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.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
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.
     */

    /**
     * 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 && STATE.compareAndSet
              (this, 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
                    STATE.setRelease(this, 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);
    }

    @Override
    public V resultNow() {
        switch (state()) {    // Future.State
            case SUCCESS:
                @SuppressWarnings("unchecked")
                V result = (V) outcome;
                return result;
            case FAILED:
                throw new IllegalStateException("Task completed with exception");
            case CANCELLED:
                throw new IllegalStateException("Task was cancelled");
            default:
                throw new IllegalStateException("Task has not completed");
        }
    }

    @Override
    public Throwable exceptionNow() {
        switch (state()) {    // Future.State
            case SUCCESS:
                throw new IllegalStateException("Task completed with a result");
            case FAILED:
                Object x = outcome;
                return (Throwable) x;
            case CANCELLED:
                throw new IllegalStateException("Task was cancelled");
            default:
                throw new IllegalStateException("Task has not completed");
        }
    }

    @Override
    public State state() {
        int s = state;
        while (s == COMPLETING) {
            // waiting for transition to NORMAL or EXCEPTIONAL
            Thread.yield();
            s = state;
        }
        switch (s) {
            case NORMAL:
                return State.SUCCESS;
            case EXCEPTIONAL:
                return State.FAILED;
            case CANCELLED:
            case INTERRUPTING:
            case INTERRUPTED:
                return State.CANCELLED;
            default:
                return State.RUNNING;
        }
    }

    /**
     * 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 (STATE.compareAndSet(this, NEW, COMPLETING)) {
            outcome = v;
            STATE.setRelease(this, 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 (STATE.compareAndSet(this, NEW, COMPLETING)) {
            outcome = t;
            STATE.setRelease(this, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

    public void run() {
        if (state != NEW ||
            !RUNNER.compareAndSet(this, 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 ||
            !RUNNER.compareAndSet(this, 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 (WAITERS.weakCompareAndSet(this, 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 = WAITERS.weakCompareAndSet(this, 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 (!WAITERS.compareAndSet(this, 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;
    }

