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 0.  The run state
     * transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
     * method setCompletion. During setCompletion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (while interrupting the runner).  State values
     * are ordered and set to powers of two to simplify checks.
     */
    private volatile int state;
    private static final int COMPLETING   = 0x01;
    private static final int INTERRUPTING = 0x02;
    private static final int NORMAL       = 0x04;
    private static final int EXCEPTIONAL  = 0x08;
    private static final int CANCELLED    = 0x10;

    /** 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 COMPLETING or INTERRUPTING to
     * establish precedence.  This intentionally stalls (just via
     * yields) in (uncommon) cases of concurrent calls during
     * cancellation until state is set, to avoid surprising users
     * during cancellation races.
     *
     * @param x the outcome
     * @param mode the completion state value
     * @return true if this call caused transition from 0 to completed
     */
    private boolean setCompletion(Object x, int mode) {
        Thread r = runner;
        if (r == Thread.currentThread()) // null out runner on completion
            UNSAFE.putObject(this, runnerOffset, r = null); // nonvolatile OK
        int next = ((mode == INTERRUPTING) ? // set up transient states
                    (r != null) ? INTERRUPTING : CANCELLED :
                    (x != null) ? COMPLETING : mode);
        for (;;) {
            int s = state;
            if (s == 0) {
                if (UNSAFE.compareAndSwapInt(this, stateOffset, 0, next)) {
                    if (next == INTERRUPTING) {
                        Thread t = runner; // recheck
                        if (t != null)
                            t.interrupt();
                        state = CANCELLED;
                    }
                    else if (next == COMPLETING) {
                        outcome = x;
                        state = mode;
                    }
                    if (waiters != null)
                        releaseAll();
                    done();
                    return true;
                }
            }
            else if (s == INTERRUPTING)
                Thread.yield(); // wait out pending cancellation interrupt
            else
                return false;
        }
    }

    /**
     * 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 | INTERRUPTING)) != 0)
            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 | INTERRUPTING)) != 0;
    }

    public boolean isDone() {
        return state != 0;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return state == 0 &&
            setCompletion(null, mayInterruptIfRunning ?
                          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() {
        Thread r = Thread.currentThread();
        if (state == 0 &&
            UNSAFE.compareAndSwapObject(this, runnerOffset, null, r)) {
            V result;
            try {
                result = callable.call();
            } catch (Throwable ex) {
                setException(ex);
                return;
            }
            set(result);
        }
    }

    /**
     * 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() {
        Thread r = Thread.currentThread();
        if (state != 0 ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset, null, r))
            return false;
        try {
            callable.call(); // don't set result
        } catch (Throwable ex) {
            setException(ex);
            return false;
        }
        runner = null;
        for (;;) {
            int s = state;
            if (s == 0)
                return true;
            if (s != INTERRUPTING)
                return false;
            Thread.yield(); // wait out pending cancellation interrupt
        }
    }

    /**
     * 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:	1.64
Committed:	Fri Jun 17 20:46:43 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.63:	+56 -44 lines
Log Message:	style update
#	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			* The run state of this task, initially 0. The run state
49			* transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
50			* method setCompletion. During setCompletion, state may take on
51			* transient values of COMPLETING (while outcome is being set) or
52	jsr166	1.64	* INTERRUPTING (while interrupting the runner). State values
53	dl	1.62	* are ordered and set to powers of two to simplify checks.
54			*/
55			private volatile int state;
56			private static final int COMPLETING = 0x01;
57			private static final int INTERRUPTING = 0x02;
58			private static final int NORMAL = 0x04;
59			private static final int EXCEPTIONAL = 0x08;
60			private static final int CANCELLED = 0x10;
61
62	jsr166	1.64	/** The underlying callable */
63			private final Callable<V> callable;
64	dl	1.62	/** The result to return or exception to throw from get() */
65			private Object outcome; // non-volatile, protected by state reads/writes
66			/** The thread running the callable; CASed during run() */
67			private volatile Thread runner;
68			/** Treiber stack of waiting threads */
69			private volatile WaitNode waiters;
70
71			/**
72			* Sets completion status, unless already completed. If
73			* necessary, we first set state to COMPLETING or INTERRUPTING to
74	jsr166	1.64	* establish precedence. This intentionally stalls (just via
75	dl	1.62	* yields) in (uncommon) cases of concurrent calls during
76			* cancellation until state is set, to avoid surprising users
77			* during cancellation races.
