/* * 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; /** * 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. * *

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. * *

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 main 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 * ideally 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. Subdividable tasks should also * not perform blocking IO, and should ideally access variables that * are completely independent of those accessed by other running * tasks. These guidelines are loosely 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. * *

It is possible to define and use ForkJoinTasks that may block, * but doing do requires three further considerations: (1) Completion * of few if any other tasks should be dependent on a task * that blocks on external synchronization or IO. Event-style async * tasks that are never joined often fall into this category. (2) To * minimize resource impact, tasks should be small; ideally performing * only the (possibly) blocking action. (3) Unless the {@link * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly * blocked tasks is known to be less than the pool's {@link * ForkJoinPool#getParallelism} level, the pool cannot guarantee that * enough threads will be available to ensure progress or good * performance. * *

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. * *

In the most typical usages, a fork-join pair act like a call * (fork) and return (join) from a parallel recursive function. As is * the case with other forms of recursive calls, returns (joins) * should be performed innermost-first. For example, {@code a.fork(); * b.fork(); b.join(); a.join();} is likely to be substantially more * efficient than joining {@code a} before {@code b}. * *

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}. * *

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. To support such usages a * ForkJoinTask may be atomically tagged with a {@code * short} value using {@link #setForkJoinTaskTag} or {@link * #compareAndSetForkJoinTaskTag} and checked using {@link * #getForkJoinTaskTag}. The ForkJoinTask implementation does not * use these {@code protected} methods or tags for any purpose, but * they may be of use in the construction of specialized subclasses. * For example, parallel graph traversals can use the supplied methods * to avoid revisiting nodes/tasks that have already been processed. * Also, completion based designs can use them to record that subtasks * have completed. (Method names for tagging are bulky in part to * encourage definition of methods that reflect their usage patterns.) * *

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 * 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. * *