    // VarHandle mechanics
    private static final VarHandle STATE;
    private static final VarHandle RUNNER;
    private static final VarHandle WAITERS;
    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            STATE = l.findVarHandle(FutureTask.class, "state", int.class);
            RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class);
            WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(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:	1.121
Committed:	Fri May 6 16:03:05 2022 UTC (2 years ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.120:	+53 -0 lines
Log Message:	sync with openjdk
#	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.lang.invoke.MethodHandles;
10	import java.lang.invoke.VarHandle;
11	import java.util.concurrent.locks.LockSupport;
12
13	/**
14	* A cancellable asynchronous computation. This class provides a base
15	* implementation of {@link Future}, with methods to start and cancel
16	* a computation, query to see if the computation is complete, and
17	* retrieve the result of the computation. The result can only be
18	* retrieved when the computation has completed; the {@code get}
19	* methods will block if the computation has not yet completed. Once
20	* the computation has completed, the computation cannot be restarted
21	* or cancelled (unless the computation is invoked using
22	* {@link #runAndReset}).
23	*
24	* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
25	* {@link Runnable} object. Because {@code FutureTask} implements
26	* {@code Runnable}, a {@code FutureTask} can be submitted to an
27	* {@link Executor} for execution.
28	*
29	* <p>In addition to serving as a standalone class, this class provides
30	* {@code protected} functionality that may be useful when creating
31	* customized task classes.
32	*
33	* @since 1.5
34	* @author Doug Lea
35	* @param <V> The result type returned by this FutureTask's {@code get} methods
36	*/
37	public class FutureTask<V> implements RunnableFuture<V> {
38	/*
39	* Revision notes: This differs from previous versions of this
40	* class that relied on AbstractQueuedSynchronizer, mainly to
41	* avoid surprising users about retaining interrupt status during
42	* cancellation races. Sync control in the current design relies
43	* on a "state" field updated via CAS to track completion, along
44	* with a simple Treiber stack to hold waiting threads.
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 && STATE.compareAndSet
137	(this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
138	return false;
139	try { // in case call to interrupt throws exception
140	if (mayInterruptIfRunning) {
141	try {
142	Thread t = runner;
143	if (t != null)
144	t.interrupt();
145	} finally { // final state
146	STATE.setRelease(this, INTERRUPTED);
147	}
148	}
149	} finally {
150	finishCompletion();
151	}
152	return true;
153	}
154
155	/**
156	* @throws CancellationException {@inheritDoc}
157	*/
158	public V get() throws InterruptedException, ExecutionException {
159	int s = state;
160	if (s <= COMPLETING)
161	s = awaitDone(false, 0L);
162	return report(s);
163	}
164
165	/**
166	* @throws CancellationException {@inheritDoc}
167	*/
168	public V get(long timeout, TimeUnit unit)
169	throws InterruptedException, ExecutionException, TimeoutException {
170	if (unit == null)
171	throw new NullPointerException();
172	int s = state;
173	if (s <= COMPLETING &&
174	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
175	throw new TimeoutException();
176	return report(s);
177	}
178
179	@Override
180	public V resultNow() {
181	switch (state()) { // Future.State
182	case SUCCESS:
183	@SuppressWarnings("unchecked")
184	V result = (V) outcome;
185	return result;
186	case FAILED:
187	throw new IllegalStateException("Task completed with exception");
188	case CANCELLED:
189	throw new IllegalStateException("Task was cancelled");
190	default:
191	throw new IllegalStateException("Task has not completed");
192	}
193	}
194
195	@Override
196	public Throwable exceptionNow() {
197	switch (state()) { // Future.State
198	case SUCCESS:
199	throw new IllegalStateException("Task completed with a result");
200	case FAILED:
201	Object x = outcome;
202	return (Throwable) x;
203	case CANCELLED:
204	throw new IllegalStateException("Task was cancelled");
205	default:
206	throw new IllegalStateException("Task has not completed");
207	}
208	}
209
210	@Override
211	public State state() {
212	int s = state;
213	while (s == COMPLETING) {
214	// waiting for transition to NORMAL or EXCEPTIONAL
215	Thread.yield();
216	s = state;
217	}
218	switch (s) {
219	case NORMAL:
220	return State.SUCCESS;
221	case EXCEPTIONAL:
222	return State.FAILED;
223	case CANCELLED:
224	case INTERRUPTING:
225	case INTERRUPTED:
226	return State.CANCELLED;
227	default:
228	return State.RUNNING;
229	}
230	}
231
232	/**
233	* Protected method invoked when this task transitions to state
