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 0.  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 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 0 to completed
     */
    private boolean setCompletion(Object x, int mode) {
        Thread r = runner;
        if (r == Thread.currentThread()) // null out runner on completion
            UNSAFE.putObject(this, runnerOffset, r = null); // nonvolatile OK
        int next = ((mode == INTERRUPTING) ? // set up transient states
                    (r != null) ? INTERRUPTING : CANCELLED :
                    (x != null) ? COMPLETING : mode);
        for (;;) {
            int s = state;
            if (s == 0) {
                if (UNSAFE.compareAndSwapInt(this, stateOffset, 0, next)) {
                    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;
                }
            }
            else if (s == INTERRUPTING)
                Thread.yield(); // wait out pending cancellation interrupt
            else
                return false;
        }
    }

    /**
     * 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 != 0;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return state == 0 &&
            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() {
        Thread r = Thread.currentThread();
        if (state == 0 &&
            UNSAFE.compareAndSwapObject(this, runnerOffset, null, r)) {
            V result;
            try {
                result = callable.call();
            } catch (Throwable ex) {
                setException(ex);
                return;
            }
            set(result);
        }
    }

    /**
     * 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() {
        Thread r = Thread.currentThread();
        if (state != 0 ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset, null, r))
            return false;
        try {
            callable.call(); // don't set result
        } catch (Throwable ex) {
            setException(ex);
            return false;
        }
        runner = null;
        for (;;) {
            int s = state;
            if (s == 0)
                return true;
            if (s != INTERRUPTING)
                return false;
            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:	1.64
Committed:	Fri Jun 17 20:46:43 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.63:	+56 -44 lines
Log Message:	style update
#	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 0. 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
53	* are ordered and set to powers of two to simplify checks.
54	*/
55	private volatile int state;
56	private static final int COMPLETING = 0x01;
57	private static final int INTERRUPTING = 0x02;
58	private static final int NORMAL = 0x04;
59	private static final int EXCEPTIONAL = 0x08;
60	private static final int CANCELLED = 0x10;
61
62	/** The underlying callable */
63	private final Callable<V> callable;
64	/** The result to return or exception to throw from get() */
65	private Object outcome; // non-volatile, protected by state reads/writes
66	/** The thread running the callable; CASed during run() */
67	private volatile Thread runner;
68	/** Treiber stack of waiting threads */
69	private volatile WaitNode waiters;
70
71	/**
72	* Sets completion status, unless already completed. If
73	* necessary, we first set state to COMPLETING or INTERRUPTING to
74	* establish precedence. This intentionally stalls (just via
75	* yields) in (uncommon) cases of concurrent calls during
76	* cancellation until state is set, to avoid surprising users
77	* during cancellation races.
78	*
79	* @param x the outcome
80	* @param mode the completion state value
81	* @return true if this call caused transition from 0 to completed
82	*/
83	private boolean setCompletion(Object x, int mode) {
84	Thread r = runner;
85	if (r == Thread.currentThread()) // null out runner on completion
86	UNSAFE.putObject(this, runnerOffset, r = null); // nonvolatile OK
87	int next = ((mode == INTERRUPTING) ? // set up transient states
88	(r != null) ? INTERRUPTING : CANCELLED :
89	(x != null) ? COMPLETING : mode);
90	for (;;) {
91	int s = state;
92	if (s == 0) {
93	if (UNSAFE.compareAndSwapInt(this, stateOffset, 0, next)) {
94	if (next == INTERRUPTING) {
95	Thread t = runner; // recheck
96	if (t != null)
97	t.interrupt();
98	state = CANCELLED;
99	}
100	else if (next == COMPLETING) {
101	outcome = x;
102	state = mode;
103	}
104	if (waiters != null)
105	releaseAll();
106	done();
107	return true;
108	}
109	}
110	else if (s == INTERRUPTING)
111	Thread.yield(); // wait out pending cancellation interrupt
112	else
113	return false;
114	}
115	}
116
117	/**
118	* Returns result or throws exception for completed task.
