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 &&
              U.compareAndSwapInt(this, STATE, NEW,
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            return false;
        try {    // in case call to interrupt throws exception
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    U.putOrderedInt(this, STATE, INTERRUPTED);
                }
            }
        } finally {
            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 (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = v;
            U.putOrderedInt(this, STATE, 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 (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = t;
            U.putOrderedInt(this, STATE, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

    public void run() {
        if (state != NEW ||
            !U.compareAndSwapObject(this, RUNNER, 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 {@code true} if successfully run and reset
     */
    protected boolean runAndReset() {
        if (state != NEW ||
            !U.compareAndSwapObject(this, RUNNER, 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 (U.compareAndSwapObject(this, WAITERS, 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 or at timeout
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        // The code below is very delicate, to achieve these goals:
        // - call nanoTime exactly once for each call to park
        // - if nanos <= 0, return promptly without allocation or nanoTime
        // - if nanos == Long.MIN_VALUE, don't underflow
        // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
        //   and we suffer a spurious wakeup, we will do no worse than
        //   to park-spin for a while
        long startTime = 0L;    // Special value 0L means not yet parked
        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) {
                if (timed && nanos <= 0L)
                    return s;
                q = new WaitNode();
            }
            else if (!queued)
                queued = U.compareAndSwapObject(this, WAITERS,
                                                q.next = waiters, q);
            else if (timed) {
                final long parkNanos;
                if (startTime == 0L) { // first time
                    startTime = System.nanoTime();
                    if (startTime == 0L)
                        startTime = 1L;
                    parkNanos = nanos;
                } else {
                    long elapsed = System.nanoTime() - startTime;
                    if (elapsed >= nanos) {
                        removeWaiter(q);
                        return state;
                    }
                    parkNanos = nanos - elapsed;
                }
                LockSupport.parkNanos(this, parkNanos);
            }
            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 (!U.compareAndSwapObject(this, WAITERS, q, s))
                        continue retry;
                }
                break;
            }
        }
    }

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

}
Revision:	1.2
Committed:	Fri Oct 3 23:48:09 2014 UTC (9 years, 8 months ago) by jsr166
Branch:	MAIN
Changes since 1.1:	+48 -35 lines
Log Message:	backport fixes from src/main
#	User	Rev	Content
1	dl	1.1	/*
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	jsr166	1.2	U.compareAndSwapInt(this, STATE, NEW,
138	dl	1.1	mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
139			return false;
140			try { // in case call to interrupt throws exception
141			if (mayInterruptIfRunning) {
142			try {
143			Thread t = runner;
144			if (t != null)
145			t.interrupt();
146			} finally { // final state
147	jsr166	1.2	U.putOrderedInt(this, STATE, INTERRUPTED);
148	dl	1.1	}
149			}
150			} finally {
151			finishCompletion();
152			}
153			return true;
154			}
155
156			/**
157			* @throws CancellationException {@inheritDoc}
158			*/
159			public V get() throws InterruptedException, ExecutionException {
160			int s = state;
161			if (s <= COMPLETING)
162			s = awaitDone(false, 0L);
163			return report(s);
164			}
165
166			/**
167			* @throws CancellationException {@inheritDoc}
168			*/
169			public V get(long timeout, TimeUnit unit)
170			throws InterruptedException, ExecutionException, TimeoutException {
171			if (unit == null)
172			throw new NullPointerException();
173			int s = state;
174			if (s <= COMPLETING &&
175			(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
176			throw new TimeoutException();
177			return report(s);
178			}
179
180			/**
181			* Protected method invoked when this task transitions to state
182			* {@code isDone} (whether normally or via cancellation). The
183			* default implementation does nothing. Subclasses may override
184			* this method to invoke completion callbacks or perform
185			* bookkeeping. Note that you can query status inside the
186			* implementation of this method to determine whether this task
187			* has been cancelled.
188			*/
189			protected void done() { }
190
191			/**
192			* Sets the result of this future to the given value unless
193			* this future has already been set or has been cancelled.
