util/concurrent/Exchanger.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.locks.*;

/**
 * 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;DataBuffer&gt; exchanger = new Exchanger();
 *   DataBuffer initialEmptyBuffer = ... a made-up type
 *   DataBuffer initialFullBuffer = ...
 *
 *   class FillingLoop implements Runnable {
 *     public void run() {
 *       DataBuffer 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() {
 *       DataBuffer 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>
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The type of objects that may be exchanged
 */
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();
        }
    }

    /**
     * Creates 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.
     * 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:	1.15
Committed:	Tue Apr 26 01:17:18 2005 UTC (19 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.14:	+28 -30 lines
Log Message:	doc fixes
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.14	* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/licenses/publicdomain
5	dl	1.2	*/
6
7	tim	1.1	package java.util.concurrent;
8	dl	1.4	import java.util.concurrent.locks.*;
9	tim	1.1
10			/**
11	dl	1.12	* A synchronization point at which two threads can exchange objects.
12			* Each thread presents some object on entry to the {@link #exchange
13			* exchange} method, and receives the object presented by the other
14			* thread on return.
15	tim	1.1	*
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	dl	1.9	* Exchanger<DataBuffer> exchanger = new Exchanger();
25			* DataBuffer initialEmptyBuffer = ... a made-up type
26			* DataBuffer initialFullBuffer = ...
27	tim	1.1	*
28			* class FillingLoop implements Runnable {
29			* public void run() {
30	dl	1.9	* DataBuffer currentBuffer = initialEmptyBuffer;
31	tim	1.1	* 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	dl	1.9	* DataBuffer currentBuffer = initialFullBuffer;
44	tim	1.1	* 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			* @since 1.5
62	dl	1.3	* @author Doug Lea
63	dl	1.11	* @param <V> The type of objects that may be exchanged
64	tim	1.1	*/
65			public class Exchanger<V> {
66	dl	1.2	private final ReentrantLock lock = new ReentrantLock();
67	dl	1.13	private final Condition taken = lock.newCondition();
68	dl	1.2
69			/** Holder for the item being exchanged */
70			private V item;
71	jsr166	1.15
72	dl	1.2	/**
73			* Arrival count transitions from 0 to 1 to 2 then back to 0
74			* during an exchange.
75			*/
76			private int arrivalCount;
77
78	dl	1.3	/**
79			* Main exchange function, handling the different policy variants.
80			*/
81	dl	1.2	private V doExchange(V x, boolean timed, long nanos) throws InterruptedException, TimeoutException {
82			lock.lock();
83			try {
84			V other;
85	dl	1.5
86			// If arrival count already at two, we must wait for
87			// a previous pair to finish and reset the count;
88			while (arrivalCount == 2) {
89			if (!timed)
90			taken.await();
91	jsr166	1.15	else if (nanos > 0)
92	dl	1.5	nanos = taken.awaitNanos(nanos);
93	jsr166	1.15	else
94	dl	1.5	throw new TimeoutException();
95			}
96
97	dl	1.2	int count = ++arrivalCount;
98
99			// If item is already waiting, replace it and signal other thread
100	jsr166	1.15	if (count == 2) {
101	dl	1.2	other = item;
102			item = x;
103			taken.signal();
104			return other;
105			}
106
107			// Otherwise, set item and wait for another thread to
108			// replace it and signal us.
109
110			item = x;
111			InterruptedException interrupted = null;
112	jsr166	1.15	try {
113	dl	1.2	while (arrivalCount != 2) {
114			if (!timed)
115			taken.await();
116	jsr166	1.15	else if (nanos > 0)
117	dl	1.2	nanos = taken.awaitNanos(nanos);
118	jsr166	1.15	else
119	dl	1.2	break; // timed out
120			}
121	tim	1.7	} catch (InterruptedException ie) {
122	dl	1.2	interrupted = ie;
123			}
124
125	dl	1.5	// Get and reset item and count after the wait.
126			// (We need to do this even if wait was aborted.)
