+ }
+ }
+ if (ex != null)
+ UNSAFE.throwException(ex);
}
/**
- * Main execution method used by worker threads. Invokes
- * base computation unless already complete
+ * Forks all tasks in the specified collection, returning when
+ * {@code isDone} holds for each task or an (unchecked) exception
+ * is encountered, in which case the exception is rethrown. If
+ * more than one task encounters an exception, then this method
+ * throws any one of these exceptions. If any task encounters an
+ * exception, others may be cancelled. However, the execution
+ * status of individual tasks is not guaranteed upon exceptional
+ * return. The status of each task may be obtained using {@link
+ * #getException()} and related methods to check if they have been
+ * cancelled, completed normally or exceptionally, or left
+ * unprocessed.
+ *
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @param tasks the collection of tasks
+ * @return the tasks argument, to simplify usage
+ * @throws NullPointerException if tasks or any element are null
*/
- final void quietlyExec() {
- if (status >= 0) {
- try {
- if (!exec())
- return;
- } catch(Throwable rex) {
- setDoneExceptionally(rex);
- return;
+ public static This method may be overridden in subclasses, but if so, must
+ * still ensure that these minimal properties hold. In particular,
+ * the {@code cancel} method itself must not throw exceptions.
+ *
+ * This method is designed to be invoked by other
+ * tasks. To terminate the current task, you can just return or
+ * throw an unchecked exception from its computation method, or
+ * invoke {@link #completeExceptionally}.
+ *
+ * @param mayInterruptIfRunning this value is ignored in the
+ * default implementation because tasks are not
+ * cancelled via interruption
+ *
+ * @return {@code true} if this task is now cancelled
*/
- private boolean tryQuietlyInvoke() {
- try {
- if (!exec())
- return false;
- } catch (Throwable rex) {
- setDoneExceptionally(rex);
- return false;
- }
- setNormalCompletion();
- return true;
+ public boolean cancel(boolean mayInterruptIfRunning) {
+ setCompletion(CANCELLED);
+ return status == CANCELLED;
}
/**
- * Cancel, ignoring any exceptions it throws
+ * 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 cancelIgnoreExceptions() {
+ final void cancelIgnoringExceptions() {
try {
cancel(false);
- } catch(Throwable ignore) {
+ } catch (Throwable ignore) {
}
}
- // 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. This
- * method may be invoked only from within other ForkJoinTask
- * computations. Attempts to invoke in other contexts result in
- * exceptions or errors including ClassCastException.
+ * Cancels if current thread is a terminating worker thread,
+ * ignoring any exceptions thrown by cancel.
*/
- public final void fork() {
- ((ForkJoinWorkerThread)(Thread.currentThread())).pushTask(this);
+ final void cancelIfTerminating() {
+ Thread t = Thread.currentThread();
+ if ((t instanceof ForkJoinWorkerThread) &&
+ ((ForkJoinWorkerThread) t).isTerminating()) {
+ try {
+ cancel(false);
+ } catch (Throwable ignore) {
+ }
+ }
+ }
+
+ public final boolean isDone() {
+ return status < 0;
+ }
+
+ public final boolean isCancelled() {
+ return status == CANCELLED;
}
/**
- * Returns the result of the computation when it is ready.
- * This method differs from get in that abnormal
- * completion results in RuntimeExceptions or Errors, not
- * ExecutionExceptions.
+ * Returns {@code true} if this task threw an exception or was cancelled.
*
- * @return the computed result
+ * @return {@code true} if this task threw an exception or was cancelled
*/
- public final V join() {
- ForkJoinWorkerThread w = getWorker();
- if (w == null || status < 0 || !w.unpushTask(this) || !tryExec())
- reportException(awaitDone(w, true));
- return getRawResult();
+ public final boolean isCompletedAbnormally() {
+ return status < NORMAL;
}
- public final V get() throws InterruptedException, ExecutionException {
- ForkJoinWorkerThread w = getWorker();
- if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke())
- awaitDone(w, true);
- return reportFutureResult();
+ /**
+ * 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;
}
- public final V get(long timeout, TimeUnit unit)
- throws InterruptedException, ExecutionException, TimeoutException {
- ForkJoinWorkerThread w = getWorker();
- if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke())
- awaitDone(w, unit.toNanos(timeout));
- return reportTimedFutureResult();
+ /**
+ * Returns the exception thrown by the base computation, or a
+ * {@code CancellationException} if cancelled, or {@code null} if
+ * none or if the method has not yet completed.
+ *
+ * @return the exception, or {@code null} if none
+ */
+ public final Throwable getException() {
+ int s = status;
+ return ((s >= NORMAL) ? null :
+ (s == CANCELLED) ? new CancellationException() :
+ exceptionMap.get(this));
}
/**
- * Possibly executes other tasks until this task is ready, then
- * returns the result of the computation. This method may be more
- * efficient than join, but is only applicable when there
- * are no potemtial 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).