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 {@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 implements Future, 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 (anded with * DONE_MASK) 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, mainly by arranging that every synchronized * block performs a wait, notifyAll or both. * * These control bits occupy only (some of) the upper half (16 * bits) of status field. The lower bits are used for user-defined * tags. */ /** The run status of this task */ volatile int status; // accessed directly by pool and workers static final int DONE_MASK = 0xf0000000; // mask out non-completion bits static final int NORMAL = 0xf0000000; // must be negative static final int CANCELLED = 0xc0000000; // must be < NORMAL static final int EXCEPTIONAL = 0x80000000; // must be < CANCELLED static final int SIGNAL = 0x00010000; // must be >= 1 << 16 static final int SMASK = 0x0000ffff; // short bits for tags /** * Marks completion and wakes up threads waiting to join this * task. * * @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 (U.compareAndSwapInt(this, STATUS, s, s | completion)) { if ((s >>> 16) != 0) synchronized (this) { notifyAll(); } return completion; } } } /** * Primary execution method for stolen tasks. Unless done, calls * exec and records status if completed, but doesn't wait for * completion otherwise. * * @return status on exit from this method */ final int doExec() { int s; boolean completed; if ((s = status) >= 0) { try { completed = exec(); } catch (Throwable rex) { return setExceptionalCompletion(rex); } if (completed) s = setCompletion(NORMAL); } return s; } /** * Tries to set SIGNAL status. Used by ForkJoinPool. Other * variants are directly incorporated into externalAwaitDone etc. * * @return true if successful */ final boolean trySetSignal() { int s; return U.compareAndSwapInt(this, STATUS, s = status, s | SIGNAL); } /** * Blocks a non-worker-thread until completion. * @return status upon completion */ private int externalAwaitDone() { boolean interrupted = false; int s; while ((s = status) >= 0) { if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) { synchronized (this) { if (status >= 0) { try { wait(); } catch (InterruptedException ie) { interrupted = true; } } else notifyAll(); } } } if (interrupted) Thread.currentThread().interrupt(); return s; } /** * Blocks a non-worker-thread until completion or interruption. */ private int externalInterruptibleAwaitDone() throws InterruptedException { int s; if (Thread.interrupted()) throw new InterruptedException(); while ((s = status) >= 0) { if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) { synchronized (this) { if (status >= 0) wait(); else notifyAll(); } } } return s; } /** * Implementation for join, get, quietlyJoin. Directly handles * only cases of already-completed, external wait, and * unfork+exec. Others are relayed to ForkJoinPool.awaitJoin. * * @return status upon completion */ private int doJoin() { int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w; if ((s = status) >= 0) { if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) { if (!(w = (wt = (ForkJoinWorkerThread)t).workQueue). tryUnpush(this) || (s = doExec()) >= 0) s = wt.pool.awaitJoin(w, this); } else s = externalAwaitDone(); } return s; } /** * Implementation for invoke, quietlyInvoke. * * @return status upon completion */ private int doInvoke() { int s; Thread t; ForkJoinWorkerThread wt; if ((s = doExec()) >= 0) { if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) s = (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this); else s = externalAwaitDone(); } return s; } // 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 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> { 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); } /** * 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. */ static final void cancelIgnoringExceptions(ForkJoinTask t) { if (t != null && t.status >= 0) { try { t.cancel(false); } catch (Throwable ignore) { } } } /** * 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 & DONE_MASK) != EXCEPTIONAL) return null; int h = System.identityHashCode(this); ExceptionNode e; final ReentrantLock lock = exceptionTableLock; lock.lock(); try { expungeStaleExceptions(); ExceptionNode[] t = exceptionTable; e = t[h & (t.length - 1)]; while (e != null && e.get() != this) e = e.next; } finally { lock.unlock(); } Throwable ex; if (e == null || (ex = e.ex) == null) return null; if (e.thrower != Thread.currentThread().getId()) { Class ec = ex.getClass(); try { Constructor noArgCtor = null; Constructor[] cs = ec.getConstructors();// public ctors only for (int i = 0; i < cs.length; ++i) { Constructor c = cs[i]; Class[] ps = c.getParameterTypes(); if (ps.length == 0) noArgCtor = c; else if (ps.length == 1 && ps[0] == Throwable.class) return (Throwable)(c.newInstance(ex)); } if (noArgCtor != null) { Throwable wx = (Throwable)(noArgCtor.newInstance()); wx.initCause(ex); return wx; } } catch (Exception ignore) { } } return ex; } /** * Poll stale refs and remove them. Call only while holding lock. */ private static void expungeStaleExceptions() { for (Object x; (x = exceptionTableRefQueue.poll()) != null;) { if (x instanceof ExceptionNode) { ForkJoinTask key = ((ExceptionNode)x).get(); ExceptionNode[] t = exceptionTable; int i = System.identityHashCode(key) & (t.length - 1); ExceptionNode e = t[i]; ExceptionNode pred = null; while (e != null) { ExceptionNode next = e.next; if (e == x) { if (pred == null) t[i] = next; else pred.next = next; break; } pred = e; e = next; } } } } /** * If lock is available, poll stale refs and remove them. * Called from ForkJoinPool when pools become quiescent. */ static final void helpExpungeStaleExceptions() { final ReentrantLock lock = exceptionTableLock; if (lock.tryLock()) { try { expungeStaleExceptions(); } finally { lock.unlock(); } } } /** * Throws exception, if any, associated with the given status. */ private void reportException(int s) { Throwable ex = ((s == CANCELLED) ? new CancellationException() : (s == EXCEPTIONAL) ? getThrowableException() : null); if (ex != null) U.throwException(ex); } // 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}. * *

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 fork() { ((ForkJoinWorkerThread)Thread.currentThread()).workQueue.push(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 not cause the * method to abruptly return by throwing {@code * InterruptedException}. * * @return the computed result */ public final V join() { int s; if ((s = doJoin() & DONE_MASK) != NORMAL) reportException(s); 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() { int s; if ((s = doInvoke() & DONE_MASK) != NORMAL) reportException(s); 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. * *

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) { int s1, s2; t2.fork(); if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL) t1.reportException(s1); if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL) t2.reportException(s2); } /** * 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 * 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) U.throwException(ex); } /** * Forks all tasks in the specified collection, returning when * {@code isDone} holds for each task or an (unchecked) exception * is encountered, in which case the exception is rethrown. If * more than one task encounters an exception, then this method * throws any one of these exceptions. If any task encounters an * exception, others may be cancelled. However, the execution * status of individual tasks is not guaranteed upon exceptional * return. The status of each task may be obtained using {@link * #getException()} and related methods to check if they have been * cancelled, completed normally or exceptionally, or left * unprocessed. * *

