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/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;

/**
 * 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 <tt>get</tt>
 * method will block if the computation has not yet completed.  Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled.
 *
 * <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or
 * {@link java.lang.Runnable} object.  Because <tt>FutureTask</tt>
 * implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be
 * submitted to an {@link Executor} for execution.
 *
 * <p>In addition to serving as a standalone class, this class provides
 * <tt>protected</tt> 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 <tt>get</tt> method
 */
public class FutureTask<V> implements RunnableFuture<V> {
    /** Synchronization control for FutureTask */
    private final Sync sync;

    /**
     * Creates a <tt>FutureTask</tt> that will, upon running, execute the
     * given <tt>Callable</tt>.
     *
     * @param  callable the callable task
     * @throws NullPointerException if callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        sync = new Sync(callable);
    }

    /**
     * Creates a <tt>FutureTask</tt> that will, upon running, execute the
     * given <tt>Runnable</tt>, and arrange that <tt>get</tt> 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:
     * <tt>Future&lt;?&gt; f = new FutureTask&lt;Object&gt;(runnable, null)</tt>
     * @throws NullPointerException if runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        sync = new Sync(Executors.callable(runnable, result));
    }

    public boolean isCancelled() {
        return sync.innerIsCancelled();
    }

    public boolean isDone() {
        return sync.innerIsDone();
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return sync.innerCancel(mayInterruptIfRunning);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        return sync.innerGet();
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        return sync.innerGet(unit.toNanos(timeout));
    }

    /**
     * Protected method invoked when this task transitions to state
     * <tt>isDone</tt> (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.
     * This method is invoked internally by the <tt>run</tt> method
     * upon successful completion of the computation.
     * @param v the value
     */
    protected void set(V v) {
        sync.innerSet(v);
    }

    /**
     * Causes this future to report an <tt>ExecutionException</tt>
     * with the given throwable as its cause, unless this Future has
     * already been set or has been cancelled.
     * This method is invoked internally by the <tt>run</tt> method
     * upon failure of the computation.
     * @param t the cause of failure
     */
    protected void setException(Throwable t) {
        sync.innerSetException(t);
    }

    // The following (duplicated) doc comment can be removed once
    //
    // 6270645: Javadoc comments should be inherited from most derived
    //          superinterface or superclass
    // is fixed.
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    public void run() {
        sync.innerRun();
    }

    /**
     * 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() {
        return sync.innerRunAndReset();
    }

    /**
     * Synchronization control for FutureTask. Note that this must be
     * a non-static inner class in order to invoke the protected
     * <tt>done</tt> method. For clarity, all inner class support
     * methods are same as outer, prefixed with "inner".
     *
     * Uses AQS sync state to represent run status
     */
    private final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -7828117401763700385L;

        /** State value representing that task is ready to run */
        private static final int READY     = 0;
        /** State value representing that task is running */
        private static final int RUNNING   = 1;
        /** State value representing that task ran */
        private static final int RAN       = 2;
        /** State value representing that task was cancelled */
        private static final int CANCELLED = 4;

        /** The underlying callable */
        private final Callable<V> callable;
        /** The result to return from get() */
        private V result;
        /** The exception to throw from get() */
        private Throwable exception;

        /**
         * The thread running task. When nulled after set/cancel, this
         * indicates that the results are accessible.  Must be
         * volatile, to ensure visibility upon completion.
         */
        private volatile Thread runner;

        Sync(Callable<V> callable) {
            this.callable = callable;
        }

        private boolean ranOrCancelled(int state) {
            return (state & (RAN | CANCELLED)) != 0;
        }

        /**
         * Implements AQS base acquire to succeed if ran or cancelled
         */
        protected int tryAcquireShared(int ignore) {
            return innerIsDone() ? 1 : -1;
        }

        /**
         * Implements AQS base release to always signal after setting
         * final done status by nulling runner thread.
         */
        protected boolean tryReleaseShared(int ignore) {
            runner = null;
            return true;
        }

        boolean innerIsCancelled() {
            return getState() == CANCELLED;
        }

        boolean innerIsDone() {
            return ranOrCancelled(getState()) && runner == null;
        }