194			*
195			* <p>This method is invoked internally by the {@link #run} method
196			* upon successful completion of the computation.
197			*
198			* @param v the value
199			*/
200			protected void set(V v) {
201	jsr166	1.2	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
202	dl	1.1	outcome = v;
203	jsr166	1.2	U.putOrderedInt(this, STATE, NORMAL); // final state
204	dl	1.1	finishCompletion();
205			}
206			}
207
208			/**
209			* Causes this future to report an {@link ExecutionException}
210			* with the given throwable as its cause, unless this future has
211			* already been set or has been cancelled.
212			*
213			* <p>This method is invoked internally by the {@link #run} method
214			* upon failure of the computation.
215			*
216			* @param t the cause of failure
217			*/
218			protected void setException(Throwable t) {
219	jsr166	1.2	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
220	dl	1.1	outcome = t;
221	jsr166	1.2	U.putOrderedInt(this, STATE, EXCEPTIONAL); // final state
222	dl	1.1	finishCompletion();
223			}
224			}
225
226			public void run() {
227			if (state != NEW \|\|
228	jsr166	1.2	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
229	dl	1.1	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	jsr166	1.2	* @return {@code true} if successfully run and reset
266	dl	1.1	*/
267			protected boolean runAndReset() {
268			if (state != NEW \|\|
269	jsr166	1.2	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
270	dl	1.1	return false;
271			boolean ran = false;
272			int s = state;
273			try {
274			Callable<V> c = callable;
275			if (c != null && s == NEW) {
276			try {
277			c.call(); // don't set result
278			ran = true;
279			} catch (Throwable ex) {
280			setException(ex);
281			}
282			}
283			} finally {
284			// runner must be non-null until state is settled to
285			// prevent concurrent calls to run()
286			runner = null;
287			// state must be re-read after nulling runner to prevent
288			// leaked interrupts
289			s = state;
290			if (s >= INTERRUPTING)
291			handlePossibleCancellationInterrupt(s);
292			}
293			return ran && s == NEW;
294			}
295
296			/**
297			* Ensures that any interrupt from a possible cancel(true) is only
298			* delivered to a task while in run or runAndReset.
299			*/
300			private void handlePossibleCancellationInterrupt(int s) {
301			// It is possible for our interrupter to stall before getting a
302			// chance to interrupt us. Let's spin-wait patiently.
303			if (s == INTERRUPTING)
304			while (state == INTERRUPTING)
305			Thread.yield(); // wait out pending interrupt
306
307			// assert state == INTERRUPTED;
308
309			// We want to clear any interrupt we may have received from
310			// cancel(true). However, it is permissible to use interrupts
311			// as an independent mechanism for a task to communicate with
312			// its caller, and there is no way to clear only the
313			// cancellation interrupt.
314			//
315			// Thread.interrupted();
316			}
317
318			/**
319			* Simple linked list nodes to record waiting threads in a Treiber
320			* stack. See other classes such as Phaser and SynchronousQueue
321			* for more detailed explanation.
322			*/
323			static final class WaitNode {
324			volatile Thread thread;
325			volatile WaitNode next;
326			WaitNode() { thread = Thread.currentThread(); }
327			}
328
329			/**
330			* Removes and signals all waiting threads, invokes done(), and
331			* nulls out callable.
332			*/
333			private void finishCompletion() {
334			// assert state > COMPLETING;
335			for (WaitNode q; (q = waiters) != null;) {
336	jsr166	1.2	if (U.compareAndSwapObject(this, WAITERS, q, null)) {
337	dl	1.1	for (;;) {
338			Thread t = q.thread;
339			if (t != null) {
340			q.thread = null;
341			LockSupport.unpark(t);
342			}
343			WaitNode next = q.next;
344			if (next == null)
345			break;
346			q.next = null; // unlink to help gc
347			q = next;
348			}
349			break;
350			}
351			}
352
353			done();
354
355			callable = null; // to reduce footprint
356			}
357
358			/**
359			* Awaits completion or aborts on interrupt or timeout.
