util/concurrent/FutureTask.java

/*
 * 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);
    }

    @Override
    public V resultNow() {
        switch (state()) {    // Future.State
            case SUCCESS:
                @SuppressWarnings("unchecked")
                V result = (V) outcome;
                return result;
            case FAILED:
                throw new IllegalStateException("Task completed with exception");
            case CANCELLED:
                throw new IllegalStateException("Task was cancelled");
            default:
                throw new IllegalStateException("Task has not completed");
        }
    }

    @Override
    public Throwable exceptionNow() {
        switch (state()) {    // Future.State
            case SUCCESS:
                throw new IllegalStateException("Task completed with a result");
            case FAILED:
                Object x = outcome;
                return (Throwable) x;
            case CANCELLED:
                throw new IllegalStateException("Task was cancelled");
            default:
                throw new IllegalStateException("Task has not completed");
        }
    }

    @Override
    public State state() {
        int s = state;
        while (s == COMPLETING) {
            // waiting for transition to NORMAL or EXCEPTIONAL
            Thread.yield();
            s = state;
        }
        switch (s) {
            case NORMAL:
                return State.SUCCESS;
            case EXCEPTIONAL:
                return State.FAILED;
            case CANCELLED:
            case INTERRUPTING:
            case INTERRUPTED:
                return State.CANCELLED;
            default:
                return State.RUNNING;
        }
    }

    /**
     * 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;
    }

}
Revision:	1.121
Committed:	Fri May 6 16:03:05 2022 UTC (2 years ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.120:	+53 -0 lines
Log Message:	sync with openjdk
#	User	Rev	Content
1	tim	1.1	/*
2	dl	1.2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.23	* Expert Group and released to the public domain, as explained at
4	jsr166	1.59	* http://creativecommons.org/publicdomain/zero/1.0/
5	tim	1.1	*/
6
7			package java.util.concurrent;
8	jsr166	1.108
9	dl	1.115	import java.lang.invoke.MethodHandles;
10			import java.lang.invoke.VarHandle;
11	dl	1.62	import java.util.concurrent.locks.LockSupport;
12	dl	1.13
13	tim	1.1	/**
14	dl	1.8	* A cancellable asynchronous computation. This class provides a base
15			* implementation of {@link Future}, with methods to start and cancel
16			* a computation, query to see if the computation is complete, and
17	dl	1.4	* retrieve the result of the computation. The result can only be
18	jsr166	1.64	* retrieved when the computation has completed; the {@code get}
19			* methods will block if the computation has not yet completed. Once
20	dl	1.8	* the computation has completed, the computation cannot be restarted
21	jsr166	1.64	* or cancelled (unless the computation is invoked using
22			* {@link #runAndReset}).
23	tim	1.1	*
24	jsr166	1.64	* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
25			* {@link Runnable} object. Because {@code FutureTask} implements
26			* {@code Runnable}, a {@code FutureTask} can be submitted to an
27			* {@link Executor} for execution.
28	tim	1.1	*
29	dl	1.14	* <p>In addition to serving as a standalone class, this class provides
30	jsr166	1.64	* {@code protected} functionality that may be useful when creating
31	dl	1.14	* customized task classes.
32			*
33	tim	1.1	* @since 1.5
34	dl	1.4	* @author Doug Lea
35	jsr166	1.64	* @param <V> The result type returned by this FutureTask's {@code get} methods
36	tim	1.1	*/
37	peierls	1.39	public class FutureTask<V> implements RunnableFuture<V> {
38	dl	1.62	/*
39			* Revision notes: This differs from previous versions of this
40			* class that relied on AbstractQueuedSynchronizer, mainly to
41			* avoid surprising users about retaining interrupt status during
42			* cancellation races. Sync control in the current design relies
43			* on a "state" field updated via CAS to track completion, along
44			* with a simple Treiber stack to hold waiting threads.
