Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * 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.lang.invoke.MethodHandles; import java.lang.invoke.VarHandle; import java.util.concurrent.locks.LockSupport; /** * A cancellable asynchronous computation. This class provides a base * implementation of {@link Future}, with methods to start and cancel * a computation, query to see if the computation is complete, and * retrieve the result of the computation. The result can only be * retrieved when the computation has completed; the {@code get} * methods will block if the computation has not yet completed. Once * the computation has completed, the computation cannot be restarted * or cancelled (unless the computation is invoked using * {@link #runAndReset}). * * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or * {@link Runnable} object. Because {@code FutureTask} implements * {@code Runnable}, a {@code FutureTask} can be submitted to an * {@link Executor} for execution. * * <p>In addition to serving as a standalone class, this class provides * {@code protected} functionality that may be useful when creating * customized task classes. * * @since 1.5 * @author Doug Lea * @param <V> The result type returned by this FutureTask's {@code get} methods */ public class FutureTask<V> implements RunnableFuture<V> { /* * Revision notes: This differs from previous versions of this * class that relied on AbstractQueuedSynchronizer, mainly to * avoid surprising users about retaining interrupt status during * cancellation races. Sync control in the current design relies * on a "state" field updated via CAS to track completion, along * with a simple Treiber stack to hold waiting threads. */ /** * The run state of this task, initially NEW. The run state * transitions to a terminal state only in methods set, * setException, and cancel. During completion, state may take on * transient values of COMPLETING (while outcome is being set) or * INTERRUPTING (only while interrupting the runner to satisfy a * cancel(true)). Transitions from these intermediate to final * states use cheaper ordered/lazy writes because values are unique * and cannot be further modified. * * Possible state transitions: * NEW -> COMPLETING -> NORMAL * NEW -> COMPLETING -> EXCEPTIONAL * NEW -> CANCELLED * NEW -> INTERRUPTING -> INTERRUPTED */ private volatile int state; private static final int NEW = 0; private static final int COMPLETING = 1; private static final int NORMAL = 2; private static final int EXCEPTIONAL = 3; private static final int CANCELLED = 4; private static final int INTERRUPTING = 5; private static final int INTERRUPTED = 6; /** The underlying callable; nulled out after running */ private Callable<V> callable; /** The result to return or exception to throw from get() */ private Object outcome; // non-volatile, protected by state reads/writes /** The thread running the callable; CASed during run() */ private volatile Thread runner; /** Treiber stack of waiting threads */ private volatile WaitNode waiters; /** * Returns result or throws exception for completed task. * * @param s completed state value */ @SuppressWarnings("unchecked") private V report(int s) throws ExecutionException { Object x = outcome; if (s == NORMAL) return (V) x; if (s >= CANCELLED) throw new CancellationException(); throw new ExecutionException((Throwable) x); } /** * Creates a {@code FutureTask} that will, upon running, execute the * given {@code Callable}. * * @param callable the callable task * @throws NullPointerException if the callable is null */ public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable } /** * Creates a {@code FutureTask} that will, upon running, execute the * given {@code Runnable}, and arrange that {@code get} will return the * given result on successful completion. * * @param runnable the runnable task * @param result the result to return on successful completion. If * you don't need a particular result, consider using * constructions of the form: * {@code Future<?> f = new FutureTask<Void>(runnable, null)} * @throws NullPointerException if the runnable is null */ public FutureTask(Runnable runnable, V result) { this.callable = Executors.callable(runnable, result); this.state = NEW; // ensure visibility of callable } public boolean isCancelled() { return state >= CANCELLED; } public boolean isDone() { return state != NEW; } public boolean cancel(boolean mayInterruptIfRunning) { if (!