util/concurrent/FutureTask.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

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 {
    /**
     * Special value for "runner" indicating task is cancelled
     */
    private static final Object CANCELLED = new Object();

    /**
     * Special value for "runner" indicating task is completed
     */
    private static final Object DONE = new Object();

    /** 
     * Holds the run-state, taking on values:
     *   null              = not yet started,
     *   [some thread ref] = running,
     *   DONE              = completed normally,
     *   CANCELLED         = cancelled (may or may not have ever run).
     */
    private volatile Object runner;

    /*
     * For simplicity of use, we can use either a Runnable or a
     * Callable as basis for run method. So one or the other of these
     * will be null.
     */

    /** The runnable; if non-null, then callable is null */
    private final Runnable runnable;
    /** The callable; if non-null, then runnable is null */
    private final Callable<V> callable;

    /** The result to return from get() */
    private V result;
    /** The exception to throw from get() */
    private Throwable exception;

    private final ReentrantLock lock = new ReentrantLock();
    private final ReentrantLock.ConditionObject accessible = lock.newCondition();

    /**
     * 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();
        this.callable = callable;
        this.runnable = null;
    }

    /**
     * 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) {
        if (runnable == null)
            throw new NullPointerException();
        this.runnable = runnable;
        this.result = result;
        this.callable = null;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        lock.lock();
        try {
            Object r = runner;
            if (r == DONE || r == CANCELLED)
                return false;
            if (mayInterruptIfRunning && r != null && r instanceof Thread)
                ((Thread)r).interrupt();
            runner = CANCELLED;
        }
        finally{
            lock.unlock();
        }
        done();
        return true;
    }
    
    public boolean isCancelled() {
        return runner == CANCELLED;
    }
    
    public boolean isDone() {
        Object r = runner;
        return r == DONE || r == CANCELLED;
    }

    /**
     * Waits if necessary for the call to <tt>callable.call</tt> 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 {
        lock.lock();
        try {
            while (!isDone())
                accessible.await();
            if (isCancelled())
                throw new CancellationException();
            else if (exception != null)
                throw new ExecutionException(exception);
            else
                return result;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Waits if necessary for at most the given time for the call to
     * <tt>callable.call</tt> to complete, and then retrieves its
     * result.
     *
     * @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 {
        lock.lock();
        try {
            if (!isDone()) {
                long nanos = unit.toNanos(timeout);
                do {
                    if (nanos <= 0)
                        throw new TimeoutException();
                    nanos = accessible.awaitNanos(nanos);
                } while (!isDone());
            }
            if (isCancelled())
                throw new CancellationException();
            else if (exception != null)
                throw new ExecutionException(exception);
            else
                return result;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Sets the result of this Future to the given value.
     * @param v the value
     */ 
    protected void set(V v) {
        lock.lock();
        try {
            result = v;
            setDone();
            accessible.signalAll();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Causes this future to report an <tt>ExecutionException</tt>
     * with the given throwable as its cause.
     * @param t the cause of failure.
     */ 
    protected void setException(Throwable t) {
        lock.lock();
        try {
            exception = t;
            setDone();
            accessible.signalAll();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Sets the state of this task to Cancelled.
     */
    protected void setCancelled() {
        lock.lock();
        try {
            runner = CANCELLED;
        }
        finally{
            lock.unlock();
        }
    }
    
    /**
     * Sets the state of this task to Done, unless already in a
     * Cancelled state, in which case Cancelled status is preserved.
     */
    protected void setDone() {
        lock.lock();
        try {
            Object r = runner;
            if (r == DONE || r == CANCELLED) 
                return;
            runner = DONE;
        }
        finally{
            lock.unlock();
        }
        done();
    }

    /**
     * Attempts to set the state of this task to Running, succeeding
     * only if the state is currently NOT Done, Running, or Cancelled.
     * @return true if successful
     */ 
    protected boolean setRunning() {
        lock.lock();
        try {
            if (runner != null)
                return false;
            runner = Thread.currentThread();
            return true;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Sets this Future to the results of computation
     */
    public void run() {
        if (setRunning()) {
            try {
                try {
                    if (runnable != null)
                        runnable.run();
                    else if (callable != null)
                        set(callable.call());
                } catch(Throwable ex) {
                    setException(ex);
                }
            } finally {
                setDone();
            }
        }
    }

    /**
     * Sets this Future to the results of computation and then resets
     * to initial state. This may be useful for tasks that must
     * execute more than once.
     */
    protected void runAndReset() {
        if (setRunning()) {
            try {
                try {
                    if (runnable != null)
                        runnable.run();
                    else if (callable != null)
                        set(callable.call());
                } catch(Throwable ex) {
                    setException(ex);
                }
            } finally {
                reset();
            }
        }
    }

