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: 2003/06/24 14:34:47 $
 * @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:	1.4
Committed:	Tue Jul 8 00:46:33 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
Changes since 1.3:	+2 -1 lines
Log Message:	Locks in subpackage; fairness params added
#	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: 2003/06/24 14:34:47 $
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	int count = ++arrivalCount;
92
93	// If item is already waiting, replace it and signal other thread
94	if (count == 2) {
95	other = item;
96	item = x;
97	taken.signal();
98	return other;
99	}
100
101	// Otherwise, set item and wait for another thread to
102	// replace it and signal us.
103
104	item = x;
105	InterruptedException interrupted = null;
106	try {
107	while (arrivalCount != 2) {
108	if (!timed)
109	taken.await();
110	else if (nanos > 0)
111	nanos = taken.awaitNanos(nanos);
112	else
113	break; // timed out
114	}
115	}
116	catch (InterruptedException ie) {
117	interrupted = ie;
118	}
119
120	// get and reset item and count after the wait.
121	other = item;
122	item = null;
123	count = arrivalCount;
124	arrivalCount = 0;
125
126	// If the other thread replaced item, then we must
127	// continue even if cancelled.
128	if (count == 2) {
129	if (interrupted != null)
130	Thread.currentThread().interrupt();
131	return other;
132	}
133
134	// Otherwise, no one is waiting for us, so we can just back out
135	if (interrupted != null) {
136	taken.signal(); // propagate to any other waiting thread
137	throw interrupted;
138	}
139	else // must be timeout
140	throw new TimeoutException();
141	}
142	finally {
143	lock.unlock();
144	}
145	}
146
147	/**
148	* Create a new Exchanger
149	**/
150	public Exchanger() {
151	}
152
153	/**
154	* Waits for another thread to arrive at this exchange point (unless
155	* it is {@link Thread#interrupt interrupted}),
156	* and then transfers the given object to it, receiving its object
157	* in return.
158	* <p>If another thread is already waiting at the exchange point then
159	* it is resumed for thread scheduling purposes and receives the object
160	* passed in by the current thread. The current thread returns immediately,
161	* receiving the object passed to the exchange by that other thread.
162	* <p>If no other thread is already waiting at the exchange then the
163	* current thread is disabled for thread scheduling purposes and lies
164	* dormant until one of two things happens:
165	* <ul>
166	* <li>Some other thread enters the exchange; or
167	* <li>Some other thread {@link Thread#interrupt interrupts} the current
168	* thread.
169	* </ul>
170	* <p>If the current thread:
171	* <ul>
172	* <li>has its interrupted status set on entry to this method; or
173	* <li>is {@link Thread#interrupt interrupted} while waiting
174	* for the exchange,
175	* </ul>
176	* then {@link InterruptedException} is thrown and the current thread's
177	* interrupted status is cleared.
178	*
179	* @param x the object to exchange
180	* @return the object provided by the other thread.
181	* @throws InterruptedException if current thread was interrupted
182	* while waiting
183	**/
184	public V exchange(V x) throws InterruptedException {
185	try {
186	return doExchange(x, false, 0);
187	}
188	catch (TimeoutException cannotHappen) {
189	throw new Error(cannotHappen);
190	}
191	}
192
193	/**
194	* Waits for another thread to arrive at this exchange point (unless
195	* it is {@link Thread#interrupt interrupted}, or the specified waiting
196	* time elapses),
197	* and then transfers the given object to it, receiving its object
198	* in return.
199	*
200	* <p>If another thread is already waiting at the exchange point then
201	* it is resumed for thread scheduling purposes and receives the object
202	* passed in by the current thread. The current thread returns immediately,
203	* receiving the object passed to the exchange by that other thread.
204	*
205	* <p>If no other thread is already waiting at the exchange then the
206	* current thread is disabled for thread scheduling purposes and lies
207	* dormant until one of three things happens:
208	* <ul>
209	* <li>Some other thread enters the exchange; or
210	* <li>Some other thread {@link Thread#interrupt interrupts} the current
211	* thread; or
212	* <li>The specified waiting time elapses.
213	* </ul>
214	* <p>If the current thread:
215	* <ul>
216	* <li>has its interrupted status set on entry to this method; or
217	* <li>is {@link Thread#interrupt interrupted} while waiting
218	* for the exchange,
219	* </ul>
220	* then {@link InterruptedException} is thrown and the current thread's
221	* interrupted status is cleared.
222	*
223	* <p>If the specified waiting time elapses then {@link TimeoutException}
224	* is thrown.
225	* The given waiting time is a best-effort lower bound. If the time is
226	* less than or equal to zero, the method will not wait at all.
227	*
228	* @param x the object to exchange
229	* @param timeout the maximum time to wait
230	* @param unit the time unit of the <tt>timeout</tt> argument.
231	* @return the object provided by the other thread.
232	* @throws InterruptedException if current thread was interrupted
233	* while waiting
234	* @throws TimeoutException if the specified waiting time elapses before
235	* another thread enters the exchange.
236	**/
237	public V exchange(V x, long timeout, TimeUnit unit)
238	throws InterruptedException, TimeoutException {
239	return doExchange(x, true, unit.toNanos(timeout));
240	}
241
242	}
243
244