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 UNDECIDED.  The run state
     * transitions to a terminal state only in method setCompletion.
     * During setCompletion, 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)).
     * State values are ordered and set to powers of two to simplify
     * checks.
     *
     * Possible state transitions:
     * UNDECIDED -> COMPLETING -> NORMAL
     * UNDECIDED -> COMPLETING -> EXCEPTIONAL
     * UNDECIDED -> CANCELLED
     * UNDECIDED -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int UNDECIDED    = 0x00;
    private static final int COMPLETING   = 0x01;
    private static final int NORMAL       = 0x02;
    private static final int EXCEPTIONAL  = 0x04;
    private static final int CANCELLED    = 0x08;
    private static final int INTERRUPTING = 0x10;
    private static final int INTERRUPTED  = 0x20;

    /** The underlying callable */
    private final 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;

    /**
     * Sets completion status, unless already completed.  If
     * necessary, we first set state to transient states COMPLETING
     * or INTERRUPTING to establish precedence.
     *
     * @param x the outcome
     * @param mode the completion state value
     * @return true if this call caused transition from UNDECIDED to completed
     */
    private boolean setCompletion(Object x, int mode) {
        // set up transient states
        int next = (x != null) ? COMPLETING : mode;
        if (!UNSAFE.compareAndSwapInt(this, stateOffset, UNDECIDED, next))
            return false;
        if (next == INTERRUPTING) {
            Thread t = runner; // recheck to avoid leaked interrupt
            if (t != null)
                t.interrupt();
            state = INTERRUPTED;
        }
        else if (next == COMPLETING) {
            outcome = x;
            state = mode;
        }
        if (waiters != null)
            releaseAll();
        done();
        return true;
    }

    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    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 callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = 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 runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
    }

    public boolean isCancelled() {
        return state >= CANCELLED;
    }

    public boolean isDone() {
        return state != UNDECIDED;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return state == UNDECIDED &&
            setCompletion(null, (mayInterruptIfRunning && runner != null) ?
                          INTERRUPTING : CANCELLED);
    }

    /**
     * @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 {
        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) {
        setCompletion(v, NORMAL);
    }

    /**
     * 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) {
        setCompletion(t, EXCEPTIONAL);
    }

    public void run() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;

        try {
            V result;
            try {
                result = callable.call();
            } catch (Throwable ex) {
                setException(ex);
                return;
            }
            set(result);
        } finally {
            runner = null;
            int s = state;
            if (s >= INTERRUPTING) {
                while ((s = state) == INTERRUPTING)
                    Thread.yield(); // wait out pending cancellation interrupt
                Thread.interrupted(); // clear any interrupt from cancel(true)
            }
        }
    }

    /**
     * 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 true if successfully run and reset
     */
    protected boolean runAndReset() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;

        try {
            try {
                callable.call(); // don't set result
                return (state == UNDECIDED);
            } catch (Throwable ex) {
                setException(ex);
                return false;
            }
        } finally {
            runner = null;
            int s = state;
            if (s >= INTERRUPTING) {
                while ((s = state) == INTERRUPTING)
                    Thread.yield(); // wait out pending cancellation interrupt
                Thread.interrupted(); // clear any interrupt from cancel(true)
            }
        }
    }

    /**
     * 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;
        WaitNode next;
    }

    /**
     * Removes and signals all waiting threads.
     */
    private void releaseAll() {
        WaitNode q;
        while ((q = waiters) != null) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        return;
                    q.next = null; // unlink to help gc
                    q = next;
                }
            }
        }
    }

    /**
     * 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
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        long last = timed ? System.nanoTime() : 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 (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (q.thread == null)
                q.thread = Thread.currentThread();
            else if (timed) {
                long now = System.nanoTime();
                if ((nanos -= (now - last)) <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                last = now;
                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 or concurrent calls to removeWaiter.
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            WaitNode pred = null;
            WaitNode q = waiters;
            while (q != null) {
                WaitNode next = node.next;
                if (q != node) {
                    pred = q;
                    q = next;
                }
                else if (pred != null) {
                    pred.next = next;
                    break;
                }
                else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q, next))
                    break;
                else { // restart on CAS failure
                    pred = null;
                    q = waiters;
                }
            }
        }
    }

