util/concurrent/Exchanger.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.*;

/**
 * An <tt>Exchanger</tt> provides a synchronization point at which two threads
 * can exchange objects.  Each thread presents some object on entry to
 * the {@link #exchange exchange} method, and receives the object presented by
 * the other thread on return.
 *
 * <p><b>Sample Usage:</b>
 * Here are the highlights of a class that uses an <tt>Exchanger</tt> to
 * swap buffers between threads so that the thread filling the
 * buffer gets a freshly
 * emptied one when it needs it, handing off the filled one to
 * the thread emptying the buffer.
 * <pre>
 * class FillAndEmpty {
 *   Exchanger&lt;Buffer&gt; exchanger = new Exchanger();
 *   Buffer initialEmptyBuffer = ... a made-up type
 *   Buffer initialFullBuffer = ...
 *
 *   class FillingLoop implements Runnable {
 *     public void run() {
 *       Buffer currentBuffer = initialEmptyBuffer;
 *       try {
 *         while (currentBuffer != null) {
 *           addToBuffer(currentBuffer);
 *           if (currentBuffer.full())
 *             currentBuffer = exchanger.exchange(currentBuffer);
 *         }
 *       }
 *       catch (InterruptedException ex) { ... handle ... }
 *     }
 *   }
 *
 *   class EmptyingLoop implements Runnable {
 *     public void run() {
 *       Buffer currentBuffer = initialFullBuffer;
 *       try {
 *         while (currentBuffer != null) {
 *           takeFromBuffer(currentBuffer);
 *           if (currentBuffer.empty())
 *             currentBuffer = exchanger.exchange(currentBuffer);
 *         }
 *       }
 *       catch (InterruptedException ex) { ... handle ...}
 *     }
 *   }
 *
 *   void start() {
 *     new Thread(new FillingLoop()).start();
 *     new Thread(new EmptyingLoop()).start();
 *   }
 * }
 * </pre>
 *
 * @fixme change example to use a bounded queue?
 *
 * @since 1.5
 * @spec JSR-166
 * @revised $Date: 2003/07/08 00:46:33 $
 * @editor $Author: dl $
 * @author Doug Lea
 */
public class Exchanger<V> {
    private final ReentrantLock lock = new ReentrantLock();
    private final Condition taken = lock.newCondition();

    /** Holder for the item being exchanged */
    private V item;
    
    /**
     * Arrival count transitions from 0 to 1 to 2 then back to 0
     * during an exchange.
     */
    private int arrivalCount;

    /**
     * Main exchange function, handling the different policy variants.
     */
    private V doExchange(V x, boolean timed, long nanos) throws InterruptedException, TimeoutException {
        lock.lock();
        try {
            V other;

            // If arrival count already at two, we must wait for
            // a previous pair to finish and reset the count;
            while (arrivalCount == 2) {
                if (!timed)
                    taken.await();
                else if (nanos > 0) 
                    nanos = taken.awaitNanos(nanos);
                else 
                    throw new TimeoutException();
            }

            int count = ++arrivalCount;

            // If item is already waiting, replace it and signal other thread
            if (count == 2) { 
                other = item;
                item = x;
                taken.signal();
                return other;
            }

            // Otherwise, set item and wait for another thread to
            // replace it and signal us.

            item = x;
            InterruptedException interrupted = null;
            try { 
                while (arrivalCount != 2) {
                    if (!timed)
                        taken.await();
                    else if (nanos > 0) 
                        nanos = taken.awaitNanos(nanos);
                    else 
                        break; // timed out
                }
            }
            catch (InterruptedException ie) {
                interrupted = ie;
            }

            // Get and reset item and count after the wait.
            // (We need to do this even if wait was aborted.)
            other = item;
            item = null;
            count = arrivalCount;
            arrivalCount = 0; 
            taken.signal();
            
            // If the other thread replaced item, then we must
            // continue even if cancelled.
            if (count == 2) {
                if (interrupted != null)
                    Thread.currentThread().interrupt();
                return other;
            }

