util/concurrent/FutureTask.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;

/**
 * A cancellable asynchronous computation.  This class provides a base
 * implementation of {@link Future}, with methods to start and cancel
 * a computation, query to see if the computation is complete, and
 * retrieve the result of the computation.  The result can only be
 * retrieved when the computation has completed; the {@code get}
 * methods will block if the computation has not yet completed.  Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled (unless the computation is invoked using
 * {@link #runAndReset}).
 *
 * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
 * {@link Runnable} object.  Because {@code FutureTask} implements
 * {@code Runnable}, a {@code FutureTask} can be submitted to an
 * {@link Executor} for execution.
 *
 * <p>In addition to serving as a standalone class, this class provides
 * {@code protected} functionality that may be useful when creating
 * customized task classes.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this FutureTask's {@code get} methods
 */
public class FutureTask<V> implements RunnableFuture<V> {
    /*
     * Revision notes: This differs from previous versions of this
     * class that relied on AbstractQueuedSynchronizer, mainly to
     * avoid surprising users about retaining interrupt status during
     * cancellation races. Sync control in the current design relies
     * on a "state" field updated via CAS to track completion, along
     * with a simple Treiber stack to hold waiting threads.
     *
     * Style note: As usual, we bypass overhead of using
     * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
     */

    /**
     * The run state of this task, initially NEW.  The run state
     * transitions to a terminal state only in methods set,
     * setException, and cancel.  During completion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (only while interrupting the runner to satisfy a
     * cancel(true)). Transitions from these intermediate to final
     * states use cheaper ordered/lazy writes because values are unique
     * and cannot be further modified.
     *
     * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;

    /** The underlying callable; nulled out after running */
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;

    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of 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 the runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

    public boolean isCancelled() {
        return state >= CANCELLED;
    }

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

    public boolean cancel(boolean mayInterruptIfRunning) {
        if (state != NEW)
            return false;
        if (mayInterruptIfRunning) {
            if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
                return false;
            try {       // in case call to interrupt throws exception
                Thread t = runner;
                if (t != null)
                    t.interrupt();
            } finally { // final state
                UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); 
            }
        }
        else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
            return false;
        finishCompletion();
        return true;
    }

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

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        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) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

    /**
     * Causes this future to report an {@link ExecutionException}
     * with the given throwable as its cause, unless this future has
     * already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon failure of the computation.
     *
     * @param t the cause of failure
     */
    protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

    /**
     * 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 != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        boolean ran = false;
        int s = state;
        try {
            Callable<V> c = callable;
            if (c != null && s == NEW) {
                try {
                    c.call(); // don't set result
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
        return ran && s == NEW;
    }

    /**
     * Ensures that any interrupt from a possible cancel(true) is only
     * delivered to a task while in run or runAndReset.
     */
    private void handlePossibleCancellationInterrupt(int s) {
        // It is possible for our interrupter to stall before getting a
        // chance to interrupt us.  Let's spin-wait patiently.
        if (s == INTERRUPTING)
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending interrupt

        // assert state == INTERRUPTED;

        // We want to clear any interrupt we may have received from
        // cancel(true).  However, it is permissible to use interrupts
        // as an independent mechanism for a task to communicate with
        // its caller, and there is no way to clear only the
        // cancellation interrupt.
        //
        // Thread.interrupted();
    }

    /**
     * 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, invokes done(), and
     * nulls out callable.
     */
    private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }

        done();

        callable = null;        // to reduce footprint
    }

    /**
     * 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 {
        final long deadline = timed ? System.nanoTime() + nanos : 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 (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

    /**
     * Tries to unlink a timed-out or interrupted wait node to avoid
     * accumulating garbage.  Internal nodes are simply unspliced
     * without CAS since it is harmless if they are traversed anyway
     * by releasers.  To avoid effects of unsplicing from already
     * removed nodes, the list is retraversed in case of an apparent
     * race.  This is slow when there are a lot of nodes, but we don't
     * expect lists to be long enough to outweigh higher-overhead
     * schemes.
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            retry:
            for (;;) {          // restart on removeWaiter race
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    s = q.next;
                    if (q.thread != null)
                        pred = q;
                    else if (pred != null) {
                        pred.next = s;
                        if (pred.thread == null) // check for race
                            continue retry;
                    }
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
                        continue retry;
                }
                break;
            }
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long stateOffset;
    private static final long runnerOffset;
    private static final long waitersOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = FutureTask.class;
            stateOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("state"));
            runnerOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("runner"));
            waitersOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

