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);
    }

    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:	1.43
Committed:	Thu Jul 21 03:03:22 2005 UTC (18 years, 10 months ago) by jsr166
Branch:	MAIN
Changes since 1.42:	+7 -1 lines
Log Message:	CancellationException
#	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	public void run() {
129	sync.innerRun();
130	}
131
132	/**
133	* Executes the computation without setting its result, and then
134	* resets this Future to initial state, failing to do so if the
135	* computation encounters an exception or is cancelled. This is
136	* designed for use with tasks that intrinsically execute more
137	* than once.
138	* @return true if successfully run and reset
139	*/
140	protected boolean runAndReset() {
141	return sync.innerRunAndReset();
142	}
143
144	/**
145	* Synchronization control for FutureTask. Note that this must be
146	* a non-static inner class in order to invoke the protected
147	* <tt>done</tt> method. For clarity, all inner class support
148	* methods are same as outer, prefixed with "inner".
149	*
150	* Uses AQS sync state to represent run status
151	*/
152	private final class Sync extends AbstractQueuedSynchronizer {
153	private static final long serialVersionUID = -7828117401763700385L;
154
155	/** State value representing that task is running */
156	private static final int RUNNING = 1;
157	/** State value representing that task ran */
158	private static final int RAN = 2;
159	/** State value representing that task was cancelled */
160	private static final int CANCELLED = 4;
161
162	/** The underlying callable */
163	private final Callable<V> callable;
164	/** The result to return from get() */
165	private V result;
166	/** The exception to throw from get() */
167	private Throwable exception;
168
169	/**
170	* The thread running task. When nulled after set/cancel, this
171	* indicates that the results are accessible. Must be
172	* volatile, to ensure visibility upon completion.
173	*/
174	private volatile Thread runner;
175
176	Sync(Callable<V> callable) {
177	this.callable = callable;
178	}
179
180	private boolean ranOrCancelled(int state) {
181	return (state & (RAN \| CANCELLED)) != 0;
182	}
183
184	/**
185	* Implements AQS base acquire to succeed if ran or cancelled
186	*/
187	protected int tryAcquireShared(int ignore) {
188	return innerIsDone()? 1 : -1;
189	}
190
191	/**
192	* Implements AQS base release to always signal after setting
193	* final done status by nulling runner thread.
194	*/
195	protected boolean tryReleaseShared(int ignore) {
196	runner = null;
197	return true;
198	}
199
200	boolean innerIsCancelled() {
201	return getState() == CANCELLED;
202	}
203
204	boolean innerIsDone() {
205	return ranOrCancelled(getState()) && runner == null;
206	}
207
208	V innerGet() throws InterruptedException, ExecutionException {
209	acquireSharedInterruptibly(0);
210	if (getState() == CANCELLED)
211	throw new CancellationException();
212	if (exception != null)
213	throw new ExecutionException(exception);
214	return result;
215	}
216
217	V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
218	if (!tryAcquireSharedNanos(0, nanosTimeout))
219	throw new TimeoutException();
220	if (getState() == CANCELLED)
221	throw new CancellationException();
222	if (exception != null)
223	throw new ExecutionException(exception);
224	return result;
225	}
226
227	void innerSet(V v) {
228	for (;;) {
229	int s = getState();
230	if (ranOrCancelled(s))
231	return;
232	if (compareAndSetState(s, RAN))
233	break;
234	}
235	result = v;
236	releaseShared(0);
237	done();
238	}
239
240	void innerSetException(Throwable t) {
241	for (;;) {
242	int s = getState();
243	if (ranOrCancelled(s))
244	return;
245	if (compareAndSetState(s, RAN))
246	break;
247	}
248	exception = t;
249	result = null;
250	releaseShared(0);
251	done();
252	}
253
254	boolean innerCancel(boolean mayInterruptIfRunning) {
255	for (;;) {
256	int s = getState();
257	if (ranOrCancelled(s))
258	return false;
259	if (compareAndSetState(s, CANCELLED))
260	break;
261	}
262	if (mayInterruptIfRunning) {
263	Thread r = runner;
264	if (r != null)
265	r.interrupt();
266	}
267	releaseShared(0);
268	done();
269	return true;
270	}
271
272	void innerRun() {
273	if (!compareAndSetState(0, RUNNING))
274	return;
275	try {
276	runner = Thread.currentThread();
277	if (getState() == RUNNING) // recheck after setting thread
278	innerSet(callable.call());
279	else
280	releaseShared(0); // cancel
281	} catch (Throwable ex) {
282	innerSetException(ex);
283	}
284	}
285
286	boolean innerRunAndReset() {
287	if (!compareAndSetState(0, RUNNING))
288	return false;
289	try {
290	runner = Thread.currentThread();
291	if (getState() == RUNNING)
292	callable.call(); // don't set result
293	runner = null;
294	return compareAndSetState(RUNNING, 0);
295	} catch (Throwable ex) {
296	innerSetException(ex);
297	return false;
298	}
299	}
300	}
301	}