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
     */
    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;
            Thread t = runner;
            if (t != null)
                t.interrupt();
            UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
        }
        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;
        return report((s <= COMPLETING) ? awaitDone(false, 0L) : 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())) {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result = null;
                boolean ran = false;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
            runner = null;
            if (state >= INTERRUPTING) {
                while (state == INTERRUPTING)
                    Thread.yield();   // wait out pending interrupt
                Thread.interrupted(); // clear interrupt from cancel(true)
            }
        }
    }

    /**
     * Executes the computation without setting its result, and then
     * resets this future to initial state, failing to do so if the
     * computation encounters an exception or is cancelled.  This is
     * designed for use with tasks that intrinsically execute more
     * than once.
     *
     * @return true if successfully run and reset
     */
    protected boolean runAndReset() {
        boolean rerun = false; // true if this task can be re-run
        if (state == NEW &&
            UNSAFE.compareAndSwapObject(this, runnerOffset,
                                        null, Thread.currentThread())) {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                try {
                    c.call(); // don't set result
                    rerun = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
            runner = null;
            int s = state;
            if (s != NEW) {
                rerun = false;
                if (s >= INTERRUPTING) {
                    while (state == INTERRUPTING)
                        Thread.yield();   // wait out pending interrupt
                    Thread.interrupted(); // clear interrupt from cancel(true)
                }
            }
        }
        return rerun;
    }

    /**
     * Simple linked list nodes to record waiting threads in a Treiber
     * stack.  See other classes such as Phaser and SynchronousQueue
     * for more detailed explanation.
     */
    static final class WaitNode {
        volatile Thread thread;
        volatile WaitNode next;
        WaitNode() { thread = Thread.currentThread(); }
    }

    /**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out callable.
     */
    private void finishCompletion() {
        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 {
        long last = timed ? System.nanoTime() : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                long now = System.nanoTime();
                if ((nanos -= (now - last)) <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                last = now;
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

    /**
     * Tries to unlink a timed-out or interrupted wait node to avoid
     * accumulating garbage.  Internal nodes are simply unspliced
     * without CAS since it is harmless if they are traversed anyway
     * by releasers.  To avoid effects of unsplicing from already
     * removed nodes, the list is retraversed 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:	1.85
Committed:	Sun Jun 19 03:21:58 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.84:	+7 -6 lines
Log Message:	null callable only in finishCompletion
#	User	Rev	Content
1	tim	1.1	/*
2	dl	1.2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.23	* Expert Group and released to the public domain, as explained at
4	jsr166	1.59	* http://creativecommons.org/publicdomain/zero/1.0/
5	tim	1.1	*/
6
7			package java.util.concurrent;
8	dl	1.62	import java.util.concurrent.locks.LockSupport;
9	dl	1.13
10	tim	1.1	/**
11	dl	1.8	* 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	dl	1.4	* retrieve the result of the computation. The result can only be
15	jsr166	1.64	* retrieved when the computation has completed; the {@code get}
16			* methods will block if the computation has not yet completed. Once
17	dl	1.8	* the computation has completed, the computation cannot be restarted
18	jsr166	1.64	* or cancelled (unless the computation is invoked using
19			* {@link #runAndReset}).
20	tim	1.1	*
21	jsr166	1.64	* <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	tim	1.1	*
26	dl	1.14	* <p>In addition to serving as a standalone class, this class provides
27	jsr166	1.64	* {@code protected} functionality that may be useful when creating
28	dl	1.14	* customized task classes.