234	* {@code isDone} (whether normally or via cancellation). The
235	* default implementation does nothing. Subclasses may override
236	* this method to invoke completion callbacks or perform
237	* bookkeeping. Note that you can query status inside the
238	* implementation of this method to determine whether this task
239	* has been cancelled.
240	*/
241	protected void done() { }
242
243	/**
244	* Sets the result of this future to the given value unless
245	* this future has already been set or has been cancelled.
246	*
247	* <p>This method is invoked internally by the {@link #run} method
248	* upon successful completion of the computation.
249	*
250	* @param v the value
251	*/
252	protected void set(V v) {
253	if (STATE.compareAndSet(this, NEW, COMPLETING)) {
254	outcome = v;
255	STATE.setRelease(this, NORMAL); // final state
256	finishCompletion();
257	}
258	}
259
260	/**
261	* Causes this future to report an {@link ExecutionException}
262	* with the given throwable as its cause, unless this future has
263	* already been set or has been cancelled.
264	*
265	* <p>This method is invoked internally by the {@link #run} method
266	* upon failure of the computation.
267	*
268	* @param t the cause of failure
269	*/
270	protected void setException(Throwable t) {
271	if (STATE.compareAndSet(this, NEW, COMPLETING)) {
272	outcome = t;
273	STATE.setRelease(this, EXCEPTIONAL); // final state
274	finishCompletion();
275	}
276	}
277
278	public void run() {
279	if (state != NEW \|\|
280	!RUNNER.compareAndSet(this, null, Thread.currentThread()))
281	return;
282	try {
283	Callable<V> c = callable;
284	if (c != null && state == NEW) {
285	V result;
286	boolean ran;
287	try {
288	result = c.call();
289	ran = true;
290	} catch (Throwable ex) {
291	result = null;
292	ran = false;
293	setException(ex);
294	}
295	if (ran)
296	set(result);
297	}
298	} finally {
299	// runner must be non-null until state is settled to
300	// prevent concurrent calls to run()
301	runner = null;
302	// state must be re-read after nulling runner to prevent
303	// leaked interrupts
304	int s = state;
305	if (s >= INTERRUPTING)
306	handlePossibleCancellationInterrupt(s);
307	}
308	}
309
310	/**
311	* Executes the computation without setting its result, and then
312	* resets this future to initial state, failing to do so if the
313	* computation encounters an exception or is cancelled. This is
314	* designed for use with tasks that intrinsically execute more
315	* than once.
316	*
317	* @return {@code true} if successfully run and reset
318	*/
319	protected boolean runAndReset() {
320	if (state != NEW \|\|
321	!RUNNER.compareAndSet(this, null, Thread.currentThread()))
322	return false;
323	boolean ran = false;
324	int s = state;
325	try {
326	Callable<V> c = callable;
327	if (c != null && s == NEW) {
328	try {
329	c.call(); // don't set result
330	ran = true;
331	} catch (Throwable ex) {
332	setException(ex);
333	}
334	}
335	} finally {
336	// runner must be non-null until state is settled to
337	// prevent concurrent calls to run()
338	runner = null;
339	// state must be re-read after nulling runner to prevent
340	// leaked interrupts
341	s = state;
342	if (s >= INTERRUPTING)
343	handlePossibleCancellationInterrupt(s);
344	}
345	return ran && s == NEW;
346	}
347
348	/**
349	* Ensures that any interrupt from a possible cancel(true) is only
350	* delivered to a task while in run or runAndReset.
351	*/
352	private void handlePossibleCancellationInterrupt(int s) {
353	// It is possible for our interrupter to stall before getting a
354	// chance to interrupt us. Let's spin-wait patiently.
355	if (s == INTERRUPTING)
356	while (state == INTERRUPTING)
357	Thread.yield(); // wait out pending interrupt
358
359	// assert state == INTERRUPTED;
360
361	// We want to clear any interrupt we may have received from
362	// cancel(true). However, it is permissible to use interrupts
363	// as an independent mechanism for a task to communicate with
364	// its caller, and there is no way to clear only the
365	// cancellation interrupt.
366	//
367	// Thread.interrupted();
368	}
369
370	/**
371	* Simple linked list nodes to record waiting threads in a Treiber
372	* stack. See other classes such as Phaser and SynchronousQueue
373	* for more detailed explanation.
374	*/
375	static final class WaitNode {
376	volatile Thread thread;
377	volatile WaitNode next;
378	WaitNode() { thread = Thread.currentThread(); }
379	}
380
381	/**
382	* Removes and signals all waiting threads, invokes done(), and
383	* nulls out callable.
384	*/
385	private void finishCompletion() {
386	// assert state > COMPLETING;
387	for (WaitNode q; (q = waiters) != null;) {
388	if (WAITERS.weakCompareAndSet(this, q, null)) {
389	for (;;) {
390	Thread t = q.thread;
391	if (t != null) {
392	q.thread = null;
393	LockSupport.unpark(t);
394	}
395	WaitNode next = q.next;
396	if (next == null)
