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 cancellable actions running in the 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
     */
    public CancellableTask(Runnable r) {
        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 beofre 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());
    }

    public void run() {
        if (setRunning()) {
            try {
                runnable.run();
            } finally {
                setDone();
            }
        }
    }

    /**
     * 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 granularity 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 futue 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:	1.13
Committed:	Sun Aug 24 14:47:31 2003 UTC (20 years, 9 months ago) by dl
Branch:	MAIN
Changes since 1.12:	+69 -2 lines
Log Message:	Javadoc clarifications
#	User	Rev	Content
1	dl	1.1	/*
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	dl	1.6	import java.util.concurrent.locks.*;
10	dl	1.1
11			/**
12			* Base class for cancellable actions running in the Executor
13			* framework. In addition to serving as a standalone class, this
14			* provides <tt>protected</tt> functionality that may be useful when
15			* creating customized task classes.
16	dl	1.5	* @since 1.5
17			* @author Doug Lea
18	dl	1.1	*/
19
20			public class CancellableTask implements Cancellable, Runnable {
21			/**
22			* Holds the run-state, taking on values:
23			* null = not yet started,
24			* [some thread ref] = running,
25			* DONE = completed normally,
26			* CANCELLED = cancelled (may or may not have ever run).
27			* Transitions use atomic updates.
28			*/
29			private volatile Object runner;
30
31	dl	1.5	/**
32	dl	1.1	* Special value for "runner" indicating task is completed
33			*/
34			private static final Object DONE = new Object();
35
36	dl	1.5	/**
37	dl	1.1	* Special value for "runner" indicating task is cancelled
38			*/
39			private static final Object CANCELLED = new Object();
40
41			private static AtomicReferenceFieldUpdater<CancellableTask, Object>
42	dl	1.9	runnerUpdater =
43			AtomicReferenceFieldUpdater.newUpdater
44			(CancellableTask.class, Object.class, "runner");
45	dl	1.1
46			/**
47			* The runnable underlying this task
48			*/
49			private volatile Runnable runnable;
50
51			/**
52	brian	1.4	* Creates a new CancellableTask which invokes the given
53			* <tt>Runnable</tt> when executed.
54	dl	1.5	* @param r the runnable action
55	dl	1.1	*/
56			public CancellableTask(Runnable r) {
57			this.runnable = r;
58			}
59
60			/**
61			* Creates a new CancellableTask without a runnable action, which
62			* must be set using <tt>setRunnable</tt> before use. This is
63			* intended for use in subclasses that must complete superclass
64			* construction beofre establishing the runnable action.
65			*/
66			protected CancellableTask() {
67			}
68
69
70			public boolean cancel(boolean mayInterruptIfRunning) {
71			Object r = runner;
72			if (r == DONE \|\| r == CANCELLED)
73			return false;
74
75			if (mayInterruptIfRunning &&
76			r != null &&
77			r instanceof Thread &&
78	dl	1.3	runnerUpdater.compareAndSet(this, r, CANCELLED))
79	dl	1.1
80			((Thread)r).interrupt();
81	dl	1.3	else
82	dl	1.1	runnerUpdater.set(this, CANCELLED);
83			return true;
84			}
85
86			public boolean isCancelled() {
87			return runner == CANCELLED;
88			}
89
90			public boolean isDone() {
91			Object r = runner;
92			return r == DONE \|\| r == CANCELLED;
93			}
94
95			/**
96			* Return the Runnable forming the basis of this task.
97	dl	1.5	* @return the runnable action
98	tim	1.10	* @see #setRunnable
99	dl	1.1	*/
100			protected Runnable getRunnable() {
101			return runnable;
102			}
103
104			/**
105			* Set the Runnable forming the basis of this task.
106	dl	1.5	* @param r the runnable action
107	tim	1.10	* @see #getRunnable
108	dl	1.1	*/
109			protected void setRunnable(Runnable r) {
110			runnable = r;
111			}
112
113			/**
114	tim	1.10	* Set the state of this task to Cancelled.
115	dl	1.1	*/
116			protected void setCancelled() {
117			runnerUpdater.set(this, CANCELLED);
118			}
119
120			/**
121			* Set the state of this task to Done, unless already
122			* in a Cancelled state, in which Cancelled status is preserved.
123			*
124			*/
125			protected void setDone() {
126			for (;;) {
127			Object r = runner;
128			if (r == DONE \|\| r == CANCELLED)
129			return;
130			if (runnerUpdater.compareAndSet(this, r, DONE))
131			return;
132			}
133			}
134
135			/**
136			* Attempt to set the state of this task to Running, succeeding
137			* only if the state is currently NOT Done, Running, or Cancelled.
138	dl	1.2	* @return true if successful
139	dl	1.1	*/
140			protected boolean setRunning() {
141			return runnerUpdater.compareAndSet(this, null, Thread.currentThread());
142			}
143
144			public void run() {
145			if (setRunning()) {
146			try {
147			runnable.run();
148	tim	1.11	} finally {
149	dl	1.1	setDone();
150			}
151			}
152			}
153
154			/**
155			* Implementation of Future methods under the control of a current
156	dl	1.13	* <tt>CancellableTask</tt>, which it relies on for methods
157			* <tt>isDone</tt>, <tt>isCancelled</tt> and <tt>cancel</tt>. This
158			* class is split into an inner class to permit Future support to
159			* be mixed-in with other flavors of tasks. Normally, such a
160			* class will delegate <tt>Future</tt> <tt>get</tt> methods to the
161			* <tt>InnerCancellableFuture</tt>, and internally arrange that
162			* <tt>set</tt> methods be invoked when computations are ready.
