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 (;;) {
            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING)
                // We may have already promised (via isDone) that we are done
                // so never return empty-handed or throw InterruptedException
                Thread.yield();
            else if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }
            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;
                }
                // nanoTime may be slow; recheck before parking
                if (state < COMPLETING)
                    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 = sun.misc.Unsafe.getUnsafe();
    private static final long STATE;
    private static final long RUNNER;
    private static final long WAITERS;
    static {
        try {
            STATE = U.objectFieldOffset
                (FutureTask.class.getDeclaredField("state"));
            RUNNER = U.objectFieldOffset
                (FutureTask.class.getDeclaredField("runner"));
            WAITERS = U.objectFieldOffset
                (FutureTask.class.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }

        // Reduce the risk of rare disastrous classloading in first call to
        // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
        Class<?> ensureLoaded = LockSupport.class;
    }

}
Revision:	1.5
Committed:	Mon Mar 23 18:56:40 2015 UTC (9 years, 2 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.4:	+4 -0 lines
Log Message:	JDK-8074773: Reduce the risk of rare disastrous classloading in first call to LockSupport.park
#	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	jsr166	1.3
9	dl	1.1	import java.util.concurrent.locks.LockSupport;
10
11			/**
12			* A cancellable asynchronous computation. This class provides a base
13			* implementation of {@link Future}, with methods to start and cancel
14			* a computation, query to see if the computation is complete, and
15			* retrieve the result of the computation. The result can only be
16			* retrieved when the computation has completed; the {@code get}
17			* methods will block if the computation has not yet completed. Once
18			* the computation has completed, the computation cannot be restarted
19			* or cancelled (unless the computation is invoked using
20			* {@link #runAndReset}).
21			*
22			* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
23			* {@link Runnable} object. Because {@code FutureTask} implements
24			* {@code Runnable}, a {@code FutureTask} can be submitted to an
25			* {@link Executor} for execution.
26			*
27			* <p>In addition to serving as a standalone class, this class provides
28			* {@code protected} functionality that may be useful when creating
29			* customized task classes.
30			*
31			* @since 1.5
32			* @author Doug Lea
33			* @param <V> The result type returned by this FutureTask's {@code get} methods
34			*/
35			public class FutureTask<V> implements RunnableFuture<V> {
36			/*
37			* Revision notes: This differs from previous versions of this
38			* class that relied on AbstractQueuedSynchronizer, mainly to
39			* avoid surprising users about retaining interrupt status during
40			* cancellation races. Sync control in the current design relies
41			* on a "state" field updated via CAS to track completion, along
42			* with a simple Treiber stack to hold waiting threads.
43			*
44			* Style note: As usual, we bypass overhead of using
45			* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
46			*/
47
48			/**
49			* The run state of this task, initially NEW. The run state
50			* transitions to a terminal state only in methods set,
51			* setException, and cancel. During completion, state may take on
52			* transient values of COMPLETING (while outcome is being set) or
53			* INTERRUPTING (only while interrupting the runner to satisfy a
54			* cancel(true)). Transitions from these intermediate to final
55			* states use cheaper ordered/lazy writes because values are unique
56			* and cannot be further modified.
57			*
58			* Possible state transitions:
59			* NEW -> COMPLETING -> NORMAL
60			* NEW -> COMPLETING -> EXCEPTIONAL
61			* NEW -> CANCELLED
62			* NEW -> INTERRUPTING -> INTERRUPTED
63			*/
64			private volatile int state;
65			private static final int NEW = 0;
66			private static final int COMPLETING = 1;
67			private static final int NORMAL = 2;
68			private static final int EXCEPTIONAL = 3;
69			private static final int CANCELLED = 4;
70			private static final int INTERRUPTING = 5;
71			private static final int INTERRUPTED = 6;
72
73			/** The underlying callable; nulled out after running */
74			private Callable<V> callable;
75			/** The result to return or exception to throw from get() */
76			private Object outcome; // non-volatile, protected by state reads/writes
77			/** The thread running the callable; CASed during run() */
78			private volatile Thread runner;
79			/** Treiber stack of waiting threads */
80			private volatile WaitNode waiters;
81
82			/**
83			* Returns result or throws exception for completed task.
