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 UNDECIDED.  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 UNDECIDED    = 0x00;
    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 underlying callable */
    private final 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 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 transition from UNDECIDED 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 = state;
            if (s == UNDECIDED) {
                if (UNSAFE.compareAndSwapInt(this, stateOffset,
                                             UNDECIDED, 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 pending cancellation interrupt
            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 {@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;
    }

    /**
     * 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);
    }

    public boolean isCancelled() {
        return (state & (CANCELLED | INTERRUPTING)) != 0;
    }

    public boolean isDone() {
        return state != UNDECIDED;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return state == UNDECIDED &&
            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 == UNDECIDED &&
            UNSAFE.compareAndSwapObject(this, runnerOffset,
                                        null, Thread.currentThread())) {
            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() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        try {
            callable.call(); // don't set result
        } catch (Throwable ex) {
            setException(ex);
            return false;
        }
        runner = null;
        for (;;) {
            int s = state;
            if (s == UNDECIDED)
                return true;
            if (s != INTERRUPTING)
                return false;
            Thread.yield(); // wait out pending cancellation interrupt
        }
    }

    /**
     * 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 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 (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);
        }
    }

    /**
     * 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 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:	1.67
Committed:	Fri Jun 17 21:07:12 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.66:	+14 -12 lines
Log Message:	reintroduce a constant, UNDECIDED, for state 0
#	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 UNDECIDED. The run state
49	* transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
50	* method setCompletion. During setCompletion, state may take on
51	* transient values of COMPLETING (while outcome is being set) or
52	* INTERRUPTING (while interrupting the runner). State values are
53	* ordered and set to powers of two to simplify checks.
54	*/
55	private volatile int state;
56	private static final int UNDECIDED = 0x00;
57	private static final int COMPLETING = 0x01;
58	private static final int INTERRUPTING = 0x02;
59	private static final int NORMAL = 0x04;
60	private static final int EXCEPTIONAL = 0x08;
61	private static final int CANCELLED = 0x10;
62
63	/** The underlying callable */
64	private final Callable<V> callable;
65	/** The result to return or exception to throw from get() */
66	private Object outcome; // non-volatile, protected by state reads/writes
67	/** The thread running the callable; CASed during run() */
68	private volatile Thread runner;
69	/** Treiber stack of waiting threads */
70	private volatile WaitNode waiters;
71
72	/**
73	* Sets completion status, unless already completed. If
74	* necessary, we first set state to COMPLETING or INTERRUPTING to
75	* establish precedence. This intentionally stalls (just via
76	* yields) in (uncommon) cases of concurrent calls during
77	* cancellation until state is set, to avoid surprising users
78	* during cancellation races.
79	*
80	* @param x the outcome
81	* @param mode the completion state value
82	* @return true if this call caused transition from UNDECIDED to completed
83	*/
84	private boolean setCompletion(Object x, int mode) {
85	Thread r = runner;
86	if (r == Thread.currentThread()) // null out runner on completion
87	UNSAFE.putObject(this, runnerOffset, r = null); // nonvolatile OK
88	int next = ((mode == INTERRUPTING) ? // set up transient states