- * @return the computed result
+ * 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 final V helpJoin() {
- ForkJoinWorkerThread w = (ForkJoinWorkerThread)(Thread.currentThread());
- if (status < 0 || !w.unpushTask(this) || !tryExec())
- reportException(w.helpJoinTask(this));
- return getRawResult();
+ public void completeExceptionally(Throwable ex) {
+ setExceptionalCompletion((ex instanceof RuntimeException) ||
+ (ex instanceof Error) ? ex :
+ new RuntimeException(ex));
}
/**
- * Performs this task, awaits its completion if necessary, and
- * return its result.
- * @throws Throwable (a RuntimeException, Error, or unchecked
- * exception) if the underlying computation did so.
- * @return the computed result
+ * 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 final V invoke() {
- if (status >= 0 && tryExec())
- return getRawResult();
+ public void complete(V value) {
+ try {
+ setRawResult(value);
+ } catch (Throwable rex) {
+ setExceptionalCompletion(rex);
+ return;
+ }
+ setCompletion(NORMAL);
+ }
+
+ public final V get() throws InterruptedException, ExecutionException {
+ quietlyJoin();
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ int s = status;
+ 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();
+ }
+
+ public final V get(long timeout, TimeUnit unit)
+ throws InterruptedException, ExecutionException, TimeoutException {
+ Thread t = Thread.currentThread();
+ ForkJoinPool pool;
+ if (t instanceof ForkJoinWorkerThread) {
+ ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
+ if (status >= 0 && w.unpushTask(this))
+ quietlyExec();
+ pool = w.pool;
+ }
else
- return join();
+ pool = null;
+ /*
+ * Timed wait loop intermixes cases for FJ (pool != null) and
+ * non FJ threads. For FJ, decrement pool count but don't try
+ * for replacement; increment count on completion. For non-FJ,
+ * deal with interrupts. This is messy, but a little less so
+ * than is splitting the FJ and nonFJ cases.
+ */
+ boolean interrupted = false;
+ boolean dec = false; // true if pool count decremented
+ long nanos = unit.toNanos(timeout);
+ for (;;) {
+ if (pool == null && Thread.interrupted()) {
+ interrupted = true;
+ break;
+ }
+ int s = status;
+ if (s < 0)
+ break;
+ if (UNSAFE.compareAndSwapInt(this, statusOffset, s, SIGNAL)) {
+ long startTime = System.nanoTime();
+ long nt; // wait time
+ while (status >= 0 &&
+ (nt = nanos - (System.nanoTime() - startTime)) > 0) {
+ if (pool != null && !dec)
+ dec = pool.tryDecrementRunningCount();
+ else {
+ long ms = nt / 1000000;
+ int ns = (int) (nt % 1000000);
+ try {
+ synchronized(this) {
+ if (status >= 0)
+ wait(ms, ns);
+ }
+ } catch (InterruptedException ie) {
+ if (pool != null)
+ cancelIfTerminating();
+ else {
+ interrupted = true;
+ break;
+ }
+ }
+ }
+ }
+ break;
+ }
+ }
+ if (pool != null && dec)
+ pool.incrementRunningCount();
+ if (interrupted)
+ throw new InterruptedException();
+ int es = status;
+ if (es != NORMAL) {
+ Throwable ex;
+ if (es == CANCELLED)
+ throw new CancellationException();
+ if (es == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
+ throw new ExecutionException(ex);
+ throw new TimeoutException();
+ }
+ return getRawResult();
}
/**
- * Joins this task, without returning its result or throwing an
+ * 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() {
- if (status >= 0) {
- ForkJoinWorkerThread w = getWorker();
- if (w == null || !w.unpushTask(this) || !tryQuietlyInvoke())
- awaitDone(w, true);
+ Thread t;
+ if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
+ ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
+ if (status >= 0) {
+ if (w.unpushTask(this)) {
+ boolean completed;
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ setExceptionalCompletion(rex);
+ return;
+ }
+ if (completed) {
+ setCompletion(NORMAL);
+ return;
+ }
+ }
+ w.joinTask(this);
+ }
}
+ else
+ externalAwaitDone();
}
/**
- * Possibly executes other tasks until this task is ready.
+ * Commences performing this task and awaits its completion if
+ * necessary, without returning its result or throwing its
+ * exception.
*/
- public final void quietlyHelpJoin() {
+ public final void quietlyInvoke() {
if (status >= 0) {
- ForkJoinWorkerThread w =
- (ForkJoinWorkerThread)(Thread.currentThread());
- if (!w.unpushTask(this) || !tryQuietlyInvoke())
- w.helpJoinTask(this);
+ boolean completed;
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ setExceptionalCompletion(rex);
+ return;
+ }
+ if (completed)
+ setCompletion(NORMAL);
+ else
+ quietlyJoin();
}
}
/**
- * Performs 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.
