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 highly order-dependent 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    = 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 */
    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:	1.70
Committed:	Fri Jun 17 22:54:47 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.69:	+8 -9 lines
Log Message:	use ordinary ordered integers for state values
#	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	jsr166	1.70	* State values are highly order-dependent to simplify checks.
54	jsr166	1.69	*
55			* Possible state transitions:
56			* UNDECIDED -> COMPLETING -> NORMAL
57			* UNDECIDED -> COMPLETING -> EXCEPTIONAL
58			* UNDECIDED -> CANCELLED
59			* UNDECIDED -> INTERRUPTING -> INTERRUPTED
60	dl	1.62	*/
61			private volatile int state;
62	jsr166	1.70	private static final int UNDECIDED = 0;
63			private static final int COMPLETING = 1;
64			private static final int NORMAL = 2;
65			private static final int EXCEPTIONAL = 3;
66			private static final int CANCELLED = 4;
67			private static final int INTERRUPTING = 5;
68			private static final int INTERRUPTED = 6;
69	dl	1.62
70	jsr166	1.64	/** The underlying callable */
71			private final Callable<V> callable;
72	dl	1.62	/** The result to return or exception to throw from get() */
73			private Object outcome; // non-volatile, protected by state reads/writes
74			/** The thread running the callable; CASed during run() */
75			private volatile Thread runner;
76			/** Treiber stack of waiting threads */
77			private volatile WaitNode waiters;
78
79			/**
80			* Sets completion status, unless already completed. If
81	jsr166	1.69	* necessary, we first set state to transient states COMPLETING
82			* or INTERRUPTING to establish precedence.
83	dl	1.62	*
84			* @param x the outcome
85			* @param mode the completion state value
86	jsr166	1.67	* @return true if this call caused transition from UNDECIDED to completed
87	dl	1.62	*/
88			private boolean setCompletion(Object x, int mode) {
89	jsr166	1.69	// set up transient states
90			int next = (x != null) ? COMPLETING : mode;
91			if (!UNSAFE.compareAndSwapInt(this, stateOffset, UNDECIDED, next))
92	jsr166	1.68	return false;
93			if (next == INTERRUPTING) {
94	jsr166	1.69	Thread t = runner; // recheck to avoid leaked interrupt
95	jsr166	1.68	if (t != null)
96			t.interrupt();
97	jsr166	1.69	state = INTERRUPTED;
98	jsr166	1.68	}
99			else if (next == COMPLETING) {
100			outcome = x;
101			state = mode;
102	dl	1.62	}
103	jsr166	1.68	if (waiters != null)
104			releaseAll();
105			done();
106			return true;
107	dl	1.62	}
108
109			/**
110	jsr166	1.64	* Returns result or throws exception for completed task.
111			*
112	dl	1.62	* @param s completed state value
113			*/
114			private V report(int s) throws ExecutionException {
115			Object x = outcome;
116			if (s == NORMAL)
117			return (V)x;
118	jsr166	1.69	if (s >= CANCELLED)
119	dl	1.62	throw new CancellationException();
120			throw new ExecutionException((Throwable)x);
121			}
122	dl	1.11
123	tim	1.1	/**
124	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
125			* given {@code Callable}.
126	tim	1.1	*
127			* @param callable the callable task
128	dl	1.9	* @throws NullPointerException if callable is null
129	tim	1.1	*/
130			public FutureTask(Callable<V> callable) {
131	dl	1.9	if (callable == null)
132			throw new NullPointerException();
133	dl	1.62	this.callable = callable;
134	tim	1.1	}
135
136			/**
137	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
138			* given {@code Runnable}, and arrange that {@code get} will return the
139	tim	1.1	* given result on successful completion.
140			*
141	jsr166	1.54	* @param runnable the runnable task
142	tim	1.1	* @param result the result to return on successful completion. If
143	dl	1.9	* you don't need a particular result, consider using
144	dl	1.16	* constructions of the form:
145	jsr166	1.58	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
146	dl	1.9	* @throws NullPointerException if runnable is null
147	tim	1.1	*/
148	dl	1.15	public FutureTask(Runnable runnable, V result) {
149	dl	1.62	this.callable = Executors.callable(runnable, result);
150	dl	1.20	}
151
152			public boolean isCancelled() {
153	jsr166	1.69	return state >= CANCELLED;
154	dl	1.20	}
155	jsr166	1.35
156	dl	1.20	public boolean isDone() {
157	jsr166	1.67	return state != UNDECIDED;
158	dl	1.13	}
159
160			public boolean cancel(boolean mayInterruptIfRunning) {
161	jsr166	1.67	return state == UNDECIDED &&
162	jsr166	1.69	setCompletion(null, (mayInterruptIfRunning && runner != null) ?