29			*
30	tim	1.1	* @since 1.5
31	dl	1.4	* @author Doug Lea
32	jsr166	1.64	* @param <V> The result type returned by this FutureTask's {@code get} methods
33	tim	1.1	*/
34	peierls	1.39	public class FutureTask<V> implements RunnableFuture<V> {
35	dl	1.62	/*
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	jsr166	1.73	* The run state of this task, initially NEW. The run state
49	dl	1.78	* 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	jsr166	1.69	*
57			* Possible state transitions:
58	jsr166	1.73	* NEW -> COMPLETING -> NORMAL
59			* NEW -> COMPLETING -> EXCEPTIONAL
60			* NEW -> CANCELLED
61			* NEW -> INTERRUPTING -> INTERRUPTED
62	dl	1.62	*/
63			private volatile int state;
64	jsr166	1.73	private static final int NEW = 0;
65	jsr166	1.70	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	dl	1.62
72	dl	1.78	/** The underlying callable; nulled out after running */
73	dl	1.77	private Callable<V> callable;
74	dl	1.62	/** 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	jsr166	1.64	* Returns result or throws exception for completed task.
83			*
84	dl	1.62	* @param s completed state value
85			*/
86			private V report(int s) throws ExecutionException {
87			Object x = outcome;
88			if (s == NORMAL)
89			return (V)x;
90	jsr166	1.69	if (s >= CANCELLED)
91	dl	1.62	throw new CancellationException();
92			throw new ExecutionException((Throwable)x);
93			}
94	dl	1.11
95	tim	1.1	/**
96	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
97			* given {@code Callable}.
98	tim	1.1	*
99			* @param callable the callable task
100	jsr166	1.79	* @throws NullPointerException if the callable is null
101	tim	1.1	*/
102			public FutureTask(Callable<V> callable) {
103	dl	1.9	if (callable == null)
104			throw new NullPointerException();
105	dl	1.62	this.callable = callable;
106	jsr166	1.85	this.state = NEW; // ensure visibility of callable
107	tim	1.1	}
108
109			/**
110	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
111			* given {@code Runnable}, and arrange that {@code get} will return the
112	tim	1.1	* given result on successful completion.
113			*
114	jsr166	1.54	* @param runnable the runnable task
115	tim	1.1	* @param result the result to return on successful completion. If
116	dl	1.9	* you don't need a particular result, consider using
117	dl	1.16	* constructions of the form:
118	jsr166	1.58	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
119	jsr166	1.79	* @throws NullPointerException if the runnable is null
120	tim	1.1	*/
121	dl	1.15	public FutureTask(Runnable runnable, V result) {
122	dl	1.62	this.callable = Executors.callable(runnable, result);
123	jsr166	1.85	this.state = NEW; // ensure visibility of callable
124	dl	1.20	}
125
126			public boolean isCancelled() {
127	jsr166	1.69	return state >= CANCELLED;
128	dl	1.20	}
129	jsr166	1.35
130	dl	1.20	public boolean isDone() {
131	jsr166	1.73	return state != NEW;
132	dl	1.13	}
133
134			public boolean cancel(boolean mayInterruptIfRunning) {
135	dl	1.78	if (state != NEW)
136			return false;
137			if (mayInterruptIfRunning) {
138			if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
139			return false;
140			Thread t = runner;
141			if (t != null)
142			t.interrupt();
143			UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
144			}
145			else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
146			return false;
147			finishCompletion();
148			return true;
149	dl	1.13	}
150	jsr166	1.35
151	jsr166	1.43	/**
152			* @throws CancellationException {@inheritDoc}
153			*/
154	dl	1.2	public V get() throws InterruptedException, ExecutionException {
155	jsr166	1.64	int s = state;
156	jsr166	1.83	return report((s <= COMPLETING) ? awaitDone(false, 0L) : s);
157	tim	1.1	}
158
159	jsr166	1.43	/**
160			* @throws CancellationException {@inheritDoc}
161			*/
162	dl	1.2	public V get(long timeout, TimeUnit unit)
163	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
164	jsr166	1.82	if (unit == null)
165			throw new NullPointerException();
166	jsr166	1.64	int s = state;
167			if (s <= COMPLETING &&
168	jsr166	1.82	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
169	dl	1.62	throw new TimeoutException();
170			return report(s);
171	tim	1.1	}
172
173			/**
174	dl	1.20	* Protected method invoked when this task transitions to state
175	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
176	dl	1.20	* default implementation does nothing. Subclasses may override
177			* this method to invoke completion callbacks or perform
178			* bookkeeping. Note that you can query status inside the
179			* implementation of this method to determine whether this task
180			* has been cancelled.