+ * Possibly executes tasks until the pool hosting the current task
+ * {@link ForkJoinPool#isQuiescent is quiescent}. This method may
+ * be of use in designs in which many tasks are forked, but none
+ * are explicitly joined, instead executing them until all are
+ * processed.
+ *
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
*/
- public final void quietlyInvoke() {
- if (status >= 0 && !tryQuietlyInvoke())
- quietlyJoin();
+ public static void helpQuiesce() {
+ ((ForkJoinWorkerThread) Thread.currentThread())
+ .helpQuiescePool();
}
/**
- * Returns true if the computation performed by this task has
- * completed (or has been cancelled).
- * @return true if this computation has completed
+ * 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 final boolean isDone() {
- return status < 0;
+ public void reinitialize() {
+ if (status == EXCEPTIONAL)
+ exceptionMap.remove(this);
+ status = 0;
}
/**
- * Returns true if this task was cancelled.
- * @return true if this task was cancelled
+ * 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 final boolean isCancelled() {
- return (status & COMPLETION_MASK) == CANCELLED;
+ public static ForkJoinPool getPool() {
+ Thread t = Thread.currentThread();
+ return (t instanceof ForkJoinWorkerThread) ?
+ ((ForkJoinWorkerThread) t).pool : null;
}
/**
- * Returns true if this task threw an exception or was cancelled
- * @return true if this task threw an exception or was cancelled
+ * 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 final boolean completedAbnormally() {
- return (status & COMPLETION_MASK) < NORMAL;
+ public static boolean inForkJoinPool() {
+ return Thread.currentThread() instanceof ForkJoinWorkerThread;
}
/**
- * Returns the exception thrown by the base computation, or a
- * CancellationException if cancelled, or null if none or if the
- * method has not yet completed.
- * @return the exception, or null if none
+ * Tries to unschedule this task for execution. This method will
+ * typically succeed if this task is the most recently forked task
+ * by the current thread, and has not commenced executing in
+ * another thread. This method may be useful when arranging
+ * alternative local processing of tasks that could have been, but
+ * were not, stolen.
+ *
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return {@code true} if unforked
*/
- public final Throwable getException() {
- int s = status & COMPLETION_MASK;
- if (s >= NORMAL)
- return null;
- if (s == CANCELLED)
- return new CancellationException();
- return exceptionMap.get(this);
+ public boolean tryUnfork() {
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .unpushTask(this);
}
/**
- * Asserts that the results of this task's computation will not be
- * used. If a cancellation occurs before this task is processed,
- * then its compute method will not be executed,
- * isCancelled will report true, and join will
- * result in a CancellationException being thrown. Otherwise, when
- * cancellation races with completion, there are no guarantees
- * about whether isCancelled will report true, whether
- * join will return normally or via an exception, or
- * whether these behaviors will remain consistent upon repeated
- * invocation.
+ * Returns an estimate of the number of tasks that have been
+ * forked by the current worker thread but not yet executed. This
+ * value may be useful for heuristic decisions about whether to
+ * fork other tasks.
*
- * This method may be overridden in subclasses, but if so, must
- * still ensure that these minimal properties hold. In particular,
- * the cancel method itself must not throw exceptions.
+ * 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}.
*
- * This method is designed to be invoked by other
- * tasks. To terminate the current task, you can just return or
- * throw an unchecked exception from its computation method, or
- * invoke completeExceptionally(someException).
+ * @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.
*
- * @param mayInterruptIfRunning this value is ignored in the
- * default implementation because tasks are not in general
- * cancelled via interruption.
+ * 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 true if this task is now cancelled
+ * @return the surplus number of tasks, which may be negative
*/
- public boolean cancel(boolean mayInterruptIfRunning) {
- setCompletion(CANCELLED);
- return (status & COMPLETION_MASK) == CANCELLED;
+ public static int getSurplusQueuedTaskCount() {
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .getEstimatedSurplusTaskCount();
}
+ // Extension methods
+
/**
- * Completes this task abnormally, and if not already aborted or
- * cancelled, causes it to throw the given exception upon
- * 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. This
- * method is overridable, but overridden versions must invoke
- * super implementation to maintain guarantees.
- * @param ex the exception to throw. If this exception is
- * not a RuntimeException or Error, the actual exception thrown
- * will be a RuntimeException with cause ex.
+ * 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 void completeExceptionally(Throwable ex) {
- setDoneExceptionally((ex instanceof RuntimeException) ||
- (ex instanceof Error)? ex :
- new RuntimeException(ex));
- }
+ public abstract V getRawResult();
/**
- * Completes this task, and if not already aborted or cancelled,
- * returning a null result upon 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.
