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.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.
     *
     * Style note: As usual, we bypass overhead of using
     * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
     */

    /**
     * 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 &&
              U.compareAndSwapInt(this, STATE, 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
                    U.putOrderedInt(this, STATE, 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 (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = v;
            U.putOrderedInt(this, STATE, 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 (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = t;
            U.putOrderedInt(this, STATE, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

    public void run() {
        if (state != NEW ||
            !U.compareAndSwapObject(this, RUNNER, 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 ||
            !U.compareAndSwapObject(this, RUNNER, 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 (U.compareAndSwapObject(this, WAITERS, 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 {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = U.compareAndSwapObject(this, WAITERS,
                                                q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            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 (!U.compareAndSwapObject(this, WAITERS, q, s))
                        continue retry;
                }
                break;
            }
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final long STATE;
    private static final long RUNNER;
    private static final long WAITERS;
    static {
        try {
            U = sun.misc.Unsafe.getUnsafe();
            Class<?> k = FutureTask.class;
            STATE   = U.objectFieldOffset(k.getDeclaredField("state"));
            RUNNER  = U.objectFieldOffset(k.getDeclaredField("runner"));
            WAITERS = U.objectFieldOffset(k.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

}
Revision:	1.105
Committed:	Thu Aug 21 01:56:10 2014 UTC (9 years, 9 months ago) by jsr166
Branch:	MAIN
Changes since 1.104:	+1 -1 lines
Log Message:	javadoc clarification
#	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	dl	1.62	import java.util.concurrent.locks.LockSupport;
9	dl	1.13
10	tim	1.1	/**
11	dl	1.8	* A cancellable asynchronous computation. This class provides a base
12			* implementation of {@link Future}, with methods to start and cancel
13			* a computation, query to see if the computation is complete, and
14	dl	1.4	* retrieve the result of the computation. The result can only be
15	jsr166	1.64	* retrieved when the computation has completed; the {@code get}
16			* methods will block if the computation has not yet completed. Once
17	dl	1.8	* the computation has completed, the computation cannot be restarted
18	jsr166	1.64	* or cancelled (unless the computation is invoked using
19			* {@link #runAndReset}).
20	tim	1.1	*
21	jsr166	1.64	* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
22			* {@link Runnable} object. Because {@code FutureTask} implements
23			* {@code Runnable}, a {@code FutureTask} can be submitted to an
24			* {@link Executor} for execution.
25	tim	1.1	*
26	dl	1.14	* <p>In addition to serving as a standalone class, this class provides
27	jsr166	1.64	* {@code protected} functionality that may be useful when creating
28	dl	1.14	* customized task classes.
29			*
30	tim	1.1	* @since 1.5
31	dl	1.4	* @author Doug Lea
32	jsr166	1.64	* @param <V> The result type returned by this FutureTask's {@code get} methods
33	tim	1.1	*/
34	peierls	1.39	public class FutureTask<V> implements RunnableFuture<V> {
35	dl	1.62	/*
36			* Revision notes: This differs from previous versions of this
37			* class that relied on AbstractQueuedSynchronizer, mainly to
38			* avoid surprising users about retaining interrupt status during
39			* cancellation races. Sync control in the current design relies
40			* on a "state" field updated via CAS to track completion, along
41			* with a simple Treiber stack to hold waiting threads.
