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 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) {
                    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) {
                    releaseShared(0);
                    return;
                }
                if (compareAndSetState(s, RAN)) {
                    exception = t;
                    result = null;
                    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(0, RUNNING))
                return;
            try {
                runner = Thread.currentThread();
                if (getState() == RUNNING) // recheck after setting thread
                    innerSet(callable.call());
                else
                    releaseShared(0); // cancel
            } catch (Throwable ex) {
                innerSetException(ex);
            }
        }

        boolean innerRunAndReset() {
            if (!compareAndSetState(0, RUNNING))
                return false;
            try {
                runner = Thread.currentThread();
                if (getState() == RUNNING)
                    callable.call(); // don't set result
                runner = null;
                return compareAndSetState(RUNNING, 0);
            } catch (Throwable ex) {
                innerSetException(ex);
                return false;
            }
        }
    }
}
Revision:	1.50
Committed:	Sun May 28 13:46:16 2006 UTC (18 years ago) by dl
Branch:	MAIN
Changes since 1.49:	+26 -19 lines
Log Message:	Avoid stale thread reference
#	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 running */
164	private static final int RUNNING = 1;
165	/** State value representing that task ran */
166	private static final int RAN = 2;
167	/** State value representing that task was cancelled */
168	private static final int CANCELLED = 4;
169
170	/** The underlying callable */
171	private final Callable<V> callable;
172	/** The result to return from get() */
173	private V result;
174	/** The exception to throw from get() */
175	private Throwable exception;
176
177	/**
178	* The thread running task. When nulled after set/cancel, this
179	* indicates that the results are accessible. Must be
180	* volatile, to ensure visibility upon completion.
181	*/
182	private volatile Thread runner;
183
184	Sync(Callable<V> callable) {
185	this.callable = callable;
186	}
187
188	private boolean ranOrCancelled(int state) {
189	return (state & (RAN \| CANCELLED)) != 0;
190	}
191
192	/**
193	* Implements AQS base acquire to succeed if ran or cancelled
194	*/
195	protected int tryAcquireShared(int ignore) {
196	return innerIsDone()? 1 : -1;
197	}
198
199	/**
200	* Implements AQS base release to always signal after setting
201	* final done status by nulling runner thread.
202	*/
203	protected boolean tryReleaseShared(int ignore) {
204	runner = null;
205	return true;
206	}
207
208	boolean innerIsCancelled() {
209	return getState() == CANCELLED;
210	}
211
212	boolean innerIsDone() {
213	return ranOrCancelled(getState()) && runner == null;
214	}
215
216	V innerGet() throws InterruptedException, ExecutionException {
217	acquireSharedInterruptibly(0);
218	if (getState() == CANCELLED)
219	throw new CancellationException();
220	if (exception != null)
221	throw new ExecutionException(exception);
222	return result;
223	}
224
225	V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
226	if (!tryAcquireSharedNanos(0, nanosTimeout))
227	throw new TimeoutException();
228	if (getState() == CANCELLED)
229	throw new CancellationException();
230	if (exception != null)
231	throw new ExecutionException(exception);
232	return result;
233	}
234
235	void innerSet(V v) {
236	for (;;) {
237	int s = getState();
238	if (s == RAN)
239	return;
240	if (s == CANCELLED) {
241	releaseShared(0);
242	return;
243	}
244	if (compareAndSetState(s, RAN)) {
245	result = v;
246	releaseShared(0);
247	done();
248	return;
249	}
250	}
251	}
252
253	void innerSetException(Throwable t) {
254	for (;;) {
255	int s = getState();
256	if (s == RAN)
257	return;
258	if (s == CANCELLED) {
259	releaseShared(0);
260	return;
261	}
262	if (compareAndSetState(s, RAN)) {
263	exception = t;
264	result = null;
265	releaseShared(0);
266	done();
267	return;
268	}
269	}
270	}
271
272	boolean innerCancel(boolean mayInterruptIfRunning) {
273	for (;;) {
274	int s = getState();
275	if (ranOrCancelled(s))
276	return false;
277	if (compareAndSetState(s, CANCELLED))
278	break;
279	}
280	if (mayInterruptIfRunning) {
281	Thread r = runner;
282	if (r != null)
283	r.interrupt();
284	}
285	releaseShared(0);
286	done();
287	return true;
288	}
289
290	void innerRun() {
291	if (!compareAndSetState(0, RUNNING))
292	return;
293	try {
294	runner = Thread.currentThread();
295	if (getState() == RUNNING) // recheck after setting thread
296	innerSet(callable.call());
297	else
298	releaseShared(0); // cancel
299	} catch (Throwable ex) {
300	innerSetException(ex);
301	}
302	}
303
304	boolean innerRunAndReset() {
305	if (!compareAndSetState(0, RUNNING))
306	return false;
307	try {
308	runner = Thread.currentThread();
309	if (getState() == RUNNING)
310	callable.call(); // don't set result
311	runner = null;
312	return compareAndSetState(RUNNING, 0);
313	} catch (Throwable ex) {
314	innerSetException(ex);
315	return false;
316	}
317	}
318	}
319	}