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
     */
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL) {
            @SuppressWarnings("unchecked") V v = (V)x;
            return v;
        }
        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)
            return false;
        if (mayInterruptIfRunning) {
            if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
                return false;
            Thread t = runner;
            if (t != null)
                t.interrupt();
            UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
        }
        else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
            return false;
        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 (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, 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 (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                try {
                    result = c.call();
                } catch (Throwable ex) {
                    setException(ex);
                    return;
                }
                set(result);
            }
        } finally {
            runner = null;
            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 true if successfully run and reset
     */
    protected boolean runAndReset() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                try {
                    c.call(); // don't set result
                    return state == NEW;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
            return false;
        } finally {
            runner = null;
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

    /**
     * Ensures that any interrupt from a possible cancel(true) does
     * not leak into subsequent code.
     */
    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 ((s = state) == INTERRUPTING)
                Thread.yield(); // wait out pending interrupt
        }
        // assert state == INTERRUPTED;
        // Clear any interrupt we may have received.
        Thread.interrupted();   // clear 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;
        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 (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)
                        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
     */
    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 (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.  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 (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
                        continue retry;
                }
                break;
            }
        }
    }

    // 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:	1.91
Committed:	Sun Jun 19 16:00:03 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.90:	+3 -1 lines
Log Message:	coding style
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.LockSupport;
9
10	/**
11	* 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	* retrieve the result of the computation. The result can only be
15	* retrieved when the computation has completed; the {@code get}
16	* methods will block if the computation has not yet completed. Once
17	* the computation has completed, the computation cannot be restarted
18	* or cancelled (unless the computation is invoked using
19	* {@link #runAndReset}).
20	*
21	* <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	*
26	* <p>In addition to serving as a standalone class, this class provides
27	* {@code protected} functionality that may be useful when creating
28	* customized task classes.
29	*
30	* @since 1.5
31	* @author Doug Lea
32	* @param <V> The result type returned by this FutureTask's {@code get} methods
33	*/
34	public class FutureTask<V> implements RunnableFuture<V> {
35	/*
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 NEW. The run state
49	* 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	*
57	* Possible state transitions:
58	* NEW -> COMPLETING -> NORMAL
59	* NEW -> COMPLETING -> EXCEPTIONAL
60	* NEW -> CANCELLED
61	* NEW -> INTERRUPTING -> INTERRUPTED
62	*/
63	private volatile int state;
64	private static final int NEW = 0;
65	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
72	/** The underlying callable; nulled out after running */
73	private Callable<V> callable;
74	/** 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	* Returns result or throws exception for completed task.
83	*
84	* @param s completed state value
85	*/
86	private V report(int s) throws ExecutionException {
87	Object x = outcome;
88	if (s == NORMAL) {
89	@SuppressWarnings("unchecked") V v = (V)x;
90	return v;
91	}
92	if (s >= CANCELLED)
93	throw new CancellationException();
94	throw new ExecutionException((Throwable)x);
95	}
96
97	/**
98	* Creates a {@code FutureTask} that will, upon running, execute the
99	* given {@code Callable}.
100	*
101	* @param callable the callable task
102	* @throws NullPointerException if the callable is null
103	*/
104	public FutureTask(Callable<V> callable) {
105	if (callable == null)
106	throw new NullPointerException();
107	this.callable = callable;
108	this.state = NEW; // ensure visibility of callable
109	}
110
111	/**
112	* Creates a {@code FutureTask} that will, upon running, execute the
113	* given {@code Runnable}, and arrange that {@code get} will return the
114	* given result on successful completion.
