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 method setCompletion.
     * During setCompletion, 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)).
     * State values are highly order-dependent to simplify checks.
     *
     * Possible state transitions:
     * NEW -> NORMAL
     * 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; normally nulled out after use */
    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;

    /**
     * Sets completion status, unless already completed.  If
     * necessary, we first set state to transient states COMPLETING
     * or INTERRUPTING to establish precedence.
     *
     * @param x the outcome
     * @param mode the completion state value
     * @return true if this call caused transition from NEW to completed
     */
    private boolean setCompletion(Object x, int mode) {
        // set up transient states
        int next = (x != null) ? COMPLETING : mode;
        if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, next))
            return false;
        if (next == INTERRUPTING) {
            // Must check after CAS to avoid missed interrupt
            Thread t = runner;
            if (t != null)
                t.interrupt();
            state = INTERRUPTED;
        }
        else if (next == COMPLETING) {
            outcome = x;
            state = mode;
        }
        if (waiters != null)
            releaseAll();
        done();
        return true;
    }

    /**
     * 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 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 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) {
        return state == NEW &&
            setCompletion(null,
                          mayInterruptIfRunning ? INTERRUPTING : CANCELLED);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        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) {
        setCompletion(v, NORMAL);
    }

    /**
     * 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) {
        setCompletion(t, EXCEPTIONAL);
    }

    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.
     */
    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 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 until no cancelled nodes
     * are found.  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;
            for (WaitNode pred = null, q = waiters; q != null;) {
                WaitNode next = q.next;
                if (q.thread != null) {
                    pred = q;
                    q = next;
                }
                else {
                    if (pred != null)
                        pred.next = next;
                    else
                        UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                    q, next);
                    pred = null; // restart until clean sweep
                    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:	1.77
Committed:	Sat Jun 18 11:34:54 2011 UTC (12 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.76:	+46 -44 lines
Log Message:	Null out callable after use
#	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 method setCompletion.
50	* During setCompletion, state may take on transient values of
51	* COMPLETING (while outcome is being set) or INTERRUPTING (only
52	* while interrupting the runner to satisfy a cancel(true)).
53	* State values are highly order-dependent to simplify checks.
54	*
55	* Possible state transitions:
56	* NEW -> NORMAL
57	* NEW -> COMPLETING -> NORMAL
58	* NEW -> COMPLETING -> EXCEPTIONAL
59	* NEW -> CANCELLED
60	* NEW -> INTERRUPTING -> INTERRUPTED
61	*/
62	private volatile int state;
63	private static final int NEW = 0;
64	private static final int COMPLETING = 1;
65	private static final int NORMAL = 2;
66	private static final int EXCEPTIONAL = 3;
67	private static final int CANCELLED = 4;
68	private static final int INTERRUPTING = 5;
69	private static final int INTERRUPTED = 6;
70
71	/** The underlying callable; normally nulled out after use */
72	private Callable<V> callable;
73	/** The result to return or exception to throw from get() */
74	private Object outcome; // non-volatile, protected by state reads/writes
75	/** The thread running the callable; CASed during run() */
76	private volatile Thread runner;
77	/** Treiber stack of waiting threads */
78	private volatile WaitNode waiters;
79
80	/**
81	* Sets completion status, unless already completed. If
82	* necessary, we first set state to transient states COMPLETING
83	* or INTERRUPTING to establish precedence.
84	*
85	* @param x the outcome
86	* @param mode the completion state value
87	* @return true if this call caused transition from NEW to completed
88	*/
89	private boolean setCompletion(Object x, int mode) {
90	// set up transient states
91	int next = (x != null) ? COMPLETING : mode;
92	if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, next))
93	return false;
94	if (next == INTERRUPTING) {
95	// Must check after CAS to avoid missed interrupt
96	Thread t = runner;
97	if (t != null)
98	t.interrupt();
99	state = INTERRUPTED;
100	}
101	else if (next == COMPLETING) {
102	outcome = x;
103	state = mode;
104	}
105	if (waiters != null)
106	releaseAll();
107	done();
108	return true;
109	}
110
111	/**
112	* Returns result or throws exception for completed task.
