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 UNDECIDED.  The run state
     * transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
     * method setCompletion.  During setCompletion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (while interrupting the runner).  State values are
     * ordered and set to powers of two to simplify checks.
     */
    private volatile int state;
    private static final int UNDECIDED    = 0x00;
    private static final int COMPLETING   = 0x01;
    private static final int INTERRUPTING = 0x02;
    private static final int NORMAL       = 0x04;
    private static final int EXCEPTIONAL  = 0x08;
    private static final int CANCELLED    = 0x10;

    /** The underlying callable */
    private final 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;

    /**
     * Sets completion status, unless already completed.  If
     * necessary, we first set state to COMPLETING or INTERRUPTING to
     * establish precedence.  This intentionally stalls (just via
     * yields) in (uncommon) cases of concurrent calls during
     * cancellation until state is set, to avoid surprising users
     * during cancellation races.
     *
     * @param x the outcome
     * @param mode the completion state value
     * @return true if this call caused transition from UNDECIDED to completed
     */
    private boolean setCompletion(Object x, int mode) {
        int next = ((mode == INTERRUPTING) ? // set up transient states
                    (runner != null) ? INTERRUPTING : CANCELLED :
                    (x != null) ? COMPLETING : mode);
        if (!UNSAFE.compareAndSwapInt(this, stateOffset,
                                      UNDECIDED, next))
            return false;
        if (next == INTERRUPTING) {
            Thread t = runner; // recheck
            if (t != null)
                t.interrupt();
            state = CANCELLED;
        }
        else if (next == COMPLETING) {
            outcome = x;
            state = mode;
        }
        if (waiters != null)
            releaseAll();
        done();
        return true;
    }

    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if ((s & (CANCELLED | INTERRUPTING)) != 0)
            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 callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = 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 runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
    }

    public boolean isCancelled() {
        return (state & (CANCELLED | INTERRUPTING)) != 0;
    }

    public boolean isDone() {
        return state != UNDECIDED;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return state == UNDECIDED &&
            setCompletion(null, mayInterruptIfRunning ?
                          INTERRUPTING : CANCELLED);
    }

    /**
     * @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 {
        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) {
        setCompletion(v, NORMAL);
    }

    /**
     * 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) {
        setCompletion(t, EXCEPTIONAL);
    }

    public void run() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;

        try {
            V result;
            try {
                result = callable.call();
            } catch (Throwable ex) {
                setException(ex);
                return;
            }
            set(result);
        } finally {
            runner = null;
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending cancellation interrupt
        }
    }

    /**
     * Executes the computation without setting its result, and then
     * resets this future to initial state, failing to do so if the
     * computation encounters an exception or is cancelled.  This is
     * designed for use with tasks that intrinsically execute more
     * than once.
     *
     * @return true if successfully run and reset
     */
    protected boolean runAndReset() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;

        try {
            try {
                callable.call(); // don't set result
                return (state == UNDECIDED);
            } catch (Throwable ex) {
                setException(ex);
                return false;
            }
        } finally {
            runner = null;
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending cancellation interrupt
        }
    }

    /**
     * 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;
        WaitNode next;
    }

    /**
     * Removes and signals all waiting threads.
     */
    private void releaseAll() {
        WaitNode q;
        while ((q = waiters) != null) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        return;
                    q.next = null; // unlink to help gc
                    q = next;
                }
            }
        }
    }

    /**
     * Awaits completion or aborts on interrupt or timeout.
     *
     * @param timed true if use timed waits
     * @param nanos time to wait, if timed
     * @return state upon completion
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        long last = timed ? System.nanoTime() : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (q.thread == null)
                q.thread = Thread.currentThread();
            else if (timed) {
                long now = System.nanoTime();
                if ((nanos -= (now - last)) <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                last = now;
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

    /**
     * Tries to unlink a timed-out or interrupted wait node to avoid
     * accumulating garbage.  Internal nodes are simply unspliced
     * without CAS since it is harmless if they are traversed anyway
     * by releasers or concurrent calls to removeWaiter.
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            WaitNode pred = null;
            WaitNode q = waiters;
            while (q != null) {
                WaitNode next = node.next;
                if (q != node) {
                    pred = q;
                    q = next;
                }
                else if (pred != null) {
                    pred.next = next;
                    break;
                }
                else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q, next))
                    break;
                else { // restart on CAS failure
                    pred = null;
                    q = waiters;
                }
            }
        }
    }