115	*
116	* @param runnable the runnable task
117	* @param result the result to return on successful completion. If
118	* you don't need a particular result, consider using
119	* constructions of the form:
120	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
121	* @throws NullPointerException if the runnable is null
122	*/
123	public FutureTask(Runnable runnable, V result) {
124	this.callable = Executors.callable(runnable, result);
125	this.state = NEW; // ensure visibility of callable
126	}
127
128	public boolean isCancelled() {
129	return state >= CANCELLED;
130	}
131
132	public boolean isDone() {
133	return state != NEW;
134	}
135
136	public boolean cancel(boolean mayInterruptIfRunning) {
137	if (state != NEW)
138	return false;
139	if (mayInterruptIfRunning) {
140	if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
141	return false;
142	Thread t = runner;
143	if (t != null)
144	t.interrupt();
145	UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
146	}
147	else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
148	return false;
149	finishCompletion();
150	return true;
151	}
152
153	/**
154	* @throws CancellationException {@inheritDoc}
155	*/
156	public V get() throws InterruptedException, ExecutionException {
157	int s = state;
158	if (s <= COMPLETING)
159	s = awaitDone(false, 0L);
160	return report(s);
161	}
162
163	/**
164	* @throws CancellationException {@inheritDoc}
165	*/
166	public V get(long timeout, TimeUnit unit)
167	throws InterruptedException, ExecutionException, TimeoutException {
168	if (unit == null)
169	throw new NullPointerException();
170	int s = state;
171	if (s <= COMPLETING &&
172	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
173	throw new TimeoutException();
174	return report(s);
175	}
176
177	/**
178	* Protected method invoked when this task transitions to state
179	* {@code isDone} (whether normally or via cancellation). The
180	* default implementation does nothing. Subclasses may override
181	* this method to invoke completion callbacks or perform
182	* bookkeeping. Note that you can query status inside the
183	* implementation of this method to determine whether this task
184	* has been cancelled.
185	*/
186	protected void done() { }
187
188	/**
189	* Sets the result of this future to the given value unless
190	* this future has already been set or has been cancelled.
191	*
192	* <p>This method is invoked internally by the {@link #run} method
193	* upon successful completion of the computation.
194	*
195	* @param v the value
196	*/
197	protected void set(V v) {
198	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
199	outcome = v;
200	UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
201	finishCompletion();
202	}
203	}
204
205	/**
206	* Causes this future to report an {@link ExecutionException}
207	* with the given throwable as its cause, unless this future has
208	* already been set or has been cancelled.
209	*
210	* <p>This method is invoked internally by the {@link #run} method
211	* upon failure of the computation.
212	*
213	* @param t the cause of failure
214	*/
215	protected void setException(Throwable t) {
216	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
217	outcome = t;
218	UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
219	finishCompletion();
220	}
221	}
222
223	public void run() {
224	if (state != NEW \|\|
225	!UNSAFE.compareAndSwapObject(this, runnerOffset,
226	null, Thread.currentThread()))
227	return;
228	try {
229	Callable<V> c = callable;
230	if (c != null && state == NEW) {
231	V result;
232	try {
233	result = c.call();
234	} catch (Throwable ex) {
235	setException(ex);
236	return;
237	}
238	set(result);
239	}
240	} finally {
241	runner = null;
242	int s = state;
243	if (s >= INTERRUPTING)
244	handlePossibleCancellationInterrupt(s);
245	}
246	}
247
248	/**
249	* Executes the computation without setting its result, and then
250	* resets this future to initial state, failing to do so if the
251	* computation encounters an exception or is cancelled. This is
252	* designed for use with tasks that intrinsically execute more
253	* than once.
254	*
255	* @return true if successfully run and reset
256	*/
257	protected boolean runAndReset() {
258	if (state != NEW \|\|
259	!UNSAFE.compareAndSwapObject(this, runnerOffset,
260	null, Thread.currentThread()))
261	return false;
262	try {
263	Callable<V> c = callable;
264	if (c != null && state == NEW) {
265	try {
266	c.call(); // don't set result
267	return state == NEW;
268	} catch (Throwable ex) {
269	setException(ex);
270	}
271	}
272	return false;
273	} finally {
274	runner = null;
275	int s = state;
276	if (s >= INTERRUPTING)
277	handlePossibleCancellationInterrupt(s);
278	}
279	}
280
281	/**
282	* Ensures that any interrupt from a possible cancel(true) does
283	* not leak into subsequent code.
284	*/
285	private void handlePossibleCancellationInterrupt(int s) {
286	// It is possible for our interrupter to stall before getting a
287	// chance to interrupt us. Let's spin-wait patiently.