45			*/
46
47			/**
48	jsr166	1.73	* The run state of this task, initially NEW. The run state
49	dl	1.78	* transitions to a terminal state only in methods set,
50			* setException, and cancel. During completion, state may take on
51			* transient values of COMPLETING (while outcome is being set) or
52			* INTERRUPTING (only while interrupting the runner to satisfy a
53			* cancel(true)). Transitions from these intermediate to final
54			* states use cheaper ordered/lazy writes because values are unique
55			* and cannot be further modified.
56	jsr166	1.69	*
57			* Possible state transitions:
58	jsr166	1.73	* NEW -> COMPLETING -> NORMAL
59			* NEW -> COMPLETING -> EXCEPTIONAL
60			* NEW -> CANCELLED
61			* NEW -> INTERRUPTING -> INTERRUPTED
62	dl	1.62	*/
63			private volatile int state;
64	jsr166	1.73	private static final int NEW = 0;
65	jsr166	1.70	private static final int COMPLETING = 1;
66			private static final int NORMAL = 2;
67			private static final int EXCEPTIONAL = 3;
68			private static final int CANCELLED = 4;
69			private static final int INTERRUPTING = 5;
70			private static final int INTERRUPTED = 6;
71	dl	1.62
72	dl	1.78	/** The underlying callable; nulled out after running */
73	dl	1.77	private Callable<V> callable;
74	dl	1.62	/** The result to return or exception to throw from get() */
75			private Object outcome; // non-volatile, protected by state reads/writes
76			/** The thread running the callable; CASed during run() */
77			private volatile Thread runner;
78			/** Treiber stack of waiting threads */
79			private volatile WaitNode waiters;
80
81			/**
82	jsr166	1.64	* Returns result or throws exception for completed task.
83			*
84	dl	1.62	* @param s completed state value
85			*/
86	jsr166	1.98	@SuppressWarnings("unchecked")
87	dl	1.62	private V report(int s) throws ExecutionException {
88			Object x = outcome;
89	jsr166	1.98	if (s == NORMAL)
90			return (V)x;
91	jsr166	1.69	if (s >= CANCELLED)
92	dl	1.62	throw new CancellationException();
93			throw new ExecutionException((Throwable)x);
94			}
95	dl	1.11
96	tim	1.1	/**
97	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
98			* given {@code Callable}.
99	tim	1.1	*
100			* @param callable the callable task
101	jsr166	1.79	* @throws NullPointerException if the callable is null
102	tim	1.1	*/
103			public FutureTask(Callable<V> callable) {
104	dl	1.9	if (callable == null)
105			throw new NullPointerException();
106	dl	1.62	this.callable = callable;
107	jsr166	1.85	this.state = NEW; // ensure visibility of callable
108	tim	1.1	}
109
110			/**
111	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
112			* given {@code Runnable}, and arrange that {@code get} will return the
113	tim	1.1	* given result on successful completion.
