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 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:
     * UNDECIDED -> COMPLETING -> NORMAL
     * UNDECIDED -> COMPLETING -> EXCEPTIONAL
     * UNDECIDED -> CANCELLED
     * UNDECIDED -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int UNDECIDED    = 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 */
    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 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 UNDECIDED to completed
     */
    private boolean setCompletion(Object x, int mode) {
        // set up transient states
        int next = (x != null) ? COMPLETING : mode;
        if (!UNSAFE.compareAndSwapInt(this, stateOffset, UNDECIDED, next))
            return false;
        if (next == INTERRUPTING) {
            Thread t = runner; // recheck to avoid leaked interrupt
            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;
    }

    /**
     * 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;
    }

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

    public boolean cancel(boolean mayInterruptIfRunning) {
        return state == UNDECIDED &&
            setCompletion(null, (mayInterruptIfRunning && runner != null) ?
                          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()))
            return;

        try {
            V result;
            try {
                result = callable.call();
            } catch (Throwable ex) {
                setException(ex);
                return;
            }
            set(result);
        } finally {
            runner = null;
            int s = state;
            if (s >= INTERRUPTING) {
                while ((s = state) == INTERRUPTING)
                    Thread.yield(); // wait out pending cancellation interrupt
                Thread.interrupted(); // clear any 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() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;

        try {
            try {
                callable.call(); // don't set result
                return (state == UNDECIDED);
            } catch (Throwable ex) {
                setException(ex);
                return false;
            }
        } finally {
            runner = null;
            int s = state;
            if (s >= INTERRUPTING) {
                while ((s = state) == INTERRUPTING)
                    Thread.yield(); // wait out pending cancellation interrupt
                Thread.interrupted(); // clear any interrupt from cancel(true)
            }
        }
    }