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

}
Revision:	1.69
Committed:	Fri Jun 17 22:45:07 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.68:	+39 -28 lines
Log Message:	introduce INTERRUPTING->INTERRUPTED state transition
#	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	1.67	* The run state of this task, initially UNDECIDED. The run state
49	jsr166	1.69	* transitions to a terminal state only in method setCompletion.
50			* During setCompletion, state may take on transient values of
51			* COMPLETING (while outcome is being set) or INTERRUPTING (only
52			* while interrupting the runner to satisfy a cancel(true)).
53			* State values are ordered and set to powers of two to simplify
54			* checks.
55			*
56			* Possible state transitions:
57			* UNDECIDED -> COMPLETING -> NORMAL
58			* UNDECIDED -> COMPLETING -> EXCEPTIONAL
59			* UNDECIDED -> CANCELLED
60			* UNDECIDED -> INTERRUPTING -> INTERRUPTED
61	dl	1.62	*/
62			private volatile int state;
63	jsr166	1.67	private static final int UNDECIDED = 0x00;
64	dl	1.62	private static final int COMPLETING = 0x01;
65	jsr166	1.69	private static final int NORMAL = 0x02;
66			private static final int EXCEPTIONAL = 0x04;
67			private static final int CANCELLED = 0x08;
68			private static final int INTERRUPTING = 0x10;
69			private static final int INTERRUPTED = 0x20;
70	dl	1.62
71	jsr166	1.64	/** The underlying callable */
72			private final Callable<V> callable;
73	dl	1.62	/** The result to return or exception to throw from get() */
74			private Object outcome; // non-volatile, protected by state reads/writes
75			/** The thread running the callable; CASed during run() */
76			private volatile Thread runner;
77			/** Treiber stack of waiting threads */
78			private volatile WaitNode waiters;
79
80			/**
81			* Sets completion status, unless already completed. If
82	jsr166	1.69	* necessary, we first set state to transient states COMPLETING
83			* or INTERRUPTING to establish precedence.
84	dl	1.62	*
85			* @param x the outcome
86			* @param mode the completion state value
87	jsr166	1.67	* @return true if this call caused transition from UNDECIDED to completed
88	dl	1.62	*/
89			private boolean setCompletion(Object x, int mode) {
90	jsr166	1.69	// set up transient states
91			int next = (x != null) ? COMPLETING : mode;
92			if (!UNSAFE.compareAndSwapInt(this, stateOffset, UNDECIDED, next))
93	jsr166	1.68	return false;
94			if (next == INTERRUPTING) {
95	jsr166	1.69	Thread t = runner; // recheck to avoid leaked interrupt
96	jsr166	1.68	if (t != null)
97			t.interrupt();
98	jsr166	1.69	state = INTERRUPTED;
99	jsr166	1.68	}
100			else if (next == COMPLETING) {
101			outcome = x;
102			state = mode;
103	dl	1.62	}
104	jsr166	1.68	if (waiters != null)
105			releaseAll();
106			done();
107			return true;
108	dl	1.62	}
109
110			/**
111	jsr166	1.64	* Returns result or throws exception for completed task.
112			*
113	dl	1.62	* @param s completed state value
114			*/
115			private V report(int s) throws ExecutionException {
116			Object x = outcome;
117			if (s == NORMAL)
118			return (V)x;
119	jsr166	1.69	if (s >= CANCELLED)
120	dl	1.62	throw new CancellationException();
121			throw new ExecutionException((Throwable)x);
122			}
123	dl	1.11
124	tim	1.1	/**
125	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
126			* given {@code Callable}.
127	tim	1.1	*
128			* @param callable the callable task
129	dl	1.9	* @throws NullPointerException if callable is null
130	tim	1.1	*/
131			public FutureTask(Callable<V> callable) {
132	dl	1.9	if (callable == null)
133			throw new NullPointerException();
134	dl	1.62	this.callable = callable;
135	tim	1.1	}
136
137			/**
138	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
139			* given {@code Runnable}, and arrange that {@code get} will return the
140	tim	1.1	* given result on successful completion.
141			*
142	jsr166	1.54	* @param runnable the runnable task
143	tim	1.1	* @param result the result to return on successful completion. If
144	dl	1.9	* you don't need a particular result, consider using
145	dl	1.16	* constructions of the form:
146	jsr166	1.58	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
147	dl	1.9	* @throws NullPointerException if runnable is null
