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
#	User	Rev	Content
1	dl	1.2	/*
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	tim	1.1	package java.util.concurrent;
8	dl	1.4	import java.util.concurrent.locks.*;
9	tim	1.1
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	dl	1.4	* @revised $Date: 2003/06/24 14:34:47 $
68	dl	1.3	* @editor $Author: dl $
69			* @author Doug Lea
70	tim	1.1	*/
71			public class Exchanger<V> {
72	dl	1.2	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	dl	1.3	/**
85			* Main exchange function, handling the different policy variants.
86			*/
87	dl	1.2	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	tim	1.1
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	dl	1.3	* @throws InterruptedException if current thread was interrupted
182			* while waiting
183	tim	1.1	**/
184			public V exchange(V x) throws InterruptedException {
185	dl	1.2	try {
186			return doExchange(x, false, 0);
187			}
188			catch (TimeoutException cannotHappen) {
189			throw new Error(cannotHappen);
190			}
191	tim	1.1	}
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	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
231	tim	1.1	* @return the object provided by the other thread.
232	dl	1.3	* @throws InterruptedException if current thread was interrupted
233			* while waiting
234	tim	1.1	* @throws TimeoutException if the specified waiting time elapses before
235			* another thread enters the exchange.
236			**/
237	dl	1.2	public V exchange(V x, long timeout, TimeUnit unit)
238	tim	1.1	throws InterruptedException, TimeoutException {
239	dl	1.2	return doExchange(x, true, unit.toNanos(timeout));
240	tim	1.1	}
241
242			}
243
244