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 Future<V>, Runnable {
    /** 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);
    }

    public V get() throws InterruptedException, ExecutionException {
        return sync.innerGet();
    }

    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.
     * @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.
     * @param t the cause of failure.
     */
    protected void setException(Throwable t) {
        sync.innerSetException(t);
    }

    /**
     * Sets this Future to the result of 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 {
        /** 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();
                innerSet(callable.call());
            } catch (Throwable ex) {
                innerSetException(ex);
            }
        }

        boolean innerRunAndReset() {
            if (!compareAndSetState(0, RUNNING))
                return false;
            try {
                runner = Thread.currentThread();
                callable.call(); // don't set result
                runner = null;
                return compareAndSetState(RUNNING, 0);
            } catch (Throwable ex) {
                innerSetException(ex);
                return false;
            }
        }
    }
}
Revision:	1.35
Committed:	Mon May 2 18:38:53 2005 UTC (19 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.34:	+14 -14 lines
Log Message:	remove trailing whitespace
#	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	dl	1.13	public class FutureTask<V> implements Future<V>, Runnable {
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	dl	1.2	public V get() throws InterruptedException, ExecutionException {
79	dl	1.27	return sync.innerGet();
80	tim	1.1	}
81
82	dl	1.2	public V get(long timeout, TimeUnit unit)
83	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
84	dl	1.27	return sync.innerGet(unit.toNanos(timeout));
85	tim	1.1	}
86
87			/**
88	dl	1.20	* Protected method invoked when this task transitions to state
89			* <tt>isDone</tt> (whether normally or via cancellation). The
90			* default implementation does nothing. Subclasses may override
91			* this method to invoke completion callbacks or perform
92			* bookkeeping. Note that you can query status inside the
93			* implementation of this method to determine whether this task
94			* has been cancelled.
95			*/
96			protected void done() { }
97
98			/**
99			* Sets the result of this Future to the given value unless
100	dl	1.29	* this future has already been set or has been cancelled.
101	tim	1.1	* @param v the value
102	jsr166	1.35	*/
103	dl	1.2	protected void set(V v) {
104	dl	1.27	sync.innerSet(v);
105	tim	1.1	}
106
107			/**
108	dl	1.13	* Causes this future to report an <tt>ExecutionException</tt>
109	dl	1.20	* with the given throwable as its cause, unless this Future has
110	dl	1.24	* already been set or has been cancelled.
111	dl	1.13	* @param t the cause of failure.
112	jsr166	1.35	*/
113	dl	1.2	protected void setException(Throwable t) {
114	dl	1.27	sync.innerSetException(t);
115	tim	1.1	}
116	jsr166	1.35
117	dl	1.14	/**
118	dl	1.24	* Sets this Future to the result of computation unless
119			* it has been cancelled.
120			*/
121			public void run() {
122	dl	1.27	sync.innerRun();
123	dl	1.24	}
124
125			/**
126	dl	1.30	* Executes the computation without setting its result, and then
127			* resets this Future to initial state, failing to do so if the
128	dl	1.24	* computation encounters an exception or is cancelled. This is
129			* designed for use with tasks that intrinsically execute more
130			* than once.
131			* @return true if successfully run and reset
132			*/
133			protected boolean runAndReset() {
134	dl	1.27	return sync.innerRunAndReset();
135	dl	1.14	}
136	dl	1.3
137	dl	1.14	/**
138	dl	1.24	* Synchronization control for FutureTask. Note that this must be
139	dl	1.30	* a non-static inner class in order to invoke the protected
140	dl	1.24	* <tt>done</tt> method. For clarity, all inner class support
141	dl	1.27	* methods are same as outer, prefixed with "inner".
142	dl	1.24	*
143			* Uses AQS sync state to represent run status
144	dl	1.20	*/