84			*
85			* @param s completed state value
86			*/
87			@SuppressWarnings("unchecked")
88			private V report(int s) throws ExecutionException {
89			Object x = outcome;
90			if (s == NORMAL)
91			return (V)x;
92			if (s >= CANCELLED)
93			throw new CancellationException();
94			throw new ExecutionException((Throwable)x);
95			}
96
97			/**
98			* Creates a {@code FutureTask} that will, upon running, execute the
99			* given {@code Callable}.
100			*
101			* @param callable the callable task
102			* @throws NullPointerException if the callable is null
103			*/
104			public FutureTask(Callable<V> callable) {
105			if (callable == null)
106			throw new NullPointerException();
107			this.callable = callable;
108			this.state = NEW; // ensure visibility of callable
109			}
110
111			/**
112			* Creates a {@code FutureTask} that will, upon running, execute the
113			* given {@code Runnable}, and arrange that {@code get} will return the
114			* given result on successful completion.
115			*
116			* @param runnable the runnable task
117			* @param result the result to return on successful completion. If
118			* you don't need a particular result, consider using
119			* constructions of the form:
120			* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
121			* @throws NullPointerException if the runnable is null
122			*/
123			public FutureTask(Runnable runnable, V result) {
124			this.callable = Executors.callable(runnable, result);
125			this.state = NEW; // ensure visibility of callable
126			}
127
128			public boolean isCancelled() {
129			return state >= CANCELLED;
130			}
131
132			public boolean isDone() {
133			return state != NEW;
134			}
135
136			public boolean cancel(boolean mayInterruptIfRunning) {
137			if (!(state == NEW &&
138	jsr166	1.2	U.compareAndSwapInt(this, STATE, NEW,
139	dl	1.1	mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
140			return false;
141			try { // in case call to interrupt throws exception
142			if (mayInterruptIfRunning) {
143			try {
144			Thread t = runner;
145			if (t != null)
146			t.interrupt();
147			} finally { // final state
148	jsr166	1.2	U.putOrderedInt(this, STATE, INTERRUPTED);
149	dl	1.1	}
150			}
151			} finally {
152			finishCompletion();
153			}
154			return true;
155			}
156
157			/**
158			* @throws CancellationException {@inheritDoc}
159			*/
160			public V get() throws InterruptedException, ExecutionException {
161			int s = state;
162			if (s <= COMPLETING)
163			s = awaitDone(false, 0L);
164			return report(s);
165			}
166
167			/**
168			* @throws CancellationException {@inheritDoc}
169			*/
170			public V get(long timeout, TimeUnit unit)
171			throws InterruptedException, ExecutionException, TimeoutException {
172			if (unit == null)
173			throw new NullPointerException();
174			int s = state;
175			if (s <= COMPLETING &&
176			(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
177			throw new TimeoutException();
178			return report(s);
179			}
180
181			/**
182			* Protected method invoked when this task transitions to state
183			* {@code isDone} (whether normally or via cancellation). The
184			* default implementation does nothing. Subclasses may override
185			* this method to invoke completion callbacks or perform
186			* bookkeeping. Note that you can query status inside the
187			* implementation of this method to determine whether this task
188			* has been cancelled.
189			*/
190			protected void done() { }
191
192			/**
193			* Sets the result of this future to the given value unless
194			* this future has already been set or has been cancelled.
195			*
196			* <p>This method is invoked internally by the {@link #run} method
197			* upon successful completion of the computation.
198			*
199			* @param v the value
200			*/
201			protected void set(V v) {
202	jsr166	1.2	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
203	dl	1.1	outcome = v;
204	jsr166	1.2	U.putOrderedInt(this, STATE, NORMAL); // final state
205	dl	1.1	finishCompletion();
206			}
207			}
208
209			/**
210			* Causes this future to report an {@link ExecutionException}
211			* with the given throwable as its cause, unless this future has
212			* already been set or has been cancelled.
213			*
214			* <p>This method is invoked internally by the {@link #run} method
215			* upon failure of the computation.