}
Revision:	1.100
Committed:	Fri Dec 14 12:29:51 2012 UTC (11 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.99:	+7 -4 lines
Log Message:	Guard against security exceptions
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.LockSupport;
9
10	/**
11	* A cancellable asynchronous computation. This class provides a base
12	* implementation of {@link Future}, with methods to start and cancel
13	* a computation, query to see if the computation is complete, and
14	* retrieve the result of the computation. The result can only be
15	* retrieved when the computation has completed; the {@code get}
16	* methods will block if the computation has not yet completed. Once
17	* the computation has completed, the computation cannot be restarted
18	* or cancelled (unless the computation is invoked using
19	* {@link #runAndReset}).
20	*
21	* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
22	* {@link Runnable} object. Because {@code FutureTask} implements
23	* {@code Runnable}, a {@code FutureTask} can be submitted to an
24	* {@link Executor} for execution.
25	*
26	* <p>In addition to serving as a standalone class, this class provides
27	* {@code protected} functionality that may be useful when creating
28	* customized task classes.
29	*
30	* @since 1.5
31	* @author Doug Lea
32	* @param <V> The result type returned by this FutureTask's {@code get} methods
33	*/
34	public class FutureTask<V> implements RunnableFuture<V> {
35	/*
36	* Revision notes: This differs from previous versions of this
37	* class that relied on AbstractQueuedSynchronizer, mainly to
38	* avoid surprising users about retaining interrupt status during
39	* cancellation races. Sync control in the current design relies
40	* on a "state" field updated via CAS to track completion, along
41	* with a simple Treiber stack to hold waiting threads.
42	*
43	* Style note: As usual, we bypass overhead of using
44	* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
45	*/
46
47	/**
48	* The run state of this task, initially NEW. The run state
49	* transitions to a terminal state only in methods set,
50	* setException, and cancel. During completion, state may take on
51	* transient values of COMPLETING (while outcome is being set) or
52	* INTERRUPTING (only while interrupting the runner to satisfy a
53	* cancel(true)). Transitions from these intermediate to final
54	* states use cheaper ordered/lazy writes because values are unique
55	* and cannot be further modified.
56	*
57	* Possible state transitions:
58	* NEW -> COMPLETING -> NORMAL
59	* NEW -> COMPLETING -> EXCEPTIONAL
60	* NEW -> CANCELLED
61	* NEW -> INTERRUPTING -> INTERRUPTED
62	*/
63	private volatile int state;
64	private static final int NEW = 0;
65	private static final int COMPLETING = 1;
66	private static final int NORMAL = 2;
67	private static final int EXCEPTIONAL = 3;
68	private static final int CANCELLED = 4;
69	private static final int INTERRUPTING = 5;
70	private static final int INTERRUPTED = 6;
71
72	/** The underlying callable; nulled out after running */
73	private Callable<V> callable;
74	/** The result to return or exception to throw from get() */
75	private Object outcome; // non-volatile, protected by state reads/writes
76	/** The thread running the callable; CASed during run() */
77	private volatile Thread runner;
78	/** Treiber stack of waiting threads */
79	private volatile WaitNode waiters;
80
81	/**
82	* Returns result or throws exception for completed task.
83	*
84	* @param s completed state value
85	*/
86	@SuppressWarnings("unchecked")
87	private V report(int s) throws ExecutionException {
88	Object x = outcome;
89	if (s == NORMAL)
90	return (V)x;
91	if (s >= CANCELLED)
92	throw new CancellationException();
93	throw new ExecutionException((Throwable)x);
94	}
95
96	/**
97	* Creates a {@code FutureTask} that will, upon running, execute the
98	* given {@code Callable}.
99	*
100	* @param callable the callable task
101	* @throws NullPointerException if the callable is null
102	*/
103	public FutureTask(Callable<V> callable) {
104	if (callable == null)
105	throw new NullPointerException();
106	this.callable = callable;
107	this.state = NEW; // ensure visibility of callable
108	}
109
110	/**
111	* Creates a {@code FutureTask} that will, upon running, execute the
112	* given {@code Runnable}, and arrange that {@code get} will return the
113	* given result on successful completion.
114	*
115	* @param runnable the runnable task
116	* @param result the result to return on successful completion. If
117	* you don't need a particular result, consider using
118	* constructions of the form:
119	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
120	* @throws NullPointerException if the runnable is null
121	*/