181			*/
182			protected void done() { }
183
184			/**
185	jsr166	1.64	* Sets the result of this future to the given value unless
186	dl	1.29	* this future has already been set or has been cancelled.
187	jsr166	1.64	*
188			* <p>This method is invoked internally by the {@link #run} method
189	dl	1.40	* upon successful completion of the computation.
190	jsr166	1.64	*
191	tim	1.1	* @param v the value
192	jsr166	1.35	*/
193	dl	1.2	protected void set(V v) {
194	dl	1.78	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
195			outcome = v;
196			UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
197			finishCompletion();
198			}
199	tim	1.1	}
200
201			/**
202	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
203			* with the given throwable as its cause, unless this future has
204	dl	1.24	* already been set or has been cancelled.
205	jsr166	1.64	*
206			* <p>This method is invoked internally by the {@link #run} method
207	dl	1.40	* upon failure of the computation.
208	jsr166	1.64	*
209	jsr166	1.41	* @param t the cause of failure
210	jsr166	1.35	*/
211	dl	1.2	protected void setException(Throwable t) {
212	dl	1.78	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
213			outcome = t;
214			UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
215			finishCompletion();
216			}
217	tim	1.1	}
218	jsr166	1.35
219	dl	1.24	public void run() {
220	dl	1.77	if (state == NEW &&
221			UNSAFE.compareAndSwapObject(this, runnerOffset,
222			null, Thread.currentThread())) {
223			Callable<V> c = callable;
224			if (c != null && state == NEW) {
225			V result = null;
226			boolean ran = false;
227			try {
228			result = c.call();
229			ran = true;
230			} catch (Throwable ex) {
231			setException(ex);
232			}
233			if (ran)
234			set(result);
235	dl	1.62	}
236	jsr166	1.68	runner = null;
237	dl	1.77	if (state >= INTERRUPTING) {
238			while (state == INTERRUPTING)
239			Thread.yield(); // wait out pending interrupt
240			Thread.interrupted(); // clear interrupt from cancel(true)
241	jsr166	1.69	}
242	dl	1.62	}
243	dl	1.24	}
244
245			/**
246	dl	1.30	* Executes the computation without setting its result, and then
247	jsr166	1.64	* resets this future to initial state, failing to do so if the
248	dl	1.24	* computation encounters an exception or is cancelled. This is
249			* designed for use with tasks that intrinsically execute more
250			* than once.
251	jsr166	1.64	*
252	dl	1.24	* @return true if successfully run and reset
253			*/
254			protected boolean runAndReset() {
255	dl	1.77	boolean rerun = false; // true if this task can be re-run
256			if (state == NEW &&
257			UNSAFE.compareAndSwapObject(this, runnerOffset,
258			null, Thread.currentThread())) {
259			Callable<V> c = callable;
260			if (c != null && state == NEW) {
261			try {
262			c.call(); // don't set result
263			rerun = true;
264			} catch (Throwable ex) {
265			setException(ex);
266			}
267	jsr166	1.68	}
268			runner = null;
269	jsr166	1.69	int s = state;
270	dl	1.77	if (s != NEW) {
271			rerun = false;
272			if (s >= INTERRUPTING) {
273			while (state == INTERRUPTING)
274			Thread.yield(); // wait out pending interrupt
275			Thread.interrupted(); // clear interrupt from cancel(true)
276			}
277	jsr166	1.69	}
278	dl	1.62	}
279	dl	1.77	return rerun;
280	dl	1.14	}
281	dl	1.3
282	dl	1.14	/**
283	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
284	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
285	dl	1.62	* for more detailed explanation.