148	tim	1.1	*/
149	dl	1.15	public FutureTask(Runnable runnable, V result) {
150	dl	1.62	this.callable = Executors.callable(runnable, result);
151	dl	1.20	}
152
153			public boolean isCancelled() {
154	jsr166	1.69	return state >= CANCELLED;
155	dl	1.20	}
156	jsr166	1.35
157	dl	1.20	public boolean isDone() {
158	jsr166	1.67	return state != UNDECIDED;
159	dl	1.13	}
160
161			public boolean cancel(boolean mayInterruptIfRunning) {
162	jsr166	1.67	return state == UNDECIDED &&
163	jsr166	1.69	setCompletion(null, (mayInterruptIfRunning && runner != null) ?
164	dl	1.62	INTERRUPTING : CANCELLED);
165	dl	1.13	}
166	jsr166	1.35
167	jsr166	1.43	/**
168			* @throws CancellationException {@inheritDoc}
169			*/
170	dl	1.2	public V get() throws InterruptedException, ExecutionException {
171	jsr166	1.64	int s = state;
172			if (s <= COMPLETING)
173			s = awaitDone(false, 0L);
174			return report(s);
175	tim	1.1	}
176
177	jsr166	1.43	/**
178			* @throws CancellationException {@inheritDoc}
179			*/
180	dl	1.2	public V get(long timeout, TimeUnit unit)
181	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
182	jsr166	1.64	int s = state;
183			if (s <= COMPLETING &&
184			(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
185	dl	1.62	throw new TimeoutException();
186			return report(s);
187	tim	1.1	}
188
189			/**
190	dl	1.20	* Protected method invoked when this task transitions to state
191	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
192	dl	1.20	* default implementation does nothing. Subclasses may override
193			* this method to invoke completion callbacks or perform
194			* bookkeeping. Note that you can query status inside the
195			* implementation of this method to determine whether this task
196			* has been cancelled.
197			*/
198			protected void done() { }
199
200			/**
201	jsr166	1.64	* Sets the result of this future to the given value unless
202	dl	1.29	* this future has already been set or has been cancelled.
203	jsr166	1.64	*
204			* <p>This method is invoked internally by the {@link #run} method
205	dl	1.40	* upon successful completion of the computation.
206	jsr166	1.64	*
207	tim	1.1	* @param v the value
208	jsr166	1.35	*/
209	dl	1.2	protected void set(V v) {
210	dl	1.62	setCompletion(v, NORMAL);
211	tim	1.1	}
212
213			/**
214	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
215			* with the given throwable as its cause, unless this future has
216	dl	1.24	* already been set or has been cancelled.
217	jsr166	1.64	*
218			* <p>This method is invoked internally by the {@link #run} method
219	dl	1.40	* upon failure of the computation.
220	jsr166	1.64	*
221	jsr166	1.41	* @param t the cause of failure
222	jsr166	1.35	*/
223	dl	1.2	protected void setException(Throwable t) {
224	dl	1.62	setCompletion(t, EXCEPTIONAL);
225	tim	1.1	}
226	jsr166	1.35
227	dl	1.24	public void run() {
228	jsr166	1.68	if (state != UNDECIDED \|\|
229			!UNSAFE.compareAndSwapObject(this, runnerOffset,
230			null, Thread.currentThread()))
231			return;
232
233			try {
234	dl	1.62	V result;
235			try {
236			result = callable.call();
237			} catch (Throwable ex) {
238			setException(ex);
239			return;
240			}
241			set(result);
242	jsr166	1.68	} finally {
243			runner = null;
244	jsr166	1.69	int s = state;
245			if (s >= INTERRUPTING) {
246			while ((s = state) == INTERRUPTING)
247			Thread.yield(); // wait out pending cancellation interrupt
248			Thread.interrupted(); // clear any interrupt from cancel(true)
249			}
250	dl	1.62	}
251	dl	1.24	}
252
253			/**
254	dl	1.30	* Executes the computation without setting its result, and then
255	jsr166	1.64	* resets this future to initial state, failing to do so if the
256	dl	1.24	* computation encounters an exception or is cancelled. This is
257			* designed for use with tasks that intrinsically execute more
258			* than once.
259	jsr166	1.64	*
260	dl	1.24	* @return true if successfully run and reset
261			*/
262			protected boolean runAndReset() {
263	jsr166	1.67	if (state != UNDECIDED \|\|
264	jsr166	1.66	!UNSAFE.compareAndSwapObject(this, runnerOffset,
265			null, Thread.currentThread()))
266	dl	1.62	return false;
267	jsr166	1.68
268	dl	1.62	try {
269	jsr166	1.68	try {
270			callable.call(); // don't set result
