util/concurrent/CancellableTask.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.atomic.*;
import java.util.concurrent.locks.*;

/**
 * Base class for {@link Cancellable} {@link java.lang.Runnable} 
 * actions within the {@link Executor} framework. In addition to
 * serving as a standalone class, this provides <tt>protected</tt>
 * functionality that may be useful when creating customized task
 * classes.
 * @since 1.5
 * @author Doug Lea
 */

public class CancellableTask implements Cancellable, Runnable {
    /** 
     * 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).
     * Transitions use atomic updates.
     */
    private volatile Object runner;

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

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

    private static AtomicReferenceFieldUpdater<CancellableTask, Object> 
        runnerUpdater = 
        AtomicReferenceFieldUpdater.newUpdater
        (CancellableTask.class, Object.class, "runner");

    /**
     * The runnable underlying this task
     */
    private volatile Runnable runnable;

    /**
     * Creates a new CancellableTask which invokes the given
     * <tt>Runnable</tt> when executed.
     * @param r the runnable action
     * @throws NullPointerException if runnable is null
     */
    public CancellableTask(Runnable r) {
        if (r == null)
            throw new NullPointerException();
        this.runnable = r;
    }

    /**
     * Creates a new CancellableTask without a runnable action, which
     * must be set using <tt>setRunnable</tt> before use.  This is
     * intended for use in subclasses that must complete superclass
     * construction before establishing the runnable action.
     */
    protected CancellableTask() {
    }


    public boolean cancel(boolean mayInterruptIfRunning) {
        Object r = runner;
        if (r == DONE || r == CANCELLED)
            return false;

        if (mayInterruptIfRunning &&
            r != null &&
            r instanceof Thread &&
            runnerUpdater.compareAndSet(this, r, CANCELLED)) 

            ((Thread)r).interrupt();
        else 
            runnerUpdater.set(this, CANCELLED);
        return true;
    }
    
    public boolean isCancelled() {
        return runner == CANCELLED;
    }
    
    public boolean isDone() {
        Object r = runner;
        return r == DONE || r == CANCELLED;
    }

    /**
     * Return the Runnable forming the basis of this task.
     * @return the runnable action
     * @see #setRunnable
     */
    protected Runnable getRunnable() {
        return runnable;
    }

    /**
     * Set the Runnable forming the basis of this task.
     * @param r the runnable action
     * @see #getRunnable
     */
    protected void setRunnable(Runnable r) {
        runnable = r;
    }

    /**
     * Set the state of this task to Cancelled.
     */
    protected void setCancelled() {
        runnerUpdater.set(this, CANCELLED);
    }
    
    /**
     * Set the state of this task to Done, unless already
     * in a Cancelled state, in which Cancelled status is preserved.
     *
     */
    protected void setDone() {
        for (;;) {
            Object r = runner;
            if (r == DONE || r == CANCELLED) 
                return;
            if (runnerUpdater.compareAndSet(this, r, DONE))
                return;
        }
    }

    /**
     * Attempt 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() {
        return runnerUpdater.compareAndSet(this, null, Thread.currentThread());
    }

    /**
     * Runs the runnable if not cancelled, maintaining run status.
     */
    public void run() {
        if (setRunning()) {
            try {
                runnable.run();
            } finally {
                setDone();
            }
        }
    }

    /**
     * Reset 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() {
        for (;;) {
            Object r = runner;
            if (r == CANCELLED) 
                return false;
            if (runnerUpdater.compareAndSet(this, r, null))
                return true;
        }
    }