286	dl	1.20	*/
287	dl	1.62	static final class WaitNode {
288			volatile Thread thread;
289	dl	1.76	volatile WaitNode next;
290			WaitNode() { thread = Thread.currentThread(); }
291	dl	1.62	}
292	dl	1.42
293	dl	1.62	/**
294	jsr166	1.85	* Removes and signals all waiting threads, invokes done(), and
295			* nulls out callable.
296	dl	1.62	*/
297	dl	1.78	private void finishCompletion() {
298	jsr166	1.81	for (WaitNode q; (q = waiters) != null;) {
299	dl	1.62	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
300			for (;;) {
301			Thread t = q.thread;
302			if (t != null) {
303			q.thread = null;
304			LockSupport.unpark(t);
305			}
306			WaitNode next = q.next;
307			if (next == null)
308	dl	1.78	break;
309	dl	1.62	q.next = null; // unlink to help gc
310			q = next;
311			}
312	dl	1.78	break;
313	dl	1.62	}
314	dl	1.24	}
315	jsr166	1.85
316	dl	1.78	done();
317	jsr166	1.85
318			callable = null; // to reduce footprint
319	dl	1.62	}
320	dl	1.24
321	dl	1.62	/**
322	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
323			*
324	dl	1.62	* @param timed true if use timed waits
325	jsr166	1.64	* @param nanos time to wait, if timed
326	dl	1.62	* @return state upon completion
327			*/
328			private int awaitDone(boolean timed, long nanos)
329			throws InterruptedException {
330	jsr166	1.63	long last = timed ? System.nanoTime() : 0L;
331	dl	1.62	WaitNode q = null;
332			boolean queued = false;
333	jsr166	1.64	for (;;) {
334	dl	1.62	if (Thread.interrupted()) {
335			removeWaiter(q);
336			throw new InterruptedException();
337			}
338	jsr166	1.64
339			int s = state;
340			if (s > COMPLETING) {
341	dl	1.62	if (q != null)
342			q.thread = null;
343			return s;
344			}
345			else if (q == null)
346			q = new WaitNode();
347			else if (!queued)
348			queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
349			q.next = waiters, q);
350			else if (timed) {
351			long now = System.nanoTime();
352			if ((nanos -= (now - last)) <= 0L) {
353			removeWaiter(q);
354			return state;
355	dl	1.50	}
356	dl	1.62	last = now;
357			LockSupport.parkNanos(this, nanos);
358	dl	1.50	}
359	dl	1.62	else
360			LockSupport.park(this);
361	dl	1.24	}
362	dl	1.62	}
363	dl	1.24
364	dl	1.62	/**
365	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
366			* accumulating garbage. Internal nodes are simply unspliced
367	dl	1.62	* without CAS since it is harmless if they are traversed anyway
368	jsr166	1.81	* by releasers. To avoid effects of unsplicing from already
369			* removed nodes, the list is retraversed in case of an apparent
370			* race. This is slow when there are a lot of nodes, but we don't
371			* expect lists to be long enough to outweigh higher-overhead
372			* schemes.
373	dl	1.62	*/
374			private void removeWaiter(WaitNode node) {
375			if (node != null) {
376			node.thread = null;
377	jsr166	1.81	retry:
378			for (;;) { // restart on removeWaiter race
379			for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
380			s = q.next;
381			if (q.thread != null)
382			pred = q;
383			else if (pred != null) {
384			pred.next = s;
385			if (pred.thread == null) // check for race
386			continue retry;
387			}
388			else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
389			q, s))
390			continue retry;
391	jsr166	1.55	}
392	jsr166	1.81	break;
393	jsr166	1.56	}
394	dl	1.14	}
395	dl	1.62	}
396	dl	1.14
397	dl	1.62	// 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	dl	1.14	}
415	dl	1.15	}
416	dl	1.62
417	dl	1.15	}