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Diffstat (limited to 'luni/src/main/java/java/util/concurrent/ForkJoinTask.java')
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diff --git a/luni/src/main/java/java/util/concurrent/ForkJoinTask.java b/luni/src/main/java/java/util/concurrent/ForkJoinTask.java new file mode 100644 index 0000000..86a29d7 --- /dev/null +++ b/luni/src/main/java/java/util/concurrent/ForkJoinTask.java @@ -0,0 +1,1357 @@ +/* + * 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.List; +import java.util.RandomAccess; +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.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; +import libcore.util.SneakyThrow; + +/** + * 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 + * @hide + * @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<? extends Throwable> 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) + SneakyThrow.sneakyThrow(ex); // android-changed + 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 = t.getException(); + } + } + if (ex != null) + SneakyThrow.sneakyThrow(ex); // android-changed + } + + /** + * 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 = t.getException(); + } + } + if (ex != null) + SneakyThrow.sneakyThrow(ex); // android-changed + return tasks; + } + + /** + * Attempts to cancel execution of this task. This attempt will + * fail if the task has already completed or could not be + * cancelled for some other reason. If successful, and this task + * has not started when {@code cancel} is called, execution of + * this task is suppressed. After this method returns + * successfully, unless there is an intervening call to {@link + * #reinitialize}, subsequent calls to {@link #isCancelled}, + * {@link #isDone}, and {@code cancel} will return {@code true} + * and calls to {@link #join} and related methods will result in + * {@code CancellationException}. + * + * <p>This method may be overridden in subclasses, but if so, must + * still ensure that these properties hold. In particular, the + * {@code cancel} method itself must not throw exceptions. + * + * <p>This method is designed to be invoked by <em>other</em> + * tasks. To terminate the current task, you can just return or + * throw an unchecked exception from its computation method, or + * invoke {@link #completeExceptionally}. + * + * @param mayInterruptIfRunning this value has no effect in the + * default implementation because interrupts are not used to + * control cancellation. + * + * @return {@code true} if this task is now cancelled + */ + public boolean cancel(boolean mayInterruptIfRunning) { + return setCompletion(CANCELLED) == CANCELLED; + } + + /** + * Cancels, ignoring any exceptions thrown by cancel. Used during + * worker and pool shutdown. Cancel is spec'ed not to throw any + * exceptions, but if it does anyway, we have no recourse during + * shutdown, so guard against this case. + */ + final void cancelIgnoringExceptions() { + try { + cancel(false); + } catch (Throwable ignore) { + } + } + + public final boolean isDone() { + return status < 0; + } + + public final boolean isCancelled() { + return status == CANCELLED; + } + + /** + * Returns {@code true} if this task threw an exception or was cancelled. + * + * @return {@code true} if this task threw an exception or was cancelled + */ + public final boolean isCompletedAbnormally() { + return status < NORMAL; + } + + /** + * Returns {@code true} if this task completed without throwing an + * exception and was not cancelled. + * + * @return {@code true} if this task completed without throwing an + * exception and was not cancelled + */ + public final boolean isCompletedNormally() { + return status == NORMAL; + } + + /** + * Returns the exception thrown by the base computation, or a + * {@code CancellationException} if cancelled, or {@code null} if + * none or if the method has not yet completed. + * + * @return the exception, or {@code null} if none + */ + public final Throwable getException() { + int s = status; + return ((s >= NORMAL) ? null : + (s == CANCELLED) ? new CancellationException() : + getThrowableException()); + } + + /** + * Completes this task abnormally, and if not already aborted or + * cancelled, causes it to throw the given exception upon + * {@code join} and related operations. This method may be used + * to induce exceptions in asynchronous tasks, or to force + * completion of tasks that would not otherwise complete. Its use + * in other situations is discouraged. This method is + * overridable, but overridden versions must invoke {@code super} + * implementation to maintain guarantees. + * + * @param ex the exception to throw. If this exception is not a + * {@code RuntimeException} or {@code Error}, the actual exception + * thrown will be a {@code RuntimeException} with cause {@code ex}. + */ + public void completeExceptionally(Throwable ex) { + setExceptionalCompletion((ex instanceof RuntimeException) || + (ex instanceof Error) ? ex : + new RuntimeException(ex)); + } + + /** + * Completes this task, and if not already aborted or cancelled, + * returning the given value as the result of subsequent + * invocations of {@code join} and related operations. This method + * may be used to provide results for asynchronous tasks, or to + * provide alternative handling for tasks that would not otherwise + * complete normally. Its use in other situations is + * discouraged. This method is overridable, but overridden + * versions must invoke {@code super} implementation to maintain + * guarantees. + * + * @param value the result value for this task + */ + public void complete(V value) { + try { + setRawResult(value); + } catch (Throwable rex) { + setExceptionalCompletion(rex); + return; + } + setCompletion(NORMAL); + } + + /** + * Waits if necessary for the computation to complete, and then + * retrieves its result. + * + * @return the computed result + * @throws CancellationException if the computation was cancelled + * @throws ExecutionException if the computation threw an + * exception + * @throws InterruptedException if the current thread is not a + * member of a ForkJoinPool and was interrupted while waiting + */ + public final V get() throws InterruptedException, ExecutionException { + int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ? + doJoin() : externalInterruptibleAwaitDone(0L); + Throwable ex; + if (s == CANCELLED) + throw new CancellationException(); + if (s == EXCEPTIONAL && (ex = getThrowableException()) != null) + throw new ExecutionException(ex); + return getRawResult(); + } + + /** + * Waits if necessary for at most the given time for the computation + * to complete, and then retrieves its result, if available. + * + * @param timeout the maximum time to wait + * @param unit the time unit of the timeout argument + * @return the computed result + * @throws CancellationException if the computation was cancelled + * @throws ExecutionException if the computation threw an + * exception + * @throws InterruptedException if the current thread is not a + * member of a ForkJoinPool and was interrupted while waiting + * @throws TimeoutException if the wait timed out + */ + public final V get(long timeout, TimeUnit unit) + throws InterruptedException, ExecutionException, TimeoutException { + Thread t = Thread.currentThread(); + if (t instanceof ForkJoinWorkerThread) { + ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; + long nanos = unit.toNanos(timeout); + if (status >= 0) { + boolean completed = false; + if (w.unpushTask(this)) { + try { + completed = exec(); + } catch (Throwable rex) { + setExceptionalCompletion(rex); + } + } + if (completed) + setCompletion(NORMAL); + else if (status >= 0 && nanos > 0) + w.pool.timedAwaitJoin(this, nanos); + } + } + else { + long millis = unit.toMillis(timeout); + if (millis > 0) + externalInterruptibleAwaitDone(millis); + } + int s = status; + if (s != NORMAL) { + Throwable ex; + if (s == CANCELLED) + throw new CancellationException(); + if (s != EXCEPTIONAL) + throw new TimeoutException(); + if ((ex = getThrowableException()) != null) + throw new ExecutionException(ex); + } + return getRawResult(); + } + + /** + * Joins this task, without returning its result or throwing its + * exception. This method may be useful when processing + * collections of tasks when some have been cancelled or otherwise + * known to have aborted. + */ + public final void quietlyJoin() { + doJoin(); + } + + /** + * Commences performing this task and awaits its completion if + * necessary, without returning its result or throwing its + * exception. + */ + public final void quietlyInvoke() { + doInvoke(); + } + + /** + * Possibly executes tasks until the pool hosting the current task + * {@link ForkJoinPool#isQuiescent is quiescent}. This method may + * be of use in designs in which many tasks are forked, but none + * are explicitly joined, instead executing them until all are + * processed. + * + * <p>This method may be invoked only from within {@code + * ForkJoinPool} computations (as may be determined using method + * {@link #inForkJoinPool}). Attempts to invoke in other contexts + * result in exceptions or errors, possibly including {@code + * ClassCastException}. + */ + public static void helpQuiesce() { + ((ForkJoinWorkerThread) Thread.currentThread()) + .helpQuiescePool(); + } + + /** + * Resets the internal bookkeeping state of this task, allowing a + * subsequent {@code fork}. This method allows repeated reuse of + * this task, but only if reuse occurs when this task has either + * never been forked, or has been forked, then completed and all + * outstanding joins of this task have also completed. Effects + * under any other usage conditions are not guaranteed. + * This method may be useful when executing + * pre-constructed trees of subtasks in loops. + * + * <p>Upon completion of this method, {@code isDone()} reports + * {@code false}, and {@code getException()} reports {@code + * null}. However, the value returned by {@code getRawResult} is + * unaffected. To clear this value, you can invoke {@code + * setRawResult(null)}. + */ + public void reinitialize() { + if (status == EXCEPTIONAL) + clearExceptionalCompletion(); + else + status = 0; + } + + /** + * Returns the pool hosting the current task execution, or null + * if this task is executing outside of any ForkJoinPool. + * + * @see #inForkJoinPool + * @return the pool, or {@code null} if none + */ + public static ForkJoinPool getPool() { + Thread t = Thread.currentThread(); + return (t instanceof ForkJoinWorkerThread) ? + ((ForkJoinWorkerThread) t).pool : null; + } + + /** + * Returns {@code true} if the current thread is a {@link + * ForkJoinWorkerThread} executing