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 <= 0L, 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;
            }
        }
    }

    /**
     * Returns a string representation of this FutureTask.
     *
     * @implSpec
     * The default implementation returns a string identifying this
     * FutureTask, as well as its completion state.  The state, in
     * brackets, contains one of the strings {@code "Completed Normally"},
     * {@code "Completed Exceptionally"}, {@code "Cancelled"}, or {@code
     * "Not completed"}.
     *
     * @return a string representation of this FutureTask
     */
    public String toString() {
        final String status;
        switch (state) {
        case NORMAL:
            status = "[Completed normally]";
            break;
        case EXCEPTIONAL:
            status = "[Completed exceptionally: " + outcome + "]";
            break;
        case CANCELLED:
        case INTERRUPTING:
        case INTERRUPTED:
            status = "[Cancelled]";
            break;
        default:
            final Callable<?> callable = this.callable;
            status = (callable == null)
                ? "[Not completed]"
                : "[Not completed, task = " + callable + "]";
        }
        return super.toString() + status;
    }

    // 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 (ReflectiveOperationException 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:	1.2
Committed:	Tue Sep 26 03:44:53 2017 UTC (6 years, 7 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+35 -0 lines
Log Message:	backport 8186265: Make toString() methods of "task" objects more useful
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8
9	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	U.compareAndSwapInt(this, STATE, NEW,
139	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	U.putOrderedInt(this, STATE, INTERRUPTED);
149	}
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	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
203	outcome = v;
204	U.putOrderedInt(this, STATE, NORMAL); // final state
205	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	if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
221	outcome = t;
222	U.putOrderedInt(this, STATE, EXCEPTIONAL); // final state
223	finishCompletion();
224	}
225	}
226
227	public void run() {
228	if (state != NEW \|\|
229	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
230	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	* @return {@code true} if successfully run and reset
267	*/
268	protected boolean runAndReset() {
269	if (state != NEW \|\|
270	!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
271	return false;
272	boolean ran = false;
273	int s = state;
274	try {
275	Callable<V> c = callable;
276	if (c != null && s == NEW) {
277	try {
278	c.call(); // don't set result
279	ran = true;
280	} catch (Throwable ex) {
281	setException(ex);
282	}
283	}
284	} finally {
285	// runner must be non-null until state is settled to
286	// prevent concurrent calls to run()
287	runner = null;
288	// state must be re-read after nulling runner to prevent
289	// leaked interrupts
290	s = state;
291	if (s >= INTERRUPTING)
292	handlePossibleCancellationInterrupt(s);
293	}
294	return ran && s == NEW;
295	}
296
297	/**
298	* Ensures that any interrupt from a possible cancel(true) is only
299	* delivered to a task while in run or runAndReset.
300	*/
301	private void handlePossibleCancellationInterrupt(int s) {
302	// It is possible for our interrupter to stall before getting a
303	// chance to interrupt us. Let's spin-wait patiently.
304	if (s == INTERRUPTING)
305	while (state == INTERRUPTING)
306	Thread.yield(); // wait out pending interrupt
307
308	// assert state == INTERRUPTED;
309
310	// We want to clear any interrupt we may have received from
311	// cancel(true). However, it is permissible to use interrupts
312	// as an independent mechanism for a task to communicate with
313	// its caller, and there is no way to clear only the
314	// cancellation interrupt.
315	//
316	// Thread.interrupted();
317	}
318
319	/**
320	* Simple linked list nodes to record waiting threads in a Treiber
321	* stack. See other classes such as Phaser and SynchronousQueue
322	* for more detailed explanation.
323	*/
324	static final class WaitNode {
325	volatile Thread thread;
326	volatile WaitNode next;
327	WaitNode() { thread = Thread.currentThread(); }
328	}
329
330	/**
331	* Removes and signals all waiting threads, invokes done(), and
332	* nulls out callable.
333	*/
334	private void finishCompletion() {
335	// assert state > COMPLETING;
336	for (WaitNode q; (q = waiters) != null;) {
337	if (U.compareAndSwapObject(this, WAITERS, q, null)) {
338	for (;;) {
339	Thread t = q.thread;
340	if (t != null) {
341	q.thread = null;
342	LockSupport.unpark(t);
343	}
344	WaitNode next = q.next;
345	if (next == null)
346	break;
347	q.next = null; // unlink to help gc
348	q = next;
349	}
350	break;
351	}
352	}
353
354	done();
355
356	callable = null; // to reduce footprint
357	}
358
359	/**
360	* Awaits completion or aborts on interrupt or timeout.
361	*
362	* @param timed true if use timed waits
363	* @param nanos time to wait, if timed
364	* @return state upon completion or at timeout
365	*/
366	private int awaitDone(boolean timed, long nanos)
367	throws InterruptedException {
368	// The code below is very delicate, to achieve these goals:
369	// - call nanoTime exactly once for each call to park
370	// - if nanos <= 0L, 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	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	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	Thread.yield();
389	else if (Thread.interrupted()) {
390	removeWaiter(q);
391	throw new InterruptedException();
392	}
393	else if (q == null) {
394	if (timed && nanos <= 0L)
395	return s;
396	q = new WaitNode();
397	}
398	else if (!queued)
399	queued = U.compareAndSwapObject(this, WAITERS,
400	q.next = waiters, q);
401	else if (timed) {
402	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	}
416	// nanoTime may be slow; recheck before parking
417	if (state < COMPLETING)
418	LockSupport.parkNanos(this, parkNanos);
419	}
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	else if (!U.compareAndSwapObject(this, WAITERS, q, s))
450	continue retry;
451	}
452	break;
453	}
454	}
455	}
456
457	/**
458	* Returns a string representation of this FutureTask.
459	*
460	* @implSpec
461	* The default implementation returns a string identifying this
462	* FutureTask, as well as its completion state. The state, in
463	* brackets, contains one of the strings {@code "Completed Normally"},
464	* {@code "Completed Exceptionally"}, {@code "Cancelled"}, or {@code
465	* "Not completed"}.
466	*
467	* @return a string representation of this FutureTask
468	*/
469	public String toString() {
470	final String status;
471	switch (state) {
472	case NORMAL:
473	status = "[Completed normally]";
474	break;
475	case EXCEPTIONAL:
476	status = "[Completed exceptionally: " + outcome + "]";
477	break;
478	case CANCELLED:
479	case INTERRUPTING:
480	case INTERRUPTED:
481	status = "[Cancelled]";
482	break;
483	default:
484	final Callable<?> callable = this.callable;
485	status = (callable == null)
486	? "[Not completed]"
487	: "[Not completed, task = " + callable + "]";
488	}
489	return super.toString() + status;
490	}
491
492	// Unsafe mechanics
493	private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
494	private static final long STATE;
495	private static final long RUNNER;
496	private static final long WAITERS;
497	static {
498	try {
499	STATE = U.objectFieldOffset
500	(FutureTask.class.getDeclaredField("state"));
501	RUNNER = U.objectFieldOffset
502	(FutureTask.class.getDeclaredField("runner"));
503	WAITERS = U.objectFieldOffset
504	(FutureTask.class.getDeclaredField("waiters"));
505	} catch (ReflectiveOperationException e) {
506	throw new Error(e);
507	}
508
509	// Reduce the risk of rare disastrous classloading in first call to
510	// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
511	Class<?> ensureLoaded = LockSupport.class;
512	}
513
514	}