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;DataBuffer&gt; exchanger = new Exchanger();
 *   DataBuffer initialEmptyBuffer = ... a made-up type
 *   DataBuffer initialFullBuffer = ...
 *
 *   class FillingLoop implements Runnable {
 *     public void run() {
 *       DataBuffer 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() {
 *       DataBuffer 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>
 *
 * @since 1.5
 * @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.
     * 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:	1.9
Committed:	Sun Aug 31 13:33:13 2003 UTC (20 years, 9 months ago) by dl
Branch:	MAIN
Changes since 1.8:	+5 -10 lines
Log Message:	Removed non-standard tags and misc javadoc cleanup
#	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<DataBuffer> exchanger = new Exchanger();
25	* DataBuffer initialEmptyBuffer = ... a made-up type
26	* DataBuffer initialFullBuffer = ...
27	*
28	* class FillingLoop implements Runnable {
29	* public void run() {
30	* DataBuffer currentBuffer = initialEmptyBuffer;
31	* try {
32	* while (currentBuffer != null) {
33	* addToBuffer(currentBuffer);
34	* if (currentBuffer.full())
35	* currentBuffer = exchanger.exchange(currentBuffer);
36	* }
37	* } catch (InterruptedException ex) { ... handle ... }
38	* }
39	* }
40	*
41	* class EmptyingLoop implements Runnable {
42	* public void run() {
43	* DataBuffer currentBuffer = initialFullBuffer;
44	* try {
45	* while (currentBuffer != null) {
46	* takeFromBuffer(currentBuffer);
47	* if (currentBuffer.empty())
48	* currentBuffer = exchanger.exchange(currentBuffer);
49	* }
50	* } catch (InterruptedException ex) { ... handle ...}
51	* }
52	* }
53	*
54	* void start() {
55	* new Thread(new FillingLoop()).start();
56	* new Thread(new EmptyingLoop()).start();
57	* }
58	* }
59	* </pre>
60	*
61	* @since 1.5
62	* @author Doug Lea
63	*/
64	public class Exchanger<V> {
65	private final ReentrantLock lock = new ReentrantLock();
66	private final Condition taken = lock.newCondition();
67
68	/** Holder for the item being exchanged */
69	private V item;
70
71	/**
72	* Arrival count transitions from 0 to 1 to 2 then back to 0
73	* during an exchange.
74	*/
75	private int arrivalCount;
76
77	/**
78	* Main exchange function, handling the different policy variants.
79	*/
80	private V doExchange(V x, boolean timed, long nanos) throws InterruptedException, TimeoutException {
81	lock.lock();
82	try {
83	V other;
84
85	// If arrival count already at two, we must wait for
86	// a previous pair to finish and reset the count;
87	while (arrivalCount == 2) {
88	if (!timed)
89	taken.await();
90	else if (nanos > 0)
91	nanos = taken.awaitNanos(nanos);
92	else
93	throw new TimeoutException();
94	}
95
96	int count = ++arrivalCount;
97
98	// If item is already waiting, replace it and signal other thread
99	if (count == 2) {
100	other = item;
101	item = x;
102	taken.signal();
103	return other;
104	}
105
106	// Otherwise, set item and wait for another thread to
107	// replace it and signal us.
108
109	item = x;
110	InterruptedException interrupted = null;
111	try {
112	while (arrivalCount != 2) {
113	if (!timed)
114	taken.await();
115	else if (nanos > 0)
116	nanos = taken.awaitNanos(nanos);
117	else
118	break; // timed out
119	}
120	} catch (InterruptedException ie) {
121	interrupted = ie;
122	}
123
124	// Get and reset item and count after the wait.
125	// (We need to do this even if wait was aborted.)
126	other = item;
127	item = null;
128	count = arrivalCount;
129	arrivalCount = 0;
130	taken.signal();
131
132	// If the other thread replaced item, then we must
133	// continue even if cancelled.
