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.*; // for javadoc (till 6280605 is fixed)
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.
 *
 * <p> Memory visibility effects: State changes to the resulting value
 * or exception <a
 * href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a call to <tt>get()</tt>.
 *
 * @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 (ranOrCancelled(s))
                    return;
                if (compareAndSetState(s, RAN))
                    break;
            }
            result = v;
            releaseShared(0);
            done();
        }

        void innerSetException(Throwable t) {
            for (;;) {
                int s = getState();
                if (ranOrCancelled(s))
                    return;
                if (compareAndSetState(s, RAN))
                    break;
            }
            exception = t;
            result = null;
            releaseShared(0);
            done();
        }

        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:	1.45
Committed:	Fri Sep 2 01:03:08 2005 UTC (18 years, 9 months ago) by brian
Branch:	MAIN
Changes since 1.44:	+5 -0 lines
Log Message:	Happens-before markup
#	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	jsr166	1.43	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
9	dl	1.13	import java.util.concurrent.locks.*;
10
11	tim	1.1	/**
12	dl	1.8	* A cancellable asynchronous computation. This class provides a base
13			* implementation of {@link Future}, with methods to start and cancel
14			* a computation, query to see if the computation is complete, and
15	dl	1.4	* retrieve the result of the computation. The result can only be
16			* retrieved when the computation has completed; the <tt>get</tt>
17			* method will block if the computation has not yet completed. Once
18	dl	1.8	* the computation has completed, the computation cannot be restarted
19			* or cancelled.
20	tim	1.1	*
21	dl	1.8	* <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or
22			* {@link java.lang.Runnable} object. Because <tt>FutureTask</tt>
23			* implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be
24			* submitted to an {@link Executor} for execution.
25	tim	1.1	*
26	dl	1.14	* <p>In addition to serving as a standalone class, this class provides
27			* <tt>protected</tt> functionality that may be useful when creating
28			* customized task classes.
29			*
30	brian	1.45	* <p> Memory visibility effects: State changes to the resulting value
31			* or exception <a
32			* href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
33			* actions following a call to <tt>get()</tt>.
34			*
35	tim	1.1	* @since 1.5
36	dl	1.4	* @author Doug Lea
37	dl	1.12	* @param <V> The result type returned by this FutureTask's <tt>get</tt> method
38	tim	1.1	*/
39	peierls	1.39	public class FutureTask<V> implements RunnableFuture<V> {
40	dl	1.24	/** Synchronization control for FutureTask */
41			private final Sync sync;
42	dl	1.11
43	tim	1.1	/**
44	dl	1.31	* Creates a <tt>FutureTask</tt> that will upon running, execute the
45	tim	1.1	* given <tt>Callable</tt>.
46			*
47			* @param callable the callable task
48	dl	1.9	* @throws NullPointerException if callable is null
49	tim	1.1	*/
50			public FutureTask(Callable<V> callable) {
51	dl	1.9	if (callable == null)
52			throw new NullPointerException();
53	dl	1.24	sync = new Sync(callable);
54	tim	1.1	}
55
56			/**
57	dl	1.31	* Creates a <tt>FutureTask</tt> that will upon running, execute the
58	tim	1.1	* given <tt>Runnable</tt>, and arrange that <tt>get</tt> will return the
59			* given result on successful completion.
