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 ordered and set to powers of two 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    = 0x00;
    private static final int COMPLETING   = 0x01;
    private static final int NORMAL       = 0x02;
    private static final int EXCEPTIONAL  = 0x04;
    private static final int CANCELLED    = 0x08;
    private static final int INTERRUPTING = 0x10;
    private static final int INTERRUPTED  = 0x20;

    /** 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:	1.69
Committed:	Fri Jun 17 22:45:07 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.68:	+39 -28 lines
Log Message:	introduce INTERRUPTING->INTERRUPTED state transition
#	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 ordered and set to powers of two to simplify
54	* checks.
55	*
56	* Possible state transitions:
57	* UNDECIDED -> COMPLETING -> NORMAL
58	* UNDECIDED -> COMPLETING -> EXCEPTIONAL
59	* UNDECIDED -> CANCELLED
60	* UNDECIDED -> INTERRUPTING -> INTERRUPTED
61	*/
62	private volatile int state;
63	private static final int UNDECIDED = 0x00;
64	private static final int COMPLETING = 0x01;
65	private static final int NORMAL = 0x02;
66	private static final int EXCEPTIONAL = 0x04;
67	private static final int CANCELLED = 0x08;
68	private static final int INTERRUPTING = 0x10;
69	private static final int INTERRUPTED = 0x20;
70
71	/** The underlying callable */
72	private final 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 UNDECIDED 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, UNDECIDED, next))
93	return false;
94	if (next == INTERRUPTING) {
95	Thread t = runner; // recheck to avoid leaked interrupt
96	if (t != null)
97	t.interrupt();
98	state = INTERRUPTED;
99	}
100	else if (next == COMPLETING) {
101	outcome = x;
102	state = mode;
103	}
104	if (waiters != null)
105	releaseAll();
106	done();
107	return true;
108	}
109
110	/**
111	* Returns result or throws exception for completed task.
112	*
113	* @param s completed state value
114	*/
115	private V report(int s) throws ExecutionException {
116	Object x = outcome;
117	if (s == NORMAL)
118	return (V)x;
119	if (s >= CANCELLED)
120	throw new CancellationException();
121	throw new ExecutionException((Throwable)x);
122	}
123
124	/**
125	* Creates a {@code FutureTask} that will, upon running, execute the
126	* given {@code Callable}.
127	*
128	* @param callable the callable task
129	* @throws NullPointerException if callable is null
130	*/
131	public FutureTask(Callable<V> callable) {
132	if (callable == null)
133	throw new NullPointerException();
134	this.callable = callable;
135	}
136
137	/**
138	* Creates a {@code FutureTask} that will, upon running, execute the
139	* given {@code Runnable}, and arrange that {@code get} will return the
140	* given result on successful completion.
141	*
142	* @param runnable the runnable task
143	* @param result the result to return on successful completion. If
144	* you don't need a particular result, consider using
145	* constructions of the form:
146	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
147	* @throws NullPointerException if runnable is null
148	*/
149	public FutureTask(Runnable runnable, V result) {
150	this.callable = Executors.callable(runnable, result);
151	}
152
153	public boolean isCancelled() {
154	return state >= CANCELLED;
155	}
156
157	public boolean isDone() {
158	return state != UNDECIDED;
159	}
160
161	public boolean cancel(boolean mayInterruptIfRunning) {
162	return state == UNDECIDED &&
163	setCompletion(null, (mayInterruptIfRunning && runner != null) ?
164	INTERRUPTING : CANCELLED);
165	}
166
167	/**
168	* @throws CancellationException {@inheritDoc}
169	*/
170	public V get() throws InterruptedException, ExecutionException {
171	int s = state;
172	if (s <= COMPLETING)
173	s = awaitDone(false, 0L);
174	return report(s);
175	}
176
177	/**
178	* @throws CancellationException {@inheritDoc}
179	*/
180	public V get(long timeout, TimeUnit unit)
181	throws InterruptedException, ExecutionException, TimeoutException {
182	int s = state;
183	if (s <= COMPLETING &&
184	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
185	throw new TimeoutException();
186	return report(s);
187	}
188
189	/**
190	* Protected method invoked when this task transitions to state
191	* {@code isDone} (whether normally or via cancellation). The
192	* default implementation does nothing. Subclasses may override
193	* this method to invoke completion callbacks or perform
194	* bookkeeping. Note that you can query status inside the
195	* implementation of this method to determine whether this task
196	* has been cancelled.
197	*/
198	protected void done() { }
199
200	/**
201	* Sets the result of this future to the given value unless
202	* this future has already been set or has been cancelled.
203	*
204	* <p>This method is invoked internally by the {@link #run} method
205	* upon successful completion of the computation.
206	*
207	* @param v the value
208	*/
209	protected void set(V v) {
210	setCompletion(v, NORMAL);
211	}
212
213	/**
214	* Causes this future to report an {@link ExecutionException}
215	* with the given throwable as its cause, unless this future has
216	* already been set or has been cancelled.
217	*
218	* <p>This method is invoked internally by the {@link #run} method
219	* upon failure of the computation.