134	if (count == 2) {
135	if (interrupted != null)
136	Thread.currentThread().interrupt();
137	return other;
138	}
139
140	// If no one is waiting for us, we can back out
141	if (interrupted != null)
142	throw interrupted;
143	else // must be timeout
144	throw new TimeoutException();
145	} finally {
146	lock.unlock();
147	}
148	}
149
150	/**
151	* Create a new Exchanger.
152	**/
153	public Exchanger() {
154	}
155
156	/**
157	* Waits for another thread to arrive at this exchange point (unless
158	* it is {@link Thread#interrupt interrupted}),
159	* and then transfers the given object to it, receiving its object
160	* in return.
161	* <p>If another thread is already waiting at the exchange point then
162	* it is resumed for thread scheduling purposes and receives the object
163	* passed in by the current thread. The current thread returns immediately,
164	* receiving the object passed to the exchange by that other thread.
165	* <p>If no other thread is already waiting at the exchange then the
166	* current thread is disabled for thread scheduling purposes and lies
167	* dormant until one of two things happens:
168	* <ul>
169	* <li>Some other thread enters the exchange; or
170	* <li>Some other thread {@link Thread#interrupt interrupts} the current
171	* thread.
172	* </ul>
173	* <p>If the current thread:
174	* <ul>
175	* <li>has its interrupted status set on entry to this method; or
176	* <li>is {@link Thread#interrupt interrupted} while waiting
177	* for the exchange,
178	* </ul>
179	* then {@link InterruptedException} is thrown and the current thread's
180	* interrupted status is cleared.
181	*
182	* @param x the object to exchange
183	* @return the object provided by the other thread.
184	* @throws InterruptedException if current thread was interrupted
185	* while waiting
186	**/
187	public V exchange(V x) throws InterruptedException {
188	try {
189	return doExchange(x, false, 0);
190	} catch (TimeoutException cannotHappen) {
191	throw new Error(cannotHappen);
192	}
193	}
194
195	/**
196	* Waits for another thread to arrive at this exchange point (unless
197	* it is {@link Thread#interrupt interrupted}, or the specified waiting
198	* time elapses),
199	* and then transfers the given object to it, receiving its object
200	* in return.
201	*
202	* <p>If another thread is already waiting at the exchange point then
203	* it is resumed for thread scheduling purposes and receives the object
204	* passed in by the current thread. The current thread returns immediately,
205	* receiving the object passed to the exchange by that other thread.
206	*
207	* <p>If no other thread is already waiting at the exchange then the
208	* current thread is disabled for thread scheduling purposes and lies
209	* dormant until one of three things happens:
210	* <ul>
211	* <li>Some other thread enters the exchange; or
212	* <li>Some other thread {@link Thread#interrupt interrupts} the current
213	* thread; or
214	* <li>The specified waiting time elapses.
215	* </ul>
216	* <p>If the current thread:
217	* <ul>
218	* <li>has its interrupted status set on entry to this method; or
219	* <li>is {@link Thread#interrupt interrupted} while waiting
220	* for the exchange,
221	* </ul>
222	* then {@link InterruptedException} is thrown and the current thread's
223	* interrupted status is cleared.
224	*
225	* <p>If the specified waiting time elapses then {@link TimeoutException}
226	* is thrown.
227	* If the time is
228	* less than or equal to zero, the method will not wait at all.
229	*
230	* @param x the object to exchange
231	* @param timeout the maximum time to wait
232	* @param unit the time unit of the <tt>timeout</tt> argument.
233	* @return the object provided by the other thread.
234	* @throws InterruptedException if current thread was interrupted
235	* while waiting
236	* @throws TimeoutException if the specified waiting time elapses before
237	* another thread enters the exchange.
238	**/
239	public V exchange(V x, long timeout, TimeUnit unit)
240	throws InterruptedException, TimeoutException {
241	return doExchange(x, true, unit.toNanos(timeout));
242	}
243
244	}
245
246