271			return (state == UNDECIDED);
272			} catch (Throwable ex) {
273			setException(ex);
274	dl	1.62	return false;
275	jsr166	1.68	}
276			} finally {
277			runner = null;
278	jsr166	1.69	int s = state;
279			if (s >= INTERRUPTING) {
280			while ((s = state) == INTERRUPTING)
281			Thread.yield(); // wait out pending cancellation interrupt
282			Thread.interrupted(); // clear any interrupt from cancel(true)
283			}
284	dl	1.62	}
285	dl	1.14	}
286	dl	1.3
287	dl	1.14	/**
288	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
289	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
290	dl	1.62	* for more detailed explanation.
291	dl	1.20	*/
292	dl	1.62	static final class WaitNode {
293			volatile Thread thread;
294			WaitNode next;
295			}
296	dl	1.42
297	dl	1.62	/**
298	jsr166	1.64	* Removes and signals all waiting threads.
299	dl	1.62	*/
300			private void releaseAll() {
301			WaitNode q;
302			while ((q = waiters) != null) {
303			if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
304			for (;;) {
305			Thread t = q.thread;
306			if (t != null) {
307			q.thread = null;
308			LockSupport.unpark(t);
309			}
310			WaitNode next = q.next;
311			if (next == null)
312			return;
313			q.next = null; // unlink to help gc
314			q = next;
315			}
316			}
317	dl	1.24	}
318	dl	1.62	}
319	dl	1.24
320	dl	1.62	/**
321	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
322			*
323	dl	1.62	* @param timed true if use timed waits
324	jsr166	1.64	* @param nanos time to wait, if timed
325	dl	1.62	* @return state upon completion
326			*/
327			private int awaitDone(boolean timed, long nanos)
328			throws InterruptedException {
329	jsr166	1.63	long last = timed ? System.nanoTime() : 0L;
330	dl	1.62	WaitNode q = null;
331			boolean queued = false;
332	jsr166	1.64	for (;;) {
333	dl	1.62	if (Thread.interrupted()) {
334			removeWaiter(q);
335			throw new InterruptedException();
336			}
337	jsr166	1.64
338			int s = state;
339			if (s > COMPLETING) {
340	dl	1.62	if (q != null)
341			q.thread = null;
342			return s;
343			}
344			else if (q == null)
345			q = new WaitNode();
346			else if (!queued)
347			queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
348			q.next = waiters, q);
349			else if (q.thread == null)
350			q.thread = Thread.currentThread();
351			else if (timed) {
352			long now = System.nanoTime();
353			if ((nanos -= (now - last)) <= 0L) {
354			removeWaiter(q);
355			return state;
356	dl	1.50	}
357	dl	1.62	last = now;
358			LockSupport.parkNanos(this, nanos);
359	dl	1.50	}
360	dl	1.62	else
361			LockSupport.park(this);
362	dl	1.24	}
363	dl	1.62	}
364	dl	1.24
365	dl	1.62	/**
366	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
367			* accumulating garbage. Internal nodes are simply unspliced
368	dl	1.62	* without CAS since it is harmless if they are traversed anyway
369			* by releasers or concurrent calls to removeWaiter.
370			*/
371			private void removeWaiter(WaitNode node) {
372			if (node != null) {
373			node.thread = null;
374			WaitNode pred = null;
375			WaitNode q = waiters;
376			while (q != null) {
377			WaitNode next = node.next;
378			if (q != node) {
379			pred = q;
380			q = next;
381	dl	1.50	}
382	dl	1.62	else if (pred != null) {
383			pred.next = next;
384			break;
385	dl	1.50	}
386	dl	1.62	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
387			q, next))
388	jsr166	1.56	break;
389	dl	1.62	else { // restart on CAS failure
390			pred = null;
391			q = waiters;
392	jsr166	1.55	}
393	jsr166	1.56	}
394	dl	1.14	}
395	dl	1.62	}
396	dl	1.14
397	dl	1.62	// Unsafe mechanics
398			private static final sun.misc.Unsafe UNSAFE;
399			private static final long stateOffset;
400			private static final long runnerOffset;
401			private static final long waitersOffset;
402			static {
403			try {
404			UNSAFE = sun.misc.Unsafe.getUnsafe();
405			Class<?> k = FutureTask.class;
406			stateOffset = UNSAFE.objectFieldOffset
407			(k.getDeclaredField("state"));
408			runnerOffset = UNSAFE.objectFieldOffset
409			(k.getDeclaredField("runner"));
410			waitersOffset = UNSAFE.objectFieldOffset
411			(k.getDeclaredField("waiters"));
412			} catch (Exception e) {
413			throw new Error(e);
414	dl	1.14	}
415	dl	1.15	}
416	dl	1.62
417	dl	1.15	}