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 "runState" indicating task is cancelled
     */
    private static final Object CANCELLED = new Object();

    /**
     * Special value for "runState" 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 runState;

    /** 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;

    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;
    }

    /**
     * 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) {
        this.callable = Executors.callable(runnable, result);
    }

    public boolean isCancelled() {
        return runState == CANCELLED;
    }
    
    public boolean isDone() {
        Object r = runState;
        return r == DONE || r == CANCELLED;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        lock.lock();
        try {
            Object r = runState;
            if (r == DONE || r == CANCELLED)
                return false;
            runState = CANCELLED;
            accessible.signalAll();
            if (mayInterruptIfRunning && r != null && r instanceof Thread)
                ((Thread)r).interrupt();
        }
        finally{
            lock.unlock();
        }
        done();
        return true;
    }
    
    /**
     * 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 {
            for (;;) {
                Object r = runState;
                if (r == CANCELLED)
                    throw new CancellationException();
                if (r == DONE) {
                    if (exception != null)
                        throw new ExecutionException(exception);
                    else
                        return result;
                }
                accessible.await();
            }
        } 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 {
        long nanos = unit.toNanos(timeout);
        lock.lock();
        try {
            for (;;) {
                Object r = runState;
                if (r == CANCELLED)
                    throw new CancellationException();
                if (r == DONE) {
                    if (exception != null)
                        throw new ExecutionException(exception);
                    else
                        return result;
                }
                if (nanos <= 0)
                    throw new TimeoutException();
                nanos = accessible.awaitNanos(nanos);
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * 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 been cancelled
     * @param v the value
     */ 
    protected void set(V v) {
        lock.lock();
        try {
            if (runState == CANCELLED) 
                return;
            result = v;
            accessible.signalAll();
            runState = DONE;
        } finally {
            lock.unlock();
        }
        done();
    }

    /**
     * Causes this future to report an <tt>ExecutionException</tt>
     * with the given throwable as its cause, unless this Future has
     * been cancelled.
     * @param t the cause of failure.
     */ 
    protected void setException(Throwable t) {
        lock.lock();
        try {
            if (runState == CANCELLED) 
                return;
            exception = t;
            accessible.signalAll();
            runState = 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
     */ 
   private boolean setRunning() {
        lock.lock();
        try {
            if (runState != null)
                return false;
            runState = Thread.currentThread();
            return true;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * 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
     */
    private boolean reset() {
        lock.lock();
        try {
            if (runState == CANCELLED) 
                return false;
            runState = null;
            return true;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Sets this Future to the result of computation unless
     * it has been cancelled.
     */
    public void run() {
        if (setRunning()) {
            try {
                set(callable.call());
            } catch(Throwable ex) {
                setException(ex);
            }
        }
    }

    /**
     * Executes the computation 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() {
        if (setRunning()) {
            try {
                // don't bother to set result; it can't be accessed
                callable.call();
                return reset();
            } catch(Throwable ex) {
                setException(ex);
            } 
        }
        return false;
    }