    /**
     * 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:	1.70
Committed:	Fri Jun 17 22:54:47 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.69:	+8 -9 lines
Log Message:	use ordinary ordered integers for state values
#	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 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	* UNDECIDED -> COMPLETING -> NORMAL
57	* UNDECIDED -> COMPLETING -> EXCEPTIONAL
58	* UNDECIDED -> CANCELLED
59	* UNDECIDED -> INTERRUPTING -> INTERRUPTED
60	*/
61	private volatile int state;
62	private static final int UNDECIDED = 0;
63	private static final int COMPLETING = 1;
64	private static final int NORMAL = 2;
65	private static final int EXCEPTIONAL = 3;
66	private static final int CANCELLED = 4;
67	private static final int INTERRUPTING = 5;
68	private static final int INTERRUPTED = 6;
69
70	/** The underlying callable */
71	private final Callable<V> callable;
72	/** The result to return or exception to throw from get() */
73	private Object outcome; // non-volatile, protected by state reads/writes
74	/** The thread running the callable; CASed during run() */
75	private volatile Thread runner;
76	/** Treiber stack of waiting threads */
77	private volatile WaitNode waiters;
78
79	/**
80	* Sets completion status, unless already completed. If
81	* necessary, we first set state to transient states COMPLETING
82	* or INTERRUPTING to establish precedence.
83	*
84	* @param x the outcome
85	* @param mode the completion state value
86	* @return true if this call caused transition from UNDECIDED to completed
87	*/
88	private boolean setCompletion(Object x, int mode) {
89	// set up transient states
90	int next = (x != null) ? COMPLETING : mode;
91	if (!UNSAFE.compareAndSwapInt(this, stateOffset, UNDECIDED, next))
92	return false;
93	if (next == INTERRUPTING) {
94	Thread t = runner; // recheck to avoid leaked interrupt
95	if (t != null)
96	t.interrupt();
97	state = INTERRUPTED;
98	}
99	else if (next == COMPLETING) {
100	outcome = x;
101	state = mode;
102	}
103	if (waiters != null)
104	releaseAll();
105	done();
106	return true;
107	}
108
109	/**
110	* Returns result or throws exception for completed task.
111	*
112	* @param s completed state value
113	*/
114	private V report(int s) throws ExecutionException {
115	Object x = outcome;
116	if (s == NORMAL)
117	return (V)x;
118	if (s >= CANCELLED)
119	throw new CancellationException();
120	throw new ExecutionException((Throwable)x);
121	}
122
123	/**
124	* Creates a {@code FutureTask} that will, upon running, execute the
125	* given {@code Callable}.
126	*
127	* @param callable the callable task
128	* @throws NullPointerException if callable is null
129	*/
130	public FutureTask(Callable<V> callable) {
131	if (callable == null)
132	throw new NullPointerException();
133	this.callable = callable;
134	}
135
136	/**
137	* Creates a {@code FutureTask} that will, upon running, execute the
138	* given {@code Runnable}, and arrange that {@code get} will return the
139	* given result on successful completion.
140	*
141	* @param runnable the runnable task
142	* @param result the result to return on successful completion. If
143	* you don't need a particular result, consider using
144	* constructions of the form:
145	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
146	* @throws NullPointerException if runnable is null
147	*/
148	public FutureTask(Runnable runnable, V result) {
149	this.callable = Executors.callable(runnable, result);
150	}
151
152	public boolean isCancelled() {
153	return state >= CANCELLED;
154	}
155
156	public boolean isDone() {
157	return state != UNDECIDED;
158	}
159
160	public boolean cancel(boolean mayInterruptIfRunning) {
161	return state == UNDECIDED &&
162	setCompletion(null, (mayInterruptIfRunning && runner != null) ?
163	INTERRUPTING : CANCELLED);
164	}
165
166	/**
167	* @throws CancellationException {@inheritDoc}
168	*/
169	public V get() throws InterruptedException, ExecutionException {
170	int s = state;
171	if (s <= COMPLETING)
172	s = awaitDone(false, 0L);
173	return report(s);
174	}
175
176	/**
177	* @throws CancellationException {@inheritDoc}
178	*/
179	public V get(long timeout, TimeUnit unit)
180	throws InterruptedException, ExecutionException, TimeoutException {
181	int s = state;
182	if (s <= COMPLETING &&
183	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
184	throw new TimeoutException();
185	return report(s);
186	}
187
188	/**
189	* Protected method invoked when this task transitions to state
190	* {@code isDone} (whether normally or via cancellation). The
191	* default implementation does nothing. Subclasses may override
192	* this method to invoke completion callbacks or perform
193	* bookkeeping. Note that you can query status inside the
194	* implementation of this method to determine whether this task
195	* has been cancelled.
196	*/
197	protected void done() { }
198
199	/**
200	* Sets the result of this future to the given value unless
201	* this future has already been set or has been cancelled.
202	*
203	* <p>This method is invoked internally by the {@link #run} method
204	* upon successful completion of the computation.
205	*
206	* @param v the value
207	*/
208	protected void set(V v) {
209	setCompletion(v, NORMAL);
210	}
211
212	/**
213	* Causes this future to report an {@link ExecutionException}
214	* with the given throwable as its cause, unless this future has
215	* already been set or has been cancelled.
216	*
217	* <p>This method is invoked internally by the {@link #run} method
218	* upon failure of the computation.
219	*
220	* @param t the cause of failure
221	*/
222	protected void setException(Throwable t) {
223	setCompletion(t, EXCEPTIONAL);
224	}
225
226	public void run() {
227	if (state != UNDECIDED \|\|
228	!UNSAFE.compareAndSwapObject(this, runnerOffset,
229	null, Thread.currentThread()))
230	return;
231
232	try {
233	V result;
234	try {
235	result = callable.call();
236	} catch (Throwable ex) {
237	setException(ex);
238	return;
239	}
240	set(result);
241	} finally {
242	runner = null;
243	int s = state;
244	if (s >= INTERRUPTING) {
245	while ((s = state) == INTERRUPTING)
246	Thread.yield(); // wait out pending cancellation interrupt
247	Thread.interrupted(); // clear any interrupt from cancel(true)
248	}
249	}
250	}
251
252	/**
253	* Executes the computation without setting its result, and then
254	* resets this future to initial state, failing to do so if the
255	* computation encounters an exception or is cancelled. This is
256	* designed for use with tasks that intrinsically execute more
257	* than once.
258	*
259	* @return true if successfully run and reset
260	*/
261	protected boolean runAndReset() {
262	if (state != UNDECIDED \|\|
263	!UNSAFE.compareAndSwapObject(this, runnerOffset,
264	null, Thread.currentThread()))
265	return false;
266
267	try {
268	try {
269	callable.call(); // don't set result
270	return (state == UNDECIDED);
271	} catch (Throwable ex) {
272	setException(ex);
273	return false;
274	}
275	} finally {
276	runner = null;
277	int s = state;
278	if (s >= INTERRUPTING) {
279	while ((s = state) == INTERRUPTING)
280	Thread.yield(); // wait out pending cancellation interrupt
281	Thread.interrupted(); // clear any interrupt from cancel(true)
282	}
283	}
284	}
285
286	/**
287	* Simple linked list nodes to record waiting threads in a Treiber
288	* stack. See other classes such as Phaser and SynchronousQueue
289	* for more detailed explanation.
290	*/
291	static final class WaitNode {
292	volatile Thread thread;
293	WaitNode next;
294	}
295
296	/**
297	* Removes and signals all waiting threads.
298	*/
299	private void releaseAll() {
300	WaitNode q;
301	while ((q = waiters) != null) {
302	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
303	for (;;) {
304	Thread t = q.thread;
305	if (t != null) {
306	q.thread = null;
307	LockSupport.unpark(t);
308	}
309	WaitNode next = q.next;
310	if (next == null)
311	return;
312	q.next = null; // unlink to help gc
313	q = next;
314	}
315	}
316	}
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 (q.thread == null)
349	q.thread = Thread.currentThread();
350	else if (timed) {
351	long now = System.nanoTime();
352	if ((nanos -= (now - last)) <= 0L) {
353	removeWaiter(q);
354	return state;
355	}
356	last = now;
357	LockSupport.parkNanos(this, nanos);
358	}
359	else
360	LockSupport.park(this);
361	}
362	}
363
364	/**
365	* Tries to unlink a timed-out or interrupted wait node to avoid
366	* accumulating garbage. Internal nodes are simply unspliced
367	* without CAS since it is harmless if they are traversed anyway
368	* by releasers or concurrent calls to removeWaiter.
369	*/
370	private void removeWaiter(WaitNode node) {
371	if (node != null) {
372	node.thread = null;
373	WaitNode pred = null;
374	WaitNode q = waiters;
375	while (q != null) {
376	WaitNode next = node.next;
377	if (q != node) {
378	pred = q;
379	q = next;
380	}
381	else if (pred != null) {
382	pred.next = next;
383	break;
384	}
385	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
386	q, next))
387	break;
388	else { // restart on CAS failure
389	pred = null;
390	q = waiters;
391	}
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	}