216			*
217			* @param t the cause of failure
218			*/
219			protected void setException(Throwable t) {
220	jsr166	1.2	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
221	dl	1.1	outcome = t;
222	jsr166	1.2	U.putOrderedInt(this, STATE, EXCEPTIONAL); // final state
223	dl	1.1	finishCompletion();
224			}
225			}
226
227			public void run() {
228			if (state != NEW \|\|
229	jsr166	1.2	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
230	dl	1.1	return;
231			try {
232			Callable<V> c = callable;
233			if (c != null && state == NEW) {
234			V result;
235			boolean ran;
236			try {
237			result = c.call();
238			ran = true;
239			} catch (Throwable ex) {
240			result = null;
241			ran = false;
242			setException(ex);
243			}
244			if (ran)
245			set(result);
246			}
247			} finally {
248			// runner must be non-null until state is settled to
249			// prevent concurrent calls to run()
250			runner = null;
251			// state must be re-read after nulling runner to prevent
252			// leaked interrupts
253			int s = state;
254			if (s >= INTERRUPTING)
255			handlePossibleCancellationInterrupt(s);
256			}
257			}
258
259			/**
260			* Executes the computation without setting its result, and then
261			* resets this future to initial state, failing to do so if the
262			* computation encounters an exception or is cancelled. This is
263			* designed for use with tasks that intrinsically execute more
264			* than once.
265			*
266	jsr166	1.2	* @return {@code true} if successfully run and reset
267	dl	1.1	*/
268			protected boolean runAndReset() {
269			if (state != NEW \|\|
270	jsr166	1.2	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
271	dl	1.1	return false;
272			boolean ran = false;
273			int s = state;
274			try {
275			Callable<V> c = callable;
276			if (c != null && s == NEW) {
277			try {
278			c.call(); // don't set result
279			ran = true;
280			} catch (Throwable ex) {
281			setException(ex);
282			}
283			}
284			} finally {
285			// runner must be non-null until state is settled to
286			// prevent concurrent calls to run()
287			runner = null;
288			// state must be re-read after nulling runner to prevent
289			// leaked interrupts
290			s = state;
291			if (s >= INTERRUPTING)
292			handlePossibleCancellationInterrupt(s);
293			}
294			return ran && s == NEW;
295			}
296
297			/**
298			* Ensures that any interrupt from a possible cancel(true) is only
299			* delivered to a task while in run or runAndReset.
300			*/
301			private void handlePossibleCancellationInterrupt(int s) {
302			// It is possible for our interrupter to stall before getting a
303			// chance to interrupt us. Let's spin-wait patiently.
304			if (s == INTERRUPTING)
305			while (state == INTERRUPTING)
306			Thread.yield(); // wait out pending interrupt
307
308			// assert state == INTERRUPTED;
309
310			// We want to clear any interrupt we may have received from
311			// cancel(true). However, it is permissible to use interrupts
312			// as an independent mechanism for a task to communicate with
313			// its caller, and there is no way to clear only the
314			// cancellation interrupt.
315			//
316			// Thread.interrupted();
317			}
318
319			/**
320			* Simple linked list nodes to record waiting threads in a Treiber
321			* stack. See other classes such as Phaser and SynchronousQueue
322			* for more detailed explanation.
323			*/
324			static final class WaitNode {
325			volatile Thread thread;
326			volatile WaitNode next;
327			WaitNode() { thread = Thread.currentThread(); }
328			}
329
330			/**
331			* Removes and signals all waiting threads, invokes done(), and
332			* nulls out callable.
333			*/
334			private void finishCompletion() {
335			// assert state > COMPLETING;
336			for (WaitNode q; (q = waiters) != null;) {
337	jsr166	1.2	if (U.compareAndSwapObject(this, WAITERS, q, null)) {
338	dl	1.1	for (;;) {
339			Thread t = q.thread;
340			if (t != null) {
341			q.thread = null;
342			LockSupport.unpark(t);
343			}
344			WaitNode next = q.next;
345			if (next == null)
346			break;
347			q.next = null; // unlink to help gc
348			q = next;
349			}
350			break;
351			}
352			}
353
354			done();
355
356			callable = null; // to reduce footprint
357			}
358
359			/**
360			* Awaits completion or aborts on interrupt or timeout.