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

}
Revision:	1.68
Committed:	Fri Jun 17 22:07:24 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.67:	+38 -44 lines
Log Message:	only reset runner to null after state is decided
#	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	import java.util.concurrent.locks.LockSupport;
9
10	/**
11	* A cancellable asynchronous computation. This class provides a base
12	* implementation of {@link Future}, with methods to start and cancel
13	* a computation, query to see if the computation is complete, and
14	* retrieve the result of the computation. The result can only be
15	* retrieved when the computation has completed; the {@code get}
16	* methods will block if the computation has not yet completed. Once
17	* the computation has completed, the computation cannot be restarted
18	* or cancelled (unless the computation is invoked using
19	* {@link #runAndReset}).
20	*
21	* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
22	* {@link Runnable} object. Because {@code FutureTask} implements
23	* {@code Runnable}, a {@code FutureTask} can be submitted to an
24	* {@link Executor} for execution.
25	*
26	* <p>In addition to serving as a standalone class, this class provides
27	* {@code protected} functionality that may be useful when creating
28	* customized task classes.
29	*
30	* @since 1.5
31	* @author Doug Lea
32	* @param <V> The result type returned by this FutureTask's {@code get} methods
33	*/
34	public class FutureTask<V> implements RunnableFuture<V> {
35	/*
36	* Revision notes: This differs from previous versions of this
37	* class that relied on AbstractQueuedSynchronizer, mainly to
38	* avoid surprising users about retaining interrupt status during
39	* cancellation races. Sync control in the current design relies
40	* on a "state" field updated via CAS to track completion, along
41	* with a simple Treiber stack to hold waiting threads.
42	*
43	* Style note: As usual, we bypass overhead of using
44	* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
45	*/
46
47	/**
48	* The run state of this task, initially UNDECIDED. The run state
49	* transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
50	* method setCompletion. During setCompletion, state may take on
51	* transient values of COMPLETING (while outcome is being set) or
52	* INTERRUPTING (while interrupting the runner). State values are
53	* ordered and set to powers of two to simplify checks.
54	*/
55	private volatile int state;
56	private static final int UNDECIDED = 0x00;
57	private static final int COMPLETING = 0x01;
58	private static final int INTERRUPTING = 0x02;
59	private static final int NORMAL = 0x04;
60	private static final int EXCEPTIONAL = 0x08;
61	private static final int CANCELLED = 0x10;
62
63	/** The underlying callable */
64	private final Callable<V> callable;
65	/** The result to return or exception to throw from get() */
66	private Object outcome; // non-volatile, protected by state reads/writes
67	/** The thread running the callable; CASed during run() */
68	private volatile Thread runner;
69	/** Treiber stack of waiting threads */
70	private volatile WaitNode waiters;
71
72	/**
73	* Sets completion status, unless already completed. If
74	* necessary, we first set state to COMPLETING or INTERRUPTING to
75	* establish precedence. This intentionally stalls (just via
76	* yields) in (uncommon) cases of concurrent calls during
77	* cancellation until state is set, to avoid surprising users
78	* during cancellation races.
79	*
80	* @param x the outcome
81	* @param mode the completion state value
82	* @return true if this call caused transition from UNDECIDED to completed
83	*/
84	private boolean setCompletion(Object x, int mode) {
85	int next = ((mode == INTERRUPTING) ? // set up transient states
86	(runner != null) ? INTERRUPTING : CANCELLED :
87	(x != null) ? COMPLETING : mode);
88	if (!UNSAFE.compareAndSwapInt(this, stateOffset,
89	UNDECIDED, next))
90	return false;
91	if (next == INTERRUPTING) {
92	Thread t = runner; // recheck
93	if (t != null)
94	t.interrupt();
95	state = CANCELLED;
96	}
97	else if (next == COMPLETING) {