(state == NEW && STATE.compareAndSet(this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED))) return false; try { // in case call to interrupt throws exception if (mayInterruptIfRunning) { try { Thread t = runner; if (t != null) t.interrupt(); } finally { // final state STATE.setRelease(this, INTERRUPTED); } } } finally { finishCompletion(); } return true; } /** * @throws CancellationException {@inheritDoc} */ public V get() throws InterruptedException, ExecutionException { int s = state; if (s <= COMPLETING) s = awaitDone(false, 0L); return report(s); } /** * @throws CancellationException {@inheritDoc} */ public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { if (unit == null) throw new NullPointerException(); int s = state; if (s <= COMPLETING && (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING) throw new TimeoutException(); return report(s); } /** * Protected method invoked when this task transitions to state * {@code isDone} (whether normally or via cancellation). The * default implementation does nothing. Subclasses may override * this method to invoke completion callbacks or perform * bookkeeping. Note that you can query status inside the * implementation of this method to determine whether this task * has been cancelled. */ protected void done() { } /** * Sets the result of this future to the given value unless * this future has already been set or has been cancelled. * * <p>This method is invoked internally by the {@link #run} method * upon successful completion of the computation. * * @param v the value */ protected void set(V v) { if (STATE.compareAndSet(this, NEW, COMPLETING)) { outcome = v; STATE.setRelease(this, NORMAL); // final state finishCompletion(); } } /** * Causes this future to report an {@link ExecutionException} * with the given throwable as its cause, unless this future has * already been set or has been cancelled. * * <p>This method is invoked internally by the {@link #run} method * upon failure of the computation. * * @param t the cause of failure */ protected void setException(Throwable t) { if (STATE.compareAndSet(this, NEW, COMPLETING)) { outcome = t; STATE.setRelease(this, EXCEPTIONAL); // final state finishCompletion(); } } public void run() { if (state != NEW || !RUNNER.compareAndSet(this, null, Thread.currentThread())) return; try { Callable<V> c = callable; if (c != null && state == NEW) { V result; boolean ran; try { result = c.call(); ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex); } if (ran) set(result); } } finally { // runner must be non-null until state is settled to // prevent concurrent calls to run() runner = null; // state must be re-read after nulling runner to prevent // leaked interrupts int s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } } /** * Executes the computation without setting its result, and then * resets this future to initial state, failing to do so if the * computation encounters an exception or is cancelled. This is * designed for use with tasks that intrinsically execute more * than once. * * @return {@code true} if successfully run and reset */ protected boolean runAndReset() { if (state != NEW || !RUNNER.compareAndSet(this, null, Thread.currentThread())) return false; boolean ran = false; int s = state; try { Callable<V> c = callable; if (c != null && s == NEW) { try { c.call(); // don't set result ran = true; } catch (Throwable ex) { setException(ex); } } } finally { // runner must be non-null until state is settled to // prevent concurrent calls to run() runner = null; // state must be re-read after nulling runner to prevent // leaked interrupts s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } return ran && s == NEW; } /** * Ensures that any interrupt from a possible cancel(true) is only * delivered to a task while in run or runAndReset. */ private void handlePossibleCancellationInterrupt(int s) { // It is possible for our interrupter to stall before getting a // chance to interrupt us. Let's spin-wait patiently. if (s == INTERRUPTING) while (state == INTERRUPTING) Thread.yield(); // wait out pending interrupt // assert state == INTERRUPTED; // We want to clear any interrupt we may have received from // cancel(true). However, it is permissible to use interrupts // as an independent mechanism for a task to communicate with // its caller, and there is no way to clear only the // cancellation interrupt. // // Thread.interrupted(); } /** * Simple linked list nodes to record waiting threads in a Treiber * stack. See other classes such as Phaser and SynchronousQueue * for more detailed explanation. */ static final class WaitNode { volatile Thread thread; volatile WaitNode