360			*
361			* @param timed true if use timed waits
362			* @param nanos time to wait, if timed
363	jsr166	1.2	* @return state upon completion or at timeout
364	dl	1.1	*/
365			private int awaitDone(boolean timed, long nanos)
366			throws InterruptedException {
367	jsr166	1.2	// The code below is very delicate, to achieve these goals:
368			// - call nanoTime exactly once for each call to park
369			// - if nanos <= 0, return promptly without allocation or nanoTime
370			// - if nanos == Long.MIN_VALUE, don't underflow
371			// - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
372			// and we suffer a spurious wakeup, we will do no worse than
373			// to park-spin for a while
374			long startTime = 0L; // Special value 0L means not yet parked
375	dl	1.1	WaitNode q = null;
376			boolean queued = false;
377			for (;;) {
378			if (Thread.interrupted()) {
379			removeWaiter(q);
380			throw new InterruptedException();
381			}
382
383			int s = state;
384			if (s > COMPLETING) {
385			if (q != null)
386			q.thread = null;
387			return s;
388			}
389			else if (s == COMPLETING) // cannot time out yet
390			Thread.yield();
391	jsr166	1.2	else if (q == null) {
392			if (timed && nanos <= 0L)
393			return s;
394	dl	1.1	q = new WaitNode();
395	jsr166	1.2	}
396	dl	1.1	else if (!queued)
397	jsr166	1.2	queued = U.compareAndSwapObject(this, WAITERS,
398			q.next = waiters, q);
399	dl	1.1	else if (timed) {
400	jsr166	1.2	final long parkNanos;
401			if (startTime == 0L) { // first time
402			startTime = System.nanoTime();
403			if (startTime == 0L)
404			startTime = 1L;
405			parkNanos = nanos;
406			} else {
407			long elapsed = System.nanoTime() - startTime;
408			if (elapsed >= nanos) {
409			removeWaiter(q);
410			return state;
411			}
412			parkNanos = nanos - elapsed;
413	dl	1.1	}
414	jsr166	1.2	LockSupport.parkNanos(this, parkNanos);
415	dl	1.1	}
416			else
417			LockSupport.park(this);
418			}
419			}
420
421			/**
422			* Tries to unlink a timed-out or interrupted wait node to avoid
423			* accumulating garbage. Internal nodes are simply unspliced
424			* without CAS since it is harmless if they are traversed anyway
425			* by releasers. To avoid effects of unsplicing from already
426			* removed nodes, the list is retraversed in case of an apparent
427			* race. This is slow when there are a lot of nodes, but we don't
428			* expect lists to be long enough to outweigh higher-overhead
429			* schemes.
430			*/
431			private void removeWaiter(WaitNode node) {
432			if (node != null) {
433			node.thread = null;
434			retry:
435			for (;;) { // restart on removeWaiter race
436			for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
437			s = q.next;
438			if (q.thread != null)
439			pred = q;
440			else if (pred != null) {
441			pred.next = s;
442			if (pred.thread == null) // check for race
443			continue retry;
444			}
445	jsr166	1.2	else if (!U.compareAndSwapObject(this, WAITERS, q, s))
446	dl	1.1	continue retry;
447			}
448			break;
449			}
450			}
451			}
452
453			// Unsafe mechanics
454	jsr166	1.2	private static final sun.misc.Unsafe U;
455			private static final long STATE;
456			private static final long RUNNER;
457			private static final long WAITERS;
458	dl	1.1	static {
459			try {
460	jsr166	1.2	U = sun.misc.Unsafe.getUnsafe();
461	dl	1.1	Class<?> k = FutureTask.class;
462	jsr166	1.2	STATE = U.objectFieldOffset(k.getDeclaredField("state"));
463			RUNNER = U.objectFieldOffset(k.getDeclaredField("runner"));
464			WAITERS = U.objectFieldOffset(k.getDeclaredField("waiters"));
465	dl	1.1	} catch (Exception e) {
466			throw new Error(e);
467			}
468			}
469
470			}