78			*
79			* @param x the outcome
80			* @param mode the completion state value
81	jsr166	1.64	* @return true if this call caused transition from 0 to completed
82	dl	1.62	*/
83			private boolean setCompletion(Object x, int mode) {
84			Thread r = runner;
85			if (r == Thread.currentThread()) // null out runner on completion
86			UNSAFE.putObject(this, runnerOffset, r = null); // nonvolatile OK
87			int next = ((mode == INTERRUPTING) ? // set up transient states
88			(r != null) ? INTERRUPTING : CANCELLED :
89			(x != null) ? COMPLETING : mode);
90	jsr166	1.64	for (;;) {
91			int s = state;
92			if (s == 0) {
93	dl	1.62	if (UNSAFE.compareAndSwapInt(this, stateOffset, 0, next)) {
94			if (next == INTERRUPTING) {
95			Thread t = runner; // recheck
96			if (t != null)
97			t.interrupt();
98			state = CANCELLED;
99			}
100			else if (next == COMPLETING) {
101			outcome = x;
102			state = mode;
103			}
104			if (waiters != null)
105			releaseAll();
106			done();
107			return true;
108			}
109			}
110			else if (s == INTERRUPTING)
111	jsr166	1.64	Thread.yield(); // wait out pending cancellation interrupt
112	dl	1.62	else
113			return false;
114			}
115			}
116
117			/**
118	jsr166	1.64	* Returns result or throws exception for completed task.
119			*
120	dl	1.62	* @param s completed state value
121			*/
122			private V report(int s) throws ExecutionException {
123			Object x = outcome;
124			if (s == NORMAL)
125			return (V)x;
126			if ((s & (CANCELLED \| INTERRUPTING)) != 0)
127			throw new CancellationException();
128			throw new ExecutionException((Throwable)x);
129			}
130	dl	1.11
131	tim	1.1	/**
132	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
133			* given {@code Callable}.
134	tim	1.1	*
135			* @param callable the callable task
136	dl	1.9	* @throws NullPointerException if callable is null
137	tim	1.1	*/
138			public FutureTask(Callable<V> callable) {
139	dl	1.9	if (callable == null)
140			throw new NullPointerException();
141	dl	1.62	this.callable = callable;
142	tim	1.1	}
143
144			/**
145	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
146			* given {@code Runnable}, and arrange that {@code get} will return the
147	tim	1.1	* given result on successful completion.
148			*
149	jsr166	1.54	* @param runnable the runnable task
150	tim	1.1	* @param result the result to return on successful completion. If
151	dl	1.9	* you don't need a particular result, consider using
152	dl	1.16	* constructions of the form:
153	jsr166	1.58	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
154	dl	1.9	* @throws NullPointerException if runnable is null
155	tim	1.1	*/
156	dl	1.15	public FutureTask(Runnable runnable, V result) {
157	dl	1.62	this.callable = Executors.callable(runnable, result);
158	dl	1.20	}
159
160			public boolean isCancelled() {
161	dl	1.62	return (state & (CANCELLED \| INTERRUPTING)) != 0;
162	dl	1.20	}
163	jsr166	1.35
164	dl	1.20	public boolean isDone() {
165	dl	1.62	return state != 0;
166	dl	1.13	}
167
168			public boolean cancel(boolean mayInterruptIfRunning) {
169	dl	1.62	return state == 0 &&
170			setCompletion(null, mayInterruptIfRunning ?
