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) {
                    // 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;
                    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:	1.51
Committed:	Tue May 30 10:19:21 2006 UTC (18 years ago) by dl
Branch:	MAIN
Changes since 1.50:	+6 -0 lines
Log Message:	Add code comments explaining last change
#	User	Rev	Content
1	tim	1.1	/*
2	dl	1.2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.23	* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/licenses/publicdomain
5	tim	1.1	*/
6
7			package java.util.concurrent;
8	dl	1.13	import java.util.concurrent.locks.*;
9
10	tim	1.1	/**
11	dl	1.8	* 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	dl	1.4	* 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	dl	1.8	* the computation has completed, the computation cannot be restarted
18			* or cancelled.
19	tim	1.1	*
20	dl	1.8	* <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	tim	1.1	*
25	dl	1.14	* <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	tim	1.1	* @since 1.5
30	dl	1.4	* @author Doug Lea
31	dl	1.12	* @param <V> The result type returned by this FutureTask's <tt>get</tt> method
32	tim	1.1	*/
33	peierls	1.39	public class FutureTask<V> implements RunnableFuture<V> {
34	dl	1.24	/** Synchronization control for FutureTask */
35			private final Sync sync;
36	dl	1.11
37	tim	1.1	/**
38	dl	1.31	* Creates a <tt>FutureTask</tt> that will upon running, execute the
39	tim	1.1	* given <tt>Callable</tt>.
40			*
41			* @param callable the callable task
42	dl	1.9	* @throws NullPointerException if callable is null
43	tim	1.1	*/
44			public FutureTask(Callable<V> callable) {
45	dl	1.9	if (callable == null)
46			throw new NullPointerException();
47	dl	1.24	sync = new Sync(callable);
48	tim	1.1	}
49
50			/**
51	dl	1.31	* Creates a <tt>FutureTask</tt> that will upon running, execute the
52	tim	1.1	* 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	dl	1.9	* you don't need a particular result, consider using
58	dl	1.16	* constructions of the form:
59			* <tt>Future<?> f = new FutureTask<Object>(runnable, null)</tt>
60	dl	1.9	* @throws NullPointerException if runnable is null
61	tim	1.1	*/
62	dl	1.15	public FutureTask(Runnable runnable, V result) {
63	dl	1.24	sync = new Sync(Executors.callable(runnable, result));
64	dl	1.20	}
65
66			public boolean isCancelled() {
67	dl	1.27	return sync.innerIsCancelled();
68	dl	1.20	}
69	jsr166	1.35
70	dl	1.20	public boolean isDone() {
71	dl	1.27	return sync.innerIsDone();
72	dl	1.13	}
73
74			public boolean cancel(boolean mayInterruptIfRunning) {
75	dl	1.27	return sync.innerCancel(mayInterruptIfRunning);
76	dl	1.13	}
77	jsr166	1.35
78	jsr166	1.43	/**
79			* @throws CancellationException {@inheritDoc}
80			*/
81	dl	1.2	public V get() throws InterruptedException, ExecutionException {
82	dl	1.27	return sync.innerGet();
83	tim	1.1	}
84
85	jsr166	1.43	/**
86			* @throws CancellationException {@inheritDoc}
87			*/
88	dl	1.2	public V get(long timeout, TimeUnit unit)
89	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
90	dl	1.27	return sync.innerGet(unit.toNanos(timeout));
91	tim	1.1	}
92
93			/**
94	dl	1.20	* 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	dl	1.29	* this future has already been set or has been cancelled.
107	dl	1.40	* This method is invoked internally by the <tt>run</tt> method
108			* upon successful completion of the computation.
109	tim	1.1	* @param v the value
110	jsr166	1.35	*/
111	dl	1.2	protected void set(V v) {
112	dl	1.27	sync.innerSet(v);
113	tim	1.1	}
114
115			/**
116	dl	1.13	* Causes this future to report an <tt>ExecutionException</tt>
117	dl	1.20	* with the given throwable as its cause, unless this Future has
118	dl	1.24	* already been set or has been cancelled.
119	dl	1.40	* This method is invoked internally by the <tt>run</tt> method
120			* upon failure of the computation.
121	jsr166	1.41	* @param t the cause of failure
122	jsr166	1.35	*/
123	dl	1.2	protected void setException(Throwable t) {
124	dl	1.27	sync.innerSetException(t);
125	tim	1.1	}
126	jsr166	1.35
127	jsr166	1.44	// 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	dl	1.24	public void run() {
137	dl	1.27	sync.innerRun();
138	dl	1.24	}
139
140			/**
141	dl	1.30	* Executes the computation without setting its result, and then
142			* resets this Future to initial state, failing to do so if the
143	dl	1.24	* 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	dl	1.27	return sync.innerRunAndReset();
150	dl	1.14	}
151	dl	1.3
152	dl	1.14	/**
153	dl	1.24	* Synchronization control for FutureTask. Note that this must be
154	dl	1.30	* a non-static inner class in order to invoke the protected
155	dl	1.24	* <tt>done</tt> method. For clarity, all inner class support
156	dl	1.27	* methods are same as outer, prefixed with "inner".