163			*
164			* <p><b>Sample Usage</b>. Here are fragments of an example subclass.
165			* <pre>
166			* class MyFutureTask<V> extends CancellableTask implements Future<
167			V> {
168			*
169			* MyFutureTask(Callable<V> callable) {
170			* setRunnable(new InnerCancellableFuture<V>(callable));
171			* }
172			*
173			* public V get() throws InterruptedException, ExecutionException {
174			* return ((InnerCancellableFuture<V>)getRunnable()).get();
175			* }
176			* // (And similarly for timeout version.)
177			*
178			* void action() { // whatever action causes execution
179			* try {
180			* ((InnerCancellableFuture<V>)getRunnable()).set(compute());
181			* } catch (Exception ex) {
182			* ((InnerCancellableFuture<V>)getRunnable()).setException(ex);
183			* }
184			* }
185			* }
186			*</pre>
187	dl	1.1	*/
188			protected class InnerCancellableFuture<V> implements Future<V>, Runnable {
189			private final Callable<V> callable;
190			private final ReentrantLock lock = new ReentrantLock();
191			private final Condition accessible = lock.newCondition();
192			private V result;
193			private Throwable exception;
194	dl	1.5
195			/**
196			* Create an InnerCancellableFuture that will execute the
197			* given callable.
198			* @param callable the function to execute
199			*/
200	dl	1.1	protected InnerCancellableFuture(Callable<V> callable) {
201			this.callable = callable;
202			}
203	tim	1.12
204			public boolean cancel(boolean mayInterruptIfRunning) {
205			return CancellableTask.this.cancel(mayInterruptIfRunning);
206			}
207
208			public boolean isCancelled() {
209			return CancellableTask.this.isCancelled();
210			}
211
212	dl	1.1
213			public boolean isDone() {
214			return CancellableTask.this.isDone();
215			}
216
217	dl	1.13
218			/**
219			* Sets this Future to the results of <tt>callable.call</tt>
220			*/
221	dl	1.1	public void run() {
222			try {
223			set(callable.call());
224	tim	1.11	} catch(Throwable ex) {
225	dl	1.1	setException(ex);
226			}
227			}
228
229	dl	1.13	/**
230			* Waits if necessary for the call to <tt>callable.call</tt> to
231			* complete, and then retrieves its result.
232			*
233			* @return computed result
234			* @throws CancellationException here???
235			* @throws ExecutionException if underlying computation threw an
236			* exception
237			* @throws InterruptedException if current thread was interrupted
238			* while waiting
239			*/
240	dl	1.1	public V get() throws InterruptedException, ExecutionException {
241			lock.lock();
242			try {
243			while (!isDone())
244			accessible.await();
245			if (isCancelled())
246			throw new CancellationException();
247			else if (exception != null)
248			throw new ExecutionException(exception);
249			else
250			return result;
251	tim	1.11	} finally {
252	dl	1.1	lock.unlock();
253			}
254			}
255
256	dl	1.13	/**
257			* Waits if necessary for at most the given time for the call to
258			* <tt>callable.call</tt> to complete, and then retrieves its
259			* result.
260			*
261			* @param timeout the maximum time to wait
262			* @param granularity the time unit of the timeout argument
263			* @return computed result
264			* @throws ExecutionException if underlying computation threw an
265			* exception
266			* @throws InterruptedException if current thread was interrupted
267			* while waiting
268			* @throws TimeoutException if the wait timed out
269			*/
270	dl	1.1	public V get(long timeout, TimeUnit unit)
271			throws InterruptedException, ExecutionException, TimeoutException {
272			lock.lock();
273			try {
274			if (!isDone()) {
275			long nanos = unit.toNanos(timeout);
276			do {
277			if (nanos <= 0)
278			throw new TimeoutException();
279			nanos = accessible.awaitNanos(nanos);
280			} while (!isDone());
281			}
282			if (isCancelled())
283			throw new CancellationException();
284			else if (exception != null)
285			throw new ExecutionException(exception);
286			else
287			return result;
288	tim	1.11	} finally {
289	dl	1.1	lock.unlock();
290			}
291			}
292
293	dl	1.13	/**
294			* Sets the result of this Future to the given value.
295			* @param v the value
296			*/
297	dl	1.1	protected void set(V v) {
298			lock.lock();
299			try {
300			result = v;
301			setDone();
302			accessible.signalAll();
303	tim	1.11	} finally {
304	dl	1.1	lock.unlock();
305			}
306			}
307
308	dl	1.13	/**
309			* Causes this futue to report an <tt>ExecutionException</tt>
310			* with the given throwable as its cause.
311			* @param t the cause of failure.
312			*/
313	dl	1.1	protected void setException(Throwable t) {
314			lock.lock();
315			try {
316			exception = t;
317			setDone();
318			accessible.signalAll();
319	tim	1.11	} finally {
320	dl	1.1	lock.unlock();
321			}
322			}
323			}
324
325			}