397	break;
398	q.next = null; // unlink to help gc
399	q = next;
400	}
401	break;
402	}
403	}
404
405	done();
406
407	callable = null; // to reduce footprint
408	}
409
410	/**
411	* Awaits completion or aborts on interrupt or timeout.
412	*
413	* @param timed true if use timed waits
414	* @param nanos time to wait, if timed
415	* @return state upon completion or at timeout
416	*/
417	private int awaitDone(boolean timed, long nanos)
418	throws InterruptedException {
419	// The code below is very delicate, to achieve these goals:
420	// - call nanoTime exactly once for each call to park
421	// - if nanos <= 0L, return promptly without allocation or nanoTime
422	// - if nanos == Long.MIN_VALUE, don't underflow
423	// - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
424	// and we suffer a spurious wakeup, we will do no worse than
425	// to park-spin for a while
426	long startTime = 0L; // Special value 0L means not yet parked
427	WaitNode q = null;
428	boolean queued = false;
429	for (;;) {
430	int s = state;
431	if (s > COMPLETING) {
432	if (q != null)
433	q.thread = null;
434	return s;
435	}
436	else if (s == COMPLETING)
437	// We may have already promised (via isDone) that we are done
438	// so never return empty-handed or throw InterruptedException
439	Thread.yield();
440	else if (Thread.interrupted()) {
441	removeWaiter(q);
442	throw new InterruptedException();
443	}
444	else if (q == null) {
445	if (timed && nanos <= 0L)
446	return s;
447	q = new WaitNode();
448	}
449	else if (!queued)
450	queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
451	else if (timed) {
452	final long parkNanos;
453	if (startTime == 0L) { // first time
454	startTime = System.nanoTime();
455	if (startTime == 0L)
456	startTime = 1L;
457	parkNanos = nanos;
458	} else {
459	long elapsed = System.nanoTime() - startTime;
460	if (elapsed >= nanos) {
461	removeWaiter(q);
462	return state;
463	}
464	parkNanos = nanos - elapsed;
465	}
466	// nanoTime may be slow; recheck before parking
467	if (state < COMPLETING)
468	LockSupport.parkNanos(this, parkNanos);
469	}
470	else
471	LockSupport.park(this);
472	}
473	}
474
475	/**
476	* Tries to unlink a timed-out or interrupted wait node to avoid
477	* accumulating garbage. Internal nodes are simply unspliced
478	* without CAS since it is harmless if they are traversed anyway
479	* by releasers. To avoid effects of unsplicing from already
480	* removed nodes, the list is retraversed in case of an apparent
481	* race. This is slow when there are a lot of nodes, but we don't
482	* expect lists to be long enough to outweigh higher-overhead
483	* schemes.
484	*/
485	private void removeWaiter(WaitNode node) {
486	if (node != null) {
487	node.thread = null;
488	retry:
489	for (;;) { // restart on removeWaiter race
490	for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
491	s = q.next;
492	if (q.thread != null)
493	pred = q;
494	else if (pred != null) {
495	pred.next = s;
496	if (pred.thread == null) // check for race
497	continue retry;
498	}
499	else if (!WAITERS.compareAndSet(this, q, s))
500	continue retry;
501	}
502	break;
503	}
504	}
505	}
506
507	/**
508	* Returns a string representation of this FutureTask.
509	*
510	* @implSpec
511	* The default implementation returns a string identifying this
512	* FutureTask, as well as its completion state. The state, in
513	* brackets, contains one of the strings {@code "Completed Normally"},
514	* {@code "Completed Exceptionally"}, {@code "Cancelled"}, or {@code
515	* "Not completed"}.
516	*
517	* @return a string representation of this FutureTask
518	*/
519	public String toString() {
520	final String status;
521	switch (state) {
522	case NORMAL:
523	status = "[Completed normally]";
524	break;
525	case EXCEPTIONAL:
526	status = "[Completed exceptionally: " + outcome + "]";
527	break;
528	case CANCELLED:
529	case INTERRUPTING:
530	case INTERRUPTED:
531	status = "[Cancelled]";
532	break;
533	default:
534	final Callable<?> callable = this.callable;
535	status = (callable == null)
536	? "[Not completed]"
537	: "[Not completed, task = " + callable + "]";
538	}
539	return super.toString() + status;
540	}
541
542	// VarHandle mechanics
543	private static final VarHandle STATE;
544	private static final VarHandle RUNNER;
545	private static final VarHandle WAITERS;
546	static {
547	try {
548	MethodHandles.Lookup l = MethodHandles.lookup();
549	STATE = l.findVarHandle(FutureTask.class, "state", int.class);
550	RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class);
551	WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class);
552	} catch (ReflectiveOperationException e) {
553	throw new ExceptionInInitializerError(e);
554	}
555
556	// Reduce the risk of rare disastrous classloading in first call to
557	// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
558	Class<?> ensureLoaded = LockSupport.class;
559	}
560
561	}