220	*
221	* @param t the cause of failure
222	*/
223	protected void setException(Throwable t) {
224	setCompletion(t, EXCEPTIONAL);
225	}
226
227	public void run() {
228	if (state != UNDECIDED \|\|
229	!UNSAFE.compareAndSwapObject(this, runnerOffset,
230	null, Thread.currentThread()))
231	return;
232
233	try {
234	V result;
235	try {
236	result = callable.call();
237	} catch (Throwable ex) {
238	setException(ex);
239	return;
240	}
241	set(result);
242	} finally {
243	runner = null;
244	int s = state;
245	if (s >= INTERRUPTING) {
246	while ((s = state) == INTERRUPTING)
247	Thread.yield(); // wait out pending cancellation interrupt
248	Thread.interrupted(); // clear any interrupt from cancel(true)
249	}
250	}
251	}
252
253	/**
254	* Executes the computation without setting its result, and then
255	* resets this future to initial state, failing to do so if the
256	* computation encounters an exception or is cancelled. This is
257	* designed for use with tasks that intrinsically execute more
258	* than once.
259	*
260	* @return true if successfully run and reset
261	*/
262	protected boolean runAndReset() {
263	if (state != UNDECIDED \|\|
264	!UNSAFE.compareAndSwapObject(this, runnerOffset,
265	null, Thread.currentThread()))
266	return false;
267
268	try {
269	try {
270	callable.call(); // don't set result
271	return (state == UNDECIDED);
272	} catch (Throwable ex) {
273	setException(ex);
274	return false;
275	}
276	} finally {
277	runner = null;
278	int s = state;
279	if (s >= INTERRUPTING) {
280	while ((s = state) == INTERRUPTING)
281	Thread.yield(); // wait out pending cancellation interrupt
282	Thread.interrupted(); // clear any interrupt from cancel(true)
283	}
284	}
285	}
286
287	/**
288	* Simple linked list nodes to record waiting threads in a Treiber
289	* stack. See other classes such as Phaser and SynchronousQueue
290	* for more detailed explanation.
291	*/
292	static final class WaitNode {
293	volatile Thread thread;
294	WaitNode next;
295	}
296
297	/**
298	* Removes and signals all waiting threads.
299	*/
300	private void releaseAll() {
301	WaitNode q;
302	while ((q = waiters) != null) {
303	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
304	for (;;) {
305	Thread t = q.thread;
306	if (t != null) {
307	q.thread = null;
308	LockSupport.unpark(t);
309	}
310	WaitNode next = q.next;
311	if (next == null)
312	return;
313	q.next = null; // unlink to help gc
314	q = next;
315	}
316	}
317	}
318	}
319
320	/**
321	* Awaits completion or aborts on interrupt or timeout.
322	*
323	* @param timed true if use timed waits
324	* @param nanos time to wait, if timed
325	* @return state upon completion
326	*/
327	private int awaitDone(boolean timed, long nanos)
328	throws InterruptedException {
329	long last = timed ? System.nanoTime() : 0L;
330	WaitNode q = null;
331	boolean queued = false;
332	for (;;) {
333	if (Thread.interrupted()) {
334	removeWaiter(q);
335	throw new InterruptedException();
336	}
337
338	int s = state;
339	if (s > COMPLETING) {
340	if (q != null)
341	q.thread = null;
342	return s;
343	}
344	else if (q == null)
345	q = new WaitNode();
346	else if (!queued)
347	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
348	q.next = waiters, q);
349	else if (q.thread == null)
350	q.thread = Thread.currentThread();
351	else if (timed) {
352	long now = System.nanoTime();
353	if ((nanos -= (now - last)) <= 0L) {
354	removeWaiter(q);
355	return state;
356	}
357	last = now;
358	LockSupport.parkNanos(this, nanos);
359	}
360	else
361	LockSupport.park(this);
362	}
363	}
364
365	/**
366	* Tries to unlink a timed-out or interrupted wait node to avoid
367	* accumulating garbage. Internal nodes are simply unspliced
368	* without CAS since it is harmless if they are traversed anyway
369	* by releasers or concurrent calls to removeWaiter.
370	*/
371	private void removeWaiter(WaitNode node) {
372	if (node != null) {
373	node.thread = null;
374	WaitNode pred = null;
375	WaitNode q = waiters;
376	while (q != null) {
377	WaitNode next = node.next;
378	if (q != node) {
379	pred = q;
380	q = next;
381	}
382	else if (pred != null) {
383	pred.next = next;
384	break;
385	}
386	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
387	q, next))
388	break;
389	else { // restart on CAS failure
390	pred = null;
391	q = waiters;
392	}
393	}
394	}
395	}
396
397	// Unsafe mechanics
398	private static final sun.misc.Unsafe UNSAFE;
399	private static final long stateOffset;
400	private static final long runnerOffset;
401	private static final long waitersOffset;
402	static {
403	try {
404	UNSAFE = sun.misc.Unsafe.getUnsafe();
405	Class<?> k = FutureTask.class;
406	stateOffset = UNSAFE.objectFieldOffset
407	(k.getDeclaredField("state"));
408	runnerOffset = UNSAFE.objectFieldOffset
409	(k.getDeclaredField("runner"));
410	waitersOffset = UNSAFE.objectFieldOffset
411	(k.getDeclaredField("waiters"));
412	} catch (Exception e) {
413	throw new Error(e);
414	}
415	}
416
417	}