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