288	if (s == INTERRUPTING) {
289	while ((s = state) == INTERRUPTING)
290	Thread.yield(); // wait out pending interrupt
291	}
292	// assert state == INTERRUPTED;
293	// Clear any interrupt we may have received.
294	Thread.interrupted(); // clear interrupt from cancel(true)
295	}
296
297	/**
298	* Simple linked list nodes to record waiting threads in a Treiber
299	* stack. See other classes such as Phaser and SynchronousQueue
300	* for more detailed explanation.
301	*/
302	static final class WaitNode {
303	volatile Thread thread;
304	volatile WaitNode next;
305	WaitNode() { thread = Thread.currentThread(); }
306	}
307
308	/**
309	* Removes and signals all waiting threads, invokes done(), and
310	* nulls out callable.
311	*/
312	private void finishCompletion() {
313	// assert state > COMPLETING;
314	for (WaitNode q; (q = waiters) != null;) {
315	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
316	for (;;) {
317	Thread t = q.thread;
318	if (t != null) {
319	q.thread = null;
320	LockSupport.unpark(t);
321	}
322	WaitNode next = q.next;
323	if (next == null)
324	break;
325	q.next = null; // unlink to help gc
326	q = next;
327	}
328	break;
329	}
330	}
331
332	done();
333
334	callable = null; // to reduce footprint
335	}
336
337	/**
338	* Awaits completion or aborts on interrupt or timeout.
339	*
340	* @param timed true if use timed waits
341	* @param nanos time to wait, if timed
342	* @return state upon completion
343	*/
344	private int awaitDone(boolean timed, long nanos)
345	throws InterruptedException {
346	long last = timed ? System.nanoTime() : 0L;
347	WaitNode q = null;
348	boolean queued = false;
349	for (;;) {
350	if (Thread.interrupted()) {
351	removeWaiter(q);
352	throw new InterruptedException();
353	}
354
355	int s = state;
356	if (s > COMPLETING) {
357	if (q != null)
358	q.thread = null;
359	return s;
360	}
361	else if (q == null)
362	q = new WaitNode();
363	else if (!queued)
364	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
365	q.next = waiters, q);
366	else if (timed) {
367	long now = System.nanoTime();
368	if ((nanos -= (now - last)) <= 0L) {
369	removeWaiter(q);
370	return state;
371	}
372	last = now;
373	LockSupport.parkNanos(this, nanos);
374	}
375	else
376	LockSupport.park(this);
377	}
378	}
379
380	/**
381	* Tries to unlink a timed-out or interrupted wait node to avoid
382	* accumulating garbage. Internal nodes are simply unspliced
383	* without CAS since it is harmless if they are traversed anyway
384	* by releasers. To avoid effects of unsplicing from already
385	* removed nodes, the list is retraversed in case of an apparent
386	* race. This is slow when there are a lot of nodes, but we don't
387	* expect lists to be long enough to outweigh higher-overhead
388	* schemes.
389	*/
390	private void removeWaiter(WaitNode node) {
391	if (node != null) {
392	node.thread = null;
393	retry:
394	for (;;) { // restart on removeWaiter race
395	for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
396	s = q.next;
397	if (q.thread != null)
398	pred = q;
399	else if (pred != null) {
400	pred.next = s;
401	if (pred.thread == null) // check for race
402	continue retry;
403	}
404	else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
405	q, s))
406	continue retry;
407	}
408	break;
409	}
410	}
411	}
412
413	// Unsafe mechanics
414	private static final sun.misc.Unsafe UNSAFE;
415	private static final long stateOffset;
416	private static final long runnerOffset;
417	private static final long waitersOffset;
418	static {
419	try {
420	UNSAFE = sun.misc.Unsafe.getUnsafe();
421	Class<?> k = FutureTask.class;
422	stateOffset = UNSAFE.objectFieldOffset
423	(k.getDeclaredField("state"));
424	runnerOffset = UNSAFE.objectFieldOffset
425	(k.getDeclaredField("runner"));
426	waitersOffset = UNSAFE.objectFieldOffset
427	(k.getDeclaredField("waiters"));
428	} catch (Exception e) {
429	throw new Error(e);
430	}
431	}
432
433	}