42			*
43			* Style note: As usual, we bypass overhead of using
44			* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
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	jsr166	1.102	if (!(state == NEW &&
137	jsr166	1.104	U.compareAndSwapInt(this, STATE, NEW,
138	jsr166	1.102	mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
139	dl	1.78	return false;
140	jsr166	1.102	try { // in case call to interrupt throws exception
141			if (mayInterruptIfRunning) {
142			try {
143			Thread t = runner;
144			if (t != null)
145			t.interrupt();
146			} finally { // final state
147	jsr166	1.104	U.putOrderedInt(this, STATE, INTERRUPTED);
148	jsr166	1.102	}
149	dl	1.100	}
150	jsr166	1.102	} finally {
151			finishCompletion();
152	dl	1.78	}
153			return true;
154	dl	1.13	}
155	jsr166	1.35
156	jsr166	1.43	/**
157			* @throws CancellationException {@inheritDoc}
158			*/
159	dl	1.2	public V get() throws InterruptedException, ExecutionException {
160	jsr166	1.64	int s = state;
161	jsr166	1.91	if (s <= COMPLETING)
162			s = awaitDone(false, 0L);
163			return report(s);
164	tim	1.1	}
165
166	jsr166	1.43	/**
167			* @throws CancellationException {@inheritDoc}
168			*/
169	dl	1.2	public V get(long timeout, TimeUnit unit)
170	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
171	jsr166	1.82	if (unit == null)
172			throw new NullPointerException();
173	jsr166	1.64	int s = state;
174			if (s <= COMPLETING &&
175	jsr166	1.82	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
176	dl	1.62	throw new TimeoutException();
177			return report(s);
178	tim	1.1	}
179
180			/**
181	dl	1.20	* Protected method invoked when this task transitions to state
182	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
183	dl	1.20	* default implementation does nothing. Subclasses may override
184			* this method to invoke completion callbacks or perform
185			* bookkeeping. Note that you can query status inside the
186			* implementation of this method to determine whether this task
187			* has been cancelled.
188			*/
189			protected void done() { }
190
191			/**
192	jsr166	1.64	* Sets the result of this future to the given value unless
193	dl	1.29	* this future has already been set or has been cancelled.
194	jsr166	1.64	*
195			* <p>This method is invoked internally by the {@link #run} method
196	dl	1.40	* upon successful completion of the computation.
197	jsr166	1.64	*
198	tim	1.1	* @param v the value
199	jsr166	1.35	*/
200	dl	1.2	protected void set(V v) {
201	jsr166	1.104	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
202	dl	1.78	outcome = v;
203	jsr166	1.104	U.putOrderedInt(this, STATE, NORMAL); // final state
204	dl	1.78	finishCompletion();
205			}
206	tim	1.1	}
207
208			/**
209	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
210			* with the given throwable as its cause, unless this future has
211	dl	1.24	* already been set or has been cancelled.
212	jsr166	1.64	*
213			* <p>This method is invoked internally by the {@link #run} method
214	dl	1.40	* upon failure of the computation.
215	jsr166	1.64	*
216	jsr166	1.41	* @param t the cause of failure
217	jsr166	1.35	*/
218	dl	1.2	protected void setException(Throwable t) {
219	jsr166	1.104	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
220	dl	1.78	outcome = t;
221	jsr166	1.104	U.putOrderedInt(this, STATE, EXCEPTIONAL); // final state
222	dl	1.78	finishCompletion();
223			}
224	tim	1.1	}
225	jsr166	1.35
226	dl	1.24	public void run() {
227	jsr166	1.87	if (state != NEW \|\|
228	jsr166	1.104	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
229	jsr166	1.87	return;
230			try {
231	dl	1.77	Callable<V> c = callable;
232			if (c != null && state == NEW) {
233	jsr166	1.87	V result;
234	jsr166	1.92	boolean ran;
235	dl	1.77	try {
236			result = c.call();
237	jsr166	1.92	ran = true;
238	dl	1.77	} catch (Throwable ex) {
239	jsr166	1.92	result = null;
240			ran = false;
241	dl	1.77	setException(ex);
242			}
243	jsr166	1.92	if (ran)
244			set(result);
245	dl	1.62	}
246	jsr166	1.87	} finally {
247	jsr166	1.93	// runner must be non-null until state is settled to
248			// prevent concurrent calls to run()
249	jsr166	1.68	runner = null;
250	jsr166	1.93	// state must be re-read after nulling runner to prevent
251			// leaked interrupts
252	jsr166	1.86	int s = state;
253			if (s >= INTERRUPTING)
254			handlePossibleCancellationInterrupt(s);
255	dl	1.62	}
256	dl	1.24	}
257
258			/**
259	dl	1.30	* Executes the computation without setting its result, and then
260	jsr166	1.64	* resets this future to initial state, failing to do so if the
261	dl	1.24	* computation encounters an exception or is cancelled. This is
262			* designed for use with tasks that intrinsically execute more
263			* than once.