114			*
115	jsr166	1.54	* @param runnable the runnable task
116	tim	1.1	* @param result the result to return on successful completion. If
117	dl	1.9	* you don't need a particular result, consider using
118	dl	1.16	* constructions of the form:
119	jsr166	1.58	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
120	jsr166	1.79	* @throws NullPointerException if the runnable is null
121	tim	1.1	*/
122	dl	1.15	public FutureTask(Runnable runnable, V result) {
123	dl	1.62	this.callable = Executors.callable(runnable, result);
124	jsr166	1.85	this.state = NEW; // ensure visibility of callable
125	dl	1.20	}
126
127			public boolean isCancelled() {
128	jsr166	1.69	return state >= CANCELLED;
129	dl	1.20	}
130	jsr166	1.35
131	dl	1.20	public boolean isDone() {
132	jsr166	1.73	return state != NEW;
133	dl	1.13	}
134
135			public boolean cancel(boolean mayInterruptIfRunning) {
136	dl	1.115	if (!(state == NEW && STATE.compareAndSet
137			(this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
138	dl	1.78	return false;
139	jsr166	1.102	try { // in case call to interrupt throws exception
140			if (mayInterruptIfRunning) {
141			try {
142			Thread t = runner;
143			if (t != null)
144			t.interrupt();
145			} finally { // final state
146	dl	1.115	STATE.setRelease(this, INTERRUPTED);
147	jsr166	1.102	}
148	dl	1.100	}
149	jsr166	1.102	} finally {
150			finishCompletion();
151	dl	1.78	}
152			return true;
153	dl	1.13	}
154	jsr166	1.35
155	jsr166	1.43	/**
156			* @throws CancellationException {@inheritDoc}
157			*/
158	dl	1.2	public V get() throws InterruptedException, ExecutionException {
159	jsr166	1.64	int s = state;
160	jsr166	1.91	if (s <= COMPLETING)
161			s = awaitDone(false, 0L);
162			return report(s);
163	tim	1.1	}
164
165	jsr166	1.43	/**
166			* @throws CancellationException {@inheritDoc}
167			*/
168	dl	1.2	public V get(long timeout, TimeUnit unit)
169	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
170	jsr166	1.82	if (unit == null)
171			throw new NullPointerException();
172	jsr166	1.64	int s = state;
173			if (s <= COMPLETING &&
174	jsr166	1.82	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
175	dl	1.62	throw new TimeoutException();
176			return report(s);
177	tim	1.1	}
178
179	dl	1.121	@Override
180			public V resultNow() {
181			switch (state()) { // Future.State
182			case SUCCESS:
183			@SuppressWarnings("unchecked")
184			V result = (V) outcome;
185			return result;
186			case FAILED:
187			throw new IllegalStateException("Task completed with exception");
188			case CANCELLED:
189			throw new IllegalStateException("Task was cancelled");
190			default:
191			throw new IllegalStateException("Task has not completed");
192			}
193			}
194
195			@Override
196			public Throwable exceptionNow() {
197			switch (state()) { // Future.State
198			case SUCCESS:
199			throw new IllegalStateException("Task completed with a result");
200			case FAILED:
201			Object x = outcome;
202			return (Throwable) x;
203			case CANCELLED:
204			throw new IllegalStateException("Task was cancelled");
205			default:
206			throw new IllegalStateException("Task has not completed");
207			}
208			}
209
210			@Override
211			public State state() {
212			int s = state;
213			while (s == COMPLETING) {
214			// waiting for transition to NORMAL or EXCEPTIONAL
215			Thread.yield();
216			s = state;
217			}
218			switch (s) {
219			case NORMAL:
220			return State.SUCCESS;
221			case EXCEPTIONAL:
222			return State.FAILED;
223			case CANCELLED:
224			case INTERRUPTING:
225			case INTERRUPTED:
226			return State.CANCELLED;
227			default:
228			return State.RUNNING;
229			}
230			}
231
232	tim	1.1	/**
233	dl	1.20	* Protected method invoked when this task transitions to state
234	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
235	dl	1.20	* default implementation does nothing. Subclasses may override
236			* this method to invoke completion callbacks or perform
237			* bookkeeping. Note that you can query status inside the
238			* implementation of this method to determine whether this task
239			* has been cancelled.
240			*/
241			protected void done() { }
242
243			/**
244	jsr166	1.64	* Sets the result of this future to the given value unless
245	dl	1.29	* this future has already been set or has been cancelled.
246	jsr166	1.64	*
247			* <p>This method is invoked internally by the {@link #run} method
248	dl	1.40	* upon successful completion of the computation.
249	jsr166	1.64	*
250	tim	1.1	* @param v the value
251	jsr166	1.35	*/
252	dl	1.2	protected void set(V v) {
253	dl	1.115	if (STATE.compareAndSet(this, NEW, COMPLETING)) {
254	dl	1.78	outcome = v;
255	dl	1.115	STATE.setRelease(this, NORMAL); // final state
256	dl	1.78	finishCompletion();
257			}
258	tim	1.1	}
259
260			/**
261	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
262			* with the given throwable as its cause, unless this future has
263	dl	1.24	* already been set or has been cancelled.
