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;

    /**
     * Constructs 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);
    }

    /**
     * Constructs 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.doIsCancelled();
    }
    
    public boolean isDone() {
        return sync.doIsDone();
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return sync.doCancel(mayInterruptIfRunning);
    }
    
    /**
     * Waits if necessary for execution to complete, and then
     * retrieves its result.
     *
     * @return computed result
     * @throws CancellationException if underlying computation was
     * cancelled
     * @throws ExecutionException if underlying computation threw an
     * exception
     * @throws InterruptedException if current thread was interrupted
     * while waiting
     */
    public V get() throws InterruptedException, ExecutionException {
        return sync.doGet();
    }

    /**
     * Waits if necessary for at most the given time for execution to
     * complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return computed result
     * @throws CancellationException if underlying computation was
     * cancelled
     * @throws ExecutionException if underlying computation threw an
     * exception
     * @throws InterruptedException if current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        return sync.doGet(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.doSet(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.doSetException(t);
    }
    
    /**
     * Sets this Future to the result of computation unless
     * it has been cancelled.
     */
    public void run() {
        sync.doRun();
    }

    /**
     * Executes the computation without setting 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.doRunAndReset();
    }

    /**
     * Synchronization control for FutureTask. Note that this must be
     * a non-static inner class in order to invoke protected
     * <tt>done</tt> method. For clarity, all inner class support
     * methods are same as outer, prefixed with "do".
     *
     * 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 serve as write barrier on 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 doIsDone()? 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 doIsCancelled() {
            return getState() == CANCELLED;
        }
        
        boolean doIsDone() {
            return ranOrCancelled(getState()) && runner == null;
        }

        V doGet() throws InterruptedException, ExecutionException {
            acquireSharedInterruptibly(0);
            if (getState() == CANCELLED)
                throw new CancellationException();
            if (exception != null)
                throw new ExecutionException(exception);
            return result;
        }

        V doGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
            if (!acquireSharedTimed(0, nanosTimeout))
                throw new TimeoutException();                
            if (getState() == CANCELLED)
                throw new CancellationException();
            if (exception != null)
                throw new ExecutionException(exception);
            return result;
        }

        void doSet(V v) {
            int s = getState();
            if (ranOrCancelled(s) || !compareAndSetState(s, RAN))
                return;
            result = v;
            releaseShared(0);
            done();
        }

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

        boolean doCancel(boolean mayInterruptIfRunning) {
            int s = getState();
            if (ranOrCancelled(s) || !compareAndSetState(s, CANCELLED)) 
                return false;
            if (mayInterruptIfRunning) {
                Thread r = runner;
                if (r != null)
                    r.interrupt();
            }
            releaseShared(0);
            done();
            return true;
        }

        void doRun() {
            if (!compareAndSetState(0, RUNNING)) 
                return;
            try {
                runner = Thread.currentThread();
                doSet(callable.call());
            } catch(Throwable ex) {
                doSetException(ex);
            } 
        }