60			*
61			* @param runnable the runnable task
62			* @param result the result to return on successful completion. If
63	dl	1.9	* you don't need a particular result, consider using
64	dl	1.16	* constructions of the form:
65			* <tt>Future<?> f = new FutureTask<Object>(runnable, null)</tt>
66	dl	1.9	* @throws NullPointerException if runnable is null
67	tim	1.1	*/
68	dl	1.15	public FutureTask(Runnable runnable, V result) {
69	dl	1.24	sync = new Sync(Executors.callable(runnable, result));
70	dl	1.20	}
71
72			public boolean isCancelled() {
73	dl	1.27	return sync.innerIsCancelled();
74	dl	1.20	}
75	jsr166	1.35
76	dl	1.20	public boolean isDone() {
77	dl	1.27	return sync.innerIsDone();
78	dl	1.13	}
79
80			public boolean cancel(boolean mayInterruptIfRunning) {
81	dl	1.27	return sync.innerCancel(mayInterruptIfRunning);
82	dl	1.13	}
83	jsr166	1.35
84	jsr166	1.43	/**
85			* @throws CancellationException {@inheritDoc}
86			*/
87	dl	1.2	public V get() throws InterruptedException, ExecutionException {
88	dl	1.27	return sync.innerGet();
89	tim	1.1	}
90
91	jsr166	1.43	/**
92			* @throws CancellationException {@inheritDoc}
93			*/
94	dl	1.2	public V get(long timeout, TimeUnit unit)
95	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
96	dl	1.27	return sync.innerGet(unit.toNanos(timeout));
97	tim	1.1	}
98
99			/**
100	dl	1.20	* Protected method invoked when this task transitions to state
101			* <tt>isDone</tt> (whether normally or via cancellation). The
102			* default implementation does nothing. Subclasses may override
103			* this method to invoke completion callbacks or perform
104			* bookkeeping. Note that you can query status inside the
105			* implementation of this method to determine whether this task
106			* has been cancelled.
107			*/
108			protected void done() { }
109
110			/**
111			* Sets the result of this Future to the given value unless
112	dl	1.29	* this future has already been set or has been cancelled.
113	dl	1.40	* This method is invoked internally by the <tt>run</tt> method
114			* upon successful completion of the computation.
115	tim	1.1	* @param v the value
116	jsr166	1.35	*/
117	dl	1.2	protected void set(V v) {
118	dl	1.27	sync.innerSet(v);
119	tim	1.1	}
120
121			/**
122	dl	1.13	* Causes this future to report an <tt>ExecutionException</tt>
123	dl	1.20	* with the given throwable as its cause, unless this Future has
124	dl	1.24	* already been set or has been cancelled.
125	dl	1.40	* This method is invoked internally by the <tt>run</tt> method
126			* upon failure of the computation.
127	jsr166	1.41	* @param t the cause of failure
128	jsr166	1.35	*/
129	dl	1.2	protected void setException(Throwable t) {
130	dl	1.27	sync.innerSetException(t);
131	tim	1.1	}
132	jsr166	1.35
133	jsr166	1.44	// The following (duplicated) doc comment can be removed once
134			//
135			// 6270645: Javadoc comments should be inherited from most derived
136			// superinterface or superclass
137			// is fixed.
138			/**
139			* Sets this Future to the result of its computation
140			* unless it has been cancelled.
141			*/
142	dl	1.24	public void run() {
143	dl	1.27	sync.innerRun();
144	dl	1.24	}
145
146			/**
147	dl	1.30	* Executes the computation without setting its result, and then
148			* resets this Future to initial state, failing to do so if the
149	dl	1.24	* computation encounters an exception or is cancelled. This is
150			* designed for use with tasks that intrinsically execute more
151			* than once.
152			* @return true if successfully run and reset
153			*/
154			protected boolean runAndReset() {
155	dl	1.27	return sync.innerRunAndReset();
156	dl	1.14	}
157	dl	1.3
158	dl	1.14	/**
159	dl	1.24	* Synchronization control for FutureTask. Note that this must be
160	dl	1.30	* a non-static inner class in order to invoke the protected
161	dl	1.24	* <tt>done</tt> method. For clarity, all inner class support
162	dl	1.27	* methods are same as outer, prefixed with "inner".
163	dl	1.24	*
164			* Uses AQS sync state to represent run status
165	dl	1.20	*/
166	dl	1.24	private final class Sync extends AbstractQueuedSynchronizer {
167	dl	1.42	private static final long serialVersionUID = -7828117401763700385L;
168
169	dl	1.24	/** State value representing that task is running */
170			private static final int RUNNING = 1;
171			/** State value representing that task ran */
172			private static final int RAN = 2;
173			/** State value representing that task was cancelled */
174			private static final int CANCELLED = 4;
175
176			/** The underlying callable */
177			private final Callable<V> callable;
178			/** The result to return from get() */
179			private V result;
180			/** The exception to throw from get() */
181			private Throwable exception;
182
183	jsr166	1.35	/**
184	dl	1.24	* The thread running task. When nulled after set/cancel, this
185			* indicates that the results are accessible. Must be
186	dl	1.32	* volatile, to ensure visibility upon completion.