}

Revision:	1.20
Committed:	Mon Dec 22 16:25:20 2003 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.19:	+99 -145 lines
Log Message:	Simplify FutureTask and AbstractExecutorService internals; improve docs
#	User	Rev	Content
1	tim	1.1	/*
2	dl	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	tim	1.1	*/
6
7			package java.util.concurrent;
8	dl	1.13	import java.util.concurrent.locks.*;
9
10	tim	1.1
11			/**
12	dl	1.8	* 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	dl	1.4	* 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	dl	1.8	* the computation has completed, the computation cannot be restarted
19			* or cancelled.
20	tim	1.1	*
21	dl	1.8	* <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	tim	1.1	*
26	dl	1.14	* <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	tim	1.1	* @since 1.5
31	dl	1.4	* @author Doug Lea
32	dl	1.12	* @param <V> The result type returned by this FutureTask's <tt>get</tt> method
33	tim	1.1	*/
34	dl	1.13	public class FutureTask<V> implements Future<V>, Runnable {
35			/**
36	dl	1.20	* Special value for "runState" indicating task is cancelled
37	dl	1.13	*/
38			private static final Object CANCELLED = new Object();
39
40			/**
41	dl	1.20	* Special value for "runState" indicating task is completed
42	dl	1.13	*/
43			private static final Object DONE = new Object();
44	tim	1.1
45	dl	1.11	/**
46	dl	1.13	* 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	dl	1.11	*/
52	dl	1.20	private volatile Object runState;
53	dl	1.14
54	dl	1.20	/** The underlying callable */
55	dl	1.15	private final Callable<V> callable;
56	dl	1.14	/** The result to return from get() */
57			private V result;
58			/** The exception to throw from get() */
59			private Throwable exception;
60
61	dl	1.13	private final ReentrantLock lock = new ReentrantLock();
62			private final ReentrantLock.ConditionObject accessible = lock.newCondition();
63	dl	1.11
64	tim	1.1	/**
65			* Constructs a <tt>FutureTask</tt> that will upon running, execute the
66			* given <tt>Callable</tt>.
67			*
68			* @param callable the callable task
69	dl	1.9	* @throws NullPointerException if callable is null
70	tim	1.1	*/
71			public FutureTask(Callable<V> callable) {
72	dl	1.9	if (callable == null)
73			throw new NullPointerException();
74	dl	1.13	this.callable = callable;
75	tim	1.1	}
76
77			/**
78			* Constructs a <tt>FutureTask</tt> that will upon running, execute the
79			* given <tt>Runnable</tt>, and arrange that <tt>get</tt> will return the
80			* given result on successful completion.
81			*
82			* @param runnable the runnable task
83			* @param result the result to return on successful completion. If
84	dl	1.9	* you don't need a particular result, consider using
85	dl	1.16	* constructions of the form:
86			* <tt>Future<?> f = new FutureTask<Object>(runnable, null)</tt>
87	dl	1.9	* @throws NullPointerException if runnable is null
88	tim	1.1	*/
89	dl	1.15	public FutureTask(Runnable runnable, V result) {
90	dl	1.20	this.callable = Executors.callable(runnable, result);
91			}
92
93			public boolean isCancelled() {
94			return runState == CANCELLED;
95			}
96
97			public boolean isDone() {
98			Object r = runState;
99			return r == DONE \|\| r == CANCELLED;
100	dl	1.13	}
101
102			public boolean cancel(boolean mayInterruptIfRunning) {
103			lock.lock();
104			try {
105	dl	1.20	Object r = runState;
106	dl	1.13	if (r == DONE \|\| r == CANCELLED)
107			return false;
108	dl	1.20	runState = CANCELLED;
109			accessible.signalAll();
110	dl	1.13	if (mayInterruptIfRunning && r != null && r instanceof Thread)
111			((Thread)r).interrupt();
112			}
113			finally{
114			lock.unlock();
115			}
116			done();
117			return true;
118			}
119
120	tim	1.1	/**
121	dl	1.17	* Waits if necessary for execution to complete, and then
122			* retrieves its result.
123	tim	1.1	*
124	dl	1.13	* @return computed result
125			* @throws CancellationException if underlying computation was
126			* cancelled
127			* @throws ExecutionException if underlying computation threw an
128			* exception
129	dl	1.4	* @throws InterruptedException if current thread was interrupted
130			* while waiting
131	tim	1.1	*/
132	dl	1.2	public V get() throws InterruptedException, ExecutionException {
133	dl	1.13	lock.lock();
134			try {
135	dl	1.20	for (;;) {
136			Object r = runState;
137			if (r == CANCELLED)
138			throw new CancellationException();
139			if (r == DONE) {
140			if (exception != null)
141			throw new ExecutionException(exception);
142			else
143			return result;
144			}
145	dl	1.13	accessible.await();
146	dl	1.20	}
147	dl	1.13	} finally {
148			lock.unlock();
149			}
150	tim	1.1	}
151
152			/**
153	dl	1.17	* Waits if necessary for at most the given time for execution to