119	*
120	* @param s completed state value
121	*/
122	private V report(int s) throws ExecutionException {
123	Object x = outcome;
124	if (s == NORMAL)
125	return (V)x;
126	if ((s & (CANCELLED \| INTERRUPTING)) != 0)
127	throw new CancellationException();
128	throw new ExecutionException((Throwable)x);
129	}
130
131	/**
132	* Creates a {@code FutureTask} that will, upon running, execute the
133	* given {@code Callable}.
134	*
135	* @param callable the callable task
136	* @throws NullPointerException if callable is null
137	*/
138	public FutureTask(Callable<V> callable) {
139	if (callable == null)
140	throw new NullPointerException();
141	this.callable = callable;
142	}
143
144	/**
145	* Creates a {@code FutureTask} that will, upon running, execute the
146	* given {@code Runnable}, and arrange that {@code get} will return the
147	* given result on successful completion.
148	*
149	* @param runnable the runnable task
150	* @param result the result to return on successful completion. If
151	* you don't need a particular result, consider using
152	* constructions of the form:
153	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
154	* @throws NullPointerException if runnable is null
155	*/
156	public FutureTask(Runnable runnable, V result) {
157	this.callable = Executors.callable(runnable, result);
158	}
159
160	public boolean isCancelled() {
161	return (state & (CANCELLED \| INTERRUPTING)) != 0;
162	}
163
164	public boolean isDone() {
165	return state != 0;
166	}
167
168	public boolean cancel(boolean mayInterruptIfRunning) {
169	return state == 0 &&
170	setCompletion(null, mayInterruptIfRunning ?
171	INTERRUPTING : CANCELLED);
172	}
173
174	/**
175	* @throws CancellationException {@inheritDoc}
176	*/
177	public V get() throws InterruptedException, ExecutionException {
178	int s = state;
179	if (s <= COMPLETING)
180	s = awaitDone(false, 0L);
181	return report(s);
182	}
183
184	/**
185	* @throws CancellationException {@inheritDoc}
186	*/
187	public V get(long timeout, TimeUnit unit)
188	throws InterruptedException, ExecutionException, TimeoutException {
189	int s = state;
190	if (s <= COMPLETING &&
191	(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
192	throw new TimeoutException();
193	return report(s);
194	}
195
196	/**
197	* Protected method invoked when this task transitions to state
198	* {@code isDone} (whether normally or via cancellation). The
199	* default implementation does nothing. Subclasses may override
200	* this method to invoke completion callbacks or perform
201	* bookkeeping. Note that you can query status inside the
202	* implementation of this method to determine whether this task
203	* has been cancelled.
204	*/
205	protected void done() { }
206
207	/**
208	* Sets the result of this future to the given value unless
209	* this future has already been set or has been cancelled.
210	*
211	* <p>This method is invoked internally by the {@link #run} method
212	* upon successful completion of the computation.
213	*
214	* @param v the value
215	*/
216	protected void set(V v) {
217	setCompletion(v, NORMAL);
218	}
219
220	/**
221	* Causes this future to report an {@link ExecutionException}
222	* with the given throwable as its cause, unless this future has
223	* already been set or has been cancelled.
224	*
225	* <p>This method is invoked internally by the {@link #run} method
226	* upon failure of the computation.
227	*
228	* @param t the cause of failure
229	*/
230	protected void setException(Throwable t) {
231	setCompletion(t, EXCEPTIONAL);
232	}
233
234	public void run() {
235	Thread r = Thread.currentThread();
236	if (state == 0 &&
237	UNSAFE.compareAndSwapObject(this, runnerOffset, null, r)) {
238	V result;
239	try {
240	result = callable.call();
241	} catch (Throwable ex) {
242	setException(ex);
243	return;
244	}
245	set(result);
246	}
247	}
248
249	/**
250	* Executes the computation without setting its result, and then
251	* resets this future to initial state, failing to do so if the
252	* computation encounters an exception or is cancelled. This is
253	* designed for use with tasks that intrinsically execute more
254	* than once.