98	outcome = x;
99	state = mode;
100	}
101	if (waiters != null)
102	releaseAll();
103	done();
104	return true;
105	}
106
107	/**
108	* Returns result or throws exception for completed task.
109	*
110	* @param s completed state value
111	*/
112	private V report(int s) throws ExecutionException {
113	Object x = outcome;
114	if (s == NORMAL)
115	return (V)x;
116	if ((s & (CANCELLED \| INTERRUPTING)) != 0)
117	throw new CancellationException();
118	throw new ExecutionException((Throwable)x);
119	}
120
121	/**
122	* Creates a {@code FutureTask} that will, upon running, execute the
123	* given {@code Callable}.
124	*
125	* @param callable the callable task
126	* @throws NullPointerException if callable is null
127	*/
128	public FutureTask(Callable<V> callable) {
129	if (callable == null)
130	throw new NullPointerException();
131	this.callable = callable;
132	}
133
134	/**
135	* Creates a {@code FutureTask} that will, upon running, execute the
136	* given {@code Runnable}, and arrange that {@code get} will return the
137	* given result on successful completion.
138	*
139	* @param runnable the runnable task
140	* @param result the result to return on successful completion. If
141	* you don't need a particular result, consider using
142	* constructions of the form:
143	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
144	* @throws NullPointerException if runnable is null
145	*/
146	public FutureTask(Runnable runnable, V result) {
147	this.callable = Executors.callable(runnable, result);
148	}
149
150	public boolean isCancelled() {
151	return (state & (CANCELLED \| INTERRUPTING)) != 0;
152	}
153
154	public boolean isDone() {
155	return state != UNDECIDED;
156	}
157
158	public boolean cancel(boolean mayInterruptIfRunning) {
159	return state == UNDECIDED &&
160	setCompletion(null, mayInterruptIfRunning ?
161	INTERRUPTING : CANCELLED);
162	}
163
164	/**
165	* @throws CancellationException {@inheritDoc}
166	*/
167	public V get() throws InterruptedException, ExecutionException {
168	int s = state;
169	if (s <= COMPLETING)
170	s = awaitDone(false, 0L);
171	return report(s);
172	}
173
174	/**
175	* @throws CancellationException {@inheritDoc}
176	*/
177	public V get(long timeout, TimeUnit unit)
178	throws InterruptedException, ExecutionException, TimeoutException {
179	int s = state;
180	if (s <= COMPLETING &&
181	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
182	throw new TimeoutException();
183	return report(s);
184	}
185
186	/**
187	* Protected method invoked when this task transitions to state
188	* {@code isDone} (whether normally or via cancellation). The
189	* default implementation does nothing. Subclasses may override
190	* this method to invoke completion callbacks or perform
191	* bookkeeping. Note that you can query status inside the
192	* implementation of this method to determine whether this task
193	* has been cancelled.
194	*/
195	protected void done() { }
196
197	/**
198	* Sets the result of this future to the given value unless
199	* this future has already been set or has been cancelled.
200	*
201	* <p>This method is invoked internally by the {@link #run} method
202	* upon successful completion of the computation.
203	*
204	* @param v the value
205	*/
206	protected void set(V v) {
207	setCompletion(v, NORMAL);
208	}
209
210	/**
211	* Causes this future to report an {@link ExecutionException}
212	* with the given throwable as its cause, unless this future has
213	* already been set or has been cancelled.
214	*
215	* <p>This method is invoked internally by the {@link #run} method
216	* upon failure of the computation.
217	*
218	* @param t the cause of failure
219	*/
220	protected void setException(Throwable t) {
221	setCompletion(t, EXCEPTIONAL);
222	}
223
224	public void run() {
225	if (state != UNDECIDED \|\|
226	!UNSAFE.compareAndSwapObject(this, runnerOffset,
227	null, Thread.currentThread()))
228	return;
229
230	try {
231	V result;
232	try {
233	result = callable.call();
234	} catch (Throwable ex) {
235	setException(ex);
236	return;
237	}
238	set(result);
239	} finally {
240	runner = null;
241	while (state == INTERRUPTING)