as a ForkJoinPool computation. + * + * @return {@code true} if the current thread is a {@link + * ForkJoinWorkerThread} executing as a ForkJoinPool computation, + * or {@code false} otherwise + */ + public static boolean inForkJoinPool() { + return Thread.currentThread() instanceof ForkJoinWorkerThread; + } + + /** + * Tries to unschedule this task for execution. This method will + * typically succeed if this task is the most recently forked task + * by the current thread, and has not commenced executing in + * another thread. This method may be useful when arranging + * alternative local processing of tasks that could have been, but + * were not, stolen. + * + * <p>This method may be invoked only from within {@code + * ForkJoinPool} computations (as may be determined using method + * {@link #inForkJoinPool}). Attempts to invoke in other contexts + * result in exceptions or errors, possibly including {@code + * ClassCastException}. + * + * @return {@code true} if unforked + */ + public boolean tryUnfork() { + return ((ForkJoinWorkerThread) Thread.currentThread()) + .unpushTask(this); + } + + /** + * Returns an estimate of the number of tasks that have been + * forked by the current worker thread but not yet executed. This + * value may be useful for heuristic decisions about whether to + * fork other tasks. + * + * <p>This method may be invoked only from within {@code + * ForkJoinPool} computations (as may be determined using method + * {@link #inForkJoinPool}). Attempts to invoke in other contexts + * result in exceptions or errors, possibly including {@code + * ClassCastException}. + * + * @return the number of tasks + */ + public static int getQueuedTaskCount() { + return ((ForkJoinWorkerThread) Thread.currentThread()) + .getQueueSize(); + } + + /** + * Returns an estimate of how many more locally queued tasks are + * held by the current worker thread than there are other worker + * threads that might steal them. This value may be useful for + * heuristic decisions about whether to fork other tasks. In many + * usages of ForkJoinTasks, at steady state, each worker should + * aim to maintain a small constant surplus (for example, 3) of + * tasks, and to process computations locally if this threshold is + * exceeded. + * + * <p>This method may be invoked only from within {@code + * ForkJoinPool} computations (as may be determined using method + * {@link #inForkJoinPool}). Attempts to invoke in other contexts + * result in exceptions or errors, possibly including {@code + * ClassCastException}. + * + * @return the surplus number of tasks, which may be negative + */ + public static int getSurplusQueuedTaskCount() { + return ((ForkJoinWorkerThread) Thread.currentThread()) + .getEstimatedSurplusTaskCount(); + } + + // Extension methods + + /** + * Returns the result that would be returned by {@link #join}, even + * if this task completed abnormally, or {@code null} if this task + * is not known to have been completed. This method is designed + * to aid debugging, as well as to support extensions. Its use in + * any other context is discouraged. + * + * @return the result, or {@code null} if not completed + */ + public abstract V getRawResult(); + + /** + * Forces the given value to be returned as a result. This method + * is designed to support extensions, and should not in general be + * called otherwise. + * + * @param value the value + */ + protected abstract void setRawResult(V value); + + /** + * Immediately performs the base action of this task. This method + * is designed to support extensions, and should not in general be + * called otherwise. The return value controls whether this task + * is considered to be done normally. It may return false in + * asynchronous actions that require explicit invocations of + * {@link #complete} to become joinable. It may also throw an + * (unchecked) exception to indicate abnormal exit. + * + * @return {@code true} if completed normally + */ + protected abstract boolean exec(); + + /** + * Returns, but does not unschedule or execute, a task queued by + * the current thread but not yet executed, if one is immediately + * available. There is no guarantee that this task will actually + * be polled or executed next. Conversely, this method may return + * null even if a task exists but cannot be accessed without + * contention with other threads. This method is designed + * primarily to support extensions, and is unlikely to be useful + * otherwise. + * + * <p>This method may be invoked only from within {@code + * ForkJoinPool} computations (as may be determined using method + * {@link #inForkJoinPool}). Attempts to invoke in other contexts + * result in exceptions or errors, possibly including {@code + * ClassCastException}. + * + * @return the next task, or {@code null} if none are available + */ + protected static ForkJoinTask<?> peekNextLocalTask() { + return ((ForkJoinWorkerThread) Thread.currentThread()) + .peekTask(); + } + + /** + * Unschedules and returns, without executing, the next task + * queued by the current thread but not yet executed. This method + * is designed primarily to support extensions, and is unlikely to + * be useful otherwise. + * + * <p>This method may be invoked only from within {@code + * ForkJoinPool} computations (as may be determined using method + * {@link #inForkJoinPool}). Attempts to invoke in other contexts + * result in exceptions or errors, possibly including {@code + * ClassCastException}. + * + * @return the next task, or {@code null} if none are available + */ + protected static ForkJoinTask<?