        V innerGet() throws InterruptedException, ExecutionException {
            acquireSharedInterruptibly(0);
            if (getState() == CANCELLED)
                throw new CancellationException();
            if (exception != null)
                throw new ExecutionException(exception);
            return result;
        }

        V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
            if (!tryAcquireSharedNanos(0, nanosTimeout))
                throw new TimeoutException();
            if (getState() == CANCELLED)
                throw new CancellationException();
            if (exception != null)
                throw new ExecutionException(exception);
            return result;
        }

        void innerSet(V v) {
            for (;;) {
                int s = getState();
                if (s == RAN)
                    return;
                if (s == CANCELLED) {
                    // aggressively release to set runner to null,
                    // in case we are racing with a cancel request
                    // that will try to interrupt runner
                    releaseShared(0);
                    return;
                }
                if (compareAndSetState(s, RAN)) {
                    result = v;
                    releaseShared(0);
                    done();
                    return;
                }
            }
        }

        void innerSetException(Throwable t) {
            for (;;) {
                int s = getState();
                if (s == RAN)
                    return;
                if (s == CANCELLED) {
                    // aggressively release to set runner to null,
                    // in case we are racing with a cancel request
                    // that will try to interrupt runner
                    releaseShared(0);
                    return;
                }
                if (compareAndSetState(s, RAN)) {
                    exception = t;
                    releaseShared(0);
                    done();
                    return;
                }
            }
        }

        boolean innerCancel(boolean mayInterruptIfRunning) {
            for (;;) {
                int s = getState();
                if (ranOrCancelled(s))
                    return false;
                if (compareAndSetState(s, CANCELLED))
                    break;
            }
            if (mayInterruptIfRunning) {
                Thread r = runner;
                if (r != null)
                    r.interrupt();
            }
            releaseShared(0);
            done();
            return true;
        }

        void innerRun() {
            if (!compareAndSetState(READY, RUNNING))
                return;

            runner = Thread.currentThread();
            if (getState() == RUNNING) { // recheck after setting thread
                V result;
                try {
                    result = callable.call();
                } catch (Throwable ex) {
                    setException(ex);
                    return;
                }
                set(result);
            } else {
                releaseShared(0); // cancel
            }
        }