361			*
362			* @param timed true if use timed waits
363			* @param nanos time to wait, if timed
364	jsr166	1.2	* @return state upon completion or at timeout
365	dl	1.1	*/
366			private int awaitDone(boolean timed, long nanos)
367			throws InterruptedException {
368	jsr166	1.2	// The code below is very delicate, to achieve these goals:
369			// - call nanoTime exactly once for each call to park
370			// - if nanos <= 0, return promptly without allocation or nanoTime
371			// - if nanos == Long.MIN_VALUE, don't underflow
372			// - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
373			// and we suffer a spurious wakeup, we will do no worse than
374			// to park-spin for a while
375			long startTime = 0L; // Special value 0L means not yet parked
376	dl	1.1	WaitNode q = null;
377			boolean queued = false;
378			for (;;) {
379			int s = state;
380			if (s > COMPLETING) {
381			if (q != null)
382			q.thread = null;
383			return s;
384			}
385	jsr166	1.4	else if (s == COMPLETING)
386			// We may have already promised (via isDone) that we are done
387			// so never return empty-handed or throw InterruptedException
388	dl	1.1	Thread.yield();
389	jsr166	1.4	else if (Thread.interrupted()) {
390			removeWaiter(q);
391			throw new InterruptedException();
392			}
393	jsr166	1.2	else if (q == null) {
394			if (timed && nanos <= 0L)
395			return s;
396	dl	1.1	q = new WaitNode();
397	jsr166	1.2	}
398	dl	1.1	else if (!queued)
399	jsr166	1.2	queued = U.compareAndSwapObject(this, WAITERS,
400			q.next = waiters, q);
401	dl	1.1	else if (timed) {
402	jsr166	1.2	final long parkNanos;
403			if (startTime == 0L) { // first time
404			startTime = System.nanoTime();
405			if (startTime == 0L)
406			startTime = 1L;
407			parkNanos = nanos;
408			} else {
409			long elapsed = System.nanoTime() - startTime;
410			if (elapsed >= nanos) {
411			removeWaiter(q);
412			return state;
413			}
414			parkNanos = nanos - elapsed;
415	dl	1.1	}
416	jsr166	1.4	// nanoTime may be slow; recheck before parking
417			if (state < COMPLETING)
418			LockSupport.parkNanos(this, parkNanos);
419	dl	1.1	}
420			else
421			LockSupport.park(this);
422			}
423			}
424
425			/**
426			* Tries to unlink a timed-out or interrupted wait node to avoid
427			* accumulating garbage. Internal nodes are simply unspliced
428			* without CAS since it is harmless if they are traversed anyway
429			* by releasers. To avoid effects of unsplicing from already
430			* removed nodes, the list is retraversed in case of an apparent
431			* race. This is slow when there are a lot of nodes, but we don't
432			* expect lists to be long enough to outweigh higher-overhead
433			* schemes.
434			*/
435			private void removeWaiter(WaitNode node) {
436			if (node != null) {
437			node.thread = null;
438			retry:
439			for (;;) { // restart on removeWaiter race
440			for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
441			s = q.next;
442			if (q.thread != null)
443			pred = q;
444			else if (pred != null) {
445			pred.next = s;
446			if (pred.thread == null) // check for race
447			continue retry;
448			}
449	jsr166	1.2	else if (!U.compareAndSwapObject(this, WAITERS, q, s))
450	dl	1.1	continue retry;
451			}
452			break;
453			}
454			}
455			}
456
457			// Unsafe mechanics
458	jsr166	1.4	private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
459	jsr166	1.2	private static final long STATE;
460			private static final long RUNNER;
461			private static final long WAITERS;
462	dl	1.1	static {
463			try {
464	jsr166	1.4	STATE = U.objectFieldOffset
465			(FutureTask.class.getDeclaredField("state"));
466			RUNNER = U.objectFieldOffset
467			(FutureTask.class.getDeclaredField("runner"));
468			WAITERS = U.objectFieldOffset
469			(FutureTask.class.getDeclaredField("waiters"));
470	dl	1.1	} catch (Exception e) {
471			throw new Error(e);
472			}
473	jsr166	1.5
474			// Reduce the risk of rare disastrous classloading in first call to
475			// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
476			Class<?> ensureLoaded = LockSupport.class;
477	dl	1.1	}
478
479			}