89	(r != null) ? INTERRUPTING : CANCELLED :
90	(x != null) ? COMPLETING : mode);
91	for (;;) {
92	int s = state;
93	if (s == UNDECIDED) {
94	if (UNSAFE.compareAndSwapInt(this, stateOffset,
95	UNDECIDED, next)) {
96	if (next == INTERRUPTING) {
97	Thread t = runner; // recheck
98	if (t != null)
99	t.interrupt();
100	state = CANCELLED;
101	}
102	else if (next == COMPLETING) {
103	outcome = x;
104	state = mode;
105	}
106	if (waiters != null)
107	releaseAll();
108	done();
109	return true;
110	}
111	}
112	else if (s == INTERRUPTING)
113	Thread.yield(); // wait out pending cancellation interrupt
114	else
115	return false;
116	}
117	}
118
119	/**
120	* Returns result or throws exception for completed task.
121	*
122	* @param s completed state value
123	*/
124	private V report(int s) throws ExecutionException {
125	Object x = outcome;
126	if (s == NORMAL)
127	return (V)x;
128	if ((s & (CANCELLED \| INTERRUPTING)) != 0)
129	throw new CancellationException();
130	throw new ExecutionException((Throwable)x);
131	}
132
133	/**
134	* Creates a {@code FutureTask} that will, upon running, execute the
135	* given {@code Callable}.
136	*
137	* @param callable the callable task
138	* @throws NullPointerException if callable is null
139	*/
140	public FutureTask(Callable<V> callable) {
141	if (callable == null)
142	throw new NullPointerException();
143	this.callable = callable;
144	}
145
146	/**
147	* Creates a {@code FutureTask} that will, upon running, execute the
148	* given {@code Runnable}, and arrange that {@code get} will return the
149	* given result on successful completion.
150	*
151	* @param runnable the runnable task
152	* @param result the result to return on successful completion. If
153	* you don't need a particular result, consider using
154	* constructions of the form:
155	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
156	* @throws NullPointerException if runnable is null
157	*/
158	public FutureTask(Runnable runnable, V result) {
159	this.callable = Executors.callable(runnable, result);
160	}
161
162	public boolean isCancelled() {
163	return (state & (CANCELLED \| INTERRUPTING)) != 0;
164	}
165
166	public boolean isDone() {
167	return state != UNDECIDED;
168	}
169
170	public boolean cancel(boolean mayInterruptIfRunning) {
171	return state == UNDECIDED &&
172	setCompletion(null, mayInterruptIfRunning ?
173	INTERRUPTING : CANCELLED);
174	}
175
176	/**
177	* @throws CancellationException {@inheritDoc}
178	*/
179	public V get() throws InterruptedException, ExecutionException {
180	int s = state;
181	if (s <= COMPLETING)
182	s = awaitDone(false, 0L);
183	return report(s);
184	}
185
186	/**
187	* @throws CancellationException {@inheritDoc}
188	*/
189	public V get(long timeout, TimeUnit unit)
190	throws InterruptedException, ExecutionException, TimeoutException {
191	int s = state;
192	if (s <= COMPLETING &&
193	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
194	throw new TimeoutException();
195	return report(s);
196	}
197
198	/**
199	* Protected method invoked when this task transitions to state
200	* {@code isDone} (whether normally or via cancellation). The
201	* default implementation does nothing. Subclasses may override
202	* this method to invoke completion callbacks or perform
203	* bookkeeping. Note that you can query status inside the
204	* implementation of this method to determine whether this task
205	* has been cancelled.
206	*/
207	protected void done() { }
208
209	/**
210	* Sets the result of this future to the given value unless
211	* this future has already been set or has been cancelled.
212	*
213	* <p>This method is invoked internally by the {@link #run} method
214	* upon successful completion of the computation.
215	*
216	* @param v the value
217	*/
218	protected void set(V v) {
219	setCompletion(v, NORMAL);
220	}
221
222	/**
223	* Causes this future to report an {@link ExecutionException}
224	* with the given throwable as its cause, unless this future has
225	* already been set or has been cancelled.
226	*
227	* <p>This method is invoked internally by the {@link #run} method
228	* upon failure of the computation.
229	*
230	* @param t the cause of failure
231	*/
232	protected void setException(Throwable t) {
233	setCompletion(t, EXCEPTIONAL);
234	}
235
236	public void run() {
237	if (state == UNDECIDED &&