    /**
     * Implementation of Future methods under the control of a current
     * <tt>CancellableTask</tt>, which it relies on for methods
     * <tt>isDone</tt>, <tt>isCancelled</tt> and <tt>cancel</tt>. This
     * class is split into an inner class to permit Future support to
     * be mixed-in with other flavors of tasks.  Normally, such a
     * class will delegate <tt>Future</tt> <tt>get</tt> methods to the
     * <tt>InnerCancellableFuture</tt>, and internally arrange that
     * <tt>set</tt> methods be invoked when computations are ready.
     *
     * <p><b>Sample Usage</b>. Here are fragments of an example subclass.
     * <pre>
     *  class MyFutureTask&lt;V&gt; extends CancellableTask implements Future&lt;
V&gt; {
     *                
     *    MyFutureTask(Callable&lt;V&gt; callable) {
     *      setRunnable(new InnerCancellableFuture&lt;V&gt;(callable));
     *    }
     *
     *    public V get() throws InterruptedException, ExecutionException {
     *      return ((InnerCancellableFuture&lt;V&gt;)getRunnable()).get();
     *    }
     *    // (And similarly for timeout version.)
     *
     *    void action() { // whatever action causes execution
     *      try {
     *        ((InnerCancellableFuture&lt;V&gt;)getRunnable()).set(compute());
     *      } catch (Exception ex) {
     *        ((InnerCancellableFuture&lt;V&gt;)getRunnable()).setException(ex);
     *      }
     *   }
     * }
     *</pre>
     */
    protected class InnerCancellableFuture<V> implements Future<V>, Runnable {
        private final Callable<V> callable;
        private final ReentrantLock lock = new ReentrantLock();
        private final Condition accessible = lock.newCondition();
        private V result;
        private Throwable exception;

        /**
         * Create an InnerCancellableFuture that will execute the
         * given callable.
         * @param callable the function to execute
         */
        protected InnerCancellableFuture(Callable<V> callable) {
            this.callable = callable;
        }
        
        public boolean cancel(boolean mayInterruptIfRunning) {
            return CancellableTask.this.cancel(mayInterruptIfRunning);
        }
        
        public boolean isCancelled() {
            return CancellableTask.this.isCancelled();
        }


        public boolean isDone() {
            return CancellableTask.this.isDone();
        }


        /**
         * Sets this Future to the results of <tt>callable.call</tt>
         */
        public void run() {
            try {
                set(callable.call());
            } catch(Throwable ex) {
                setException(ex);
            }
        }

        /**
         * Waits if necessary for the call to <tt>callable.call</tt> to
         * complete, and then retrieves its result.
         *
         * @return computed result
         * @throws CancellationException here???
         * @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 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();
            }
        }
    }
   