            // If no one is waiting for us, we can back out
            if (interrupted != null) 
                throw interrupted;
            else  // must be timeout
                throw new TimeoutException();
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Create a new Exchanger
     **/
    public Exchanger() {
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * it is {@link Thread#interrupt interrupted}),
     * and then transfers the given object to it, receiving its object
     * in return.
     * <p>If another thread is already waiting at the exchange point then
     * it is resumed for thread scheduling purposes and receives the object
     * passed in by the current thread. The current thread returns immediately,
     * receiving the object passed to the exchange by that other thread.
     * <p>If no other thread is already waiting at the exchange then the 
     * current thread is disabled for thread scheduling purposes and lies
     * dormant until one of two things happens:
     * <ul>
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or 
     * <li>is {@link Thread#interrupt interrupted} while waiting
     * for the exchange, 
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * @param x the object to exchange
     * @return the object provided by the other thread.
     * @throws InterruptedException if current thread was interrupted 
     * while waiting
     **/
    public V exchange(V x) throws InterruptedException {
        try {
            return doExchange(x, false, 0);
        }
        catch (TimeoutException cannotHappen) { 
            throw new Error(cannotHappen);
        }
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * it is {@link Thread#interrupt interrupted}, or the specified waiting
     * time elapses),
     * and then transfers the given object to it, receiving its object
     * in return.
     *
     * <p>If another thread is already waiting at the exchange point then
     * it is resumed for thread scheduling purposes and receives the object
     * passed in by the current thread. The current thread returns immediately,
     * receiving the object passed to the exchange by that other thread.
     *
     * <p>If no other thread is already waiting at the exchange then the 
     * current thread is disabled for thread scheduling purposes and lies
     * dormant until one of three things happens:
     * <ul>
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or 
     * <li>is {@link Thread#interrupt interrupted} while waiting
     * for the exchange, 
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * <p>If the specified waiting time elapses then {@link TimeoutException}
     * is thrown.
     * The given waiting time is a best-effort lower bound. If the time is 
     * less than or equal to zero, the method will not wait at all.
     *
     * @param x the object to exchange
     * @param timeout the maximum time to wait
     * @param unit the time unit of the <tt>timeout</tt> argument.
     * @return the object provided by the other thread.
     * @throws InterruptedException if current thread was interrupted
     * while waiting
     * @throws TimeoutException if the specified waiting time elapses before
     * another thread enters the exchange.
     **/
    public V exchange(V x, long timeout, TimeUnit unit) 
        throws InterruptedException, TimeoutException {
        return doExchange(x, true, unit.toNanos(timeout));
    }