255	*
256	* @return true if successfully run and reset
257	*/
258	protected boolean runAndReset() {
259	Thread r = Thread.currentThread();
260	if (state != 0 \|\|
261	!UNSAFE.compareAndSwapObject(this, runnerOffset, null, r))
262	return false;
263	try {
264	callable.call(); // don't set result
265	} catch (Throwable ex) {
266	setException(ex);
267	return false;
268	}
269	runner = null;
270	for (;;) {
271	int s = state;
272	if (s == 0)
273	return true;
274	if (s != INTERRUPTING)
275	return false;
276	Thread.yield(); // wait out pending cancellation interrupt
277	}
278	}
279
280	/**
281	* Simple linked list nodes to record waiting threads in a Treiber
282	* stack. See other classes such as Phaser and SynchronousQueue
283	* for more detailed explanation.
284	*/
285	static final class WaitNode {
286	volatile Thread thread;
287	WaitNode next;
288	}
289
290	/**
291	* Removes and signals all waiting threads.
292	*/
293	private void releaseAll() {
294	WaitNode q;
295	while ((q = waiters) != null) {
296	if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
297	for (;;) {
298	Thread t = q.thread;
299	if (t != null) {
300	q.thread = null;
301	LockSupport.unpark(t);
302	}
303	WaitNode next = q.next;
304	if (next == null)
305	return;
306	q.next = null; // unlink to help gc
307	q = next;
308	}
309	}
310	}
311	}
312
313	/**
314	* Awaits completion or aborts on interrupt or timeout.
315	*
316	* @param timed true if use timed waits
317	* @param nanos time to wait, if timed
318	* @return state upon completion
319	*/
320	private int awaitDone(boolean timed, long nanos)
321	throws InterruptedException {
322	long last = timed ? System.nanoTime() : 0L;
323	WaitNode q = null;
324	boolean queued = false;
325	for (;;) {
326	if (Thread.interrupted()) {
327	removeWaiter(q);
328	throw new InterruptedException();
329	}
330
331	int s = state;
332	if (s > COMPLETING) {
333	if (q != null)
334	q.thread = null;
335	return s;
336	}
337	else if (q == null)
338	q = new WaitNode();
339	else if (!queued)
340	queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
341	q.next = waiters, q);
342	else if (q.thread == null)
343	q.thread = Thread.currentThread();
344	else if (timed) {
345	long now = System.nanoTime();
346	if ((nanos -= (now - last)) <= 0L) {
347	removeWaiter(q);
348	return state;
349	}
350	last = now;
351	LockSupport.parkNanos(this, nanos);
352	}
353	else
354	LockSupport.park(this);
355	}
356	}
357
358	/**
359	* Tries to unlink a timed-out or interrupted wait node to avoid
360	* accumulating garbage. Internal nodes are simply unspliced
361	* without CAS since it is harmless if they are traversed anyway
362	* by releasers or concurrent calls to removeWaiter.
363	*/
364	private void removeWaiter(WaitNode node) {
365	if (node != null) {
366	node.thread = null;
367	WaitNode pred = null;
368	WaitNode q = waiters;
369	while (q != null) {
370	WaitNode next = node.next;
371	if (q != node) {
372	pred = q;
373	q = next;
374	}
375	else if (pred != null) {
376	pred.next = next;
377	break;
378	}
379	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
380	q, next))
381	break;
382	else { // restart on CAS failure
383	pred = null;
384	q = waiters;
385	}
386	}
387	}
388	}
389
390	// Unsafe mechanics
391	private static final sun.misc.Unsafe UNSAFE;
392	private static final long stateOffset;
393	private static final long runnerOffset;
394	private static final long waitersOffset;
395	static {
396	try {
397	UNSAFE = sun.misc.Unsafe.getUnsafe();
398	Class<?> k = FutureTask.class;
399	stateOffset = UNSAFE.objectFieldOffset
400	(k.getDeclaredField("state"));
401	runnerOffset = UNSAFE.objectFieldOffset
402	(k.getDeclaredField("runner"));
403	waitersOffset = UNSAFE.objectFieldOffset
404	(k.getDeclaredField("waiters"));
405	} catch (Exception e) {
406	throw new Error(e);
407	}
408	}
409
410	}