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 <tt>get</tt>
 * method will block if the computation has not yet completed.  Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled.
 *
 * <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or
 * {@link java.lang.Runnable} object.  Because <tt>FutureTask</tt>
 * implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be
 * submitted to an {@link Executor} for execution.
 *
 * <p>In addition to serving as a standalone class, this class provides
 * <tt>protected</tt> 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 <tt>get</tt> method
 */
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 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;
    /** The underlying callable */
    private final Callable<V> callable;
    /** 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 transtion 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;;) {
            if ((s = state) == 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 cancellation
            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 <tt>FutureTask</tt> that will, upon running, execute the
     * given <tt>Callable</tt>.
     *
     * @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 <tt>FutureTask</tt> that will, upon running, execute the
     * given <tt>Runnable</tt>, and arrange that <tt>get</tt> 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;
        return report((s = state) > COMPLETING ? s : awaitDone(false, 0L));
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        int s;
        long nanos = unit.toNanos(timeout);
        if ((s = state) <= COMPLETING &&
            (s = awaitDone(true, nanos)) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }

    /**
     * Protected method invoked when this task transitions to state
     * <tt>isDone</tt> (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.
     * This method is invoked internally by the <tt>run</tt> 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 <tt>ExecutionException</tt>
     * with the given throwable as its cause, unless this Future has
     * already been set or has been cancelled.
     * This method is invoked internally by the <tt>run</tt> 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 racing cancellation
        }
    }

    /**
     * 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 of 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 (int s;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }
            else if ((s = state) > 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);
        }
    }

    /**
     * Try 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:	1.62
Committed:	Fri Jun 17 14:43:54 2011 UTC (12 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.61:	+251 -175 lines
Log Message:	Reimplement to avoid surprises about interrupt status on cancellation
#	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			* retrieved when the computation has completed; the <tt>get</tt>
16			* method will block if the computation has not yet completed. Once
17	dl	1.8	* the computation has completed, the computation cannot be restarted
18			* or cancelled.
19	tim	1.1	*
20	dl	1.8	* <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or
21			* {@link java.lang.Runnable} object. Because <tt>FutureTask</tt>
22			* implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be
23			* submitted to an {@link Executor} for execution.
24	tim	1.1	*
25	dl	1.14	* <p>In addition to serving as a standalone class, this class provides
26			* <tt>protected</tt> functionality that may be useful when creating
27			* customized task classes.
28			*
29	tim	1.1	* @since 1.5
30	dl	1.4	* @author Doug Lea
31	dl	1.12	* @param <V> The result type returned by this FutureTask's <tt>get</tt> method
32	tim	1.1	*/
33	peierls	1.39	public class FutureTask<V> implements RunnableFuture<V> {
34	dl	1.62	/*
35			* Revision notes: This differs from previous versions of this
36			* class that relied on AbstractQueuedSynchronizer, mainly to
37			* avoid surprising users about retaining interrupt status during
38			* cancellation races. Sync control in the current design relies
39			* on a "state" field updated via CAS to track completion, along
40			* with a simple Treiber stack to hold waiting threads.
41			*
42			* Style note: As usual, we bypass overhead of using
43			* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
44			*/
45
46			/**
47			* The run state of this task, initially 0. The run state
48			* transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
49			* method setCompletion. During setCompletion, state may take on
50			* transient values of COMPLETING (while outcome is being set) or
51			* INTERRUPTING (while interrupting the runner). State values
52			* are ordered and set to powers of two to simplify checks.
53			*/
54			private volatile int state;
55			private static final int COMPLETING = 0x01;
56			private static final int INTERRUPTING = 0x02;
57			private static final int NORMAL = 0x04;
58			private static final int EXCEPTIONAL = 0x08;
59			private static final int CANCELLED = 0x10;
60
61			/** The result to return or exception to throw from get() */
62			private Object outcome; // non-volatile, protected by state reads/writes
63			/** The thread running the callable; CASed during run() */
64			private volatile Thread runner;
65			/** The underlying callable */
66			private final Callable<V> callable;
67			/** Treiber stack of waiting threads */
68			private volatile WaitNode waiters;
69
70			/**
71			* Sets completion status, unless already completed. If
72			* necessary, we first set state to COMPLETING or INTERRUPTING to
73			* establish precedence. This intentionally stalls (just via
74			* yields) in (uncommon) cases of concurrent calls during
75			* cancellation until state is set, to avoid surprising users
76			* during cancellation races.