122	public FutureTask(Runnable runnable, V result) {
123	this.callable = Executors.callable(runnable, result);
124	this.state = NEW; // ensure visibility of callable
125	}
126
127	public boolean isCancelled() {
128	return state >= CANCELLED;
129	}
130
131	public boolean isDone() {
132	return state != NEW;
133	}
134
135	public boolean cancel(boolean mayInterruptIfRunning) {
136	if (state != NEW)
137	return false;
138	if (mayInterruptIfRunning) {
139	if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
140	return false;
141	try { // in case call to interrupt throws exception
142	Thread t = runner;
143	if (t != null)
144	t.interrupt();
145	} finally { // final state
146	UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
147	}
148	}
149	else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
150	return false;
151	finishCompletion();
152	return true;
153	}
154
155	/**
156	* @throws CancellationException {@inheritDoc}
157	*/
158	public V get() throws InterruptedException, ExecutionException {
159	int s = state;
160	if (s <= COMPLETING)
161	s = awaitDone(false, 0L);
162	return report(s);
163	}
164
165	/**
166	* @throws CancellationException {@inheritDoc}
167	*/
168	public V get(long timeout, TimeUnit unit)
169	throws InterruptedException, ExecutionException, TimeoutException {
170	if (unit == null)
171	throw new NullPointerException();
172	int s = state;
173	if (s <= COMPLETING &&
174	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
175	throw new TimeoutException();
176	return report(s);
177	}
178
179	/**
180	* Protected method invoked when this task transitions to state
181	* {@code isDone} (whether normally or via cancellation). The
182	* default implementation does nothing. Subclasses may override
183	* this method to invoke completion callbacks or perform
184	* bookkeeping. Note that you can query status inside the
185	* implementation of this method to determine whether this task
186	* has been cancelled.
187	*/
188	protected void done() { }
189
190	/**
191	* Sets the result of this future to the given value unless
192	* this future has already been set or has been cancelled.
193	*
194	* <p>This method is invoked internally by the {@link #run} method
195	* upon successful completion of the computation.
196	*
197	* @param v the value
198	*/
199	protected void set(V v) {
200	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
201	outcome = v;
202	UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
203	finishCompletion();
204	}
205	}
206
207	/**
208	* Causes this future to report an {@link ExecutionException}
209	* with the given throwable as its cause, unless this future has
210	* already been set or has been cancelled.
211	*
212	* <p>This method is invoked internally by the {@link #run} method
213	* upon failure of the computation.
214	*
215	* @param t the cause of failure
216	*/
217	protected void setException(Throwable t) {
218	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
219	outcome = t;
220	UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
221	finishCompletion();
222	}
223	}
224
225	public void run() {
226	if (state != NEW \|\|
227	!UNSAFE.compareAndSwapObject(this, runnerOffset,
228	null, Thread.currentThread()))
229	return;
230	try {
231	Callable<V> c = callable;
232	if (c != null && state == NEW) {
233	V result;
234	boolean ran;
235	try {
236	result = c.call();
237	ran = true;
238	} catch (Throwable ex) {
239	result = null;
240	ran = false;
241	setException(ex);
242	}
243	if (ran)
244	set(result);
245	}
246	} finally {
247	// runner must be non-null until state is settled to
248	// prevent concurrent calls to run()
249	runner = null;
250	// state must be re-read after nulling runner to prevent
251	// leaked interrupts
252	int s = state;
253	if (s >= INTERRUPTING)
254	handlePossibleCancellationInterrupt(s);
255	}
256	}
257
258	/**
259	* Executes the computation without setting its result, and then
260	* resets this future to initial state, failing to do so if the
261	* computation encounters an exception or is cancelled. This is
262	* designed for use with tasks that intrinsically execute more
263	* than once.
264	*
265	* @return true if successfully run and reset
266	*/
267	protected boolean runAndReset() {
268	if (state != NEW \|\|
269	!UNSAFE.compareAndSwapObject(this, runnerOffset,
270	null, Thread.currentThread()))
271	return false;
272	boolean ran = false;
273	int s = state;
274	try {
275	Callable<V> c = callable;
276	if (c != null && s == NEW) {
277	try {
278	c.call(); // don't set result
279	ran = true;
280	} catch (Throwable ex) {
281	setException(ex);
282	}
283	}
284	} finally {
285	// runner must be non-null until state is settled to
286	// prevent concurrent calls to run()
287	runner = null;
