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
#	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	1.5	* @revised $Date: 2003/07/08 00:46:33 $
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	dl	1.5
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	dl	1.2	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	dl	1.5	// Get and reset item and count after the wait.
133			// (We need to do this even if wait was aborted.)
134	dl	1.2	other = item;
135			item = null;
136			count = arrivalCount;
137			arrivalCount = 0;
138	dl	1.5	taken.signal();
139	dl	1.2
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	dl	1.5	// If no one is waiting for us, we can back out
149			if (interrupted != null)
150	dl	1.2	throw interrupted;
151			else // must be timeout
152			throw new TimeoutException();
153			}
154			finally {
155			lock.unlock();
156			}
157			}
158	tim	1.1
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	dl	1.3	* @throws InterruptedException if current thread was interrupted
194			* while waiting
195	tim	1.1	**/
196			public V exchange(V x) throws InterruptedException {
197	dl	1.2	try {
198			return doExchange(x, false, 0);
199			}
200			catch (TimeoutException cannotHappen) {
201			throw new Error(cannotHappen);
202			}
203	tim	1.1	}
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	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
243	tim	1.1	* @return the object provided by the other thread.
244	dl	1.3	* @throws InterruptedException if current thread was interrupted
245			* while waiting
246	tim	1.1	* @throws TimeoutException if the specified waiting time elapses before
247			* another thread enters the exchange.
248			**/
249	dl	1.2	public V exchange(V x, long timeout, TimeUnit unit)
250	tim	1.1	throws InterruptedException, TimeoutException {
251	dl	1.2	return doExchange(x, true, unit.toNanos(timeout));
252	tim	1.1	}
253
254			}
255
256