145	dl	1.24	private final class Sync extends AbstractQueuedSynchronizer {
146			/** State value representing that task is running */
147			private static final int RUNNING = 1;
148			/** State value representing that task ran */
149			private static final int RAN = 2;
150			/** State value representing that task was cancelled */
151			private static final int CANCELLED = 4;
152
153			/** The underlying callable */
154			private final Callable<V> callable;
155			/** The result to return from get() */
156			private V result;
157			/** The exception to throw from get() */
158			private Throwable exception;
159
160	jsr166	1.35	/**
161	dl	1.24	* The thread running task. When nulled after set/cancel, this
162			* indicates that the results are accessible. Must be
163	dl	1.32	* volatile, to ensure visibility upon completion.
164	dl	1.24	*/
165			private volatile Thread runner;
166
167			Sync(Callable<V> callable) {
168			this.callable = callable;
169			}
170
171			private boolean ranOrCancelled(int state) {
172			return (state & (RAN \| CANCELLED)) != 0;
173			}
174
175			/**
176	dl	1.26	* Implements AQS base acquire to succeed if ran or cancelled
177	dl	1.24	*/
178	dl	1.26	protected int tryAcquireShared(int ignore) {
179	dl	1.27	return innerIsDone()? 1 : -1;
180	dl	1.24	}
181
182			/**
183			* Implements AQS base release to always signal after setting
184			* final done status by nulling runner thread.
185			*/
186	dl	1.25	protected boolean tryReleaseShared(int ignore) {
187	dl	1.24	runner = null;
188	jsr166	1.35	return true;
189	dl	1.24	}
190
191	dl	1.27	boolean innerIsCancelled() {
192	dl	1.24	return getState() == CANCELLED;
193			}
194	jsr166	1.35
195	dl	1.27	boolean innerIsDone() {
196	dl	1.24	return ranOrCancelled(getState()) && runner == null;
197			}
198
199	dl	1.27	V innerGet() throws InterruptedException, ExecutionException {
200	dl	1.24	acquireSharedInterruptibly(0);
201			if (getState() == CANCELLED)
202			throw new CancellationException();
203			if (exception != null)
204			throw new ExecutionException(exception);
205			return result;
206			}
207
208	dl	1.27	V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
209	dl	1.28	if (!tryAcquireSharedNanos(0, nanosTimeout))
210	jsr166	1.35	throw new TimeoutException();
211	dl	1.24	if (getState() == CANCELLED)
212			throw new CancellationException();
213			if (exception != null)
214			throw new ExecutionException(exception);
215			return result;
216			}
217
218	dl	1.27	void innerSet(V v) {
219	dl	1.33	for (;;) {
220			int s = getState();
221			if (ranOrCancelled(s))
222			return;
223			if (compareAndSetState(s, RAN))
224			break;
225			}
226	dl	1.24	result = v;
227			releaseShared(0);
228			done();
229			}
230
231	dl	1.27	void innerSetException(Throwable t) {
232	dl	1.33	for (;;) {
233			int s = getState();
234			if (ranOrCancelled(s))
235			return;
236			if (compareAndSetState(s, RAN))
237			break;
238			}
239	dl	1.24	exception = t;
240			result = null;
241			releaseShared(0);
242			done();
243			}
244
245	dl	1.27	boolean innerCancel(boolean mayInterruptIfRunning) {
246	dl	1.33	for (;;) {
247			int s = getState();
248			if (ranOrCancelled(s))
249			return false;
250	jsr166	1.35	if (compareAndSetState(s, CANCELLED))
251	dl	1.33	break;
252			}
253	dl	1.24	if (mayInterruptIfRunning) {
254			Thread r = runner;
255			if (r != null)
256			r.interrupt();
257			}
258			releaseShared(0);
259			done();
260	dl	1.14	return true;
261			}
262
263	dl	1.27	void innerRun() {
264	jsr166	1.35	if (!compareAndSetState(0, RUNNING))
265	dl	1.24	return;
266	dl	1.14	try {
267	dl	1.24	runner = Thread.currentThread();
268	dl	1.27	innerSet(callable.call());
269	jsr166	1.34	} catch (Throwable ex) {
270	dl	1.27	innerSetException(ex);
271	jsr166	1.35	}
272	dl	1.14	}
273
274	dl	1.27	boolean innerRunAndReset() {
275	jsr166	1.35	if (!compareAndSetState(0, RUNNING))
276	dl	1.24	return false;
277	dl	1.15	try {
278	dl	1.24	runner = Thread.currentThread();
279			callable.call(); // don't set result
280			runner = null;
281			return compareAndSetState(RUNNING, 0);
282	jsr166	1.34	} catch (Throwable ex) {
283	dl	1.27	innerSetException(ex);
284	dl	1.24	return false;
285	jsr166	1.35	}
286	dl	1.14	}
287	dl	1.15	}
288			}