264	jsr166	1.64	*
265			* <p>This method is invoked internally by the {@link #run} method
266	dl	1.40	* upon failure of the computation.
267	jsr166	1.64	*
268	jsr166	1.41	* @param t the cause of failure
269	jsr166	1.35	*/
270	dl	1.2	protected void setException(Throwable t) {
271	dl	1.115	if (STATE.compareAndSet(this, NEW, COMPLETING)) {
272	dl	1.78	outcome = t;
273	dl	1.115	STATE.setRelease(this, EXCEPTIONAL); // final state
274	dl	1.78	finishCompletion();
275			}
276	tim	1.1	}
277	jsr166	1.35
278	dl	1.24	public void run() {
279	jsr166	1.87	if (state != NEW \|\|
280	dl	1.115	!RUNNER.compareAndSet(this, null, Thread.currentThread()))
281	jsr166	1.87	return;
282			try {
283	dl	1.77	Callable<V> c = callable;
284			if (c != null && state == NEW) {
285	jsr166	1.87	V result;
286	jsr166	1.92	boolean ran;
287	dl	1.77	try {
288			result = c.call();
289	jsr166	1.92	ran = true;
290	dl	1.77	} catch (Throwable ex) {
291	jsr166	1.92	result = null;
292			ran = false;
293	dl	1.77	setException(ex);
294			}
295	jsr166	1.92	if (ran)
296			set(result);
297	dl	1.62	}
298	jsr166	1.87	} finally {
299	jsr166	1.93	// runner must be non-null until state is settled to
300			// prevent concurrent calls to run()
301	jsr166	1.68	runner = null;
302	jsr166	1.93	// state must be re-read after nulling runner to prevent
303			// leaked interrupts
304	jsr166	1.86	int s = state;
305			if (s >= INTERRUPTING)
306			handlePossibleCancellationInterrupt(s);
307	dl	1.62	}
308	dl	1.24	}
309
310			/**
311	dl	1.30	* Executes the computation without setting its result, and then
312	jsr166	1.64	* resets this future to initial state, failing to do so if the
313	dl	1.24	* computation encounters an exception or is cancelled. This is
314			* designed for use with tasks that intrinsically execute more
315			* than once.
316	jsr166	1.64	*
317	jsr166	1.103	* @return {@code true} if successfully run and reset
318	dl	1.24	*/
319			protected boolean runAndReset() {
320	jsr166	1.87	if (state != NEW \|\|
321	dl	1.115	!RUNNER.compareAndSet(this, null, Thread.currentThread()))
322	jsr166	1.87	return false;
323	jsr166	1.92	boolean ran = false;
324			int s = state;
325	jsr166	1.87	try {
326	dl	1.77	Callable<V> c = callable;
327	jsr166	1.92	if (c != null && s == NEW) {
328	dl	1.77	try {
329			c.call(); // don't set result
330	jsr166	1.92	ran = true;
331	dl	1.77	} catch (Throwable ex) {
332			setException(ex);
333			}
334	jsr166	1.68	}
335	jsr166	1.87	} finally {
336	jsr166	1.93	// runner must be non-null until state is settled to
337			// prevent concurrent calls to run()
338	jsr166	1.68	runner = null;
339	jsr166	1.93	// state must be re-read after nulling runner to prevent
340			// leaked interrupts
341	jsr166	1.92	s = state;
342	jsr166	1.87	if (s >= INTERRUPTING)
343			handlePossibleCancellationInterrupt(s);
344	dl	1.62	}
345	jsr166	1.92	return ran && s == NEW;
346	dl	1.14	}
347	dl	1.3
348	dl	1.14	/**
349	jsr166	1.95	* Ensures that any interrupt from a possible cancel(true) is only
350			* delivered to a task while in run or runAndReset.