    /**
     * 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() { }

    /**
     * Resets the run state of this task to its initial state unless
     * it has been cancelled. (Note that a cancelled task cannot be
     * reset.)
     * @return true if successful
     */
    protected boolean reset() {
        lock.lock();
        try {
            if (runner == CANCELLED) 
                return false;
            runner = null;
            return true;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Return the <tt>Runnable</tt> or <tt>Callable</tt> used
     * in the constructor for this <tt>Future</tt>.
     * @return the task
     */ 
    protected Object getTask() {
        if (runnable != null)
            return runnable;
        else
            return callable;
    }

}

Revision:	1.16
Committed:	Fri Dec 19 14:42:25 2003 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.15:	+2 -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. Use, modify, and
4	* redistribute this code in any way without acknowledgement.
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.*;
9
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 Future<V>, Runnable {
35	/**
36	* Special value for "runner" indicating task is cancelled
37	*/
38	private static final Object CANCELLED = new Object();
39
40	/**
41	* Special value for "runner" indicating task is completed
42	*/
43	private static final Object DONE = new Object();
44
45	/**
46	* Holds the run-state, taking on values:
47	* null = not yet started,
48	* [some thread ref] = running,
49	* DONE = completed normally,
50	* CANCELLED = cancelled (may or may not have ever run).
51	*/
52	private volatile Object runner;
53
54	/*
55	* For simplicity of use, we can use either a Runnable or a
56	* Callable as basis for run method. So one or the other of these
57	* will be null.
58	*/
59
60	/** The runnable; if non-null, then callable is null */
61	private final Runnable runnable;
62	/** The callable; if non-null, then runnable is null */
63	private final Callable<V> callable;
64
65	/** The result to return from get() */
66	private V result;
67	/** The exception to throw from get() */
68	private Throwable exception;
69
70	private final ReentrantLock lock = new ReentrantLock();
71	private final ReentrantLock.ConditionObject accessible = lock.newCondition();
72
73	/**
74	* Constructs a <tt>FutureTask</tt> that will upon running, execute the
75	* given <tt>Callable</tt>.
76	*
77	* @param callable the callable task
78	* @throws NullPointerException if callable is null
79	*/
80	public FutureTask(Callable<V> callable) {
81	if (callable == null)
82	throw new NullPointerException();
83	this.callable = callable;
84	this.runnable = null;
85	}
86
87	/**
88	* Constructs a <tt>FutureTask</tt> that will upon running, execute the
89	* given <tt>Runnable</tt>, and arrange that <tt>get</tt> will return the
90	* given result on successful completion.
91	*
92	* @param runnable the runnable task
93	* @param result the result to return on successful completion. If
94	* you don't need a particular result, consider using
95	* constructions of the form:
96	* <tt>Future<?> f = new FutureTask<Object>(runnable, null)</tt>
97	* @throws NullPointerException if runnable is null
98	*/
99	public FutureTask(Runnable runnable, V result) {
100	if (runnable == null)
101	throw new NullPointerException();
102	this.runnable = runnable;
103	this.result = result;
104	this.callable = null;
105	}
106
107	public boolean cancel(boolean mayInterruptIfRunning) {
108	lock.lock();
109	try {
110	Object r = runner;
111	if (r == DONE \|\| r == CANCELLED)
112	return false;
113	if (mayInterruptIfRunning && r != null && r instanceof Thread)
114	((Thread)r).interrupt();
115	runner = CANCELLED;
116	}
117	finally{
118	lock.unlock();
119	}
120	done();
121	return true;
122	}
123
124	public boolean isCancelled() {
125	return runner == CANCELLED;
126	}
127
128	public boolean isDone() {
129	Object r = runner;
130	return r == DONE \|\| r == CANCELLED;
131	}
132
133	/**
134	* Waits if necessary for the call to <tt>callable.call</tt> to
135	* complete, and then retrieves its result.
136	*
137	* @return computed result
138	* @throws CancellationException if underlying computation was
139	* cancelled
140	* @throws ExecutionException if underlying computation threw an
141	* exception
142	* @throws InterruptedException if current thread was interrupted
143	* while waiting
144	*/
145	public V get() throws InterruptedException, ExecutionException {
146	lock.lock();
147	try {
148	while (!isDone())
149	accessible.await();
150	if (isCancelled())
151	throw new CancellationException();
152	else if (exception != null)
153	throw new ExecutionException(exception);
154	else
155	return result;
