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;

/**
 * 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: 2003/05/27 18:14:40 $
 * @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;
            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.
            other = item;
            item = null;
            count = arrivalCount;
            arrivalCount = 0; 
            
            // If the other thread replaced item, then we must
            // continue even if cancelled.
            if (count == 2) {
                if (interrupted != null)
                    Thread.currentThread().interrupt();
                return other;
            }

            // Otherwise, no one is waiting for us, so we can just back out
            if (interrupted != null) {
                taken.signal(); // propagate to any other waiting thread
                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:	1.3
Committed:	Tue Jun 24 14:34:47 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.2:	+10 -4 lines
Log Message:	Added missing javadoc tags; minor reformatting
#	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
9	/**
10	* An <tt>Exchanger</tt> provides a synchronization point at which two threads
11	* can exchange objects. Each thread presents some object on entry to
12	* the {@link #exchange exchange} method, and receives the object presented by
13	* the other thread on return.
14	*
15	* <p><b>Sample Usage:</b>
16	* Here are the highlights of a class that uses an <tt>Exchanger</tt> to
17	* swap buffers between threads so that the thread filling the
18	* buffer gets a freshly
19	* emptied one when it needs it, handing off the filled one to
20	* the thread emptying the buffer.
21	* <pre>
22	* class FillAndEmpty {
23	* Exchanger<Buffer> exchanger = new Exchanger();
24	* Buffer initialEmptyBuffer = ... a made-up type
25	* Buffer initialFullBuffer = ...
26	*
27	* class FillingLoop implements Runnable {
28	* public void run() {
29	* Buffer currentBuffer = initialEmptyBuffer;
30	* try {
31	* while (currentBuffer != null) {
32	* addToBuffer(currentBuffer);
33	* if (currentBuffer.full())
34	* currentBuffer = exchanger.exchange(currentBuffer);
35	* }
36	* }
37	* catch (InterruptedException ex) { ... handle ... }
38	* }
39	* }
40	*
41	* class EmptyingLoop implements Runnable {
42	* public void run() {
43	* Buffer currentBuffer = initialFullBuffer;
44	* try {
45	* while (currentBuffer != null) {
46	* takeFromBuffer(currentBuffer);
47	* if (currentBuffer.empty())
48	* currentBuffer = exchanger.exchange(currentBuffer);
49	* }
50	* }
51	* catch (InterruptedException ex) { ... handle ...}
52	* }
53	* }
54	*
55	* void start() {
56	* new Thread(new FillingLoop()).start();
57	* new Thread(new EmptyingLoop()).start();
58	* }
59	* }
60	* </pre>
61	*
62	* @fixme change example to use a bounded queue?
63	*
64	* @since 1.5
65	* @spec JSR-166
66	* @revised $Date: 2003/05/27 18:14:40 $
67	* @editor $Author: dl $
68	* @author Doug Lea
69	*/
70	public class Exchanger<V> {
71	private final ReentrantLock lock = new ReentrantLock();
72	private final Condition taken = lock.newCondition();
73
74	/** Holder for the item being exchanged */
75	private V item;
76
77	/**
78	* Arrival count transitions from 0 to 1 to 2 then back to 0
79	* during an exchange.
80	*/
81	private int arrivalCount;
82
83	/**
84	* Main exchange function, handling the different policy variants.
85	*/
86	private V doExchange(V x, boolean timed, long nanos) throws InterruptedException, TimeoutException {
87	lock.lock();
88	try {
89	V other;
90	int count = ++arrivalCount;
91
92	// If item is already waiting, replace it and signal other thread
93	if (count == 2) {
94	other = item;
95	item = x;
96	taken.signal();
97	return other;
98	}
99
100	// Otherwise, set item and wait for another thread to
101	// replace it and signal us.
102
103	item = x;
104	InterruptedException interrupted = null;
105	try {
106	while (arrivalCount != 2) {
107	if (!timed)
108	taken.await();
109	else if (nanos > 0)
110	nanos = taken.awaitNanos(nanos);
111	else
112	break; // timed out
113	}
114	}
115	catch (InterruptedException ie) {
116	interrupted = ie;
117	}
118
119	// get and reset item and count after the wait.
120	other = item;
121	item = null;
122	count = arrivalCount;
123	arrivalCount = 0;
124
