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)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;
    }

    /**
     * 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;
    }

    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())) {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result = null;
                boolean ran = false;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
                if (ran)
                    set(result);
                callable = null;      // null out upon use to reduce footprint
            }
            runner = null;
            if (state >= INTERRUPTING) {
                while (state == INTERRUPTING)
                    Thread.yield();   // wait out pending interrupt
                Thread.interrupted(); // clear interrupt from cancel(true)
            }
        }
    }

    /**
     * Executes the computation without setting its result, and then
     * resets this future to initial state, failing to do so if the
     * computation encounters an exception or is cancelled.  This is
     * designed for use with tasks that intrinsically execute more
     * than once.
     *
     * @return true if successfully run and reset
     */
    protected boolean runAndReset() {
        boolean rerun = false; // true if this task can be re-run
        if (state == NEW &&
            UNSAFE.compareAndSwapObject(this, runnerOffset,
                                        null, Thread.currentThread())) {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                try {
                    c.call(); // don't set result
                    rerun = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
            runner = null;
            int s = state;
            if (s != NEW) {
                rerun = false;
                if (s >= INTERRUPTING) {
                    while (state == INTERRUPTING)
                        Thread.yield();   // wait out pending interrupt
                    Thread.interrupted(); // clear interrupt from cancel(true)
                }
            }
            if (!rerun)
                callable = null;
        }
        return rerun;
    }

    /**
     * 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, and invokes done().
     */
    private void finishCompletion() {
        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();
    }

