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;
        return report((s <= COMPLETING) ? awaitDone(false, 0L) : 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:	1.90
Committed:	Sun Jun 19 15:50:17 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.89:	+1 -1 lines
Log Message:	tighten comment
#	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	return report((s <= COMPLETING) ? awaitDone(false, 0L) : s);
159	}
160
161	/**
162	* @throws CancellationException {@inheritDoc}
163	*/
164	public V get(long timeout, TimeUnit unit)
165	throws InterruptedException, ExecutionException, TimeoutException {
166	if (unit == null)
167	throw new NullPointerException();
168	int s = state;
169	if (s <= COMPLETING &&
170	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
171	throw new TimeoutException();
172	return report(s);
173	}
174
175	/**
176	* Protected method invoked when this task transitions to state
177	* {@code isDone} (whether normally or via cancellation). The
178	* default implementation does nothing. Subclasses may override
179	* this method to invoke completion callbacks or perform
180	* bookkeeping. Note that you can query status inside the
181	* implementation of this method to determine whether this task
182	* has been cancelled.
183	*/
184	protected void done() { }
185
186	/**
187	* Sets the result of this future to the given value unless
188	* this future has already been set or has been cancelled.
189	*
190	* <p>This method is invoked internally by the {@link #run} method
191	* upon successful completion of the computation.
192	*
193	* @param v the value
194	*/
195	protected void set(V v) {
196	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
197	outcome = v;
198	UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
199	finishCompletion();
200	}
201	}
202
203	/**
204	* Causes this future to report an {@link ExecutionException}
205	* with the given throwable as its cause, unless this future has
206	* already been set or has been cancelled.
207	*
208	* <p>This method is invoked internally by the {@link #run} method
209	* upon failure of the computation.
210	*
211	* @param t the cause of failure
212	*/
213	protected void setException(Throwable t) {
214	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
215	outcome = t;
216	UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
217	finishCompletion();
218	}
219	}
220
221	public void run() {
222	if (state != NEW \|\|
223	!UNSAFE.compareAndSwapObject(this, runnerOffset,
224	null, Thread.currentThread()))
225	return;
226	try {
227	Callable<V> c = callable;
228	if (c != null && state == NEW) {
229	V result;
230	try {
231	result = c.call();
232	} catch (Throwable ex) {
233	setException(ex);
234	return;
235	}
236	set(result);
237	}
238	} finally {
239	runner = null;
240	int s = state;
241	if (s >= INTERRUPTING)
242	handlePossibleCancellationInterrupt(s);
243	}
244	}
245
246	/**
247	* Executes the computation without setting its result, and then
248	* resets this future to initial state, failing to do so if the
249	* computation encounters an exception or is cancelled. This is
250	* designed for use with tasks that intrinsically execute more
251	* than once.
252	*
253	* @return true if successfully run and reset
254	*/
255	protected boolean runAndReset() {
256	if (state != NEW \|\|
257	!UNSAFE.compareAndSwapObject(this, runnerOffset,
258	null, Thread.currentThread()))
259	return false;
260	try {
261	Callable<V> c = callable;
262	if (c != null && state == NEW) {
263	try {
264	c.call(); // don't set result
265	return state == NEW;
266	} catch (Throwable ex) {
267	setException(ex);
268	}
269	}
270	return false;
271	} finally {
272	runner = null;
273	int s = state;
274	if (s >= INTERRUPTING)
275	handlePossibleCancellationInterrupt(s);
276	}
277	}
278
279	/**
280	* Ensures that any interrupt from a possible cancel(true) does
281	* not leak into subsequent code.
282	*/
283	private void handlePossibleCancellationInterrupt(int s) {
284	// It is possible for our interrupter to stall before getting a
285	// chance to interrupt us. Let's spin-wait patiently.
286	if (s == INTERRUPTING) {
287	while ((s = state) == INTERRUPTING)
288	Thread.yield(); // wait out pending interrupt
289	}
290	// assert state == INTERRUPTED;
291	// Clear any interrupt we may have received.
