src/share/classes/java/util/concurrent/ForkJoinTask.java
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@@ -40,10 +40,20 @@
import java.util.Collections;
import java.util.List;
import java.util.RandomAccess;
import java.util.Map;
import java.util.WeakHashMap;
+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;
/**
* 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
@@ -127,10 +137,20 @@
* 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
@@ -141,13 +161,14 @@
*
* <p>ForkJoinTasks should perform relatively small amounts of
* computation. Large tasks should be split into smaller subtasks,
* usually via recursive decomposition. As a very rough rule of thumb,
* a task should perform more than 100 and less than 10000 basic
- * computational steps. If tasks are too big, then parallelism cannot
- * improve throughput. If too small, then memory and internal task
- * maintenance overhead may overwhelm processing.
+ * 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>.
@@ -240,73 +261,91 @@
exceptionMap.put(this, rex);
setCompletion(EXCEPTIONAL);
}
/**
- * Blocks a worker thread until completion. Called only by
- * pool. Currently unused -- pool-based waits use timeout
- * version below.
- */
- final void internalAwaitDone() {
- int s; // the odd construction reduces lock bias effects
- while ((s = status) >= 0) {
- try {
- synchronized (this) {
- if (UNSAFE.compareAndSwapInt(this, statusOffset, s,SIGNAL))
- wait();
- }
- } catch (InterruptedException ie) {
- cancelIfTerminating();
- }
- }
- }
-
- /**
* Blocks a worker thread until completed or timed out. Called
* only by pool.
- *
- * @return status on exit
*/
- final int internalAwaitDone(long millis) {
- int s;
- if ((s = status) >= 0) {
- try {
+ final void internalAwaitDone(long millis, int nanos) {
+ int s = status;
+ if ((s == 0 &&
+ UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL)) ||
+ s > 0) {
+ try { // the odd construction reduces lock bias effects
synchronized (this) {
- if (UNSAFE.compareAndSwapInt(this, statusOffset, s,SIGNAL))
- wait(millis, 0);
+ if (status > 0)
+ wait(millis, nanos);
+ else
+ notifyAll();
}
} catch (InterruptedException ie) {
cancelIfTerminating();
}
- s = status;
}
- return s;
}
/**
* Blocks a non-worker-thread until completion.
*/
private void externalAwaitDone() {
- int s;
- while ((s = status) >= 0) {
- synchronized (this) {
- if (UNSAFE.compareAndSwapInt(this, statusOffset, s, SIGNAL)){
+ if (status >= 0) {
boolean interrupted = false;
- while (status >= 0) {
+ synchronized (this) {
+ for (;;) {
+ int s = status;
+ if (s == 0)
+ UNSAFE.compareAndSwapInt(this, statusOffset,
+ 0, SIGNAL);
+ else if (s < 0) {
+ notifyAll();
+ break;
+ }
+ else {
try {
wait();
} catch (InterruptedException ie) {
interrupted = true;
}
}
+ }
+ }
if (interrupted)
Thread.currentThread().interrupt();
+ }
+ }
+
+ /**
+ * Blocks a non-worker-thread until completion or interruption or timeout.
+ */
+ private void externalInterruptibleAwaitDone(boolean timed, long nanos)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ if (status >= 0) {
+ long startTime = timed ? System.nanoTime() : 0L;
+ synchronized (this) {
+ for (;;) {
+ long nt;
+ int s = status;
+ if (s == 0)
+ UNSAFE.compareAndSwapInt(this, statusOffset,
+ 0, SIGNAL);
+ else if (s < 0) {
+ notifyAll();
break;
}
+ else if (!timed)
+ wait();
+ else if ((nt = nanos - (System.nanoTime()-startTime)) > 0L)
+ wait(nt / 1000000, (int)(nt % 1000000));
+ else
+ break;
}
}
}
+ }
/**
* Unless done, calls exec and records status if completed, but
* doesn't wait for completion otherwise. Primary execution method
* for ForkJoinWorkerThread.
@@ -333,11 +372,11 @@
* any thread other than the one executing it unless preceded by a
* call to {@link #join} or related methods, or a call to {@link
* #isDone} returning {@code true}.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -347,14 +386,17 @@
.pushTask(this);
return this;
}
/**
- * Returns the result of the computation when it {@link #isDone is done}.
- * This method differs from {@link #get()} in that
+ * 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}.
+ * {@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() {
quietlyJoin();
@@ -392,11 +434,11 @@
* #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
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -420,11 +462,11 @@
* 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
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -475,11 +517,11 @@
* #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
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -527,29 +569,32 @@
return tasks;
}
/**
* Attempts to cancel execution of this task. This attempt will
- * fail if the task has already completed, has already been
- * cancelled, or could not be cancelled for some other reason. If
- * successful, and this task has not started when cancel is
- * called, execution of this task is suppressed, {@link
- * #isCancelled} will report true, and {@link #join} will result
- * in a {@code CancellationException} being thrown.