113	*
114	* @param s completed state value
115	*/
116	private V report(int s) throws ExecutionException {
117	Object x = outcome;
118	if (s == NORMAL)
119	return (V)x;
120	if (s >= CANCELLED)
121	throw new CancellationException();
122	throw new ExecutionException((Throwable)x);
123	}
124
125	/**
126	* Creates a {@code FutureTask} that will, upon running, execute the
127	* given {@code Callable}.
128	*
129	* @param callable the callable task
130	* @throws NullPointerException if callable is null
131	*/
132	public FutureTask(Callable<V> callable) {
133	if (callable == null)
134	throw new NullPointerException();
135	this.callable = callable;
136	this.state = NEW;
137	}
138
139	/**
140	* Creates a {@code FutureTask} that will, upon running, execute the
141	* given {@code Runnable}, and arrange that {@code get} will return the
142	* given result on successful completion.
143	*
144	* @param runnable the runnable task
145	* @param result the result to return on successful completion. If
146	* you don't need a particular result, consider using
147	* constructions of the form:
148	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
149	* @throws NullPointerException if runnable is null
150	*/
151	public FutureTask(Runnable runnable, V result) {
152	this.callable = Executors.callable(runnable, result);
153	this.state = NEW;
154	}
155
156	public boolean isCancelled() {
157	return state >= CANCELLED;
158	}
159
160	public boolean isDone() {
161	return state != NEW;
162	}
163
164	public boolean cancel(boolean mayInterruptIfRunning) {
165	return state == NEW &&
166	setCompletion(null,
167	mayInterruptIfRunning ? INTERRUPTING : CANCELLED);
168	}
169
170	/**
171	* @throws CancellationException {@inheritDoc}
172	*/
173	public V get() throws InterruptedException, ExecutionException {
174	int s = state;
175	if (s <= COMPLETING)
176	s = awaitDone(false, 0L);
177	return report(s);
178	}
179
180	/**
181	* @throws CancellationException {@inheritDoc}
182	*/
183	public V get(long timeout, TimeUnit unit)
184	throws InterruptedException, ExecutionException, TimeoutException {
185	int s = state;
186	if (s <= COMPLETING &&
187	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
188	throw new TimeoutException();
189	return report(s);
190	}
191
192	/**
193	* Protected method invoked when this task transitions to state
194	* {@code isDone} (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	* this future has already been set or has been cancelled.
206	*
207	* <p>This method is invoked internally by the {@link #run} method
208	* upon successful completion of the computation.
209	*
210	* @param v the value
211	*/
212	protected void set(V v) {
213	setCompletion(v, NORMAL);
214	}
215
216	/**
217	* Causes this future to report an {@link ExecutionException}
218	* with the given throwable as its cause, unless this future has
219	* already been set or has been cancelled.
220	*
221	* <p>This method is invoked internally by the {@link #run} method
222	* upon failure of the computation.
223	*
224	* @param t the cause of failure
225	*/
226	protected void setException(Throwable t) {
227	setCompletion(t, EXCEPTIONAL);
228	}
229
230	public void run() {
231	if (state == NEW &&
232	UNSAFE.compareAndSwapObject(this, runnerOffset,
233	null, Thread.currentThread())) {
234	Callable<V> c = callable;
235	if (c != null && state == NEW) {
236	V result = null;
237	boolean ran = false;
238	try {
239	result = c.call();
240	ran = true;
241	} catch (Throwable ex) {
242	setException(ex);
243	}
244	if (ran)
245	set(result);
246	callable = null; // null out upon use to reduce footprint
247	}
248	runner = null;
249	if (state >= INTERRUPTING) {
250	while (state == INTERRUPTING)
251	Thread.yield(); // wait out pending interrupt
252	Thread.interrupted(); // clear interrupt from cancel(true)
253	}
254	}
255	}
256
257	/**
258	* Executes the computation without setting its result, and then
259	* resets this future to initial state, failing to do so if the
260	* computation encounters an exception or is cancelled. This is
261	* designed for use with tasks that intrinsically execute more
262	* than once.