+ * 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 result value for this task.
+ * @param value the value
*/
- public void complete(V value) {
- try {
- setRawResult(value);
- } catch(Throwable rex) {
- setDoneExceptionally(rex);
- return;
- }
- setNormalCompletion();
- }
+ protected abstract void setRawResult(V value);
/**
- * Resets the internal bookkeeping state of this task, allowing a
- * subsequent 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, and are
- * almost surely wrong. This method may be useful when executing
- * pre-constructed trees of subtasks in loops.
+ * 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
*/
- public void reinitialize() {
- if ((status & COMPLETION_MASK) == EXCEPTIONAL)
- exceptionMap.remove(this);
- status = 0;
- }
+ protected abstract boolean exec();
/**
- * Tries to unschedule this task for execution. This method will
- * typically succeed if this task is the next task that would be
- * executed by the current thread, and will typically fail (return
- * false) otherwise. This method may be useful when arranging
- * faster local processing of tasks that could have been, but were
- * not, stolen.
- * @return true if unforked
+ * Returns, but does not unschedule or execute, a task queued by
+ * the current thread but not yet executed, if one is immediately
+ * available. There is no guarantee that this task will actually
+ * be polled or executed next. Conversely, this method may return
+ * null even if a task exists but cannot be accessed without
+ * contention with other threads. This method is designed
+ * primarily to support extensions, and is unlikely to be useful
+ * otherwise.
+ *
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return the next task, or {@code null} if none are available
*/
- public boolean tryUnfork() {
- return ((ForkJoinWorkerThread)(Thread.currentThread())).unpushTask(this);
+ protected static ForkJoinTask peekNextLocalTask() {
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .peekTask();
}
/**
- * Forks both tasks, returning when isDone holds for both
- * of them or an exception is encountered. This method may be
- * invoked only from within other ForkJoinTask
- * computations. Attempts to invoke in other contexts result in
- * exceptions or errors including ClassCastException.
- * @param t1 one task
- * @param t2 the other task
- * @throws NullPointerException if t1 or t2 are null
- * @throws RuntimeException or Error if either task did so.
+ * Unschedules and returns, without executing, the next task
+ * queued by the current thread but not yet executed. This method
+ * is designed primarily to support extensions, and is unlikely to
+ * be useful otherwise.
+ *
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return the next task, or {@code null} if none are available
*/
- public static void invokeAll(ForkJoinTaskt1, ForkJoinTask t2) {
- t2.fork();
- t1.invoke();
- t2.join();
+ protected static ForkJoinTask pollNextLocalTask() {
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .pollLocalTask();
}
/**
- * Forks the given tasks, returning when isDone holds for
- * all of them. If any task encounters an exception, others may be
- * cancelled. This method may be invoked only from within other
- * ForkJoinTask computations. Attempts to invoke in other contexts
- * result in exceptions or errors including ClassCastException.
- * @param tasks the array of tasks
- * @throws NullPointerException if tasks or any element are null.
- * @throws RuntimeException or Error if any task did so.
+ * Unschedules and returns, without executing, the next task
+ * queued by the current thread but not yet executed, if one is
+ * available, or if not available, a task that was forked by some
+ * other thread, if available. Availability may be transient, so a
+ * {@code null} result does not necessarily imply quiescence
+ * of the pool this task is operating in. This method is designed
+ * primarily to support extensions, and is unlikely to be useful
+ * otherwise.
+ *
+ * This method may be invoked only from within {@code
+ * ForkJoinTask} computations (as may be determined using method
+ * {@link #inForkJoinPool}). Attempts to invoke in other contexts
+ * result in exceptions or errors, possibly including {@code
+ * ClassCastException}.
+ *
+ * @return a task, or {@code null} if none are available
*/
- 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);
+ protected static ForkJoinTask pollTask() {
+ return ((ForkJoinWorkerThread) Thread.currentThread())
+ .pollTask();
}
/**
- * Forks all tasks in the collection, returning when
- * isDone holds for all of them. If any task encounters
- * an exception, others may be cancelled. This method may be
- * invoked only from within other ForkJoinTask
- * computations. Attempts to invoke in other contexts result in
- * exceptions or errors including ClassCastException.
- * @param tasks the collection of tasks
- * @throws NullPointerException if tasks or any element are null.
- * @throws RuntimeException or Error if any task did so.
+ * Adaptor for Runnables. This implements RunnableFuture
+ * to be compliant with AbstractExecutorService constraints
+ * when used in ForkJoinPool.
*/
- public static void invokeAll(Collection> tasks) {
- if (!(tasks instanceof List)) {
- invokeAll(tasks.toArray(new ForkJoinTask[tasks.size()]));
- return;
- }
- List> ts =
- (List>)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();
- }
+ static final class AdaptedRunnable