351	jsr166	1.86	*/
352			private void handlePossibleCancellationInterrupt(int s) {
353			// It is possible for our interrupter to stall before getting a
354			// chance to interrupt us. Let's spin-wait patiently.
355	jsr166	1.96	if (s == INTERRUPTING)
356			while (state == INTERRUPTING)
357	jsr166	1.86	Thread.yield(); // wait out pending interrupt
358	jsr166	1.96
359	jsr166	1.89	// assert state == INTERRUPTED;
360	jsr166	1.94
361	jsr166	1.95	// We want to clear any interrupt we may have received from
362			// cancel(true). However, it is permissible to use interrupts
363			// as an independent mechanism for a task to communicate with
364			// its caller, and there is no way to clear only the
365			// cancellation interrupt.
366			//
367			// Thread.interrupted();
368	jsr166	1.86	}
369
370			/**
371	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
372	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
373	dl	1.62	* for more detailed explanation.
374	dl	1.20	*/
375	dl	1.62	static final class WaitNode {
376			volatile Thread thread;
377	dl	1.76	volatile WaitNode next;
378			WaitNode() { thread = Thread.currentThread(); }
379	dl	1.62	}
380	dl	1.42
381	dl	1.62	/**
382	jsr166	1.85	* Removes and signals all waiting threads, invokes done(), and
383			* nulls out callable.
384	dl	1.62	*/
385	dl	1.78	private void finishCompletion() {
386	jsr166	1.90	// assert state > COMPLETING;
387	jsr166	1.81	for (WaitNode q; (q = waiters) != null;) {
388	jsr166	1.118	if (WAITERS.weakCompareAndSet(this, q, null)) {
389	dl	1.62	for (;;) {
390			Thread t = q.thread;
391			if (t != null) {
392			q.thread = null;
393			LockSupport.unpark(t);
394			}
395			WaitNode next = q.next;
396			if (next == null)
397	dl	1.78	break;
398	dl	1.62	q.next = null; // unlink to help gc
399			q = next;
400			}
401	dl	1.78	break;
402	dl	1.62	}
403	dl	1.24	}
404	jsr166	1.85
405	dl	1.78	done();
406	jsr166	1.85
407			callable = null; // to reduce footprint
408	dl	1.62	}
409	dl	1.24
410	dl	1.62	/**
411	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
412			*
413	dl	1.62	* @param timed true if use timed waits
414	jsr166	1.64	* @param nanos time to wait, if timed
415	jsr166	1.105	* @return state upon completion or at timeout
416	dl	1.62	*/
417			private int awaitDone(boolean timed, long nanos)
418			throws InterruptedException {
419	jsr166	1.106	// The code below is very delicate, to achieve these goals:
420			// - call nanoTime exactly once for each call to park
421	jsr166	1.113	// - if nanos <= 0L, return promptly without allocation or nanoTime
422	jsr166	1.106	// - if nanos == Long.MIN_VALUE, don't underflow
423			// - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
424			// and we suffer a spurious wakeup, we will do no worse than
425			// to park-spin for a while
426			long startTime = 0L; // Special value 0L means not yet parked
427	dl	1.62	WaitNode q = null;
428			boolean queued = false;
429	jsr166	1.64	for (;;) {
430			int s = state;
431			if (s > COMPLETING) {
432	dl	1.62	if (q != null)
433			q.thread = null;
434			return s;
435			}
436	jsr166	1.111	else if (s == COMPLETING)
437			// We may have already promised (via isDone) that we are done
438			// so never return empty-handed or throw InterruptedException
439	dl	1.97	Thread.yield();
440	jsr166	1.111	else if (Thread.interrupted()) {
441			removeWaiter(q);
442			throw new InterruptedException();
443			}
444	jsr166	1.106	else if (q == null) {
445			if (timed && nanos <= 0L)
446			return s;
447	dl	1.62	q = new WaitNode();
448	jsr166	1.106	}
449	dl	1.62	else if (!queued)
450	jsr166	1.118	queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
451	dl	1.62	else if (timed) {
452	jsr166	1.106	final long parkNanos;
