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: 2003/08/06 18:22:09 $
 * @editor $Author: tim $
 * @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:	1.7
Committed:	Fri Aug 8 20:05:07 2003 UTC (20 years, 10 months ago) by tim
Branch:	MAIN
Changes since 1.6:	+7 -12 lines
Log Message:	Scrunched catch, finally, else clauses.
#	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	tim	1.7	* } catch (InterruptedException ex) { ... handle ... }
38	tim	1.1	* }
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	tim	1.7	* } catch (InterruptedException ex) { ... handle ...}
51	tim	1.1	* }
52			* }
53			*
54			* void start() {
55			* new Thread(new FillingLoop()).start();
56			* new Thread(new EmptyingLoop()).start();
57			* }
58			* }
59			* </pre>
60			*
61			* @fixme change example to use a bounded queue?
62			*
63			* @since 1.5
64			* @spec JSR-166
65	tim	1.7	* @revised $Date: 2003/08/06 18:22:09 $
66			* @editor $Author: tim $
67	dl	1.3	* @author Doug Lea
68	tim	1.1	*/
69			public class Exchanger<V> {
70	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
71			private final Condition taken = lock.newCondition();
72
73			/** Holder for the item being exchanged */
74			private V item;
75
76			/**
77			* Arrival count transitions from 0 to 1 to 2 then back to 0
78			* during an exchange.
79			*/
80			private int arrivalCount;
81
82	dl	1.3	/**
83			* Main exchange function, handling the different policy variants.
84			*/
85	dl	1.2	private V doExchange(V x, boolean timed, long nanos) throws InterruptedException, TimeoutException {
86			lock.lock();
87			try {
88			V other;
89	dl	1.5
90			// If arrival count already at two, we must wait for
91			// a previous pair to finish and reset the count;
92			while (arrivalCount == 2) {
93			if (!timed)
94			taken.await();
95			else if (nanos > 0)
96			nanos = taken.awaitNanos(nanos);
97			else
98			throw new TimeoutException();
99			}
100
101	dl	1.2	int count = ++arrivalCount;
102
103			// If item is already waiting, replace it and signal other thread
104			if (count == 2) {
105			other = item;
106			item = x;
107			taken.signal();
108			return other;
109			}
110
111			// Otherwise, set item and wait for another thread to
112			// replace it and signal us.
113
114			item = x;
115			InterruptedException interrupted = null;
116			try {
117			while (arrivalCount != 2) {
118			if (!timed)
119			taken.await();
120			else if (nanos > 0)
121			nanos = taken.awaitNanos(nanos);
122			else
123			break; // timed out
124			}
125	tim	1.7	} catch (InterruptedException ie) {
126	dl	1.2	interrupted = ie;
127			}
128
129	dl	1.5	// Get and reset item and count after the wait.
130			// (We need to do this even if wait was aborted.)
131	dl	1.2	other = item;
132			item = null;
133			count = arrivalCount;
134			arrivalCount = 0;
135	dl	1.5	taken.signal();
136	dl	1.2
137			// If the other thread replaced item, then we must
138			// continue even if cancelled.
139			if (count == 2) {
140			if (interrupted != null)
141			Thread.currentThread().interrupt();
142			return other;
143			}
144
145	dl	1.5	// If no one is waiting for us, we can back out
146			if (interrupted != null)
147	dl	1.2	throw interrupted;
148			else // must be timeout
149			throw new TimeoutException();
150	tim	1.7	} finally {
151	dl	1.2	lock.unlock();
152			}
153			}
154	tim	1.1
155			/**
156	tim	1.6	* Create a new Exchanger.
157	tim	1.1	**/
158			public Exchanger() {
159			}
160
161			/**
162			* Waits for another thread to arrive at this exchange point (unless
163			* it is {@link Thread#interrupt interrupted}),
164			* and then transfers the given object to it, receiving its object
165			* in return.
166			* <p>If another thread is already waiting at the exchange point then
167			* it is resumed for thread scheduling purposes and receives the object
168			* passed in by the current thread. The current thread returns immediately,
169			* receiving the object passed to the exchange by that other thread.
170			* <p>If no other thread is already waiting at the exchange then the
171			* current thread is disabled for thread scheduling purposes and lies
172			* dormant until one of two things happens:
173			* <ul>
174			* <li>Some other thread enters the exchange; or
175			* <li>Some other thread {@link Thread#interrupt interrupts} the current
176			* thread.
177			* </ul>
178			* <p>If the current thread:
179			* <ul>
180			* <li>has its interrupted status set on entry to this method; or
181			* <li>is {@link Thread#interrupt interrupted} while waiting
182			* for the exchange,
183			* </ul>
184			* then {@link InterruptedException} is thrown and the current thread's
185			* interrupted status is cleared.
186			*
187			* @param x the object to exchange
188			* @return the object provided by the other thread.
189	dl	1.3	* @throws InterruptedException if current thread was interrupted
190			* while waiting
191	tim	1.1	**/
192			public V exchange(V x) throws InterruptedException {
193	dl	1.2	try {
194			return doExchange(x, false, 0);
195	tim	1.7	} catch (TimeoutException cannotHappen) {
196	dl	1.2	throw new Error(cannotHappen);
197			}
198	tim	1.1	}
199
200			/**
201			* Waits for another thread to arrive at this exchange point (unless
202			* it is {@link Thread#interrupt interrupted}, or the specified waiting
203			* time elapses),
204			* and then transfers the given object to it, receiving its object
205			* in return.
206			*
207			* <p>If another thread is already waiting at the exchange point then
208			* it is resumed for thread scheduling purposes and receives the object
209			* passed in by the current thread. The current thread returns immediately,
210			* receiving the object passed to the exchange by that other thread.
211			*
212			* <p>If no other thread is already waiting at the exchange then the
213			* current thread is disabled for thread scheduling purposes and lies
214			* dormant until one of three things happens:
215			* <ul>
216			* <li>Some other thread enters the exchange; or
217			* <li>Some other thread {@link Thread#interrupt interrupts} the current
218			* thread; or
219			* <li>The specified waiting time elapses.
220			* </ul>
221			* <p>If the current thread:
222			* <ul>
223			* <li>has its interrupted status set on entry to this method; or
224			* <li>is {@link Thread#interrupt interrupted} while waiting
225			* for the exchange,
226			* </ul>
227			* then {@link InterruptedException} is thrown and the current thread's
228			* interrupted status is cleared.
229			*
230			* <p>If the specified waiting time elapses then {@link TimeoutException}
231			* is thrown.
232			* The given waiting time is a best-effort lower bound. If the time is
233			* less than or equal to zero, the method will not wait at all.
234			*
235			* @param x the object to exchange
236			* @param timeout the maximum time to wait
237	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
238	tim	1.1	* @return the object provided by the other thread.
239	dl	1.3	* @throws InterruptedException if current thread was interrupted
240			* while waiting
241	tim	1.1	* @throws TimeoutException if the specified waiting time elapses before
242			* another thread enters the exchange.
243			**/
244	dl	1.2	public V exchange(V x, long timeout, TimeUnit unit)
245	tim	1.1	throws InterruptedException, TimeoutException {
246	dl	1.2	return doExchange(x, true, unit.toNanos(timeout));
247	tim	1.1	}
248
249			}
250
251