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

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

        boolean innerCancel(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 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:	1.32
Committed:	Sat Apr 10 14:25:24 2004 UTC (20 years, 1 month ago) by dl
Branch:	MAIN
Changes since 1.31:	+1 -1 lines
Log Message:	Documentation improvements
#	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	* Creates 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	* Creates 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.innerIsCancelled();
68	}
69
70	public boolean isDone() {
71	return sync.innerIsDone();
72	}
73
74	public boolean cancel(boolean mayInterruptIfRunning) {
75	return sync.innerCancel(mayInterruptIfRunning);
76	}
77
78	public V get() throws InterruptedException, ExecutionException {
79	return sync.innerGet();
80	}
81
82	public V get(long timeout, TimeUnit unit)
83	throws InterruptedException, ExecutionException, TimeoutException {
84	return sync.innerGet(unit.toNanos(timeout));
85	}
86
87	/**
88	* 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	* this future has already been set or has been cancelled.
101	* @param v the value
102	*/
103	protected void set(V v) {
104	sync.innerSet(v);
105	}
106
107	/**
108	* Causes this future to report an <tt>ExecutionException</tt>
109	* with the given throwable as its cause, unless this Future has
110	* already been set or has been cancelled.
111	* @param t the cause of failure.
112	*/
113	protected void setException(Throwable t) {
114	sync.innerSetException(t);
115	}
116
117	/**
118	* Sets this Future to the result of computation unless
119	* it has been cancelled.
120	*/
121	public void run() {
122	sync.innerRun();
123	}
124
125	/**
126	* Executes the computation without setting its result, and then
127	* resets this Future to initial state, failing to do so if the
128	* 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	return sync.innerRunAndReset();
135	}
136
137	/**
138	* Synchronization control for FutureTask. Note that this must be
139	* a non-static inner class in order to invoke the protected
140	* <tt>done</tt> method. For clarity, all inner class support
141	* methods are same as outer, prefixed with "inner".
142	*
143	* Uses AQS sync state to represent run status
144	*/
145	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	/**
161	* The thread running task. When nulled after set/cancel, this
162	* indicates that the results are accessible. Must be
163	* volatile, to ensure visibility upon completion.
164	*/
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	* Implements AQS base acquire to succeed if ran or cancelled
177	*/
178	protected int tryAcquireShared(int ignore) {
179	return innerIsDone()? 1 : -1;
180	}
181
182	/**
183	* Implements AQS base release to always signal after setting
184	* final done status by nulling runner thread.
185	*/
186	protected boolean tryReleaseShared(int ignore) {
187	runner = null;
188	return true;
189	}
190
191	boolean innerIsCancelled() {
192	return getState() == CANCELLED;
193	}
194
195	boolean innerIsDone() {
196	return ranOrCancelled(getState()) && runner == null;
197	}
198
199	V innerGet() throws InterruptedException, ExecutionException {
200	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	V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
209	if (!tryAcquireSharedNanos(0, nanosTimeout))
210	throw new TimeoutException();
211	if (getState() == CANCELLED)
212	throw new CancellationException();
213	if (exception != null)
214	throw new ExecutionException(exception);
215	return result;
216	}
217
218	void innerSet(V v) {
219	int s = getState();
220	if (ranOrCancelled(s) \|\| !compareAndSetState(s, RAN))
221	return;
222	result = v;
223	releaseShared(0);
224	done();
225	}
226
227	void innerSetException(Throwable t) {
228	int s = getState();
229	if (ranOrCancelled(s) \|\| !compareAndSetState(s, RAN))
230	return;
231	exception = t;
232	result = null;
233	releaseShared(0);
234	done();
235	}
236
237	boolean innerCancel(boolean mayInterruptIfRunning) {
238	int s = getState();
239	if (ranOrCancelled(s) \|\| !compareAndSetState(s, CANCELLED))
240	return false;
241	if (mayInterruptIfRunning) {
242	Thread r = runner;
243	if (r != null)
244	r.interrupt();
245	}
246	releaseShared(0);
247	done();
248	return true;
249	}
250
251	void innerRun() {
252	if (!compareAndSetState(0, RUNNING))
253	return;
254	try {
255	runner = Thread.currentThread();
256	innerSet(callable.call());
257	} catch(Throwable ex) {
258	innerSetException(ex);
259	}
260	}
261
262	boolean innerRunAndReset() {
263	if (!compareAndSetState(0, RUNNING))
264	return false;
265	try {
266	runner = Thread.currentThread();
267	callable.call(); // don't set result
268	runner = null;
269	return compareAndSetState(RUNNING, 0);
270	} catch(Throwable ex) {
271	innerSetException(ex);
272	return false;
273	}
274	}
275	}
276	}