77			*
78			* @param x the outcome
79			* @param mode the completion state value
80			* @return true if this call caused transtion from 0 to completed
81			*/
82			private boolean setCompletion(Object x, int mode) {
83			Thread r = runner;
84			if (r == Thread.currentThread()) // null out runner on completion
85			UNSAFE.putObject(this, runnerOffset, r = null); // nonvolatile OK
86			int next = ((mode == INTERRUPTING) ? // set up transient states
87			(r != null) ? INTERRUPTING : CANCELLED :
88			(x != null) ? COMPLETING : mode);
89			for (int s;;) {
90			if ((s = state) == 0) {
91			if (UNSAFE.compareAndSwapInt(this, stateOffset, 0, next)) {
92			if (next == INTERRUPTING) {
93			Thread t = runner; // recheck
94			if (t != null)
95			t.interrupt();
96			state = CANCELLED;
97			}
98			else if (next == COMPLETING) {
99			outcome = x;
100			state = mode;
101			}
102			if (waiters != null)
103			releaseAll();
104			done();
105			return true;
106			}
107			}
108			else if (s == INTERRUPTING)
109			Thread.yield(); // wait out cancellation
110			else
111			return false;
112			}
113			}
114
115			/**
116			* Returns result or throws exception for completed task
117			* @param s completed state value
118			*/
119			private V report(int s) throws ExecutionException {
120			Object x = outcome;
121			if (s == NORMAL)
122			return (V)x;
123			if ((s & (CANCELLED \| INTERRUPTING)) != 0)
124			throw new CancellationException();
125			throw new ExecutionException((Throwable)x);
126			}
127	dl	1.11
128	tim	1.1	/**
129	jsr166	1.54	* Creates a <tt>FutureTask</tt> that will, upon running, execute the
130	tim	1.1	* given <tt>Callable</tt>.
131			*
132			* @param callable the callable task
133	dl	1.9	* @throws NullPointerException if callable is null
134	tim	1.1	*/
135			public FutureTask(Callable<V> callable) {
136	dl	1.9	if (callable == null)
137			throw new NullPointerException();
138	dl	1.62	this.callable = callable;
139	tim	1.1	}
140
141			/**
142	jsr166	1.54	* Creates a <tt>FutureTask</tt> that will, upon running, execute the
143	tim	1.1	* given <tt>Runnable</tt>, and arrange that <tt>get</tt> will return the
144			* given result on successful completion.
145			*
146	jsr166	1.54	* @param runnable the runnable task
147	tim	1.1	* @param result the result to return on successful completion. If
148	dl	1.9	* you don't need a particular result, consider using
149	dl	1.16	* constructions of the form:
150	jsr166	1.58	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
151	dl	1.9	* @throws NullPointerException if runnable is null
152	tim	1.1	*/
153	dl	1.15	public FutureTask(Runnable runnable, V result) {
154	dl	1.62	this.callable = Executors.callable(runnable, result);
155	dl	1.20	}
156
157			public boolean isCancelled() {
158	dl	1.62	return (state & (CANCELLED \| INTERRUPTING)) != 0;
159	dl	1.20	}
160	jsr166	1.35
161	dl	1.20	public boolean isDone() {
162	dl	1.62	return state != 0;
163	dl	1.13	}
164
165			public boolean cancel(boolean mayInterruptIfRunning) {
166	dl	1.62	return state == 0 &&
167			setCompletion(null, mayInterruptIfRunning ?
168			INTERRUPTING : CANCELLED);
169	dl	1.13	}
170	jsr166	1.35
171	jsr166	1.43	/**
172			* @throws CancellationException {@inheritDoc}
173			*/
174	dl	1.2	public V get() throws InterruptedException, ExecutionException {
175	dl	1.62	int s;
176			return report((s = state) > COMPLETING ? s : awaitDone(false, 0L));
177	tim	1.1	}
178
179	jsr166	1.43	/**
180			* @throws CancellationException {@inheritDoc}
181			*/
182	dl	1.2	public V get(long timeout, TimeUnit unit)
183	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
184	dl	1.62	int s;
185			long nanos = unit.toNanos(timeout);
186			if ((s = state) <= COMPLETING &&
187			(s = awaitDone(true, nanos)) <= COMPLETING)
188			throw new TimeoutException();
189			return report(s);
190	tim	1.1	}
191
192			/**
193	dl	1.20	* Protected method invoked when this task transitions to state
194			* <tt>isDone</tt> (whether normally or via cancellation). The
195			* default implementation does nothing. Subclasses may override
196			* this method to invoke completion callbacks or perform
197			* bookkeeping. Note that you can query status inside the
198			* implementation of this method to determine whether this task
199			* has been cancelled.
200			*/
201			protected void done() { }
202
203			/**
204			* Sets the result of this Future to the given value unless
205	dl	1.29	* this future has already been set or has been cancelled.
206	dl	1.40	* This method is invoked internally by the <tt>run</tt> method
207			* upon successful completion of the computation.
208	tim	1.1	* @param v the value
209	jsr166	1.35	*/
210	dl	1.2	protected void set(V v) {
211	dl	1.62	setCompletion(v, NORMAL);
212	tim	1.1	}
213
214			/**
215	dl	1.13	* Causes this future to report an <tt>ExecutionException</tt>
216	dl	1.20	* with the given throwable as its cause, unless this Future has
217	dl	1.24	* already been set or has been cancelled.
218	dl	1.40	* This method is invoked internally by the <tt>run</tt> method
219			* upon failure of the computation.