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 > Collection invokeAll(Collection tasks) { if (!(tasks instanceof RandomAccess) || !(tasks instanceof List)) { invokeAll(tasks.toArray(new ForkJoinTask[tasks.size()])); return tasks; } @SuppressWarnings("unchecked") 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 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) U.throwException(ex); return tasks; } /** * Attempts to cancel execution of this task. This attempt will * fail if the task has already completed or could not be * cancelled for some other reason. If successful, and this task * has not started when {@code cancel} is called, execution of * this task is suppressed. After this method returns * successfully, unless there is an intervening call to {@link * #reinitialize}, subsequent calls to {@link #isCancelled}, * {@link #isDone}, and {@code cancel} will return {@code true} * and calls to {@link #join} and related methods will result in * {@code CancellationException}. * *

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. * *

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 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) & DONE_MASK) == CANCELLED; } public final boolean isDone() { return status < 0; } public final boolean isCancelled() { return (status & DONE_MASK) == 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 & DONE_MASK) == 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 & DONE_MASK; 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); } /** * Completes this task. The most recent value established by * {@link #setRawResult} (or {@code null}) will be returned as the * result of subsequent invocations of {@code join} and related * operations. This method may be useful when processing sets of * tasks when some do not otherwise complete normally. Its use in * other situations is discouraged. */ public final void quietlyComplete() { 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(); Throwable ex; if ((s &= DONE_MASK) == 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 { if (Thread.interrupted()) throw new InterruptedException(); // Messy in part because we measure in nanosecs, but wait in millisecs int s; long ns, ms; if ((s = status) >= 0 && (ns = unit.toNanos(timeout)) > 0L) { long deadline = System.nanoTime() + ns; ForkJoinPool p = null; ForkJoinPool.WorkQueue w = null; Thread t = Thread.currentThread(); if (t instanceof ForkJoinWorkerThread) { ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t; p = wt.pool; w = wt.workQueue; s = p.helpJoinOnce(w, this); // no retries on failure } boolean canBlock = false; boolean interrupted = false; try { while ((s = status) >= 0) { if (w != null && w.runState < 0) cancelIgnoringExceptions(this); else if (!canBlock) { if (p == null || p.tryCompensate(this, null)) canBlock = true; } else { if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) { synchronized (this) { if (status >= 0) { try { wait(ms); } catch (InterruptedException ie) { if (p == null) interrupted = true; } } else notifyAll(); } } if ((s = status) < 0 || interrupted || (ns = deadline - System.nanoTime()) <= 0L) break; } } } finally { if (p != null && canBlock) p.incrementActiveCount(); } if (interrupted) throw new InterruptedException(); } if ((s &= DONE_MASK) != 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. * *

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 wt = (ForkJoinWorkerThread)Thread.currentThread(); wt.pool.helpQuiescePool(wt.workQueue); } /** * 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. * *

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 & DONE_MASK) == 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. * *

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()) .workQueue.tryUnpush(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. * *

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()) .workQueue.queueSize(); } /** * 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. * *

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() { /* * The aim of this method is to return a cheap heuristic guide * for task partitioning when programmers, frameworks, tools, * or languages have little or no idea about task granularity. * In essence by offering this method, we ask users only about * tradeoffs in overhead vs expected throughput and its * variance, rather than how finely to partition tasks. * * In a steady state strict (tree-structured) computation, * each thread makes available for stealing enough tasks for * other threads to remain active. Inductively, if all threads * play by the same rules, each thread should make available * only a constant number of tasks. * * The minimum useful constant is just 1. But using a value of * 1 would require immediate replenishment upon each steal to * maintain enough tasks, which is infeasible. Further, * partitionings/granularities of offered tasks should * minimize steal rates, which in general means that threads * nearer the top of computation tree should generate more * than those nearer the bottom. In perfect steady state, each * thread is at approximately the same level of computation * tree. However, producing extra tasks amortizes the * uncertainty of progress and diffusion assumptions. * * So, users will want to use values larger, but not much * larger than 1 to both smooth over transient shortages and * hedge against uneven progress; as traded off against the * cost of extra task overhead. We leave the user to pick a * threshold value to compare with the results of this call to * guide decisions, but recommend values such as 3. * * When all threads are active, it is on average OK to * estimate surplus strictly locally. In steady-state, if one * thread is maintaining say 2 surplus tasks, then so are * others. So we can just use estimated queue length. * However, this strategy alone leads to serious mis-estimates * in some non-steady-state conditions (ramp-up, ramp-down, * other stalls). We can detect many of these by further * considering the number of "idle" threads, that are known to * have zero queued tasks, so compensate by a factor of * (#idle/#active) threads. */ ForkJoinWorkerThread wt = (ForkJoinWorkerThread)Thread.currentThread(); return wt.workQueue.queueSize() - wt.pool.idlePerActive(); } // 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. * *