156	} finally {
157	lock.unlock();
158	}
159	}
160
161	/**
162	* Waits if necessary for at most the given time for the call to
163	* <tt>callable.call</tt> to complete, and then retrieves its
164	* result.
165	*
166	* @param timeout the maximum time to wait
167	* @param unit the time unit of the timeout argument
168	* @return computed result
169	* @throws CancellationException if underlying computation was
170	* cancelled
171	* @throws ExecutionException if underlying computation threw an
172	* exception
173	* @throws InterruptedException if current thread was interrupted
174	* while waiting
175	* @throws TimeoutException if the wait timed out
176	*/
177	public V get(long timeout, TimeUnit unit)
178	throws InterruptedException, ExecutionException, TimeoutException {
179	lock.lock();
180	try {
181	if (!isDone()) {
182	long nanos = unit.toNanos(timeout);
183	do {
184	if (nanos <= 0)
185	throw new TimeoutException();
186	nanos = accessible.awaitNanos(nanos);
187	} while (!isDone());
188	}
189	if (isCancelled())
190	throw new CancellationException();
191	else if (exception != null)
192	throw new ExecutionException(exception);
193	else
194	return result;
195	} finally {
196	lock.unlock();
197	}
198	}
199
200	/**
201	* Sets the result of this Future to the given value.
202	* @param v the value
203	*/
204	protected void set(V v) {
205	lock.lock();
206	try {
207	result = v;
208	setDone();
209	accessible.signalAll();
210	} finally {
211	lock.unlock();
212	}
213	}
214
215	/**
216	* Causes this future to report an <tt>ExecutionException</tt>
217	* with the given throwable as its cause.
218	* @param t the cause of failure.
219	*/
220	protected void setException(Throwable t) {
221	lock.lock();
222	try {
223	exception = t;
224	setDone();
225	accessible.signalAll();
226	} finally {
227	lock.unlock();
228	}
229	}
230
231	/**
232	* Sets the state of this task to Cancelled.
233	*/
234	protected void setCancelled() {
235	lock.lock();
236	try {
237	runner = CANCELLED;
238	}
239	finally{
240	lock.unlock();
241	}
242	}
243
244	/**
245	* Sets the state of this task to Done, unless already in a
246	* Cancelled state, in which case Cancelled status is preserved.
247	*/
248	protected void setDone() {
249	lock.lock();
250	try {
251	Object r = runner;
252	if (r == DONE \|\| r == CANCELLED)
253	return;
254	runner = DONE;
255	}
256	finally{
257	lock.unlock();
258	}
259	done();
260	}
261
262	/**
263	* Attempts to set the state of this task to Running, succeeding
264	* only if the state is currently NOT Done, Running, or Cancelled.
265	* @return true if successful
266	*/
267	protected boolean setRunning() {
268	lock.lock();
269	try {
270	if (runner != null)
271	return false;
272	runner = Thread.currentThread();
273	return true;
274	}
275	finally {
276	lock.unlock();
277	}
278	}
279
280	/**
281	* Sets this Future to the results of computation
282	*/
283	public void run() {
284	if (setRunning()) {
285	try {
286	try {
287	if (runnable != null)
288	runnable.run();
289	else if (callable != null)
290	set(callable.call());
291	} catch(Throwable ex) {
292	setException(ex);
293	}
294	} finally {
295	setDone();
296	}
297	}
298	}
299
300	/**
301	* Sets this Future to the results of computation and then resets
302	* to initial state. This may be useful for tasks that must
303	* execute more than once.
304	*/
305	protected void runAndReset() {
306	if (setRunning()) {
307	try {
308	try {
309	if (runnable != null)
310	runnable.run();
311	else if (callable != null)
312	set(callable.call());
313	} catch(Throwable ex) {
314	setException(ex);
315	}
316	} finally {
317	reset();
318	}
319	}
320	}
321
322	/**
323	* Protected method invoked when this task transitions to state
324	* <tt>isDone</tt> (whether normally or via cancellation). The
325	* default implementation does nothing. Subclasses may override
326	* this method to invoke completion callbacks or perform
327	* bookkeeping. Note that you can query status inside the
328	* implementation of this method to determine whether this task
329	* has been cancelled.
330	*/
331	protected void done() { }
332
333	/**
334	* Resets the run state of this task to its initial state unless
335	* it has been cancelled. (Note that a cancelled task cannot be
336	* reset.)
337	* @return true if successful
338	*/
339	protected boolean reset() {
340	lock.lock();
341	try {
342	if (runner == CANCELLED)
343	return false;
344	runner = null;
345	return true;
346	}
347	finally {
348	lock.unlock();
349	}
350	}
351
352	/**
353	* Return the <tt>Runnable</tt> or <tt>Callable</tt> used
354	* in the constructor for this <tt>Future</tt>.
355	* @return the task
356	*/
357	protected Object getTask() {
358	if (runnable != null)
359	return runnable;
360	else
361	return callable;
362	}
363
364	}
365