157	dl	1.24	*
158			* Uses AQS sync state to represent run status
159	dl	1.20	*/
160	dl	1.24	private final class Sync extends AbstractQueuedSynchronizer {
161	dl	1.42	private static final long serialVersionUID = -7828117401763700385L;
162
163	dl	1.24	/** 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	jsr166	1.35	/**
178	dl	1.24	* The thread running task. When nulled after set/cancel, this
179			* indicates that the results are accessible. Must be
180	dl	1.32	* volatile, to ensure visibility upon completion.
181	dl	1.24	*/
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	dl	1.26	* Implements AQS base acquire to succeed if ran or cancelled
194	dl	1.24	*/
195	dl	1.26	protected int tryAcquireShared(int ignore) {
196	dl	1.27	return innerIsDone()? 1 : -1;
197	dl	1.24	}
198
199			/**
200			* Implements AQS base release to always signal after setting
201			* final done status by nulling runner thread.
202			*/
203	dl	1.25	protected boolean tryReleaseShared(int ignore) {
204	dl	1.24	runner = null;
205	jsr166	1.35	return true;
206	dl	1.24	}
207
208	dl	1.27	boolean innerIsCancelled() {
209	dl	1.24	return getState() == CANCELLED;
210			}
211	jsr166	1.35
212	dl	1.27	boolean innerIsDone() {
213	dl	1.24	return ranOrCancelled(getState()) && runner == null;
214			}
215
216	dl	1.27	V innerGet() throws InterruptedException, ExecutionException {
217	dl	1.24	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	dl	1.27	V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
226	dl	1.28	if (!tryAcquireSharedNanos(0, nanosTimeout))
227	jsr166	1.35	throw new TimeoutException();
228	dl	1.24	if (getState() == CANCELLED)
229			throw new CancellationException();
230			if (exception != null)
231			throw new ExecutionException(exception);
232			return result;
233			}
234
235	dl	1.27	void innerSet(V v) {
236	jsr166	1.38	for (;;) {
237			int s = getState();
238	dl	1.50	if (s == RAN)
239			return;
240			if (s == CANCELLED) {
241	dl	1.51	// aggressively release to set runner to null,
242			// in case we are racing with a cancel request
243			// that will try to interrupt runner
244	dl	1.50	releaseShared(0);
245			return;
246			}
247			if (compareAndSetState(s, RAN)) {
248			result = v;
249			releaseShared(0);
250			done();
251	jsr166	1.38	return;
252	dl	1.50	}
253			}
254	dl	1.24	}
255
256	dl	1.27	void innerSetException(Throwable t) {
257	jsr166	1.38	for (;;) {
258			int s = getState();
259	dl	1.50	if (s == RAN)
260			return;
261			if (s == CANCELLED) {
262	dl	1.51	// aggressively release to set runner to null,
263			// in case we are racing with a cancel request
264			// that will try to interrupt runner
265	dl	1.50	releaseShared(0);
266			return;
267			}
268			if (compareAndSetState(s, RAN)) {
269			exception = t;
270			result = null;
271			releaseShared(0);
272			done();
273	jsr166	1.38	return;
274	dl	1.50	}
275	jsr166	1.38	}
276	dl	1.24	}
277
278	dl	1.27	boolean innerCancel(boolean mayInterruptIfRunning) {
279	jsr166	1.38	for (;;) {
280			int s = getState();
281			if (ranOrCancelled(s))
282			return false;
283			if (compareAndSetState(s, CANCELLED))
284			break;
285			}
286	dl	1.24	if (mayInterruptIfRunning) {
287			Thread r = runner;
288	dl	1.50	if (r != null)
289			r.interrupt();
290	dl	1.24	}
291			releaseShared(0);
292			done();
293	dl	1.14	return true;
294			}
295
296	dl	1.27	void innerRun() {
297	jsr166	1.35	if (!compareAndSetState(0, RUNNING))
298	dl	1.24	return;
299	dl	1.14	try {
300	dl	1.24	runner = Thread.currentThread();
301	dl	1.37	if (getState() == RUNNING) // recheck after setting thread
302	dl	1.36	innerSet(callable.call());
303			else
304	dl	1.37	releaseShared(0); // cancel
305	jsr166	1.34	} catch (Throwable ex) {
306	dl	1.27	innerSetException(ex);
307	jsr166	1.35	}
308	dl	1.14	}
309
310	dl	1.27	boolean innerRunAndReset() {
311	jsr166	1.35	if (!compareAndSetState(0, RUNNING))
312	dl	1.24	return false;
313	dl	1.15	try {
314	dl	1.24	runner = Thread.currentThread();
315	dl	1.36	if (getState() == RUNNING)
316			callable.call(); // don't set result
317	dl	1.24	runner = null;
318			return compareAndSetState(RUNNING, 0);
319	jsr166	1.34	} catch (Throwable ex) {
320	dl	1.27	innerSetException(ex);
321	dl	1.24	return false;
322	jsr166	1.35	}
323	dl	1.14	}
324	dl	1.15	}
325			}