242	Thread.yield(); // wait out pending cancellation interrupt
243	}
244	}
245
246	/**
247	* Executes the computation without setting its result, and then
248	* resets this future to initial state, failing to do so if the
249	* computation encounters an exception or is cancelled. This is
250	* designed for use with tasks that intrinsically execute more
251	* than once.
252	*
253	* @return true if successfully run and reset
254	*/
255	protected boolean runAndReset() {
256	if (state != UNDECIDED \|\|
257	!UNSAFE.compareAndSwapObject(this, runnerOffset,
258	null, Thread.currentThread()))
259	return false;
260
261	try {
262	try {
263	callable.call(); // don't set result
264	return (state == UNDECIDED);
265	} catch (Throwable ex) {
266	setException(ex);
267	return false;
268	}
269	} finally {
270	runner = null;
271	while (state == INTERRUPTING)
272	Thread.yield(); // wait out pending cancellation interrupt
273	}
274	}
275
276	/**
277	* Simple linked list nodes to record waiting threads in a Treiber
278	* stack. See other classes such as Phaser and SynchronousQueue
279	* for more detailed explanation.
280	*/
281	static final class WaitNode {
282	volatile Thread thread;
283	WaitNode next;
284	}
285
286	/**
287	* Removes and signals all waiting threads.
288	*/
289	private void releaseAll() {
290	WaitNode q;
291	while ((q = waiters) != null) {
292	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
293	for (;;) {
294	Thread t = q.thread;
295	if (t != null) {
296	q.thread = null;
297	LockSupport.unpark(t);
298	}
299	WaitNode next = q.next;
300	if (next == null)
301	return;
302	q.next = null; // unlink to help gc
303	q = next;
304	}
305	}
306	}
307	}
308
309	/**
310	* Awaits completion or aborts on interrupt or timeout.
311	*
312	* @param timed true if use timed waits
313	* @param nanos time to wait, if timed
314	* @return state upon completion
315	*/
316	private int awaitDone(boolean timed, long nanos)
317	throws InterruptedException {
318	long last = timed ? System.nanoTime() : 0L;
319	WaitNode q = null;
320	boolean queued = false;
321	for (;;) {
322	if (Thread.interrupted()) {
323	removeWaiter(q);
324	throw new InterruptedException();
325	}
326
327	int s = state;
328	if (s > COMPLETING) {
329	if (q != null)
330	q.thread = null;
331	return s;
332	}
333	else if (q == null)
334	q = new WaitNode();
335	else if (!queued)
336	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
337	q.next = waiters, q);
338	else if (q.thread == null)
339	q.thread = Thread.currentThread();
340	else if (timed) {
341	long now = System.nanoTime();
342	if ((nanos -= (now - last)) <= 0L) {
343	removeWaiter(q);
344	return state;
345	}
346	last = now;
347	LockSupport.parkNanos(this, nanos);
348	}
349	else
350	LockSupport.park(this);
351	}
352	}
353
354	/**
355	* Tries to unlink a timed-out or interrupted wait node to avoid
356	* accumulating garbage. Internal nodes are simply unspliced
357	* without CAS since it is harmless if they are traversed anyway
358	* by releasers or concurrent calls to removeWaiter.
359	*/
360	private void removeWaiter(WaitNode node) {
361	if (node != null) {
362	node.thread = null;
363	WaitNode pred = null;
364	WaitNode q = waiters;
365	while (q != null) {
366	WaitNode next = node.next;
367	if (q != node) {
368	pred = q;
369	q = next;
370	}
371	else if (pred != null) {
372	pred.next = next;
373	break;
374	}
375	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
376	q, next))
377	break;
378	else { // restart on CAS failure
379	pred = null;
380	q = waiters;
381	}
382	}
383	}
384	}
385
386	// Unsafe mechanics
387	private static final sun.misc.Unsafe UNSAFE;
388	private static final long stateOffset;
389	private static final long runnerOffset;
390	private static final long waitersOffset;
391	static {
392	try {
393	UNSAFE = sun.misc.Unsafe.getUnsafe();
394	Class<?> k = FutureTask.class;
395	stateOffset = UNSAFE.objectFieldOffset
396	(k.getDeclaredField("state"));
397	runnerOffset = UNSAFE.objectFieldOffset
398	(k.getDeclaredField("runner"));
399	waitersOffset = UNSAFE.objectFieldOffset
400	(k.getDeclaredField("waiters"));
401	} catch (Exception e) {
402	throw new Error(e);
403	}
404	}
405
406	}