264	jsr166	1.64	*
265	jsr166	1.103	* @return {@code true} if successfully run and reset
266	dl	1.24	*/
267			protected boolean runAndReset() {
268	jsr166	1.87	if (state != NEW \|\|
269	jsr166	1.104	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
270	jsr166	1.87	return false;
271	jsr166	1.92	boolean ran = false;
272			int s = state;
273	jsr166	1.87	try {
274	dl	1.77	Callable<V> c = callable;
275	jsr166	1.92	if (c != null && s == NEW) {
276	dl	1.77	try {
277			c.call(); // don't set result
278	jsr166	1.92	ran = true;
279	dl	1.77	} catch (Throwable ex) {
280			setException(ex);
281			}
282	jsr166	1.68	}
283	jsr166	1.87	} finally {
284	jsr166	1.93	// runner must be non-null until state is settled to
285			// prevent concurrent calls to run()
286	jsr166	1.68	runner = null;
287	jsr166	1.93	// state must be re-read after nulling runner to prevent
288			// leaked interrupts
289	jsr166	1.92	s = state;
290	jsr166	1.87	if (s >= INTERRUPTING)
291			handlePossibleCancellationInterrupt(s);
292	dl	1.62	}
293	jsr166	1.92	return ran && s == NEW;
294	dl	1.14	}
295	dl	1.3
296	dl	1.14	/**
297	jsr166	1.95	* Ensures that any interrupt from a possible cancel(true) is only
298			* delivered to a task while in run or runAndReset.
299	jsr166	1.86	*/
300			private void handlePossibleCancellationInterrupt(int s) {
301			// It is possible for our interrupter to stall before getting a
302			// chance to interrupt us. Let's spin-wait patiently.
303	jsr166	1.96	if (s == INTERRUPTING)
304			while (state == INTERRUPTING)
305	jsr166	1.86	Thread.yield(); // wait out pending interrupt
306	jsr166	1.96
307	jsr166	1.89	// assert state == INTERRUPTED;
308	jsr166	1.94
309	jsr166	1.95	// We want to clear any interrupt we may have received from
310			// cancel(true). However, it is permissible to use interrupts
311			// as an independent mechanism for a task to communicate with
312			// its caller, and there is no way to clear only the
313			// cancellation interrupt.
314			//
315			// Thread.interrupted();
316	jsr166	1.86	}
317
318			/**
319	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
320	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
321	dl	1.62	* for more detailed explanation.
322	dl	1.20	*/
323	dl	1.62	static final class WaitNode {
324			volatile Thread thread;
325	dl	1.76	volatile WaitNode next;
326			WaitNode() { thread = Thread.currentThread(); }
327	dl	1.62	}
328	dl	1.42
329	dl	1.62	/**
330	jsr166	1.85	* Removes and signals all waiting threads, invokes done(), and
331			* nulls out callable.