453			if (startTime == 0L) { // first time
454			startTime = System.nanoTime();
455			if (startTime == 0L)
456			startTime = 1L;
457			parkNanos = nanos;
458			} else {
459			long elapsed = System.nanoTime() - startTime;
460			if (elapsed >= nanos) {
461			removeWaiter(q);
462			return state;
463			}
464			parkNanos = nanos - elapsed;
465	dl	1.50	}
466	jsr166	1.107	// nanoTime may be slow; recheck before parking
467			if (state < COMPLETING)
468			LockSupport.parkNanos(this, parkNanos);
469	dl	1.50	}
470	dl	1.62	else
471			LockSupport.park(this);
472	dl	1.24	}
473	dl	1.62	}
474	dl	1.24
475	dl	1.62	/**
476	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
477			* accumulating garbage. Internal nodes are simply unspliced
478	dl	1.62	* without CAS since it is harmless if they are traversed anyway
479	jsr166	1.81	* by releasers. To avoid effects of unsplicing from already
480			* removed nodes, the list is retraversed in case of an apparent
481			* race. This is slow when there are a lot of nodes, but we don't
482			* expect lists to be long enough to outweigh higher-overhead
483			* schemes.
484	dl	1.62	*/
485			private void removeWaiter(WaitNode node) {
486			if (node != null) {
487			node.thread = null;
488	jsr166	1.81	retry:
489			for (;;) { // restart on removeWaiter race
490			for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
491			s = q.next;
492			if (q.thread != null)
493			pred = q;
494			else if (pred != null) {
495			pred.next = s;
496			if (pred.thread == null) // check for race
497			continue retry;
498			}
499	dl	1.115	else if (!WAITERS.compareAndSet(this, q, s))
500	jsr166	1.81	continue retry;
501	jsr166	1.55	}
502	jsr166	1.81	break;
503	jsr166	1.56	}
504	dl	1.14	}
505	dl	1.62	}
506	dl	1.14
507	jsr166	1.119	/**
508			* Returns a string representation of this FutureTask.
509			*
510			* @implSpec
511			* The default implementation returns a string identifying this
512			* FutureTask, as well as its completion state. The state, in
513			* brackets, contains one of the strings {@code "Completed Normally"},
514			* {@code "Completed Exceptionally"}, {@code "Cancelled"}, or {@code
515			* "Not completed"}.
516			*
517			* @return a string representation of this FutureTask
518			*/
519			public String toString() {
520			final String status;
521			switch (state) {
522			case NORMAL:
523			status = "[Completed normally]";
524			break;
525			case EXCEPTIONAL:
526			status = "[Completed exceptionally: " + outcome + "]";
527			break;
528			case CANCELLED:
529			case INTERRUPTING:
530			case INTERRUPTED:
531			status = "[Cancelled]";
532			break;
533			default:
534			final Callable<?> callable = this.callable;
535			status = (callable == null)
536			? "[Not completed]"
537			: "[Not completed, task = " + callable + "]";
538			}
539			return super.toString() + status;
540			}
541
542	dl	1.115	// VarHandle mechanics
543			private static final VarHandle STATE;
544			private static final VarHandle RUNNER;
545			private static final VarHandle WAITERS;
546	dl	1.62	static {
547			try {
548	dl	1.115	MethodHandles.Lookup l = MethodHandles.lookup();
549			STATE = l.findVarHandle(FutureTask.class, "state", int.class);
550			RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class);
551			WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class);
552	jsr166	1.109	} catch (ReflectiveOperationException e) {
553	jsr166	1.120	throw new ExceptionInInitializerError(e);
554	dl	1.14	}
555	jsr166	1.116
556	jsr166	1.112	// Reduce the risk of rare disastrous classloading in first call to
557			// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
558			Class<?> ensureLoaded = LockSupport.class;
559	dl	1.15	}
560	dl	1.62
561	dl	1.15	}