187	dl	1.24	*/
188			private volatile Thread runner;
189
190			Sync(Callable<V> callable) {
191			this.callable = callable;
192			}
193
194			private boolean ranOrCancelled(int state) {
195			return (state & (RAN \| CANCELLED)) != 0;
196			}
197
198			/**
199	dl	1.26	* Implements AQS base acquire to succeed if ran or cancelled
200	dl	1.24	*/
201	dl	1.26	protected int tryAcquireShared(int ignore) {
202	dl	1.27	return innerIsDone()? 1 : -1;
203	dl	1.24	}
204
205			/**
206			* Implements AQS base release to always signal after setting
207			* final done status by nulling runner thread.
208			*/
209	dl	1.25	protected boolean tryReleaseShared(int ignore) {
210	dl	1.24	runner = null;
211	jsr166	1.35	return true;
212	dl	1.24	}
213
214	dl	1.27	boolean innerIsCancelled() {
215	dl	1.24	return getState() == CANCELLED;
216			}
217	jsr166	1.35
218	dl	1.27	boolean innerIsDone() {
219	dl	1.24	return ranOrCancelled(getState()) && runner == null;
220			}
221
222	dl	1.27	V innerGet() throws InterruptedException, ExecutionException {
223	dl	1.24	acquireSharedInterruptibly(0);
224			if (getState() == CANCELLED)
225			throw new CancellationException();
226			if (exception != null)
227			throw new ExecutionException(exception);
228			return result;
229			}
230
231	dl	1.27	V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
232	dl	1.28	if (!tryAcquireSharedNanos(0, nanosTimeout))
233	jsr166	1.35	throw new TimeoutException();
234	dl	1.24	if (getState() == CANCELLED)
235			throw new CancellationException();
236			if (exception != null)
237			throw new ExecutionException(exception);
238			return result;
239			}
240
241	dl	1.27	void innerSet(V v) {
242	jsr166	1.38	for (;;) {
243			int s = getState();
244			if (ranOrCancelled(s))
245			return;
246			if (compareAndSetState(s, RAN))
247			break;
248			}
249	dl	1.24	result = v;
250			releaseShared(0);
251			done();
252			}
253
254	dl	1.27	void innerSetException(Throwable t) {
255	jsr166	1.38	for (;;) {
256			int s = getState();
257			if (ranOrCancelled(s))
258			return;
259			if (compareAndSetState(s, RAN))
260			break;
261			}
262	dl	1.24	exception = t;
263			result = null;
264			releaseShared(0);
265			done();
266			}
267
268	dl	1.27	boolean innerCancel(boolean mayInterruptIfRunning) {
269	jsr166	1.38	for (;;) {
270			int s = getState();
271			if (ranOrCancelled(s))
272			return false;
273			if (compareAndSetState(s, CANCELLED))
274			break;
275			}
276	dl	1.24	if (mayInterruptIfRunning) {
277			Thread r = runner;
278			if (r != null)
279			r.interrupt();
280			}
281			releaseShared(0);
282			done();
283	dl	1.14	return true;
284			}
285
286	dl	1.27	void innerRun() {
287	jsr166	1.35	if (!compareAndSetState(0, RUNNING))
288	dl	1.24	return;
289	dl	1.14	try {
290	dl	1.24	runner = Thread.currentThread();
291	dl	1.37	if (getState() == RUNNING) // recheck after setting thread
292	dl	1.36	innerSet(callable.call());
293			else
294	dl	1.37	releaseShared(0); // cancel
295	jsr166	1.34	} catch (Throwable ex) {
296	dl	1.27	innerSetException(ex);
297	jsr166	1.35	}
298	dl	1.14	}
299
300	dl	1.27	boolean innerRunAndReset() {
301	jsr166	1.35	if (!compareAndSetState(0, RUNNING))
302	dl	1.24	return false;
303	dl	1.15	try {
304	dl	1.24	runner = Thread.currentThread();
305	dl	1.36	if (getState() == RUNNING)
306			callable.call(); // don't set result
307	dl	1.24	runner = null;
308			return compareAndSetState(RUNNING, 0);
309	jsr166	1.34	} catch (Throwable ex) {
310	dl	1.27	innerSetException(ex);
311	dl	1.24	return false;
312	jsr166	1.35	}
313	dl	1.14	}
314	dl	1.15	}
315			}