next; WaitNode() { thread = Thread.currentThread(); } } /** * Removes and signals all waiting threads, invokes done(), and * nulls out callable. */ private void finishCompletion() { // assert state > COMPLETING; for (WaitNode q; (q = waiters) != null;) { if (WAITERS.weakCompareAndSet(this, q, null)) { for (;;) { Thread t = q.thread; if (t != null) { q.thread = null; LockSupport.unpark(t); } WaitNode next = q.next; if (next == null) break; q.next = null; // unlink to help gc q = next; } break; } } done(); callable = null; // to reduce footprint } /** * Awaits completion or aborts on interrupt or timeout. * * @param timed true if use timed waits * @param nanos time to wait, if timed * @return state upon completion or at timeout */ private int awaitDone(boolean timed, long nanos) throws InterruptedException { // The code below is very delicate, to achieve these goals: // - call nanoTime exactly once for each call to park // - if nanos <= 0L, return promptly without allocation or nanoTime // - if nanos == Long.MIN_VALUE, don't underflow // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic // and we suffer a spurious wakeup, we will do no worse than // to park-spin for a while long startTime = 0L; // Special value 0L means not yet parked WaitNode q = null; boolean queued = false; for (;;) { int s = state; if (s > COMPLETING) { if (q != null) q.thread = null; return s; } else if (s == COMPLETING) // We may have already promised (via isDone) that we are done // so never return empty-handed or throw InterruptedException Thread.yield(); else if (Thread.interrupted()) { removeWaiter(q); throw new InterruptedException(); } else if (q == null) { if (timed && nanos <= 0L) return s; q = new WaitNode(); } else if (!queued) queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q); else if (timed) { final long parkNanos; if (startTime == 0L) { // first time startTime = System.nanoTime(); if (startTime == 0L) startTime = 1L; parkNanos = nanos; } else { long elapsed = System.nanoTime() - startTime; if (elapsed >= nanos) { removeWaiter(q); return state; } parkNanos = nanos - elapsed; } // nanoTime may be slow; recheck before parking if (state < COMPLETING) LockSupport.parkNanos(this, parkNanos); } else LockSupport.park(this); } } /** * Tries to unlink a timed-out or interrupted wait node to avoid * accumulating garbage. Internal nodes are simply unspliced * without CAS since it is harmless if they are traversed anyway * by releasers. To avoid effects of unsplicing from already * removed nodes, the list is retraversed in case of an apparent * race. This is slow when there are a lot of nodes, but we don't * expect lists to be long enough to outweigh higher-overhead * schemes. */ private void removeWaiter(WaitNode node) { if (node != null) { node.thread = null; retry: for (;;) { // restart on removeWaiter race for (WaitNode pred = null, q = waiters, s; q != null; q = s) { s = q.next; if (q.thread != null) pred = q; else if (pred != null) { pred.next = s; if (pred.thread == null) // check for race continue retry; } else if (!WAITERS.compareAndSet(this, q, s)) continue retry; } break; } } } /** * Returns a string representation of this FutureTask. * * @implSpec * The default implementation returns a string identifying this * FutureTask, as well as its completion state. The state, in * brackets, contains one of the strings {@code "Completed Normally"}, * {@code "Completed Exceptionally"}, {@code "Cancelled"}, or {@code * "Not completed"}. * * @return a string representation of this FutureTask */ public String toString() { final String status; switch (state) { case NORMAL: status = "[Completed normally]"; break; case EXCEPTIONAL: status = "[Completed exceptionally: " + outcome + "]"; break; case CANCELLED: case INTERRUPTING: case INTERRUPTED: status = "[Cancelled]"; break; default: final Callable<?> callable = this.callable; status = (callable == null) ? "[Not completed]" : "[Not completed, task = " + callable + "]"; } return super.toString() + status; } // VarHandle mechanics private static final VarHandle STATE; private static final VarHandle RUNNER; private static final VarHandle WAITERS; static { try { MethodHandles.Lookup l = MethodHandles.lookup(); STATE = l.findVarHandle(FutureTask.class, "state", int.class); RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class); WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } // Reduce the risk of rare disastrous classloading in first call to // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 Class<?> ensureLoaded = LockSupport.class; } }