    /**
     * 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:	1.84
Committed:	Sat Jun 18 22:24:25 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.83:	+1 -1 lines
Log Message:	whitespace
#	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	return (V)x;
90	if (s >= CANCELLED)
91	throw new CancellationException();
92	throw new ExecutionException((Throwable)x);
93	}
94
95	/**
96	* Creates a {@code FutureTask} that will, upon running, execute the
97	* given {@code Callable}.
98	*
99	* @param callable the callable task
100	* @throws NullPointerException if the callable is null
101	*/
102	public FutureTask(Callable<V> callable) {
103	if (callable == null)
104	throw new NullPointerException();
105	this.callable = callable;
106	this.state = NEW;
107	}
108
109	/**
110	* Creates a {@code FutureTask} that will, upon running, execute the
111	* given {@code Runnable}, and arrange that {@code get} will return the
112	* given result on successful completion.
113	*
114	* @param runnable the runnable task
115	* @param result the result to return on successful completion. If
116	* you don't need a particular result, consider using
117	* constructions of the form:
118	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
119	* @throws NullPointerException if the runnable is null
120	*/
121	public FutureTask(Runnable runnable, V result) {
122	this.callable = Executors.callable(runnable, result);
123	this.state = NEW;
124	}
125
126	public boolean isCancelled() {
127	return state >= CANCELLED;
128	}
129
130	public boolean isDone() {
131	return state != NEW;
132	}
133
134	public boolean cancel(boolean mayInterruptIfRunning) {
135	if (state != NEW)
136	return false;
137	if (mayInterruptIfRunning) {
138	if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
139	return false;
140	Thread t = runner;
141	if (t != null)
142	t.interrupt();
143	UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
144	}
145	else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
146	return false;
147	finishCompletion();
148	return true;
149	}
150
151	/**
152	* @throws CancellationException {@inheritDoc}
153	*/
154	public V get() throws InterruptedException, ExecutionException {
155	int s = state;
156	return report((s <= COMPLETING) ? awaitDone(false, 0L) : s);
157	}
158
159	/**
160	* @throws CancellationException {@inheritDoc}
161	*/
162	public V get(long timeout, TimeUnit unit)
163	throws InterruptedException, ExecutionException, TimeoutException {
164	if (unit == null)
165	throw new NullPointerException();
166	int s = state;
167	if (s <= COMPLETING &&
168	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
169	throw new TimeoutException();
170	return report(s);
171	}
172
173	/**
174	* Protected method invoked when this task transitions to state
175	* {@code isDone} (whether normally or via cancellation). The
176	* default implementation does nothing. Subclasses may override
177	* this method to invoke completion callbacks or perform
178	* bookkeeping. Note that you can query status inside the
179	* implementation of this method to determine whether this task
180	* has been cancelled.
181	*/
182	protected void done() { }
183
184	/**
185	* Sets the result of this future to the given value unless
186	* this future has already been set or has been cancelled.
187	*
188	* <p>This method is invoked internally by the {@link #run} method
189	* upon successful completion of the computation.
190	*
191	* @param v the value
192	*/
193	protected void set(V v) {
194	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
195	outcome = v;
196	UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
197	finishCompletion();
198	}
199	}
200
201	/**
202	* Causes this future to report an {@link ExecutionException}
203	* with the given throwable as its cause, unless this future has
204	* already been set or has been cancelled.
205	*
206	* <p>This method is invoked internally by the {@link #run} method
207	* upon failure of the computation.
208	*
209	* @param t the cause of failure
210	*/
211	protected void setException(Throwable t) {
212	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
213	outcome = t;
214	UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
215	finishCompletion();
216	}
217	}
218
219	public void run() {
220	if (state == NEW &&
221	UNSAFE.compareAndSwapObject(this, runnerOffset,
222	null, Thread.currentThread())) {
223	Callable<V> c = callable;
224	if (c != null && state == NEW) {
225	V result = null;
226	boolean ran = false;
227	try {
228	result = c.call();
229	ran = true;
230	} catch (Throwable ex) {
231	setException(ex);
232	}
233	if (ran)
234	set(result);
235	callable = null; // null out upon use to reduce footprint
236	}
237	runner = null;
238	if (state >= INTERRUPTING) {
239	while (state == INTERRUPTING)
240	Thread.yield(); // wait out pending interrupt
241	Thread.interrupted(); // clear interrupt from cancel(true)
242	}
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	boolean rerun = false; // true if this task can be re-run
257	if (state == NEW &&
258	UNSAFE.compareAndSwapObject(this, runnerOffset,
259	null, Thread.currentThread())) {
260	Callable<V> c = callable;
261	if (c != null && state == NEW) {
262	try {
263	c.call(); // don't set result
264	rerun = true;
265	} catch (Throwable ex) {
266	setException(ex);
267	}
268	}
269	runner = null;
270	int s = state;
271	if (s != NEW) {
272	rerun = false;
273	if (s >= INTERRUPTING) {
274	while (state == INTERRUPTING)
275	Thread.yield(); // wait out pending interrupt
276	Thread.interrupted(); // clear interrupt from cancel(true)
277	}
278	}
279	if (!rerun)
280	callable = null;
281	}
282	return rerun;
283	}
284
285	/**
286	* Simple linked list nodes to record waiting threads in a Treiber
287	* stack. See other classes such as Phaser and SynchronousQueue
288	* for more detailed explanation.
289	*/
290	static final class WaitNode {
291	volatile Thread thread;
292	volatile WaitNode next;
293	WaitNode() { thread = Thread.currentThread(); }
294	}
295
296	/**
297	* Removes and signals all waiting threads, and invokes done().
298	*/
299	private void finishCompletion() {
300	for (WaitNode q; (q = waiters) != null;) {
301	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
302	for (;;) {
303	Thread t = q.thread;
304	if (t != null) {
305	q.thread = null;
306	LockSupport.unpark(t);
307	}
308	WaitNode next = q.next;
309	if (next == null)
310	break;
311	q.next = null; // unlink to help gc
312	q = next;
313	}
314	break;
315	}
316	}
317	done();
318	}
319
320	/**
321	* Awaits completion or aborts on interrupt or timeout.
322	*
323	* @param timed true if use timed waits
324	* @param nanos time to wait, if timed
325	* @return state upon completion
326	*/
327	private int awaitDone(boolean timed, long nanos)
328	throws InterruptedException {
329	long last = timed ? System.nanoTime() : 0L;
330	WaitNode q = null;
331	boolean queued = false;
332	for (;;) {
333	if (Thread.interrupted()) {
334	removeWaiter(q);
335	throw new InterruptedException();
336	}
337
338	int s = state;
339	if (s > COMPLETING) {
340	if (q != null)
341	q.thread = null;
342	return s;
343	}
344	else if (q == null)
345	q = new WaitNode();
346	else if (!queued)
347	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
348	q.next = waiters, q);
349	else if (timed) {
350	long now = System.nanoTime();
351	if ((nanos -= (now - last)) <= 0L) {
352	removeWaiter(q);
353	return state;
354	}
355	last = now;
356	LockSupport.parkNanos(this, nanos);
357	}
358	else
359	LockSupport.park(this);
360	}
361	}
362
363	/**
364	* Tries to unlink a timed-out or interrupted wait node to avoid
365	* accumulating garbage. Internal nodes are simply unspliced
366	* without CAS since it is harmless if they are traversed anyway
367	* by releasers. To avoid effects of unsplicing from already
368	* removed nodes, the list is retraversed in case of an apparent
369	* race. This is slow when there are a lot of nodes, but we don't
370	* expect lists to be long enough to outweigh higher-overhead
371	* schemes.
372	*/
373	private void removeWaiter(WaitNode node) {
374	if (node != null) {
375	node.thread = null;
376	retry:
377	for (;;) { // restart on removeWaiter race
378	for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
379	s = q.next;
380	if (q.thread != null)
381	pred = q;
382	else if (pred != null) {
383	pred.next = s;
384	if (pred.thread == null) // check for race
385	continue retry;
386	}
387	else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
388	q, s))
389	continue retry;
390	}
391	break;
392	}
393	}
394	}
395
396	// Unsafe mechanics
397	private static final sun.misc.Unsafe UNSAFE;
398	private static final long stateOffset;
399	private static final long runnerOffset;
400	private static final long waitersOffset;
401	static {
402	try {
403	UNSAFE = sun.misc.Unsafe.getUnsafe();
404	Class<?> k = FutureTask.class;
405	stateOffset = UNSAFE.objectFieldOffset
406	(k.getDeclaredField("state"));
407	runnerOffset = UNSAFE.objectFieldOffset
408	(k.getDeclaredField("runner"));
409	waitersOffset = UNSAFE.objectFieldOffset
410	(k.getDeclaredField("waiters"));
411	} catch (Exception e) {
412	throw new Error(e);
413	}
414	}
415
416	}