263	*
264	* @return true if successfully run and reset
265	*/
266	protected boolean runAndReset() {
267	boolean rerun = false; // true if this task can be re-run
268	if (state == NEW &&
269	UNSAFE.compareAndSwapObject(this, runnerOffset,
270	null, Thread.currentThread())) {
271	Callable<V> c = callable;
272	if (c != null && state == NEW) {
273	try {
274	c.call(); // don't set result
275	rerun = true;
276	} catch (Throwable ex) {
277	setException(ex);
278	}
279	}
280	runner = null;
281	int s = state;
282	if (s != NEW) {
283	rerun = false;
284	if (s >= INTERRUPTING) {
285	while (state == INTERRUPTING)
286	Thread.yield(); // wait out pending interrupt
287	Thread.interrupted(); // clear interrupt from cancel(true)
288	}
289	}
290	if (!rerun)
291	callable = null;
292	}
293	return rerun;
294	}
295
296	/**
297	* Simple linked list nodes to record waiting threads in a Treiber
298	* stack. See other classes such as Phaser and SynchronousQueue
299	* for more detailed explanation.
300	*/
301	static final class WaitNode {
302	volatile Thread thread;
303	volatile WaitNode next;
304	WaitNode() { thread = Thread.currentThread(); }
305	}
306
307	/**
308	* Removes and signals all waiting threads.
309	*/
310	private void releaseAll() {
311	WaitNode q;
312	while ((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	return;
323	q.next = null; // unlink to help gc
324	q = next;
325	}
326	}
327	}
328	}
329
330	/**
331	* Awaits completion or aborts on interrupt or timeout.
332	*
333	* @param timed true if use timed waits
334	* @param nanos time to wait, if timed
335	* @return state upon completion
336	*/
337	private int awaitDone(boolean timed, long nanos)
338	throws InterruptedException {
339	long last = timed ? System.nanoTime() : 0L;
340	WaitNode q = null;
341	boolean queued = false;
342	for (;;) {
343	if (Thread.interrupted()) {
344	removeWaiter(q);
345	throw new InterruptedException();
346	}
347
348	int s = state;
349	if (s > COMPLETING) {
350	if (q != null)
351	q.thread = null;
352	return s;
353	}
354	else if (q == null)
355	q = new WaitNode();
356	else if (!queued)
357	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
358	q.next = waiters, q);
359	else if (timed) {
360	long now = System.nanoTime();
361	if ((nanos -= (now - last)) <= 0L) {
362	removeWaiter(q);
363	return state;
364	}
365	last = now;
366	LockSupport.parkNanos(this, nanos);
367	}
368	else
369	LockSupport.park(this);
370	}
371	}
372
373	/**
374	* Tries to unlink a timed-out or interrupted wait node to avoid
375	* accumulating garbage. Internal nodes are simply unspliced
376	* without CAS since it is harmless if they are traversed anyway
377	* by releasers. To avoid effects of unsplicing from already
378	* removed nodes, the list is retraversed until no cancelled nodes
379	* are found. This is slow when there are a lot of nodes, but we
380	* don't expect lists to be long enough to outweigh
381	* higher-overhead schemes.
382	*/
383	private void removeWaiter(WaitNode node) {
384	if (node != null) {
385	node.thread = null;
386	for (WaitNode pred = null, q = waiters; q != null;) {
387	WaitNode next = q.next;
388	if (q.thread != null) {
389	pred = q;
390	q = next;
391	}
392	else {
393	if (pred != null)
394	pred.next = next;
395	else
396	UNSAFE.compareAndSwapObject(this, waitersOffset,
397	q, next);
398	pred = null; // restart until clean sweep
399	q = waiters;
400	}
401	}
402	}
403	}
404
405	// Unsafe mechanics
406	private static final sun.misc.Unsafe UNSAFE;
407	private static final long stateOffset;
408	private static final long runnerOffset;
409	private static final long waitersOffset;
410	static {
411	try {
412	UNSAFE = sun.misc.Unsafe.getUnsafe();
413	Class<?> k = FutureTask.class;
414	stateOffset = UNSAFE.objectFieldOffset
415	(k.getDeclaredField("state"));
416	runnerOffset = UNSAFE.objectFieldOffset
417	(k.getDeclaredField("runner"));
418	waitersOffset = UNSAFE.objectFieldOffset
419	(k.getDeclaredField("waiters"));
420	} catch (Exception e) {
421	throw new Error(e);
422	}
423	}
424
425	}