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;
    }

    /**
     * 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;
    }

    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 {
        long nanos = unit.toNanos(timeout);
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, nanos)) <= 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);
                callable = null;       // null out upon use to reduce footprint
            }
            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)
                }
            }
            if (!rerun)
                callable = null;
        }
        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, and invokes done().
     */
    private void finishCompletion() {
        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)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
        done();
    }

    /**
     * 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 until no cancelled nodes
     * are found.  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;
            for (WaitNode pred = null, q = waiters; q != null;) {
                WaitNode next = q.next;
                if (q.thread != null) {
                    pred = q;
                    q = next;
                }
                else {
                    if (pred != null)
                        pred.next = next;
                    else
                        UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                    q, next);
                    pred = null; // restart until clean sweep
                    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:	1.80
Committed:	Sat Jun 18 15:20:30 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.79:	+1 -2 lines
Log Message:	small javadoc tidying
#	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	dl	1.77	this.state = NEW;
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	dl	1.77	this.state = NEW;
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	dl	1.78	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	dl	1.78	long nanos = unit.toNanos(timeout);
165	jsr166	1.64	int s = state;
166			if (s <= COMPLETING &&
167	dl	1.78	(s = awaitDone(true, nanos)) <= COMPLETING)
168	dl	1.62	throw new TimeoutException();
169			return report(s);
170	tim	1.1	}
171
172			/**
173	dl	1.20	* Protected method invoked when this task transitions to state
174	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
175	dl	1.20	* default implementation does nothing. Subclasses may override
176			* this method to invoke completion callbacks or perform
177			* bookkeeping. Note that you can query status inside the
178			* implementation of this method to determine whether this task
179			* has been cancelled.
180			*/
181			protected void done() { }
182
183			/**
184	jsr166	1.64	* Sets the result of this future to the given value unless
185	dl	1.29	* this future has already been set or has been cancelled.
186	jsr166	1.64	*
187			* <p>This method is invoked internally by the {@link #run} method
188	dl	1.40	* upon successful completion of the computation.
189	jsr166	1.64	*
190	tim	1.1	* @param v the value
191	jsr166	1.35	*/
192	dl	1.2	protected void set(V v) {
193	dl	1.78	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
194			outcome = v;
195			UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
196			finishCompletion();
197			}
198	tim	1.1	}
199
200			/**
201	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
202			* with the given throwable as its cause, unless this future has
203	dl	1.24	* already been set or has been cancelled.
204	jsr166	1.64	*
205			* <p>This method is invoked internally by the {@link #run} method
206	dl	1.40	* upon failure of the computation.
207	jsr166	1.64	*
208	jsr166	1.41	* @param t the cause of failure
209	jsr166	1.35	*/
210	dl	1.2	protected void setException(Throwable t) {
211	dl	1.78	if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
212			outcome = t;
213			UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
214			finishCompletion();
215			}
216	tim	1.1	}
217	jsr166	1.35
218	dl	1.24	public void run() {
219	dl	1.77	if (state == NEW &&
220			UNSAFE.compareAndSwapObject(this, runnerOffset,
221			null, Thread.currentThread())) {
222			Callable<V> c = callable;
223			if (c != null && state == NEW) {
