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 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 {
        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 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 {
        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 intriniscally
     * 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:	1.17
Committed:	Sat Dec 20 18:57:32 2003 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.16:	+5 -6 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 execution to complete, and then
135	* 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 execution to
163	* complete, and then retrieves its result, if available.
164	*
165	* @param timeout the maximum time to wait
166	* @param unit the time unit of the timeout argument
167	* @return computed result
168	* @throws CancellationException if underlying computation was
169	* cancelled
170	* @throws ExecutionException if underlying computation threw an
171	* exception
172	* @throws InterruptedException if current thread was interrupted
173	* while waiting
174	* @throws TimeoutException if the wait timed out
175	*/
176	public V get(long timeout, TimeUnit unit)
177	throws InterruptedException, ExecutionException, TimeoutException {
178	lock.lock();
179	try {
180	if (!isDone()) {
181	long nanos = unit.toNanos(timeout);
182	do {
183	if (nanos <= 0)
184	throw new TimeoutException();
185	nanos = accessible.awaitNanos(nanos);
186	} while (!isDone());
187	}
188	if (isCancelled())
189	throw new CancellationException();
190	else if (exception != null)
191	throw new ExecutionException(exception);
192	else
193	return result;
194	} finally {
195	lock.unlock();
196	}
197	}
198
199	/**
200	* Sets the result of this Future to the given value.
201	* @param v the value
202	*/
203	protected void set(V v) {
204	lock.lock();
205	try {
206	result = v;
207	setDone();
208	accessible.signalAll();
209	} finally {
210	lock.unlock();
211	}
212	}
213
214	/**
215	* Causes this future to report an <tt>ExecutionException</tt>
216	* with the given throwable as its cause.
217	* @param t the cause of failure.
218	*/
219	protected void setException(Throwable t) {
220	lock.lock();
221	try {
222	exception = t;
223	setDone();
224	accessible.signalAll();
225	} finally {
226	lock.unlock();
227	}
228	}
229
230	/**
231	* Sets the state of this task to Cancelled.
232	*/
233	protected void setCancelled() {
234	lock.lock();
235	try {
236	runner = CANCELLED;
237	}
238	finally{
239	lock.unlock();
240	}
241	}
242
243	/**
244	* Sets the state of this task to Done, unless already in a
245	* Cancelled state, in which case Cancelled status is preserved.
246	*/
247	protected void setDone() {
248	lock.lock();
249	try {
250	Object r = runner;
251	if (r == DONE \|\| r == CANCELLED)
252	return;
253	runner = DONE;
254	}
255	finally{
256	lock.unlock();
257	}
258	done();
259	}
260
261	/**
262	* Attempts to set the state of this task to Running, succeeding
263	* only if the state is currently NOT Done, Running, or Cancelled.
264	* @return true if successful
265	*/
266	protected boolean setRunning() {
267	lock.lock();
268	try {
269	if (runner != null)
270	return false;
271	runner = Thread.currentThread();
272	return true;
273	}
274	finally {
275	lock.unlock();
276	}
277	}
278
279	/**
280	* Sets this Future to the results of computation
281	*/
282	public void run() {
283	if (setRunning()) {
284	try {
285	try {
286	if (runnable != null)
287	runnable.run();
288	else if (callable != null)
289	set(callable.call());
290	} catch(Throwable ex) {
291	setException(ex);
292	}
293	} finally {
294	setDone();
295	}
296	}
297	}
298
299	/**
300	* Sets this Future to the results of computation and then resets
301	* to initial state. This may be useful for tasks that intriniscally
302	* execute more than once.
303	*/
304	protected void runAndReset() {
305	if (setRunning()) {
306	try {
307	try {
308	if (runnable != null)
309	runnable.run();
310	else if (callable != null)
311	set(callable.call());
312	} catch(Throwable ex) {
313	setException(ex);
314	}
315	} finally {
316	reset();
317	}
318	}
319	}
320
321	/**
322	* Protected method invoked when this task transitions to state
323	* <tt>isDone</tt> (whether normally or via cancellation). The
324	* default implementation does nothing. Subclasses may override
325	* this method to invoke completion callbacks or perform
326	* bookkeeping. Note that you can query status inside the
327	* implementation of this method to determine whether this task
328	* has been cancelled.
329	*/
330	protected void done() { }
331
332	/**
333	* Resets the run state of this task to its initial state unless
334	* it has been cancelled. (Note that a cancelled task cannot be
335	* reset.)
336	* @return true if successful
337	*/
338	protected boolean reset() {
339	lock.lock();
340	try {
341	if (runner == CANCELLED)
342	return false;
343	runner = null;
344	return true;
345	}
346	finally {
347	lock.unlock();
348	}
349	}
350
351	/**
352	* Return the <tt>Runnable</tt> or <tt>Callable</tt> used
353	* in the constructor for this <tt>Future</tt>.
354	* @return the task
355	*/
356	protected Object getTask() {
357	if (runnable != null)
358	return runnable;
359	else
360	return callable;
361	}
362
363	}
364