288	// state must be re-read after nulling runner to prevent
289	// leaked interrupts
290	s = state;
291	if (s >= INTERRUPTING)
292	handlePossibleCancellationInterrupt(s);
293	}
294	return ran && s == NEW;
295	}
296
297	/**
298	* Ensures that any interrupt from a possible cancel(true) is only
299	* delivered to a task while in run or runAndReset.
300	*/
301	private void handlePossibleCancellationInterrupt(int s) {
302	// It is possible for our interrupter to stall before getting a
303	// chance to interrupt us. Let's spin-wait patiently.
304	if (s == INTERRUPTING)
305	while (state == INTERRUPTING)
306	Thread.yield(); // wait out pending interrupt
307
308	// assert state == INTERRUPTED;
309
310	// We want to clear any interrupt we may have received from
311	// cancel(true). However, it is permissible to use interrupts
312	// as an independent mechanism for a task to communicate with
313	// its caller, and there is no way to clear only the
314	// cancellation interrupt.
315	//
316	// Thread.interrupted();
317	}
318
319	/**
320	* Simple linked list nodes to record waiting threads in a Treiber
321	* stack. See other classes such as Phaser and SynchronousQueue
322	* for more detailed explanation.
323	*/
324	static final class WaitNode {
325	volatile Thread thread;
326	volatile WaitNode next;
327	WaitNode() { thread = Thread.currentThread(); }
328	}
329
330	/**
331	* Removes and signals all waiting threads, invokes done(), and
332	* nulls out callable.
333	*/
334	private void finishCompletion() {
335	// assert state > COMPLETING;
336	for (WaitNode q; (q = waiters) != null;) {
337	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
338	for (;;) {
339	Thread t = q.thread;
340	if (t != null) {
341	q.thread = null;
342	LockSupport.unpark(t);
343	}
344	WaitNode next = q.next;
345	if (next == null)
346	break;
347	q.next = null; // unlink to help gc
348	q = next;
349	}
350	break;
351	}
352	}
353
354	done();
355
356	callable = null; // to reduce footprint
357	}
358
359	/**
360	* Awaits completion or aborts on interrupt or timeout.
361	*
362	* @param timed true if use timed waits
363	* @param nanos time to wait, if timed
364	* @return state upon completion
365	*/
366	private int awaitDone(boolean timed, long nanos)
367	throws InterruptedException {
368	final long deadline = timed ? System.nanoTime() + nanos : 0L;
369	WaitNode q = null;
370	boolean queued = false;
371	for (;;) {
372	if (Thread.interrupted()) {
373	removeWaiter(q);
374	throw new InterruptedException();
375	}
376
377	int s = state;
378	if (s > COMPLETING) {
379	if (q != null)
380	q.thread = null;
381	return s;
382	}
383	else if (s == COMPLETING) // cannot time out yet
384	Thread.yield();
385	else if (q == null)
386	q = new WaitNode();
387	else if (!queued)
388	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
389	q.next = waiters, q);
390	else if (timed) {
391	nanos = deadline - System.nanoTime();
392	if (nanos <= 0L) {
393	removeWaiter(q);
394	return state;
395	}
396	LockSupport.parkNanos(this, nanos);
397	}
398	else
399	LockSupport.park(this);
400	}
401	}
402
403	/**
404	* Tries to unlink a timed-out or interrupted wait node to avoid
405	* accumulating garbage. Internal nodes are simply unspliced
406	* without CAS since it is harmless if they are traversed anyway
407	* by releasers. To avoid effects of unsplicing from already
408	* removed nodes, the list is retraversed in case of an apparent
409	* race. This is slow when there are a lot of nodes, but we don't
410	* expect lists to be long enough to outweigh higher-overhead
411	* schemes.
412	*/
413	private void removeWaiter(WaitNode node) {
414	if (node != null) {
415	node.thread = null;
416	retry:
417	for (;;) { // restart on removeWaiter race
418	for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
419	s = q.next;
420	if (q.thread != null)
421	pred = q;
422	else if (pred != null) {
423	pred.next = s;
424	if (pred.thread == null) // check for race
425	continue retry;
426	}
427	else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
428	q, s))
429	continue retry;
430	}
431	break;
432	}
433	}
434	}
435
436	// Unsafe mechanics
437	private static final sun.misc.Unsafe UNSAFE;
438	private static final long stateOffset;
439	private static final long runnerOffset;
440	private static final long waitersOffset;
441	static {
442	try {
443	UNSAFE = sun.misc.Unsafe.getUnsafe();
444	Class<?> k = FutureTask.class;
445	stateOffset = UNSAFE.objectFieldOffset
446	(k.getDeclaredField("state"));
447	runnerOffset = UNSAFE.objectFieldOffset
448	(k.getDeclaredField("runner"));
449	waitersOffset = UNSAFE.objectFieldOffset
450	(k.getDeclaredField("waiters"));
451	} catch (Exception e) {
452	throw new Error(e);
453	}
454	}
455
456	}