        boolean innerRunAndReset() {
            if (!compareAndSetState(READY, RUNNING))
                return false;
            try {
                runner = Thread.currentThread();
                if (getState() == RUNNING)
                    callable.call(); // don't set result
                runner = null;
                return compareAndSetState(RUNNING, READY);
            } catch (Throwable ex) {
                setException(ex);
                return false;
            }
        }
    }
}
Revision:	1.56
Committed:	Sun May 18 23:47:56 2008 UTC (16 years ago) by jsr166
Branch:	MAIN
Changes since 1.55:	+32 -32 lines
Log Message:	Sync with OpenJDK; untabify
#	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/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.*;
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 <tt>get</tt>
16	* method will block if the computation has not yet completed. Once
17	* the computation has completed, the computation cannot be restarted
18	* or cancelled.
19	*
20	* <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or
21	* {@link java.lang.Runnable} object. Because <tt>FutureTask</tt>
22	* implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be
23	* submitted to an {@link Executor} for execution.
24	*
25	* <p>In addition to serving as a standalone class, this class provides
26	* <tt>protected</tt> functionality that may be useful when creating
27	* customized task classes.
28	*
29	* @since 1.5
30	* @author Doug Lea
31	* @param <V> The result type returned by this FutureTask's <tt>get</tt> method
32	*/
33	public class FutureTask<V> implements RunnableFuture<V> {
34	/** Synchronization control for FutureTask */
35	private final Sync sync;
36
37	/**
38	* Creates a <tt>FutureTask</tt> that will, upon running, execute the
39	* given <tt>Callable</tt>.
40	*
41	* @param callable the callable task
42	* @throws NullPointerException if callable is null
43	*/
44	public FutureTask(Callable<V> callable) {
45	if (callable == null)
46	throw new NullPointerException();
47	sync = new Sync(callable);
48	}
49
50	/**
51	* Creates a <tt>FutureTask</tt> that will, upon running, execute the
52	* given <tt>Runnable</tt>, and arrange that <tt>get</tt> will return the
53	* given result on successful completion.
54	*
55	* @param runnable the runnable task
56	* @param result the result to return on successful completion. If
57	* you don't need a particular result, consider using
58	* constructions of the form:
59	* <tt>Future<?> f = new FutureTask<Object>(runnable, null)</tt>
60	* @throws NullPointerException if runnable is null
61	*/
62	public FutureTask(Runnable runnable, V result) {
63	sync = new Sync(Executors.callable(runnable, result));
64	}
65
66	public boolean isCancelled() {
67	return sync.innerIsCancelled();
68	}
69
70	public boolean isDone() {
71	return sync.innerIsDone();
72	}
73
74	public boolean cancel(boolean mayInterruptIfRunning) {
75	return sync.innerCancel(mayInterruptIfRunning);
76	}
77
78	/**
79	* @throws CancellationException {@inheritDoc}
80	*/
81	public V get() throws InterruptedException, ExecutionException {
82	return sync.innerGet();
83	}
84
85	/**
86	* @throws CancellationException {@inheritDoc}
87	*/
88	public V get(long timeout, TimeUnit unit)
89	throws InterruptedException, ExecutionException, TimeoutException {
90	return sync.innerGet(unit.toNanos(timeout));
91	}
92
93	/**
94	* Protected method invoked when this task transitions to state
95	* <tt>isDone</tt> (whether normally or via cancellation). The
96	* default implementation does nothing. Subclasses may override
97	* this method to invoke completion callbacks or perform
98	* bookkeeping. Note that you can query status inside the
99	* implementation of this method to determine whether this task
100	* has been cancelled.
101	*/
102	protected void done() { }
103
104	/**
105	* Sets the result of this Future to the given value unless
106	* this future has already been set or has been cancelled.
107	* This method is invoked internally by the <tt>run</tt> method
108	* upon successful completion of the computation.
109	* @param v the value
110	*/
111	protected void set(V v) {
112	sync.innerSet(v);
113	}
114
115	/**
116	* Causes this future to report an <tt>ExecutionException</tt>
117	* with the given throwable as its cause, unless this Future has
118	* already been set or has been cancelled.
119	* This method is invoked internally by the <tt>run</tt> method
120	* upon failure of the computation.
121	* @param t the cause of failure
122	*/
123	protected void setException(Throwable t) {
124	sync.innerSetException(t);
125	}
126
127	// The following (duplicated) doc comment can be removed once
128	//
129	// 6270645: Javadoc comments should be inherited from most derived
130	// superinterface or superclass
131	// is fixed.
132	/**
133	* Sets this Future to the result of its computation
134	* unless it has been cancelled.
135	*/
136	public void run() {
137	sync.innerRun();
138	}
139
140	/**
141	* Executes the computation without setting its result, and then