154			* complete, and then retrieves its result, if available.
155	tim	1.1	*
156			* @param timeout the maximum time to wait
157	dl	1.2	* @param unit the time unit of the timeout argument
158	dl	1.13	* @return computed result
159			* @throws CancellationException if underlying computation was
160			* cancelled
161			* @throws ExecutionException if underlying computation threw an
162			* exception
163	dl	1.4	* @throws InterruptedException if current thread was interrupted
164			* while waiting
165	tim	1.1	* @throws TimeoutException if the wait timed out
166			*/
167	dl	1.2	public V get(long timeout, TimeUnit unit)
168	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
169	dl	1.20	long nanos = unit.toNanos(timeout);
170	dl	1.13	lock.lock();
171			try {
172	dl	1.20	for (;;) {
173			Object r = runState;
174			if (r == CANCELLED)
175			throw new CancellationException();
176			if (r == DONE) {
177			if (exception != null)
178			throw new ExecutionException(exception);
179			else
180			return result;
181			}
182			if (nanos <= 0)
183			throw new TimeoutException();
184			nanos = accessible.awaitNanos(nanos);
185	dl	1.13	}
186			} finally {
187			lock.unlock();
188			}
189	tim	1.1	}
190
191			/**
192	dl	1.20	* Protected method invoked when this task transitions to state
193			* <tt>isDone</tt> (whether normally or via cancellation). The
194			* default implementation does nothing. Subclasses may override
195			* this method to invoke completion callbacks or perform
196			* bookkeeping. Note that you can query status inside the
197			* implementation of this method to determine whether this task
198			* has been cancelled.
199			*/
200			protected void done() { }
201
202			/**
203			* Sets the result of this Future to the given value unless
204			* this future has been cancelled
205	tim	1.1	* @param v the value
206	dl	1.13	*/
207	dl	1.2	protected void set(V v) {
208	dl	1.13	lock.lock();
209			try {
210	dl	1.20	if (runState == CANCELLED)
211			return;
212	dl	1.13	result = v;
213	dl	1.20	accessible.signalAll();
214			runState = DONE;
215	dl	1.13	} finally {
216			lock.unlock();
217			}
218	dl	1.20	done();
219	tim	1.1	}
220
221			/**
222	dl	1.13	* Causes this future to report an <tt>ExecutionException</tt>
223	dl	1.20	* with the given throwable as its cause, unless this Future has
224			* been cancelled.
225	dl	1.13	* @param t the cause of failure.
226			*/
227	dl	1.2	protected void setException(Throwable t) {
228	dl	1.13	lock.lock();
229			try {
230	dl	1.20	if (runState == CANCELLED)
231			return;
232	dl	1.13	exception = t;
233	dl	1.20	accessible.signalAll();
234			runState = DONE;
235	dl	1.13	} finally {
236			lock.unlock();
237			}
238	dl	1.20	done();
239	tim	1.1	}
240	dl	1.20
241	dl	1.14	/**
242	dl	1.20	* Attempts to set the state of this task to Running, succeeding
243			* only if the state is currently NOT Done, Running, or Cancelled.
244			* @return true if successful
245			*/
246			private boolean setRunning() {
247	dl	1.14	lock.lock();
248			try {
249	dl	1.20	if (runState != null)
250			return false;
251			runState = Thread.currentThread();
252			return true;
253	dl	1.14	}
254	dl	1.20	finally {
255	dl	1.14	lock.unlock();
256			}
257			}
258	dl	1.3
259	dl	1.14	/**
260	dl	1.20	* Resets the run state of this task to its initial state unless
261			* it has been cancelled. (Note that a cancelled task cannot be
262			* reset.)
263	dl	1.14	* @return true if successful
264	dl	1.20	*/
265			private boolean reset() {
266	dl	1.14	lock.lock();
267			try {
268	dl	1.20	if (runState == CANCELLED)
269	dl	1.14	return false;
270	dl	1.20	runState = null;
271	dl	1.14	return true;
272			}
273			finally {
274			lock.unlock();
275			}
276			}
277
278			/**
279	dl	1.20	* Sets this Future to the result of computation unless
280			* it has been cancelled.
281	dl	1.14	*/
282			public void run() {
283			if (setRunning()) {
284			try {
285	dl	1.20	set(callable.call());
286	dl	1.19	} catch(Throwable ex) {
287			setException(ex);
288	dl	1.14	}
289			}
290			}
291
292			/**
293	dl	1.20	* Executes the computation and then resets this Future to initial
294			* state; failing to do so if the computation encounters an
295			* exception or is cancelled. This is designed for use with tasks
296			* that intrinsically execute more than once.
297			* @return true if successfully run and reset
298	dl	1.15	*/
299	dl	1.20	protected boolean runAndReset() {
300	dl	1.15	if (setRunning()) {
301			try {
302	dl	1.20	// don't bother to set result; it can't be accessed
303			callable.call();
304			return reset();
305	dl	1.19	} catch(Throwable ex) {
306			setException(ex);
307	dl	1.20	}
308	dl	1.14	}
309	dl	1.20	return false;
310	dl	1.15	}
311	tim	1.1
312	dl	1.15	}
313	tim	1.1