292	Thread.interrupted(); // clear interrupt from cancel(true)
293	}
294
295	/**
296	* Simple linked list nodes to record waiting threads in a Treiber
297	* stack. See other classes such as Phaser and SynchronousQueue
298	* for more detailed explanation.
299	*/
300	static final class WaitNode {
301	volatile Thread thread;
302	volatile WaitNode next;
303	WaitNode() { thread = Thread.currentThread(); }
304	}
305
306	/**
307	* Removes and signals all waiting threads, invokes done(), and
308	* nulls out callable.
309	*/
310	private void finishCompletion() {
311	// assert state > COMPLETING;
312	for (WaitNode q; (q = waiters) != null;) {
313	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
314	for (;;) {
315	Thread t = q.thread;
316	if (t != null) {
317	q.thread = null;
318	LockSupport.unpark(t);
319	}
320	WaitNode next = q.next;
321	if (next == null)
322	break;
323	q.next = null; // unlink to help gc
324	q = next;
325	}
326	break;
327	}
328	}
329
330	done();
331
332	callable = null; // to reduce footprint
333	}
334
335	/**
336	* Awaits completion or aborts on interrupt or timeout.
337	*
338	* @param timed true if use timed waits
339	* @param nanos time to wait, if timed
340	* @return state upon completion
341	*/
342	private int awaitDone(boolean timed, long nanos)
343	throws InterruptedException {
344	long last = timed ? System.nanoTime() : 0L;
345	WaitNode q = null;
346	boolean queued = false;
347	for (;;) {
348	if (Thread.interrupted()) {
349	removeWaiter(q);
350	throw new InterruptedException();
351	}
352
353	int s = state;
354	if (s > COMPLETING) {
355	if (q != null)
356	q.thread = null;
357	return s;
358	}
359	else if (q == null)
360	q = new WaitNode();
361	else if (!queued)
362	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
363	q.next = waiters, q);
364	else if (timed) {
365	long now = System.nanoTime();
366	if ((nanos -= (now - last)) <= 0L) {
367	removeWaiter(q);
368	return state;
369	}
370	last = now;
371	LockSupport.parkNanos(this, nanos);
372	}
373	else
374	LockSupport.park(this);
375	}
376	}
377
378	/**
379	* Tries to unlink a timed-out or interrupted wait node to avoid
380	* accumulating garbage. Internal nodes are simply unspliced
381	* without CAS since it is harmless if they are traversed anyway
382	* by releasers. To avoid effects of unsplicing from already
383	* removed nodes, the list is retraversed in case of an apparent
384	* race. This is slow when there are a lot of nodes, but we don't
385	* expect lists to be long enough to outweigh higher-overhead
386	* schemes.
387	*/
388	private void removeWaiter(WaitNode node) {
389	if (node != null) {
390	node.thread = null;
391	retry:
392	for (;;) { // restart on removeWaiter race
393	for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
394	s = q.next;
395	if (q.thread != null)
396	pred = q;
397	else if (pred != null) {
398	pred.next = s;
399	if (pred.thread == null) // check for race
400	continue retry;
401	}
402	else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
403	q, s))
404	continue retry;
405	}
406	break;
407	}
408	}
409	}
410
411	// Unsafe mechanics
412	private static final sun.misc.Unsafe UNSAFE;
413	private static final long stateOffset;
414	private static final long runnerOffset;
415	private static final long waitersOffset;
416	static {
417	try {
418	UNSAFE = sun.misc.Unsafe.getUnsafe();
419	Class<?> k = FutureTask.class;
420	stateOffset = UNSAFE.objectFieldOffset
421	(k.getDeclaredField("state"));
422	runnerOffset = UNSAFE.objectFieldOffset
423	(k.getDeclaredField("runner"));
424	waitersOffset = UNSAFE.objectFieldOffset
425	(k.getDeclaredField("waiters"));
426	} catch (Exception e) {
427	throw new Error(e);
428	}
429	}
430
431	}