332	dl	1.62	*/
333	dl	1.78	private void finishCompletion() {
334	jsr166	1.90	// assert state > COMPLETING;
335	jsr166	1.81	for (WaitNode q; (q = waiters) != null;) {
336	jsr166	1.104	if (U.compareAndSwapObject(this, WAITERS, q, null)) {
337	dl	1.62	for (;;) {
338			Thread t = q.thread;
339			if (t != null) {
340			q.thread = null;
341			LockSupport.unpark(t);
342			}
343			WaitNode next = q.next;
344			if (next == null)
345	dl	1.78	break;
346	dl	1.62	q.next = null; // unlink to help gc
347			q = next;
348			}
349	dl	1.78	break;
350	dl	1.62	}
351	dl	1.24	}
352	jsr166	1.85
353	dl	1.78	done();
354	jsr166	1.85
355			callable = null; // to reduce footprint
356	dl	1.62	}
357	dl	1.24
358	dl	1.62	/**
359	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
360			*
361	dl	1.62	* @param timed true if use timed waits
362	jsr166	1.64	* @param nanos time to wait, if timed
363	jsr166	1.105	* @return state upon completion or at timeout
364	dl	1.62	*/
365			private int awaitDone(boolean timed, long nanos)
366			throws InterruptedException {
367	jsr166	1.99	final long deadline = timed ? System.nanoTime() + nanos : 0L;
368	dl	1.62	WaitNode q = null;
369			boolean queued = false;
370	jsr166	1.64	for (;;) {
371	dl	1.62	if (Thread.interrupted()) {
372			removeWaiter(q);
373			throw new InterruptedException();
374			}
375	jsr166	1.64
376			int s = state;
377			if (s > COMPLETING) {
378	dl	1.62	if (q != null)
379			q.thread = null;
380			return s;
381			}
382	dl	1.97	else if (s == COMPLETING) // cannot time out yet
383			Thread.yield();
384	dl	1.62	else if (q == null)
385			q = new WaitNode();
386			else if (!queued)
387	jsr166	1.104	queued = U.compareAndSwapObject(this, WAITERS,
388			q.next = waiters, q);
389	dl	1.62	else if (timed) {
390	jsr166	1.99	nanos = deadline - System.nanoTime();
391			if (nanos <= 0L) {
392	dl	1.62	removeWaiter(q);
393			return state;
394	dl	1.50	}
395	dl	1.62	LockSupport.parkNanos(this, nanos);
396	dl	1.50	}
397	dl	1.62	else
398			LockSupport.park(this);
399	dl	1.24	}
400	dl	1.62	}
401	dl	1.24
402	dl	1.62	/**
403	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
404			* accumulating garbage. Internal nodes are simply unspliced
405	dl	1.62	* without CAS since it is harmless if they are traversed anyway
406	jsr166	1.81	* by releasers. To avoid effects of unsplicing from already
407			* removed nodes, the list is retraversed in case of an apparent
408			* race. This is slow when there are a lot of nodes, but we don't
409			* expect lists to be long enough to outweigh higher-overhead
410			* schemes.
411	dl	1.62	*/
412			private void removeWaiter(WaitNode node) {
413			if (node != null) {
414			node.thread = null;
415	jsr166	1.81	retry:
416			for (;;) { // restart on removeWaiter race
417			for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
418			s = q.next;
419			if (q.thread != null)
420			pred = q;
421			else if (pred != null) {
422			pred.next = s;
423			if (pred.thread == null) // check for race
424			continue retry;
425			}
426	jsr166	1.104	else if (!U.compareAndSwapObject(this, WAITERS, q, s))
427	jsr166	1.81	continue retry;
428	jsr166	1.55	}
429	jsr166	1.81	break;
430	jsr166	1.56	}
431	dl	1.14	}
432	dl	1.62	}
433	dl	1.14
434	dl	1.62	// Unsafe mechanics
435	jsr166	1.104	private static final sun.misc.Unsafe U;
436			private static final long STATE;
437			private static final long RUNNER;
438			private static final long WAITERS;
439	dl	1.62	static {
440			try {
441	jsr166	1.104	U = sun.misc.Unsafe.getUnsafe();
442	dl	1.62	Class<?> k = FutureTask.class;
443	jsr166	1.104	STATE = U.objectFieldOffset(k.getDeclaredField("state"));
444			RUNNER = U.objectFieldOffset(k.getDeclaredField("runner"));
445			WAITERS = U.objectFieldOffset(k.getDeclaredField("waiters"));
446	dl	1.62	} catch (Exception e) {
447			throw new Error(e);
448	dl	1.14	}
449	dl	1.15	}
450	dl	1.62
451	dl	1.15	}