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 {
        long nanos = unit.toNanos(timeout);
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, nanos)) <= 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:	1.81
Committed:	Sat Jun 18 18:31:52 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.80:	+21 -21 lines
Log Message:	removeWaiters does only one traversal in the common case
#	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	long nanos = unit.toNanos(timeout);
165	int s = state;
166	if (s <= COMPLETING &&
167	(s = awaitDone(true, nanos)) <= COMPLETING)
168	throw new TimeoutException();
169	return report(s);
170	}
171
172	/**
173	* Protected method invoked when this task transitions to state
174	* {@code isDone} (whether normally or via cancellation). The
175	* default implementation does nothing. Subclasses may override
176	* this method to invoke completion callbacks or perform
177	* bookkeeping. Note that you can query status inside the
178	* implementation of this method to determine whether this task
179	* has been cancelled.
180	*/
181	protected void done() { }
182
183	/**
184	* Sets the result of this future to the given value unless
185	* this future has already been set or has been cancelled.
186	*
187	* <p>This method is invoked internally by the {@link #run} method
188	* upon successful completion of the computation.
189	*
190	* @param v the value
191	*/
192	protected void set(V v) {
193	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
194	outcome = v;
195	UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
196	finishCompletion();
197	}
198	}
199
200	/**
201	* Causes this future to report an {@link ExecutionException}
202	* with the given throwable as its cause, unless this future has
203	* already been set or has been cancelled.
204	*
205	* <p>This method is invoked internally by the {@link #run} method
206	* upon failure of the computation.
207	*
208	* @param t the cause of failure
209	*/
210	protected void setException(Throwable t) {
211	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
212	outcome = t;
213	UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
214	finishCompletion();
215	}
216	}
217
218	public void run() {
219	if (state == NEW &&
220	UNSAFE.compareAndSwapObject(this, runnerOffset,
221	null, Thread.currentThread())) {
222	Callable<V> c = callable;
223	if (c != null && state == NEW) {
224	V result = null;
225	boolean ran = false;
226	try {
227	result = c.call();
228	ran = true;
229	} catch (Throwable ex) {
230	setException(ex);
231	}
232	if (ran)
233	set(result);
234	callable = null; // null out upon use to reduce footprint
235	}
236	runner = null;
237	if (state >= INTERRUPTING) {
238	while (state == INTERRUPTING)
239	Thread.yield(); // wait out pending interrupt
240	Thread.interrupted(); // clear interrupt from cancel(true)
241	}
242	}
243	}
244
245	/**
246	* Executes the computation without setting its result, and then
247	* resets this future to initial state, failing to do so if the
248	* computation encounters an exception or is cancelled. This is
249	* designed for use with tasks that intrinsically execute more
250	* than once.
251	*
252	* @return true if successfully run and reset
253	*/
254	protected boolean runAndReset() {
255	boolean rerun = false; // true if this task can be re-run
256	if (state == NEW &&
257	UNSAFE.compareAndSwapObject(this, runnerOffset,
258	null, Thread.currentThread())) {
259	Callable<V> c = callable;
260	if (c != null && state == NEW) {
261	try {
262	c.call(); // don't set result
263	rerun = true;
264	} catch (Throwable ex) {
265	setException(ex);
266	}
267	}
268	runner = null;
269	int s = state;
270	if (s != NEW) {
271	rerun = false;
272	if (s >= INTERRUPTING) {
273	while (state == INTERRUPTING)
274	Thread.yield(); // wait out pending interrupt
275	Thread.interrupted(); // clear interrupt from cancel(true)
276	}
277	}
278	if (!rerun)
279	callable = null;
280	}
281	return rerun;
282	}
283
284	/**
285	* Simple linked list nodes to record waiting threads in a Treiber
286	* stack. See other classes such as Phaser and SynchronousQueue
287	* for more detailed explanation.
288	*/
289	static final class WaitNode {
290	volatile Thread thread;
291	volatile WaitNode next;
292	WaitNode() { thread = Thread.currentThread(); }
293	}
294
295	/**
296	* Removes and signals all waiting threads, and invokes done().
297	*/
298	private void finishCompletion() {
299	for (WaitNode q; (q = waiters) != null;) {
300	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
301	for (;;) {
302	Thread t = q.thread;
303	if (t != null) {
304	q.thread = null;
305	LockSupport.unpark(t);
306	}
307	WaitNode next = q.next;
308	if (next == null)
309	break;
310	q.next = null; // unlink to help gc
311	q = next;
312	}
313	break;
314	}
315	}
316	done();
317	}
318
319	/**
320	* Awaits completion or aborts on interrupt or timeout.
321	*
322	* @param timed true if use timed waits
323	* @param nanos time to wait, if timed
324	* @return state upon completion
325	*/
326	private int awaitDone(boolean timed, long nanos)
327	throws InterruptedException {
328	long last = timed ? System.nanoTime() : 0L;
329	WaitNode q = null;
330	boolean queued = false;
331	for (;;) {
332	if (Thread.interrupted()) {
333	removeWaiter(q);
334	throw new InterruptedException();
335	}
336
337	int s = state;
338	if (s > COMPLETING) {
339	if (q != null)
340	q.thread = null;
341	return s;
342	}
343	else if (q == null)
344	q = new WaitNode();
345	else if (!queued)
346	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
347	q.next = waiters, q);
348	else if (timed) {
349	long now = System.nanoTime();
350	if ((nanos -= (now - last)) <= 0L) {
351	removeWaiter(q);
352	return state;
353	}
354	last = now;
355	LockSupport.parkNanos(this, nanos);
356	}
357	else
358	LockSupport.park(this);
359	}
360	}
361
362	/**
363	* Tries to unlink a timed-out or interrupted wait node to avoid
364	* accumulating garbage. Internal nodes are simply unspliced
365	* without CAS since it is harmless if they are traversed anyway
366	* by releasers. To avoid effects of unsplicing from already
367	* removed nodes, the list is retraversed in case of an apparent
368	* race. This is slow when there are a lot of nodes, but we don't
369	* expect lists to be long enough to outweigh higher-overhead
370	* schemes.
371	*/
372	private void removeWaiter(WaitNode node) {
373	if (node != null) {
374	node.thread = null;
375	retry:
376	for (;;) { // restart on removeWaiter race
377	for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
378	s = q.next;
379	if (q.thread != null)
380	pred = q;
381	else if (pred != null) {
382	pred.next = s;
383	if (pred.thread == null) // check for race
384	continue retry;
385	}
386	else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
387	q, s))
388	continue retry;
389	}
390	break;
391	}
392	}
393	}
394
395	// Unsafe mechanics
396	private static final sun.misc.Unsafe UNSAFE;
397	private static final long stateOffset;
398	private static final long runnerOffset;
399	private static final long waitersOffset;
400	static {
401	try {
402	UNSAFE = sun.misc.Unsafe.getUnsafe();
403	Class<?> k = FutureTask.class;
404	stateOffset = UNSAFE.objectFieldOffset
405	(k.getDeclaredField("state"));
406	runnerOffset = UNSAFE.objectFieldOffset
407	(k.getDeclaredField("runner"));
408	waitersOffset = UNSAFE.objectFieldOffset
409	(k.getDeclaredField("waiters"));
410	} catch (Exception e) {
411	throw new Error(e);
412	}
413	}
414
415	}