}
Revision:	1.18
Committed:	Fri Sep 26 11:37:10 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.17:	+1 -1 lines
Log Message:	Spellcheck
#	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.atomic.*;
9	import java.util.concurrent.locks.*;
10
11	/**
12	* Base class for {@link Cancellable} {@link java.lang.Runnable}
13	* actions within the {@link Executor} framework. In addition to
14	* serving as a standalone class, this provides <tt>protected</tt>
15	* functionality that may be useful when creating customized task
16	* classes.
17	* @since 1.5
18	* @author Doug Lea
19	*/
20
21	public class CancellableTask implements Cancellable, Runnable {
22	/**
23	* Holds the run-state, taking on values:
24	* null = not yet started,
25	* [some thread ref] = running,
26	* DONE = completed normally,
27	* CANCELLED = cancelled (may or may not have ever run).
28	* Transitions use atomic updates.
29	*/
30	private volatile Object runner;
31
32	/**
33	* Special value for "runner" indicating task is completed
34	*/
35	private static final Object DONE = new Object();
36
37	/**
38	* Special value for "runner" indicating task is cancelled
39	*/
40	private static final Object CANCELLED = new Object();
41
42	private static AtomicReferenceFieldUpdater<CancellableTask, Object>
43	runnerUpdater =
44	AtomicReferenceFieldUpdater.newUpdater
45	(CancellableTask.class, Object.class, "runner");
46
47	/**
48	* The runnable underlying this task
49	*/
50	private volatile Runnable runnable;
51
52	/**
53	* Creates a new CancellableTask which invokes the given
54	* <tt>Runnable</tt> when executed.
55	* @param r the runnable action
56	* @throws NullPointerException if runnable is null
57	*/
58	public CancellableTask(Runnable r) {
59	if (r == null)
60	throw new NullPointerException();
61	this.runnable = r;
62	}
63
64	/**
65	* Creates a new CancellableTask without a runnable action, which
66	* must be set using <tt>setRunnable</tt> before use. This is
67	* intended for use in subclasses that must complete superclass
68	* construction before establishing the runnable action.
69	*/
70	protected CancellableTask() {
71	}
72
73
74	public boolean cancel(boolean mayInterruptIfRunning) {
75	Object r = runner;
76	if (r == DONE \|\| r == CANCELLED)
77	return false;
78
79	if (mayInterruptIfRunning &&
80	r != null &&
81	r instanceof Thread &&
82	runnerUpdater.compareAndSet(this, r, CANCELLED))
83
84	((Thread)r).interrupt();
85	else
86	runnerUpdater.set(this, CANCELLED);
87	return true;
88	}
89
90	public boolean isCancelled() {
91	return runner == CANCELLED;
92	}
93
94	public boolean isDone() {
95	Object r = runner;
96	return r == DONE \|\| r == CANCELLED;
97	}
98
99	/**
100	* Return the Runnable forming the basis of this task.
101	* @return the runnable action
102	* @see #setRunnable
103	*/
104	protected Runnable getRunnable() {
105	return runnable;
106	}
107
108	/**
109	* Set the Runnable forming the basis of this task.
110	* @param r the runnable action
111	* @see #getRunnable
112	*/
113	protected void setRunnable(Runnable r) {
114	runnable = r;
115	}
116
117	/**
118	* Set the state of this task to Cancelled.
119	*/
120	protected void setCancelled() {
121	runnerUpdater.set(this, CANCELLED);
122	}
123
124	/**
125	* Set the state of this task to Done, unless already
126	* in a Cancelled state, in which Cancelled status is preserved.
127	*
128	*/
129	protected void setDone() {
130	for (;;) {
131	Object r = runner;
132	if (r == DONE \|\| r == CANCELLED)
133	return;
134	if (runnerUpdater.compareAndSet(this, r, DONE))
135	return;
136	}
137	}
138
139	/**
140	* Attempt to set the state of this task to Running, succeeding
141	* only if the state is currently NOT Done, Running, or Cancelled.
142	* @return true if successful
143	*/
144	protected boolean setRunning() {
145	return runnerUpdater.compareAndSet(this, null, Thread.currentThread());
146	}
147
148	/**
149	* Runs the runnable if not cancelled, maintaining run status.
150	*/
151	public void run() {
152	if (setRunning()) {
153	try {
154	runnable.run();
155	} finally {
156	setDone();
157	}
158	}
159	}
160
161	/**
162	* Reset the run state of this task to its initial state unless
163	* it has been cancelled. (Note that a cancelled task cannot be
164	* reset.)
165	* @return true if successful
166	*/
167	protected boolean reset() {
168	for (;;) {
169	Object r = runner;
170	if (r == CANCELLED)
171	return false;
172	if (runnerUpdater.compareAndSet(this, r, null))
173	return true;
174	}
175	}
176
177	/**
178	* Implementation of Future methods under the control of a current
179	* <tt>CancellableTask</tt>, which it relies on for methods
180	* <tt>isDone</tt>, <tt>isCancelled</tt> and <tt>cancel</tt>. This