171			INTERRUPTING : CANCELLED);
172	dl	1.13	}
173	jsr166	1.35
174	jsr166	1.43	/**
175			* @throws CancellationException {@inheritDoc}
176			*/
177	dl	1.2	public V get() throws InterruptedException, ExecutionException {
178	jsr166	1.64	int s = state;
179			if (s <= COMPLETING)
180			s = awaitDone(false, 0L);
181			return report(s);
182	tim	1.1	}
183
184	jsr166	1.43	/**
185			* @throws CancellationException {@inheritDoc}
186			*/
187	dl	1.2	public V get(long timeout, TimeUnit unit)
188	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
189	jsr166	1.64	int s = state;
190			if (s <= COMPLETING &&
191			(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
192	dl	1.62	throw new TimeoutException();
193			return report(s);
194	tim	1.1	}
195
196			/**
197	dl	1.20	* Protected method invoked when this task transitions to state
198	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
199	dl	1.20	* default implementation does nothing. Subclasses may override
200			* this method to invoke completion callbacks or perform
201			* bookkeeping. Note that you can query status inside the
202			* implementation of this method to determine whether this task
203			* has been cancelled.
204			*/
205			protected void done() { }
206
207			/**
208	jsr166	1.64	* Sets the result of this future to the given value unless
209	dl	1.29	* this future has already been set or has been cancelled.
210	jsr166	1.64	*
211			* <p>This method is invoked internally by the {@link #run} method
212	dl	1.40	* upon successful completion of the computation.
213	jsr166	1.64	*
214	tim	1.1	* @param v the value
215	jsr166	1.35	*/
216	dl	1.2	protected void set(V v) {
217	dl	1.62	setCompletion(v, NORMAL);
218	tim	1.1	}
219
220			/**
221	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
222			* with the given throwable as its cause, unless this future has
223	dl	1.24	* already been set or has been cancelled.
224	jsr166	1.64	*
225			* <p>This method is invoked internally by the {@link #run} method
226	dl	1.40	* upon failure of the computation.
227	jsr166	1.64	*
228	jsr166	1.41	* @param t the cause of failure
229	jsr166	1.35	*/
230	dl	1.2	protected void setException(Throwable t) {
231	dl	1.62	setCompletion(t, EXCEPTIONAL);
232	tim	1.1	}
233	jsr166	1.35
234	dl	1.24	public void run() {
235	dl	1.62	Thread r = Thread.currentThread();
236			if (state == 0 &&
237			UNSAFE.compareAndSwapObject(this, runnerOffset, null, r)) {
238			V result;
239			try {
240			result = callable.call();
241			} catch (Throwable ex) {
242			setException(ex);
243			return;
244			}
245			set(result);
246			}
247	dl	1.24	}
248
249			/**
250	dl	1.30	* Executes the computation without setting its result, and then
251	jsr166	1.64	* resets this future to initial state, failing to do so if the
252	dl	1.24	* computation encounters an exception or is cancelled. This is
253			* designed for use with tasks that intrinsically execute more
254			* than once.
255	jsr166	1.64	*
256	dl	1.24	* @return true if successfully run and reset
257			*/
258			protected boolean runAndReset() {
259	dl	1.62	Thread r = Thread.currentThread();
260			if (state != 0 \|\|
261			!UNSAFE.compareAndSwapObject(this, runnerOffset, null, r))
262			return false;
263			try {
264			callable.call(); // don't set result
265			} catch (Throwable ex) {
266			setException(ex);
267			return false;
268			}
269			runner = null;
270			for (;;) {
271			int s = state;
272			if (s == 0)
273			return true;
274			if (s != INTERRUPTING)
275			return false;
276	jsr166	1.64	Thread.yield(); // wait out pending cancellation interrupt
277	dl	1.62	}
278	dl	1.14	}
279	dl	1.3
280	dl	1.14	/**
281	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
282	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
283	dl	1.62	* for more detailed explanation.
284	dl	1.20	*/
285	dl	1.62	static final class WaitNode {
286			volatile Thread thread;
287			WaitNode next;
288			}
289	dl	1.42
290	dl	1.62	/**
291	jsr166	1.64	* Removes and signals all waiting threads.