224			V result = null;
225			boolean ran = false;
226			try {
227			result = c.call();
228			ran = true;
229			} catch (Throwable ex) {
230			setException(ex);
231			}
232			if (ran)
233			set(result);
234			callable = null; // null out upon use to reduce footprint
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	1.77	if (!rerun)
279			callable = null;
280	dl	1.62	}
281	dl	1.77	return rerun;
282	dl	1.14	}
283	dl	1.3
284	dl	1.14	/**
285	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
286	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
287	dl	1.62	* for more detailed explanation.
288	dl	1.20	*/
289	dl	1.62	static final class WaitNode {
290			volatile Thread thread;
291	dl	1.76	volatile WaitNode next;
292			WaitNode() { thread = Thread.currentThread(); }
293	dl	1.62	}
294	dl	1.42
295	dl	1.62	/**
296	jsr166	1.80	* Removes and signals all waiting threads, and invokes done().
297	dl	1.62	*/
298	dl	1.78	private void finishCompletion() {
299	dl	1.62	WaitNode q;
300			while ((q = waiters) != null) {
301			if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
302			for (;;) {
303			Thread t = q.thread;
304			if (t != null) {
305			q.thread = null;
306			LockSupport.unpark(t);
307			}
308			WaitNode next = q.next;
309			if (next == null)
310	dl	1.78	break;
311	dl	1.62	q.next = null; // unlink to help gc
312			q = next;
313			}
314	dl	1.78	break;
315	dl	1.62	}
316	dl	1.24	}
317	dl	1.78	done();
318	dl	1.62	}
319	dl	1.24
320	dl	1.62	/**
321	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
322			*
323	dl	1.62	* @param timed true if use timed waits
324	jsr166	1.64	* @param nanos time to wait, if timed
325	dl	1.62	* @return state upon completion
326			*/
327			private int awaitDone(boolean timed, long nanos)
328			throws InterruptedException {
329	jsr166	1.63	long last = timed ? System.nanoTime() : 0L;
330	dl	1.62	WaitNode q = null;
331			boolean queued = false;
332	jsr166	1.64	for (;;) {
333	dl	1.62	if (Thread.interrupted()) {
334			removeWaiter(q);
335			throw new InterruptedException();
336			}
337	jsr166	1.64
338			int s = state;
339			if (s > COMPLETING) {
340	dl	1.62	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 (timed) {
350			long now = System.nanoTime();
351			if ((nanos -= (now - last)) <= 0L) {
352			removeWaiter(q);
353			return state;
354	dl	1.50	}
355	dl	1.62	last = now;
356			LockSupport.parkNanos(this, nanos);
357	dl	1.50	}
358	dl	1.62	else
359			LockSupport.park(this);
360	dl	1.24	}
361	dl	1.62	}
362	dl	1.24
363	dl	1.62	/**
364	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
365			* accumulating garbage. Internal nodes are simply unspliced
366	dl	1.62	* without CAS since it is harmless if they are traversed anyway
367	dl	1.76	* by releasers. To avoid effects of unsplicing from already
368			* removed nodes, the list is retraversed until no cancelled nodes
369			* are found. This is slow when there are a lot of nodes, but we
370			* don't expect lists to be long enough to outweigh
371			* higher-overhead schemes.
372	dl	1.62	*/
373			private void removeWaiter(WaitNode node) {
374			if (node != null) {
375			node.thread = null;
376	jsr166	1.75	for (WaitNode pred = null, q = waiters; q != null;) {
377	jsr166	1.74	WaitNode next = q.next;
378	dl	1.76	if (q.thread != null) {
379	dl	1.62	pred = q;
380			q = next;
381	dl	1.50	}
382	dl	1.76	else {
383			if (pred != null)
384			pred.next = next;
385			else
386			UNSAFE.compareAndSwapObject(this, waitersOffset,
387			q, next);
388			pred = null; // restart until clean sweep
389	dl	1.62	q = waiters;
390	jsr166	1.55	}
391	jsr166	1.56	}
392	dl	1.14	}
393	dl	1.62	}
394	dl	1.14
395	dl	1.62	// Unsafe mechanics
396			private static final sun.misc.Unsafe UNSAFE;
397			private static final long stateOffset;
398			private static final long runnerOffset;
399			private static final long waitersOffset;
400			static {
401			try {
402			UNSAFE = sun.misc.Unsafe.getUnsafe();
403			Class<?> k = FutureTask.class;
404			stateOffset = UNSAFE.objectFieldOffset
405			(k.getDeclaredField("state"));
406			runnerOffset = UNSAFE.objectFieldOffset
407			(k.getDeclaredField("runner"));
408			waitersOffset = UNSAFE.objectFieldOffset
409			(k.getDeclaredField("waiters"));
410			} catch (Exception e) {
411			throw new Error(e);
412	dl	1.14	}
413	dl	1.15	}
414	dl	1.62
415	dl	1.15	}