181	* class is split into an inner class to permit Future support to
182	* be mixed-in with other flavors of tasks. Normally, such a
183	* class will delegate <tt>Future</tt> <tt>get</tt> methods to the
184	* <tt>InnerCancellableFuture</tt>, and internally arrange that
185	* <tt>set</tt> methods be invoked when computations are ready.
186	*
187	* <p><b>Sample Usage</b>. Here are fragments of an example subclass.
188	* <pre>
189	* class MyFutureTask<V> extends CancellableTask implements Future<
190	V> {
191	*
192	* MyFutureTask(Callable<V> callable) {
193	* setRunnable(new InnerCancellableFuture<V>(callable));
194	* }
195	*
196	* public V get() throws InterruptedException, ExecutionException {
197	* return ((InnerCancellableFuture<V>)getRunnable()).get();
198	* }
199	* // (And similarly for timeout version.)
200	*
201	* void action() { // whatever action causes execution
202	* try {
203	* ((InnerCancellableFuture<V>)getRunnable()).set(compute());
204	* } catch (Exception ex) {
205	* ((InnerCancellableFuture<V>)getRunnable()).setException(ex);
206	* }
207	* }
208	* }
209	*</pre>
210	*/
211	protected class InnerCancellableFuture<V> implements Future<V>, Runnable {
212	private final Callable<V> callable;
213	private final ReentrantLock lock = new ReentrantLock();
214	private final Condition accessible = lock.newCondition();
215	private V result;
216	private Throwable exception;
217
218	/**
219	* Create an InnerCancellableFuture that will execute the
220	* given callable.
221	* @param callable the function to execute
222	*/
223	protected InnerCancellableFuture(Callable<V> callable) {
224	this.callable = callable;
225	}
226
227	public boolean cancel(boolean mayInterruptIfRunning) {
228	return CancellableTask.this.cancel(mayInterruptIfRunning);
229	}
230
231	public boolean isCancelled() {
232	return CancellableTask.this.isCancelled();
233	}
234
235
236	public boolean isDone() {
237	return CancellableTask.this.isDone();
238	}
239
240
241	/**
242	* Sets this Future to the results of <tt>callable.call</tt>
243	*/
244	public void run() {
245	try {
246	set(callable.call());
247	} catch(Throwable ex) {
248	setException(ex);
249	}
250	}
251
252	/**
253	* Waits if necessary for the call to <tt>callable.call</tt> to
254	* complete, and then retrieves its result.
255	*
256	* @return computed result
257	* @throws CancellationException here???
258	* @throws ExecutionException if underlying computation threw an
259	* exception
260	* @throws InterruptedException if current thread was interrupted
261	* while waiting
262	*/
263	public V get() throws InterruptedException, ExecutionException {
264	lock.lock();
265	try {
266	while (!isDone())
267	accessible.await();
268	if (isCancelled())
269	throw new CancellationException();
270	else if (exception != null)
271	throw new ExecutionException(exception);
272	else
273	return result;
274	} finally {
275	lock.unlock();
276	}
277	}
278
279	/**
280	* Waits if necessary for at most the given time for the call to
281	* <tt>callable.call</tt> to complete, and then retrieves its
282	* result.
283	*
284	* @param timeout the maximum time to wait
285	* @param unit the time unit of the timeout argument
286	* @return computed result
287	* @throws ExecutionException if underlying computation threw an
288	* exception
289	* @throws InterruptedException if current thread was interrupted
290	* while waiting
291	* @throws TimeoutException if the wait timed out
292	*/
293	public V get(long timeout, TimeUnit unit)
294	throws InterruptedException, ExecutionException, TimeoutException {
295	lock.lock();
296	try {
297	if (!isDone()) {
298	long nanos = unit.toNanos(timeout);
299	do {
300	if (nanos <= 0)
301	throw new TimeoutException();
302	nanos = accessible.awaitNanos(nanos);
303	} while (!isDone());
304	}
305	if (isCancelled())
306	throw new CancellationException();
307	else if (exception != null)
308	throw new ExecutionException(exception);
309	else
310	return result;
311	} finally {
312	lock.unlock();
313	}
314	}
315
316	/**
317	* Sets the result of this Future to the given value.
318	* @param v the value
319	*/
320	protected void set(V v) {
321	lock.lock();
322	try {
323	result = v;
324	setDone();
325	accessible.signalAll();
326	} finally {
327	lock.unlock();
328	}
329	}
330
331	/**
332	* Causes this future to report an <tt>ExecutionException</tt>
333	* with the given throwable as its cause.
334	* @param t the cause of failure.
335	*/
336	protected void setException(Throwable t) {
337	lock.lock();
338	try {
339	exception = t;
340	setDone();
341	accessible.signalAll();
342	} finally {
343	lock.unlock();
344	}
345	}
346	}
347
348	}