125	// If the other thread replaced item, then we must
126	// continue even if cancelled.
127	if (count == 2) {
128	if (interrupted != null)
129	Thread.currentThread().interrupt();
130	return other;
131	}
132
133	// Otherwise, no one is waiting for us, so we can just back out
134	if (interrupted != null) {
135	taken.signal(); // propagate to any other waiting thread
136	throw interrupted;
137	}
138	else // must be timeout
139	throw new TimeoutException();
140	}
141	finally {
142	lock.unlock();
143	}
144	}
145
146	/**
147	* Create a new Exchanger
148	**/
149	public Exchanger() {
150	}
151
152	/**
153	* Waits for another thread to arrive at this exchange point (unless
154	* it is {@link Thread#interrupt interrupted}),
155	* and then transfers the given object to it, receiving its object
156	* in return.
157	* <p>If another thread is already waiting at the exchange point then
158	* it is resumed for thread scheduling purposes and receives the object
159	* passed in by the current thread. The current thread returns immediately,
160	* receiving the object passed to the exchange by that other thread.
161	* <p>If no other thread is already waiting at the exchange then the
162	* current thread is disabled for thread scheduling purposes and lies
163	* dormant until one of two things happens:
164	* <ul>
165	* <li>Some other thread enters the exchange; or
166	* <li>Some other thread {@link Thread#interrupt interrupts} the current
167	* thread.
168	* </ul>
169	* <p>If the current thread:
170	* <ul>
171	* <li>has its interrupted status set on entry to this method; or
172	* <li>is {@link Thread#interrupt interrupted} while waiting
173	* for the exchange,
174	* </ul>
175	* then {@link InterruptedException} is thrown and the current thread's
176	* interrupted status is cleared.
177	*
178	* @param x the object to exchange
179	* @return the object provided by the other thread.
180	* @throws InterruptedException if current thread was interrupted
181	* while waiting
182	**/
183	public V exchange(V x) throws InterruptedException {
184	try {
185	return doExchange(x, false, 0);
186	}
187	catch (TimeoutException cannotHappen) {
188	throw new Error(cannotHappen);
189	}
190	}
191
192	/**
193	* Waits for another thread to arrive at this exchange point (unless
194	* it is {@link Thread#interrupt interrupted}, or the specified waiting
195	* time elapses),
196	* and then transfers the given object to it, receiving its object
197	* in return.
198	*
199	* <p>If another thread is already waiting at the exchange point then
200	* it is resumed for thread scheduling purposes and receives the object
201	* passed in by the current thread. The current thread returns immediately,
202	* receiving the object passed to the exchange by that other thread.
203	*
204	* <p>If no other thread is already waiting at the exchange then the
205	* current thread is disabled for thread scheduling purposes and lies
206	* dormant until one of three things happens:
207	* <ul>
208	* <li>Some other thread enters the exchange; or
209	* <li>Some other thread {@link Thread#interrupt interrupts} the current
210	* thread; or
211	* <li>The specified waiting time elapses.
212	* </ul>
213	* <p>If the current thread:
214	* <ul>
215	* <li>has its interrupted status set on entry to this method; or
216	* <li>is {@link Thread#interrupt interrupted} while waiting
217	* for the exchange,
218	* </ul>
219	* then {@link InterruptedException} is thrown and the current thread's
220	* interrupted status is cleared.
221	*
222	* <p>If the specified waiting time elapses then {@link TimeoutException}
223	* is thrown.
224	* The given waiting time is a best-effort lower bound. If the time is
225	* less than or equal to zero, the method will not wait at all.
226	*
227	* @param x the object to exchange
228	* @param timeout the maximum time to wait
229	* @param unit the time unit of the <tt>timeout</tt> argument.
230	* @return the object provided by the other thread.
231	* @throws InterruptedException if current thread was interrupted
232	* while waiting
233	* @throws TimeoutException if the specified waiting time elapses before
234	* another thread enters the exchange.
235	**/
236	public V exchange(V x, long timeout, TimeUnit unit)
237	throws InterruptedException, TimeoutException {
238	return doExchange(x, true, unit.toNanos(timeout));
239	}
240
241	}
242
243