+ * 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 minimal properties hold. In particular,
- * the {@code cancel} method itself must not throw exceptions.
+ * 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 is ignored in the
- * default implementation because tasks are not
- * cancelled via interruption
+ * @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) {
setCompletion(CANCELLED);
@@ -679,27 +724,17 @@
* exception
* @throws InterruptedException if the current thread is not a
* member of a ForkJoinPool and was interrupted while waiting
*/
public final V get() throws InterruptedException, ExecutionException {
- int s;
- if (Thread.currentThread() instanceof ForkJoinWorkerThread) {
+ Thread t = Thread.currentThread();
+ if (t instanceof ForkJoinWorkerThread)
quietlyJoin();
- s = status;
- }
- else {
- while ((s = status) >= 0) {
- synchronized (this) { // interruptible form of awaitDone
- if (UNSAFE.compareAndSwapInt(this, statusOffset,
- s, SIGNAL)) {
- while (status >= 0)
- wait();
- }
- }
- }
- }
- if (s < NORMAL) {
+ else
+ externalInterruptibleAwaitDone(false, 0L);
+ 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);
@@ -721,76 +756,22 @@
* 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 {
+ long nanos = unit.toNanos(timeout);
Thread t = Thread.currentThread();
- ForkJoinPool pool;
- if (t instanceof ForkJoinWorkerThread) {
- ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
- if (status >= 0 && w.unpushTask(this))
- quietlyExec();
- pool = w.pool;
- }
+ if (t instanceof ForkJoinWorkerThread)
+ ((ForkJoinWorkerThread)t).joinTask(this, true, nanos);
else
- pool = null;
- /*
- * Timed wait loop intermixes cases for FJ (pool != null) and
- * non FJ threads. For FJ, decrement pool count but don't try
- * for replacement; increment count on completion. For non-FJ,
- * deal with interrupts. This is messy, but a little less so
- * than is splitting the FJ and nonFJ cases.
- */
- boolean interrupted = false;
- boolean dec = false; // true if pool count decremented
- long nanos = unit.toNanos(timeout);
- for (;;) {
- if (pool == null && Thread.interrupted()) {
- interrupted = true;
- break;
- }
+ externalInterruptibleAwaitDone(true, nanos);
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) {
+ if (s != NORMAL) {
Throwable ex;
- if (es == CANCELLED)
+ if (s == CANCELLED)
throw new CancellationException();
- if (es == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
+ if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null)
throw new ExecutionException(ex);
throw new TimeoutException();
}
return getRawResult();
}
@@ -817,11 +798,11 @@
if (completed) {
setCompletion(NORMAL);
return;
}
}
- w.joinTask(this);
+ w.joinTask(this, false, 0L);
}
}
else
externalAwaitDone();
}
@@ -853,11 +834,11 @@
* be of use in designs in which many tasks are forked, but none
* are explicitly joined, instead executing them until all are
* processed.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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() {
@@ -872,10 +853,16 @@
* 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)
exceptionMap.remove(this);
status = 0;
@@ -893,15 +880,16 @@
return (t instanceof ForkJoinWorkerThread) ?
((ForkJoinWorkerThread) t).pool : null;
}
/**
- * Returns {@code true} if the current thread is executing as a
- * ForkJoinPool computation.
+ * Returns {@code true} if the current thread is a {@link
+ * ForkJoinWorkerThread} executing as a ForkJoinPool computation.
*
- * @return {@code true} if the current thread is executing as a
- * ForkJoinPool computation, or false otherwise
+ * @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;
}
@@ -912,11 +900,11 @@
* another thread. This method may be useful when arranging
* alternative local processing of tasks that could have been, but
* were not, stolen.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -931,11 +919,11 @@
* forked by the current worker thread but not yet executed. This
* value may be useful for heuristic decisions about whether to
* fork other tasks.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -954,11 +942,11 @@
* aim to maintain a small constant surplus (for example, 3) of
* tasks, and to process computations locally if this threshold is
* exceeded.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -1012,11 +1000,11 @@
* contention with other threads. This method is designed
* primarily to support extensions, and is unlikely to be useful
* otherwise.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -1031,11 +1019,11 @@
* queued by the current thread but not yet executed. This method
* is designed primarily to support extensions, and is unlikely to
* be useful otherwise.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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
@@ -1054,11 +1042,11 @@
* of the pool this task is operating in. This method is designed
* primarily to support extensions, and is unlikely to be useful
* otherwise.
*
* <p>This method may be invoked only from within {@code
- * ForkJoinTask} computations (as may be determined using method
+ * 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