> pollNextLocalTask() { + return ((ForkJoinWorkerThread) Thread.currentThread()) + .pollLocalTask(); + } + + /** + * Unschedules and returns, without executing, the next task + * queued by the current thread but not yet executed, if one is + * available, or if not available, a task that was forked by some + * other thread, if available. Availability may be transient, so a + * {@code null} result does not necessarily imply quiescence + * of the pool this task is operating in. This method is designed + * primarily to support extensions, and is unlikely to be useful + * otherwise. + * + * <p>This method may be invoked only from within {@code + * ForkJoinPool} computations (as may be determined using method + * {@link #inForkJoinPool}). Attempts to invoke in other contexts + * result in exceptions or errors, possibly including {@code + * ClassCastException}. + * + * @return a task, or {@code null} if none are available + */ + protected static ForkJoinTask<?> pollTask() { + return ((ForkJoinWorkerThread) Thread.currentThread()) + .pollTask(); + } + + /** + * Adaptor for Runnables. This implements RunnableFuture + * to be compliant with AbstractExecutorService constraints + * when used in ForkJoinPool. + */ + static final class AdaptedRunnable<T> extends ForkJoinTask<T> + implements RunnableFuture<T> { + final Runnable runnable; + final T resultOnCompletion; + T result; + AdaptedRunnable(Runnable runnable, T result) { + if (runnable == null) throw new NullPointerException(); + this.runnable = runnable; + this.resultOnCompletion = result; + } + public T getRawResult() { return result; } + public void setRawResult(T v) { result = v; } + public boolean exec() { + runnable.run(); + result = resultOnCompletion; + return true; + } + public void run() { invoke(); } + private static final long serialVersionUID = 5232453952276885070L; + } + + /** + * Adaptor for Callables + */ + static final class AdaptedCallable<T> extends ForkJoinTask<T> + implements RunnableFuture<T> { + final Callable<? extends T> callable; + T result; + AdaptedCallable(Callable<? extends T> callable) { + if (callable == null) throw new NullPointerException(); + this.callable = callable; + } + public T getRawResult() { return result; } + public void setRawResult(T v) { result = v; } + public boolean exec() { + try { + result = callable.call(); + return true; + } catch (Error err) { + throw err; + } catch (RuntimeException rex) { + throw rex; + } catch (Exception ex) { + throw new RuntimeException(ex); + } + } + public void run() { invoke(); } + private static final long serialVersionUID = 2838392045355241008L; + } + + /** + * Returns a new {@code ForkJoinTask} that performs the {@code run} + * method of the given {@code Runnable} as its action, and returns + * a null result upon {@link #join}. + * + * @param runnable the runnable action + * @return the task + */ + public static ForkJoinTask<?> adapt(Runnable runnable) { + return new AdaptedRunnable<Void>(runnable, null); + } + + /** + * Returns a new {@code ForkJoinTask} that performs the {@code run} + * method of the given {@code Runnable} as its action, and returns + * the given result upon {@link #join}. + * + * @param runnable the runnable action + * @param result the result upon completion + * @return the task + */ + public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) { + return new AdaptedRunnable<T>(runnable, result); + } + + /** + * Returns a new {@code ForkJoinTask} that performs the {@code call} + * method of the given {@code Callable} as its action, and returns + * its result upon {@link #join}, translating any checked exceptions + * encountered into {@code RuntimeException}. + * + * @param callable the callable action + * @return the task + */ + public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) { + return new AdaptedCallable<T>(callable); + } + + // Serialization support + + private static final long serialVersionUID = -7721805057305804111L; + + /** + * Saves the state to a stream (that is, serializes it). + * + * @serialData the current run status and the exception thrown + * during execution, or {@code null} if none + * @param s the stream + */ + private void writeObject(java.io.ObjectOutputStream s) + throws java.io.IOException { + s.defaultWriteObject(); + s.writeObject(getException()); + } + + /** + * Reconstitutes the instance from a stream (that is, deserializes it). + * + * @param s the stream + */ + private void readObject(java.io.ObjectInputStream s) + throws java.io.IOException, ClassNotFoundException { + s.defaultReadObject(); + Object ex = s.readObject(); + if (ex != null) + setExceptionalCompletion((Throwable)ex); + } + + // Unsafe mechanics + private static final sun.misc.Unsafe UNSAFE; + private static final long statusOffset; + static { + exceptionTableLock = new ReentrantLock(); + exceptionTableRefQueue = new ReferenceQueue<Object>(); + exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY]; + try { + UNSAFE = sun.misc.Unsafe.getUnsafe(); + statusOffset = UNSAFE.objectFieldOffset + (ForkJoinTask.class.getDeclaredField("status")); + } catch (Exception e) { + throw new Error(e); + } + } + +} |