142	* resets this Future to initial state, failing to do so if the
143	* computation encounters an exception or is cancelled. This is
144	* designed for use with tasks that intrinsically execute more
145	* than once.
146	* @return true if successfully run and reset
147	*/
148	protected boolean runAndReset() {
149	return sync.innerRunAndReset();
150	}
151
152	/**
153	* Synchronization control for FutureTask. Note that this must be
154	* a non-static inner class in order to invoke the protected
155	* <tt>done</tt> method. For clarity, all inner class support
156	* methods are same as outer, prefixed with "inner".
157	*
158	* Uses AQS sync state to represent run status
159	*/
160	private final class Sync extends AbstractQueuedSynchronizer {
161	private static final long serialVersionUID = -7828117401763700385L;
162
163	/** State value representing that task is ready to run */
164	private static final int READY = 0;
165	/** State value representing that task is running */
166	private static final int RUNNING = 1;
167	/** State value representing that task ran */
168	private static final int RAN = 2;
169	/** State value representing that task was cancelled */
170	private static final int CANCELLED = 4;
171
172	/** The underlying callable */
173	private final Callable<V> callable;
174	/** The result to return from get() */
175	private V result;
176	/** The exception to throw from get() */
177	private Throwable exception;
178
179	/**
180	* The thread running task. When nulled after set/cancel, this
181	* indicates that the results are accessible. Must be
182	* volatile, to ensure visibility upon completion.
183	*/
184	private volatile Thread runner;
185
186	Sync(Callable<V> callable) {
187	this.callable = callable;
188	}
189
190	private boolean ranOrCancelled(int state) {
191	return (state & (RAN \| CANCELLED)) != 0;
192	}
193
194	/**
195	* Implements AQS base acquire to succeed if ran or cancelled
196	*/
197	protected int tryAcquireShared(int ignore) {
198	return innerIsDone() ? 1 : -1;
199	}
200
201	/**
202	* Implements AQS base release to always signal after setting
203	* final done status by nulling runner thread.
204	*/
205	protected boolean tryReleaseShared(int ignore) {
206	runner = null;
207	return true;
208	}
209
210	boolean innerIsCancelled() {
211	return getState() == CANCELLED;
212	}
213
214	boolean innerIsDone() {
215	return ranOrCancelled(getState()) && runner == null;
216	}
217
218	V innerGet() throws InterruptedException, ExecutionException {
219	acquireSharedInterruptibly(0);
220	if (getState() == CANCELLED)
221	throw new CancellationException();
222	if (exception != null)
223	throw new ExecutionException(exception);
224	return result;
225	}
226
227	V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
228	if (!tryAcquireSharedNanos(0, nanosTimeout))
229	throw new TimeoutException();
230	if (getState() == CANCELLED)
231	throw new CancellationException();
232	if (exception != null)
233	throw new ExecutionException(exception);
234	return result;
235	}
236
237	void innerSet(V v) {
238	for (;;) {
239	int s = getState();
240	if (s == RAN)
241	return;
242	if (s == CANCELLED) {
243	// aggressively release to set runner to null,
244	// in case we are racing with a cancel request
245	// that will try to interrupt runner
246	releaseShared(0);
247	return;
248	}
249	if (compareAndSetState(s, RAN)) {
250	result = v;
251	releaseShared(0);
252	done();
253	return;
254	}
255	}
256	}
257
258	void innerSetException(Throwable t) {
259	for (;;) {
260	int s = getState();
261	if (s == RAN)
262	return;
263	if (s == CANCELLED) {
264	// aggressively release to set runner to null,
265	// in case we are racing with a cancel request
266	// that will try to interrupt runner
267	releaseShared(0);
268	return;
269	}
270	if (compareAndSetState(s, RAN)) {
271	exception = t;
272	releaseShared(0);
273	done();
274	return;
275	}
276	}
277	}
278
279	boolean innerCancel(boolean mayInterruptIfRunning) {
280	for (;;) {
281	int s = getState();
282	if (ranOrCancelled(s))
283	return false;
284	if (compareAndSetState(s, CANCELLED))
285	break;
286	}
287	if (mayInterruptIfRunning) {
288	Thread r = runner;
289	if (r != null)
290	r.interrupt();
291	}
292	releaseShared(0);
293	done();
294	return true;
295	}
296
297	void innerRun() {
298	if (!compareAndSetState(READY, RUNNING))
299	return;
300
301	runner = Thread.currentThread();
302	if (getState() == RUNNING) { // recheck after setting thread
303	V result;
304	try {
305	result = callable.call();
306	} catch (Throwable ex) {
307	setException(ex);
308	return;
309	}
310	set(result);
311	} else {
312	releaseShared(0); // cancel
313	}
314	}
315
316	boolean innerRunAndReset() {
317	if (!compareAndSetState(READY, RUNNING))
318	return false;
319	try {
320	runner = Thread.currentThread();
321	if (getState() == RUNNING)
322	callable.call(); // don't set result
323	runner = null;
324	return compareAndSetState(RUNNING, READY);
325	} catch (Throwable ex) {
326	setException(ex);
327	return false;
328	}
329	}
330	}
331	}