}


Revision:	1.5
Committed:	Wed Jul 9 23:23:17 2003 UTC (20 years, 10 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_2
Changes since 1.4:	+18 -6 lines
Log Message:	Misc performance tunings
#	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	* An <tt>Exchanger</tt> provides a synchronization point at which two threads
12	* can exchange objects. Each thread presents some object on entry to
13	* the {@link #exchange exchange} method, and receives the object presented by
14	* the other thread on return.
15	*
16	* <p><b>Sample Usage:</b>
17	* Here are the highlights of a class that uses an <tt>Exchanger</tt> to
18	* swap buffers between threads so that the thread filling the
19	* buffer gets a freshly
20	* emptied one when it needs it, handing off the filled one to
21	* the thread emptying the buffer.
22	* <pre>
23	* class FillAndEmpty {
24	* Exchanger<Buffer> exchanger = new Exchanger();
25	* Buffer initialEmptyBuffer = ... a made-up type
26	* Buffer initialFullBuffer = ...
27	*
28	* class FillingLoop implements Runnable {
29	* public void run() {
30	* Buffer currentBuffer = initialEmptyBuffer;
31	* try {
32	* while (currentBuffer != null) {
33	* addToBuffer(currentBuffer);
34	* if (currentBuffer.full())
35	* currentBuffer = exchanger.exchange(currentBuffer);
36	* }
37	* }
38	* catch (InterruptedException ex) { ... handle ... }
39	* }
40	* }
41	*
42	* class EmptyingLoop implements Runnable {
43	* public void run() {
44	* Buffer currentBuffer = initialFullBuffer;
45	* try {
46	* while (currentBuffer != null) {
47	* takeFromBuffer(currentBuffer);
48	* if (currentBuffer.empty())
49	* currentBuffer = exchanger.exchange(currentBuffer);
50	* }
51	* }
52	* catch (InterruptedException ex) { ... handle ...}
53	* }
54	* }
55	*
56	* void start() {
57	* new Thread(new FillingLoop()).start();
58	* new Thread(new EmptyingLoop()).start();
59	* }
60	* }
61	* </pre>
62	*
63	* @fixme change example to use a bounded queue?
64	*
65	* @since 1.5
66	* @spec JSR-166
67	* @revised $Date: 2003/07/08 00:46:33 $
68	* @editor $Author: dl $
69	* @author Doug Lea
70	*/
71	public class Exchanger<V> {
72	private final ReentrantLock lock = new ReentrantLock();
73	private final Condition taken = lock.newCondition();
74
75	/** Holder for the item being exchanged */
76	private V item;
77
78	/**
79	* Arrival count transitions from 0 to 1 to 2 then back to 0
80	* during an exchange.
81	*/
82	private int arrivalCount;
83
84	/**
85	* Main exchange function, handling the different policy variants.
86	*/
87	private V doExchange(V x, boolean timed, long nanos) throws InterruptedException, TimeoutException {
88	lock.lock();
89	try {
90	V other;
91
92	// If arrival count already at two, we must wait for
93	// a previous pair to finish and reset the count;
94	while (arrivalCount == 2) {
95	if (!timed)
96	taken.await();
97	else if (nanos > 0)
98	nanos = taken.awaitNanos(nanos);
99	else
100	throw new TimeoutException();
101	}
102
103	int count = ++arrivalCount;
104
105	// If item is already waiting, replace it and signal other thread
106	if (count == 2) {
107	other = item;
108	item = x;
109	taken.signal();
110	return other;
111	}
112
113	// Otherwise, set item and wait for another thread to
114	// replace it and signal us.
115
116	item = x;
117	InterruptedException interrupted = null;
118	try {
119	while (arrivalCount != 2) {
120	if (!timed)
121	taken.await();
122	else if (nanos > 0)
123	nanos = taken.awaitNanos(nanos);
124	else
125	break; // timed out
126	}
127	}
128	catch (InterruptedException ie) {
129	interrupted = ie;
130	}
131
132	// Get and reset item and count after the wait.
133	// (We need to do this even if wait was aborted.)
134	other = item;
135	item = null;
136	count = arrivalCount;
137	arrivalCount = 0;
138	taken.signal();
139
140	// If the other thread replaced item, then we must
141	// continue even if cancelled.
142	if (count == 2) {
143	if (interrupted != null)
144	Thread.currentThread().interrupt();
145	return other;
146	}
147
148	// If no one is waiting for us, we can back out
149	if (interrupted != null)
150	throw interrupted;
151	else // must be timeout
152	throw new TimeoutException();
153	}
154	finally {
155	lock.unlock();
156	}
157	}
158
159	/**
160	* Create a new Exchanger
161	**/
162	public Exchanger() {
163	}
164
165	/**
166	* Waits for another thread to arrive at this exchange point (unless
167	* it is {@link Thread#interrupt interrupted}),
168	* and then transfers the given object to it, receiving its object
169	* in return.
170	* <p>If another thread is already waiting at the exchange point then
171	* it is resumed for thread scheduling purposes and receives the object
172	* passed in by the current thread. The current thread returns immediately,
173	* receiving the object passed to the exchange by that other thread.
174	* <p>If no other thread is already waiting at the exchange then the
175	* current thread is disabled for thread scheduling purposes and lies
176	* dormant until one of two things happens:
177	* <ul>
178	* <li>Some other thread enters the exchange; or
179	* <li>Some other thread {@link Thread#interrupt interrupts} the current
180	* thread.
181	* </ul>
182	* <p>If the current thread:
183	* <ul>
184	* <li>has its interrupted status set on entry to this method; or
185	* <li>is {@link Thread#interrupt interrupted} while waiting
186	* for the exchange,
187	* </ul>
188	* then {@link InterruptedException} is thrown and the current thread's
189	* interrupted status is cleared.
190	*
191	* @param x the object to exchange
192	* @return the object provided by the other thread.
193	* @throws InterruptedException if current thread was interrupted
194	* while waiting
195	**/
196	public V exchange(V x) throws InterruptedException {
197	try {
198	return doExchange(x, false, 0);
199	}
200	catch (TimeoutException cannotHappen) {
201	throw new Error(cannotHappen);
202	}
203	}
204
205	/**
206	* Waits for another thread to arrive at this exchange point (unless
207	* it is {@link Thread#interrupt interrupted}, or the specified waiting
208	* time elapses),
209	* and then transfers the given object to it, receiving its object
210	* in return.
211	*
212	* <p>If another thread is already waiting at the exchange point then
213	* it is resumed for thread scheduling purposes and receives the object
214	* passed in by the current thread. The current thread returns immediately,
215	* receiving the object passed to the exchange by that other thread.
216	*
217	* <p>If no other thread is already waiting at the exchange then the
218	* current thread is disabled for thread scheduling purposes and lies
219	* dormant until one of three things happens:
220	* <ul>
221	* <li>Some other thread enters the exchange; or
222	* <li>Some other thread {@link Thread#interrupt interrupts} the current
223	* thread; or
224	* <li>The specified waiting time elapses.
225	* </ul>
226	* <p>If the current thread:
227	* <ul>
228	* <li>has its interrupted status set on entry to this method; or
229	* <li>is {@link Thread#interrupt interrupted} while waiting
230	* for the exchange,
231	* </ul>
232	* then {@link InterruptedException} is thrown and the current thread's
233	* interrupted status is cleared.
234	*
235	* <p>If the specified waiting time elapses then {@link TimeoutException}
236	* is thrown.
237	* The given waiting time is a best-effort lower bound. If the time is
238	* less than or equal to zero, the method will not wait at all.
239	*
240	* @param x the object to exchange
241	* @param timeout the maximum time to wait
242	* @param unit the time unit of the <tt>timeout</tt> argument.
243	* @return the object provided by the other thread.
244	* @throws InterruptedException if current thread was interrupted
245	* while waiting
246	* @throws TimeoutException if the specified waiting time elapses before
247	* another thread enters the exchange.
248	**/
249	public V exchange(V x, long timeout, TimeUnit unit)
250	throws InterruptedException, TimeoutException {
251	return doExchange(x, true, unit.toNanos(timeout));
252	}
253
254	}
255
256