163	dl	1.62	INTERRUPTING : CANCELLED);
164	dl	1.13	}
165	jsr166	1.35
166	jsr166	1.43	/**
167			* @throws CancellationException {@inheritDoc}
168			*/
169	dl	1.2	public V get() throws InterruptedException, ExecutionException {
170	jsr166	1.64	int s = state;
171			if (s <= COMPLETING)
172			s = awaitDone(false, 0L);
173			return report(s);
174	tim	1.1	}
175
176	jsr166	1.43	/**
177			* @throws CancellationException {@inheritDoc}
178			*/
179	dl	1.2	public V get(long timeout, TimeUnit unit)
180	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
181	jsr166	1.64	int s = state;
182			if (s <= COMPLETING &&
183			(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
184	dl	1.62	throw new TimeoutException();
185			return report(s);
186	tim	1.1	}
187
188			/**
189	dl	1.20	* Protected method invoked when this task transitions to state
190	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
191	dl	1.20	* default implementation does nothing. Subclasses may override
192			* this method to invoke completion callbacks or perform
193			* bookkeeping. Note that you can query status inside the
194			* implementation of this method to determine whether this task
195			* has been cancelled.
196			*/
197			protected void done() { }
198
199			/**
200	jsr166	1.64	* Sets the result of this future to the given value unless
201	dl	1.29	* this future has already been set or has been cancelled.
202	jsr166	1.64	*
203			* <p>This method is invoked internally by the {@link #run} method
204	dl	1.40	* upon successful completion of the computation.
205	jsr166	1.64	*
206	tim	1.1	* @param v the value
207	jsr166	1.35	*/
208	dl	1.2	protected void set(V v) {
209	dl	1.62	setCompletion(v, NORMAL);
210	tim	1.1	}
211
212			/**
213	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
214			* with the given throwable as its cause, unless this future has
215	dl	1.24	* already been set or has been cancelled.
216	jsr166	1.64	*
217			* <p>This method is invoked internally by the {@link #run} method
218	dl	1.40	* upon failure of the computation.
219	jsr166	1.64	*
220	jsr166	1.41	* @param t the cause of failure
221	jsr166	1.35	*/
222	dl	1.2	protected void setException(Throwable t) {
223	dl	1.62	setCompletion(t, EXCEPTIONAL);
224	tim	1.1	}
225	jsr166	1.35
226	dl	1.24	public void run() {
227	jsr166	1.68	if (state != UNDECIDED \|\|
228			!UNSAFE.compareAndSwapObject(this, runnerOffset,
229			null, Thread.currentThread()))
230			return;
231
232			try {
233	dl	1.62	V result;
234			try {
235			result = callable.call();
236			} catch (Throwable ex) {
237			setException(ex);
238			return;
239			}
240			set(result);
241	jsr166	1.68	} finally {
242			runner = null;
243	jsr166	1.69	int s = state;
244			if (s >= INTERRUPTING) {
245			while ((s = state) == INTERRUPTING)
246			Thread.yield(); // wait out pending cancellation interrupt
247			Thread.interrupted(); // clear any interrupt from cancel(true)
248			}
249	dl	1.62	}
250	dl	1.24	}
251
252			/**
253	dl	1.30	* Executes the computation without setting its result, and then
254	jsr166	1.64	* resets this future to initial state, failing to do so if the
255	dl	1.24	* computation encounters an exception or is cancelled. This is
256			* designed for use with tasks that intrinsically execute more
257			* than once.
258	jsr166	1.64	*
259	dl	1.24	* @return true if successfully run and reset
260			*/
261			protected boolean runAndReset() {
262	jsr166	1.67	if (state != UNDECIDED \|\|
263	jsr166	1.66	!UNSAFE.compareAndSwapObject(this, runnerOffset,
264			null, Thread.currentThread()))
265	dl	1.62	return false;
266	jsr166	1.68
267	dl	1.62	try {
268	jsr166	1.68	try {
269			callable.call(); // don't set result
270			return (state == UNDECIDED);
271			} catch (Throwable ex) {
272			setException(ex);
273	dl	1.62	return false;
274	jsr166	1.68	}
275			} finally {
276			runner = null;
277	jsr166	1.69	int s = state;
278			if (s >= INTERRUPTING) {
279			while ((s = state) == INTERRUPTING)
280			Thread.yield(); // wait out pending cancellation interrupt
281			Thread.interrupted(); // clear any interrupt from cancel(true)
282			}
283	dl	1.62	}
284	dl	1.14	}
285	dl	1.3
286	dl	1.14	/**
287	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
288	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
289	dl	1.62	* for more detailed explanation.