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	dl	1.62	Thread r = Thread.currentThread();
228			if (state == 0 &&
229			UNSAFE.compareAndSwapObject(this, runnerOffset, null, r)) {
230			V result;
231			try {
232			result = callable.call();
233			} catch (Throwable ex) {
234			setException(ex);
235			return;
236			}
237			set(result);
238			}
239	dl	1.24	}
240
241			/**
242	dl	1.30	* Executes the computation without setting its result, and then
243			* resets this Future to initial state, failing to do so if the
244	dl	1.24	* computation encounters an exception or is cancelled. This is
245			* designed for use with tasks that intrinsically execute more
246			* than once.
247			* @return true if successfully run and reset
248			*/
249			protected boolean runAndReset() {
250	dl	1.62	Thread r = Thread.currentThread();
251			if (state != 0 \|\|
252			!UNSAFE.compareAndSwapObject(this, runnerOffset, null, r))
253			return false;
254			try {
255			callable.call(); // don't set result
256			} catch (Throwable ex) {
257			setException(ex);
258			return false;
259			}
260			runner = null;
261			for (;;) {
262			int s = state;
263			if (s == 0)
264			return true;
265			if (s != INTERRUPTING)
266			return false;
267			Thread.yield(); // wait out racing cancellation
268			}
269	dl	1.14	}
270	dl	1.3
271	dl	1.14	/**
272	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
273			* stack. See other classes such as Phaser and SynchronousQueue
274			* for more detailed explanation.
275	dl	1.20	*/
276	dl	1.62	static final class WaitNode {
277			volatile Thread thread;
278			WaitNode next;
279			}
280	dl	1.42
281	dl	1.62	/**
282			* Removes and signals all waiting threads
283			*/
284			private void releaseAll() {
285			WaitNode q;
286			while ((q = waiters) != null) {
287			if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
288			for (;;) {
289			Thread t = q.thread;
290			if (t != null) {
291			q.thread = null;
292			LockSupport.unpark(t);
293			}
294			WaitNode next = q.next;
295			if (next == null)
296			return;
297			q.next = null; // unlink to help gc
298			q = next;
299			}
300			}
301	dl	1.24	}
302	dl	1.62	}
303	dl	1.24
304	dl	1.62	/**
305			* Awaits completion or aborts on interrupt of timeout
306			* @param timed true if use timed waits
307			* @param nanos time to wait if timed
308			* @return state upon completion
309			*/
310			private int awaitDone(boolean timed, long nanos)
311			throws InterruptedException {
312			long last = timed? System.nanoTime() : 0L;
313			WaitNode q = null;
314			boolean queued = false;
315			for (int s;;) {
316			if (Thread.interrupted()) {
317			removeWaiter(q);
318			throw new InterruptedException();
319			}
320			else if ((s = state) > COMPLETING) {
321			if (q != null)
322			q.thread = null;
323			return s;
324			}
325			else if (q == null)
326			q = new WaitNode();
327			else if (!queued)
328			queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
329			q.next = waiters, q);
330			else if (q.thread == null)
331			q.thread = Thread.currentThread();
332			else if (timed) {
333			long now = System.nanoTime();
334			if ((nanos -= (now - last)) <= 0L) {
335			removeWaiter(q);
336			return state;
337	dl	1.50	}
338	dl	1.62	last = now;
339			LockSupport.parkNanos(this, nanos);
340	dl	1.50	}
341	dl	1.62	else
342			LockSupport.park(this);
343	dl	1.24	}
344	dl	1.62	}
345	dl	1.24
346	dl	1.62	/**
347			* Try to unlink a timed-out or interrupted wait node to avoid
348			* accumulating garbage. Internal nodes are simply unspliced
349			* without CAS since it is harmless if they are traversed anyway
350			* by releasers or concurrent calls to removeWaiter.
351			*/
352			private void removeWaiter(WaitNode node) {
353			if (node != null) {
354			node.thread = null;
355			WaitNode pred = null;
356			WaitNode q = waiters;
357			while (q != null) {
358			WaitNode next = node.next;
359			if (q != node) {
360			pred = q;
361			q = next;
362	dl	1.50	}
363	dl	1.62	else if (pred != null) {
364			pred.next = next;
365			break;
366	dl	1.50	}
367	dl	1.62	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
368			q, next))
369	jsr166	1.56	break;
370	dl	1.62	else { // restart on CAS failure
371			pred = null;
372			q = waiters;
373	jsr166	1.55	}
374	jsr166	1.56	}
375	dl	1.14	}
376	dl	1.62	}
377	dl	1.14
378	dl	1.62	// Unsafe mechanics
379			private static final sun.misc.Unsafe UNSAFE;
380			private static final long stateOffset;
381			private static final long runnerOffset;
382			private static final long waitersOffset;
383			static {
384			try {
385			UNSAFE = sun.misc.Unsafe.getUnsafe();
386			Class<?> k = FutureTask.class;
387			stateOffset = UNSAFE.objectFieldOffset
388			(k.getDeclaredField("state"));
389			runnerOffset = UNSAFE.objectFieldOffset
390			(k.getDeclaredField("runner"));
391			waitersOffset = UNSAFE.objectFieldOffset
392			(k.getDeclaredField("waiters"));
393			} catch (Exception e) {
394			throw new Error(e);
395	dl	1.14	}
396	dl	1.15	}
397	dl	1.62
398	dl	1.15	}