        boolean doRunAndReset() {
            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) {
                doSetException(ex);
                return false;
            } 
        }
    }
}
Revision:	1.26
Committed:	Wed Jan 7 01:00:50 2004 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.25:	+2 -2 lines
Log Message:	replace isFirst param with isFirst method
#	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.locks.*;
9
10	/**
11	* 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	* 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	* the computation has completed, the computation cannot be restarted
18	* or cancelled.
19	*
20	* <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	*
25	* <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	* @since 1.5
30	* @author Doug Lea
31	* @param <V> The result type returned by this FutureTask's <tt>get</tt> method
32	*/
33	public class FutureTask<V> implements Future<V>, Runnable {
34	/** Synchronization control for FutureTask */
35	private final Sync sync;
36
37	/**
38	* Constructs a <tt>FutureTask</tt> that will upon running, execute the
39	* given <tt>Callable</tt>.
40	*
41	* @param callable the callable task
42	* @throws NullPointerException if callable is null
43	*/
44	public FutureTask(Callable<V> callable) {
45	if (callable == null)
46	throw new NullPointerException();
47	sync = new Sync(callable);
48	}
49
50	/**
51	* Constructs a <tt>FutureTask</tt> that will upon running, execute the
52	* 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	* you don't need a particular result, consider using
58	* constructions of the form:
59	* <tt>Future<?> f = new FutureTask<Object>(runnable, null)</tt>
60	* @throws NullPointerException if runnable is null
61	*/
62	public FutureTask(Runnable runnable, V result) {
63	sync = new Sync(Executors.callable(runnable, result));
64	}
65
66	public boolean isCancelled() {
67	return sync.doIsCancelled();
68	}
69
70	public boolean isDone() {
71	return sync.doIsDone();
72	}
73
74	public boolean cancel(boolean mayInterruptIfRunning) {
75	return sync.doCancel(mayInterruptIfRunning);
76	}
77
78	/**
79	* Waits if necessary for execution to complete, and then
80	* retrieves its result.
81	*
82	* @return computed result
83	* @throws CancellationException if underlying computation was
84	* cancelled
85	* @throws ExecutionException if underlying computation threw an
86	* exception
87	* @throws InterruptedException if current thread was interrupted
88	* while waiting
89	*/
90	public V get() throws InterruptedException, ExecutionException {
91	return sync.doGet();
92	}
93
94	/**
95	* Waits if necessary for at most the given time for execution to
96	* complete, and then retrieves its result, if available.
97	*
98	* @param timeout the maximum time to wait
99	* @param unit the time unit of the timeout argument
100	* @return computed result
101	* @throws CancellationException if underlying computation was
102	* cancelled
103	* @throws ExecutionException if underlying computation threw an
104	* exception
105	* @throws InterruptedException if current thread was interrupted
106	* while waiting
107	* @throws TimeoutException if the wait timed out
108	*/
109	public V get(long timeout, TimeUnit unit)
110	throws InterruptedException, ExecutionException, TimeoutException {
111	return sync.doGet(unit.toNanos(timeout));
112	}
113
114	/**
115	* Protected method invoked when this task transitions to state
116	* <tt>isDone</tt> (whether normally or via cancellation). The
117	* default implementation does nothing. Subclasses may override
118	* this method to invoke completion callbacks or perform
119	* bookkeeping. Note that you can query status inside the
120	* implementation of this method to determine whether this task
121	* has been cancelled.
122	*/
123	protected void done() { }
124
125	/**
126	* Sets the result of this Future to the given value unless
127	* this future has already been set or has been cancelled
128	* @param v the value
129	*/
130	protected void set(V v) {
131	sync.doSet(v);
132	}
133
134	/**
135	* Causes this future to report an <tt>ExecutionException</tt>
136	* with the given throwable as its cause, unless this Future has
137	* already been set or has been cancelled.
138	* @param t the cause of failure.
139	*/
140	protected void setException(Throwable t) {
141	sync.doSetException(t);
142	}
143
144	/**
145	* Sets this Future to the result of computation unless
146	* it has been cancelled.
147	*/
148	public void run() {
149	sync.doRun();
150	}
151
152	/**
153	* Executes the computation without setting result, and then
154	* resets this Future to initial state; failing to do so if the
155	* computation encounters an exception or is cancelled. This is
156	* designed for use with tasks that intrinsically execute more
157	* than once.
158	* @return true if successfully run and reset
159	*/
160	protected boolean runAndReset() {
161	return sync.doRunAndReset();
162	}
163
164	/**
165	* Synchronization control for FutureTask. Note that this must be
166	* a non-static inner class in order to invoke protected
167	* <tt>done</tt> method. For clarity, all inner class support
168	* methods are same as outer, prefixed with "do".
169	*
170	* Uses AQS sync state to represent run status
171	*/
172	private final class Sync extends AbstractQueuedSynchronizer {
173	/** State value representing that task is running */
174	private static final int RUNNING = 1;
175	/** State value representing that task ran */
176	private static final int RAN = 2;
177	/** State value representing that task was cancelled */
178	private static final int CANCELLED = 4;
179
180	/** The underlying callable */
181	private final Callable<V> callable;
182	/** The result to return from get() */
183	private V result;
184	/** The exception to throw from get() */
185	private Throwable exception;
186
187	/**
188	* The thread running task. When nulled after set/cancel, this
189	* indicates that the results are accessible. Must be
190	* volatile, to serve as write barrier on completion.
191	*/
192	private volatile Thread runner;
193
194	Sync(Callable<V> callable) {
195	this.callable = callable;
196	}
197
198	private boolean ranOrCancelled(int state) {
199	return (state & (RAN \| CANCELLED)) != 0;
200	}
201
202	/**
203	* Implements AQS base acquire to succeed if ran or cancelled
204	*/
205	protected int tryAcquireShared(int ignore) {
206	return doIsDone()? 1 : -1;
207	}
208
209	/**
210	* Implements AQS base release to always signal after setting
211	* final done status by nulling runner thread.
212	*/
213	protected boolean tryReleaseShared(int ignore) {
214	runner = null;
215	return true;
216	}
217
218	boolean doIsCancelled() {
219	return getState() == CANCELLED;
220	}
221
222	boolean doIsDone() {
223	return ranOrCancelled(getState()) && runner == null;
224	}
225
226	V doGet() throws InterruptedException, ExecutionException {
227	acquireSharedInterruptibly(0);
228	if (getState() == CANCELLED)
229	throw new CancellationException();
230	if (exception != null)
231	throw new ExecutionException(exception);
232	return result;
233	}
234
235	V doGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
236	if (!acquireSharedTimed(0, nanosTimeout))
237	throw new TimeoutException();
238	if (getState() == CANCELLED)
239	throw new CancellationException();
240	if (exception != null)
241	throw new ExecutionException(exception);
242	return result;
243	}
244
245	void doSet(V v) {
246	int s = getState();
247	if (ranOrCancelled(s) \|\| !compareAndSetState(s, RAN))
248	return;
249	result = v;
250	releaseShared(0);
251	done();
252	}
253
254	void doSetException(Throwable t) {
255	int s = getState();
256	if (ranOrCancelled(s) \|\| !compareAndSetState(s, RAN))
257	return;
258	exception = t;
259	result = null;
260	releaseShared(0);
261	done();
262	}
263
264	boolean doCancel(boolean mayInterruptIfRunning) {
265	int s = getState();
266	if (ranOrCancelled(s) \|\| !compareAndSetState(s, CANCELLED))
267	return false;
268	if (mayInterruptIfRunning) {
269	Thread r = runner;
270	if (r != null)
271	r.interrupt();
272	}
273	releaseShared(0);
274	done();
275	return true;
276	}
277
278	void doRun() {
279	if (!compareAndSetState(0, RUNNING))
280	return;
281	try {
282	runner = Thread.currentThread();
283	doSet(callable.call());
284	} catch(Throwable ex) {
285	doSetException(ex);
286	}
287	}
288
289	boolean doRunAndReset() {
290	if (!compareAndSetState(0, RUNNING))
291	return false;
292	try {
293	runner = Thread.currentThread();
294	callable.call(); // don't set result
295	runner = null;
296	return compareAndSetState(RUNNING, 0);
297	} catch(Throwable ex) {
298	doSetException(ex);
299	return false;
300	}
301	}
302	}
303	}