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()).workQueue.peek(); } /** * 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 * 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()) .workQueue.nextLocalTask(); } /** * 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 * 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() { ForkJoinWorkerThread wt = (ForkJoinWorkerThread)Thread.currentThread(); return wt.pool.nextTaskFor(wt.workQueue); } // tag operations /** * Returns the tag for this task. * * @return the tag for this task * @since 1.8 */ public final short getForkJoinTaskTag() { return (short)status; } /** * Atomically sets the tag value for this task. * * @param tag the tag value * @return the previous value of the tag * @since 1.8 */ public final short setForkJoinTaskTag(short tag) { for (int s;;) { if (U.compareAndSwapInt(this, STATUS, s = status, (s & ~SMASK) | (tag & SMASK))) return (short)s; } } /** * Atomically conditionally sets the tag value for this task. * Among other applications, tags can be used as visit markers * in tasks operating on graphs, as in mathods that check: {@code * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))} * before processing, otherwise exiting because the node has * already been visited. * * @param e the expected tag value * @param tag the new tag value * @return true if successful; i.e., the current value was * equal to e and is now tag. * @since 1.8 */ public final boolean compareAndSetForkJoinTaskTag(short e, short tag) { for (int s;;) { if ((short)(s = status) != e) return false; if (U.compareAndSwapInt(this, STATUS, s, (s & ~SMASK) | (tag & SMASK))) return true; } } /** * Adaptor for Runnables. This implements RunnableFuture * to be compliant with AbstractExecutorService constraints * when used in ForkJoinPool. */ static final class AdaptedRunnable extends ForkJoinTask implements RunnableFuture { final Runnable runnable; T result; AdaptedRunnable(Runnable runnable, T result) { if (runnable == null) throw new NullPointerException(); this.runnable = runnable; this.result = result; // OK to set this even before completion } public final T getRawResult() { return result; } public final void setRawResult(T v) { result = v; } public final boolean exec() { runnable.run(); return true; } public final void run() { invoke(); } private static final long serialVersionUID = 5232453952276885070L; } /** * Adaptor for Runnables without results */ static final class AdaptedRunnableAction extends ForkJoinTask implements RunnableFuture { final Runnable runnable; AdaptedRunnableAction(Runnable runnable) { if (runnable == null) throw new NullPointerException(); this.runnable = runnable; } public final Void getRawResult() { return null; } public final void setRawResult(Void v) { } public final boolean exec() { runnable.run(); return true; } public final void run() { invoke(); } private static final long serialVersionUID = 5232453952276885070L; } /** * Adaptor for Callables */ static final class AdaptedCallable extends ForkJoinTask implements RunnableFuture { final Callable callable; T result; AdaptedCallable(Callable callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; } public final T getRawResult() { return result; } public final void setRawResult(T v) { result = v; } public final 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 final 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 AdaptedRunnableAction(runnable); } /** * 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 ForkJoinTask adapt(Runnable runnable, T result) { return new AdaptedRunnable(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 ForkJoinTask adapt(Callable callable) { return new AdaptedCallable(callable); } // Serialization support private static final long serialVersionUID = -7721805057305804111L; /** * Saves this task to a stream (that is, serializes it). * * @serialData the current run status and the exception thrown * during execution, or {@code null} if none */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { s.defaultWriteObject(); s.writeObject(getException()); } /** * Reconstitutes this task from a stream (that is, deserializes it). */ 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 U; private static final long STATUS; static { exceptionTableLock = new ReentrantLock(); exceptionTableRefQueue = new ReferenceQueue(); exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY]; try { U = sun.misc.Unsafe.getUnsafe(); STATUS = U.objectFieldOffset (ForkJoinTask.class.getDeclaredField("status")); } catch (Exception e) { throw new Error(e); } } }