238	UNSAFE.compareAndSwapObject(this, runnerOffset,
239	null, Thread.currentThread())) {
240	V result;
241	try {
242	result = callable.call();
243	} catch (Throwable ex) {
244	setException(ex);
245	return;
246	}
247	set(result);
248	}
249	}
250
251	/**
252	* Executes the computation without setting its result, and then
253	* resets this future to initial state, failing to do so if the
254	* computation encounters an exception or is cancelled. This is
255	* designed for use with tasks that intrinsically execute more
256	* than once.
257	*
258	* @return true if successfully run and reset
259	*/
260	protected boolean runAndReset() {
261	if (state != UNDECIDED \|\|
262	!UNSAFE.compareAndSwapObject(this, runnerOffset,
263	null, Thread.currentThread()))
264	return false;
265	try {
266	callable.call(); // don't set result
267	} catch (Throwable ex) {
268	setException(ex);
269	return false;
270	}
271	runner = null;
272	for (;;) {
273	int s = state;
274	if (s == UNDECIDED)
275	return true;
276	if (s != INTERRUPTING)
277	return false;
278	Thread.yield(); // wait out pending cancellation interrupt
279	}
280	}
281
282	/**
283	* Simple linked list nodes to record waiting threads in a Treiber
284	* stack. See other classes such as Phaser and SynchronousQueue
285	* for more detailed explanation.
286	*/
287	static final class WaitNode {
288	volatile Thread thread;
289	WaitNode next;
290	}
291
292	/**
293	* Removes and signals all waiting threads.
294	*/
295	private void releaseAll() {
296	WaitNode q;
297	while ((q = waiters) != null) {
298	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
299	for (;;) {
300	Thread t = q.thread;
301	if (t != null) {
302	q.thread = null;
303	LockSupport.unpark(t);
304	}
305	WaitNode next = q.next;
306	if (next == null)
307	return;
308	q.next = null; // unlink to help gc
309	q = next;
310	}
311	}
312	}
313	}
314
315	/**
316	* Awaits completion or aborts on interrupt or timeout.
317	*
318	* @param timed true if use timed waits
319	* @param nanos time to wait, if timed
320	* @return state upon completion
321	*/
322	private int awaitDone(boolean timed, long nanos)
323	throws InterruptedException {
324	long last = timed ? System.nanoTime() : 0L;
325	WaitNode q = null;
326	boolean queued = false;
327	for (;;) {
328	if (Thread.interrupted()) {
329	removeWaiter(q);
330	throw new InterruptedException();
331	}
332
333	int s = state;
334	if (s > COMPLETING) {
335	if (q != null)
336	q.thread = null;
337	return s;
338	}
339	else if (q == null)
340	q = new WaitNode();
341	else if (!queued)
342	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
343	q.next = waiters, q);
344	else if (q.thread == null)
345	q.thread = Thread.currentThread();
346	else if (timed) {
347	long now = System.nanoTime();
348	if ((nanos -= (now - last)) <= 0L) {
349	removeWaiter(q);
350	return state;
351	}
352	last = now;
353	LockSupport.parkNanos(this, nanos);
354	}
355	else
356	LockSupport.park(this);
357	}
358	}
359
360	/**
361	* Tries to unlink a timed-out or interrupted wait node to avoid
362	* accumulating garbage. Internal nodes are simply unspliced
363	* without CAS since it is harmless if they are traversed anyway
364	* by releasers or concurrent calls to removeWaiter.
365	*/
366	private void removeWaiter(WaitNode node) {
367	if (node != null) {
368	node.thread = null;
369	WaitNode pred = null;
370	WaitNode q = waiters;
371	while (q != null) {
372	WaitNode next = node.next;
373	if (q != node) {
374	pred = q;
375	q = next;
376	}
377	else if (pred != null) {
378	pred.next = next;
379	break;
380	}
381	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
382	q, next))
383	break;
384	else { // restart on CAS failure
385	pred = null;
386	q = waiters;
387	}
388	}
389	}
390	}
391
392	// Unsafe mechanics
393	private static final sun.misc.Unsafe UNSAFE;
394	private static final long stateOffset;
395	private static final long runnerOffset;
396	private static final long waitersOffset;
397	static {
398	try {
399	UNSAFE = sun.misc.Unsafe.getUnsafe();
400	Class<?> k = FutureTask.class;
401	stateOffset = UNSAFE.objectFieldOffset
402	(k.getDeclaredField("state"));
403	runnerOffset = UNSAFE.objectFieldOffset
404	(k.getDeclaredField("runner"));
405	waitersOffset = UNSAFE.objectFieldOffset
406	(k.getDeclaredField("waiters"));
407	} catch (Exception e) {
408	throw new Error(e);
409	}
410	}
411
412	}