290	dl	1.20	*/
291	dl	1.62	static final class WaitNode {
292			volatile Thread thread;
293			WaitNode next;
294			}
295	dl	1.42
296	dl	1.62	/**
297	jsr166	1.64	* Removes and signals all waiting threads.
298	dl	1.62	*/
299			private void releaseAll() {
300			WaitNode q;
301			while ((q = waiters) != null) {
302			if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
303			for (;;) {
304			Thread t = q.thread;
305			if (t != null) {
306			q.thread = null;
307			LockSupport.unpark(t);
308			}
309			WaitNode next = q.next;
310			if (next == null)
311			return;
312			q.next = null; // unlink to help gc
313			q = next;
314			}
315			}
316	dl	1.24	}
317	dl	1.62	}
318	dl	1.24
319	dl	1.62	/**
320	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
321			*
322	dl	1.62	* @param timed true if use timed waits
323	jsr166	1.64	* @param nanos time to wait, if timed
324	dl	1.62	* @return state upon completion
325			*/
326			private int awaitDone(boolean timed, long nanos)
327			throws InterruptedException {
328	jsr166	1.63	long last = timed ? System.nanoTime() : 0L;
329	dl	1.62	WaitNode q = null;
330			boolean queued = false;
331	jsr166	1.64	for (;;) {
332	dl	1.62	if (Thread.interrupted()) {
333			removeWaiter(q);
334			throw new InterruptedException();
335			}
336	jsr166	1.64
337			int s = state;
338			if (s > COMPLETING) {
339	dl	1.62	if (q != null)
340			q.thread = null;
341			return s;
342			}
343			else if (q == null)
344			q = new WaitNode();
345			else if (!queued)
346			queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
347			q.next = waiters, q);
348			else if (q.thread == null)
349			q.thread = Thread.currentThread();
350			else if (timed) {
351			long now = System.nanoTime();
352			if ((nanos -= (now - last)) <= 0L) {
353			removeWaiter(q);
354			return state;
355	dl	1.50	}
356	dl	1.62	last = now;
357			LockSupport.parkNanos(this, nanos);
358	dl	1.50	}
359	dl	1.62	else
360			LockSupport.park(this);
361	dl	1.24	}
362	dl	1.62	}
363	dl	1.24
364	dl	1.62	/**
365	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
366			* accumulating garbage. Internal nodes are simply unspliced
367	dl	1.62	* without CAS since it is harmless if they are traversed anyway
368			* by releasers or concurrent calls to removeWaiter.
369			*/
370			private void removeWaiter(WaitNode node) {
371			if (node != null) {
372			node.thread = null;
373			WaitNode pred = null;
374			WaitNode q = waiters;
375			while (q != null) {
376			WaitNode next = node.next;
377			if (q != node) {
378			pred = q;
379			q = next;
380	dl	1.50	}
381	dl	1.62	else if (pred != null) {
382			pred.next = next;
383			break;
384	dl	1.50	}
385	dl	1.62	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
386			q, next))
387	jsr166	1.56	break;
388	dl	1.62	else { // restart on CAS failure
389			pred = null;
390			q = waiters;
391	jsr166	1.55	}
392	jsr166	1.56	}
393	dl	1.14	}
394	dl	1.62	}
395	dl	1.14
396	dl	1.62	// Unsafe mechanics
397			private static final sun.misc.Unsafe UNSAFE;
398			private static final long stateOffset;
399			private static final long runnerOffset;
400			private static final long waitersOffset;
401			static {
402			try {
403			UNSAFE = sun.misc.Unsafe.getUnsafe();
404			Class<?> k = FutureTask.class;
405			stateOffset = UNSAFE.objectFieldOffset
406			(k.getDeclaredField("state"));
407			runnerOffset = UNSAFE.objectFieldOffset
408			(k.getDeclaredField("runner"));
409			waitersOffset = UNSAFE.objectFieldOffset
410			(k.getDeclaredField("waiters"));
411			} catch (Exception e) {
412			throw new Error(e);
413	dl	1.14	}
414	dl	1.15	}
415	dl	1.62
416	dl	1.15	}