292	dl	1.62	*/
293			private void releaseAll() {
294			WaitNode q;
295			while ((q = waiters) != null) {
296			if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
297			for (;;) {
298			Thread t = q.thread;
299			if (t != null) {
300			q.thread = null;
301			LockSupport.unpark(t);
302			}
303			WaitNode next = q.next;
304			if (next == null)
305			return;
306			q.next = null; // unlink to help gc
307			q = next;
308			}
309			}
310	dl	1.24	}
311	dl	1.62	}
312	dl	1.24
313	dl	1.62	/**
314	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
315			*
316	dl	1.62	* @param timed true if use timed waits
317	jsr166	1.64	* @param nanos time to wait, if timed
318	dl	1.62	* @return state upon completion
319			*/
320			private int awaitDone(boolean timed, long nanos)
321			throws InterruptedException {
322	jsr166	1.63	long last = timed ? System.nanoTime() : 0L;
323	dl	1.62	WaitNode q = null;
324			boolean queued = false;
325	jsr166	1.64	for (;;) {
326	dl	1.62	if (Thread.interrupted()) {
327			removeWaiter(q);
328			throw new InterruptedException();
329			}
330	jsr166	1.64
331			int s = state;
332			if (s > COMPLETING) {
333	dl	1.62	if (q != null)
334			q.thread = null;
335			return s;
336			}
337			else if (q == null)
338			q = new WaitNode();
339			else if (!queued)
340			queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
341			q.next = waiters, q);
342			else if (q.thread == null)
343			q.thread = Thread.currentThread();
344			else if (timed) {
345			long now = System.nanoTime();
346			if ((nanos -= (now - last)) <= 0L) {
347			removeWaiter(q);
348			return state;
349	dl	1.50	}
350	dl	1.62	last = now;
351			LockSupport.parkNanos(this, nanos);
352	dl	1.50	}
353	dl	1.62	else
354			LockSupport.park(this);
355	dl	1.24	}
356	dl	1.62	}
357	dl	1.24
358	dl	1.62	/**
359	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
360			* accumulating garbage. Internal nodes are simply unspliced
361	dl	1.62	* without CAS since it is harmless if they are traversed anyway
362			* by releasers or concurrent calls to removeWaiter.
363			*/
364			private void removeWaiter(WaitNode node) {
365			if (node != null) {
366			node.thread = null;
367			WaitNode pred = null;
368			WaitNode q = waiters;
369			while (q != null) {
370			WaitNode next = node.next;
371			if (q != node) {
372			pred = q;
373			q = next;
374	dl	1.50	}
375	dl	1.62	else if (pred != null) {
376			pred.next = next;
377			break;
378	dl	1.50	}
379	dl	1.62	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
380			q, next))
381	jsr166	1.56	break;
382	dl	1.62	else { // restart on CAS failure
383			pred = null;
384			q = waiters;
385	jsr166	1.55	}
386	jsr166	1.56	}
387	dl	1.14	}
388	dl	1.62	}
389	dl	1.14
390	dl	1.62	// Unsafe mechanics
391			private static final sun.misc.Unsafe UNSAFE;
392			private static final long stateOffset;
393			private static final long runnerOffset;
394			private static final long waitersOffset;
395			static {
396			try {
397			UNSAFE = sun.misc.Unsafe.getUnsafe();
398			Class<?> k = FutureTask.class;
399			stateOffset = UNSAFE.objectFieldOffset
400			(k.getDeclaredField("state"));
401			runnerOffset = UNSAFE.objectFieldOffset
402			(k.getDeclaredField("runner"));
403			waitersOffset = UNSAFE.objectFieldOffset
404			(k.getDeclaredField("waiters"));
405			} catch (Exception e) {
406			throw new Error(e);
407	dl	1.14	}
408	dl	1.15	}
409	dl	1.62
410	dl	1.15	}