127	dl	1.2	other = item;
128			item = null;
129			count = arrivalCount;
130	jsr166	1.15	arrivalCount = 0;
131	dl	1.5	taken.signal();
132	jsr166	1.15
133	dl	1.2	// If the other thread replaced item, then we must
134			// continue even if cancelled.
135			if (count == 2) {
136			if (interrupted != null)
137			Thread.currentThread().interrupt();
138			return other;
139			}
140
141	dl	1.5	// If no one is waiting for us, we can back out
142	jsr166	1.15	if (interrupted != null)
143	dl	1.2	throw interrupted;
144			else // must be timeout
145			throw new TimeoutException();
146	tim	1.7	} finally {
147	dl	1.2	lock.unlock();
148			}
149			}
150	tim	1.1
151			/**
152	jsr166	1.15	* Creates a new Exchanger.
153			*/
154	tim	1.1	public Exchanger() {
155			}
156
157			/**
158			* Waits for another thread to arrive at this exchange point (unless
159			* it is {@link Thread#interrupt interrupted}),
160			* and then transfers the given object to it, receiving its object
161			* in return.
162			* <p>If another thread is already waiting at the exchange point then
163			* it is resumed for thread scheduling purposes and receives the object
164			* passed in by the current thread. The current thread returns immediately,
165			* receiving the object passed to the exchange by that other thread.
166	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
167	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
168			* dormant until one of two things happens:
169			* <ul>
170			* <li>Some other thread enters the exchange; or
171			* <li>Some other thread {@link Thread#interrupt interrupts} the current
172			* thread.
173			* </ul>
174			* <p>If the current thread:
175			* <ul>
176	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
177	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
178	jsr166	1.15	* for the exchange,
179	tim	1.1	* </ul>
180	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
181			* interrupted status is cleared.
182	tim	1.1	*
183			* @param x the object to exchange
184			* @return the object provided by the other thread.
185	jsr166	1.15	* @throws InterruptedException if current thread was interrupted
186	dl	1.3	* while waiting
187	jsr166	1.15	*/
188	tim	1.1	public V exchange(V x) throws InterruptedException {
189	dl	1.2	try {
190			return doExchange(x, false, 0);
191	jsr166	1.15	} catch (TimeoutException cannotHappen) {
192	dl	1.2	throw new Error(cannotHappen);
193			}
194	tim	1.1	}
195
196			/**
197			* Waits for another thread to arrive at this exchange point (unless
198			* it is {@link Thread#interrupt interrupted}, or the specified waiting
199			* time elapses),
200			* and then transfers the given object to it, receiving its object
201			* in return.
202			*
203			* <p>If another thread is already waiting at the exchange point then
204			* it is resumed for thread scheduling purposes and receives the object
205			* passed in by the current thread. The current thread returns immediately,
206			* receiving the object passed to the exchange by that other thread.
207			*
208	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
209	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
210			* dormant until one of three things happens:
211			* <ul>
212			* <li>Some other thread enters the exchange; or
213			* <li>Some other thread {@link Thread#interrupt interrupts} the current
214			* thread; or
215			* <li>The specified waiting time elapses.
216			* </ul>
217			* <p>If the current thread:
218			* <ul>
219	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
220	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
221	jsr166	1.15	* for the exchange,
222	tim	1.1	* </ul>
223	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
224			* interrupted status is cleared.
225	tim	1.1	*
226			* <p>If the specified waiting time elapses then {@link TimeoutException}
227			* is thrown.
228	jsr166	1.15	* If the time is
229	tim	1.1	* less than or equal to zero, the method will not wait at all.
230			*
231			* @param x the object to exchange
232			* @param timeout the maximum time to wait
233	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
234	tim	1.1	* @return the object provided by the other thread.
235	dl	1.3	* @throws InterruptedException if current thread was interrupted
236			* while waiting
237	tim	1.1	* @throws TimeoutException if the specified waiting time elapses before
238			* another thread enters the exchange.
239	jsr166	1.15	*/
240			public V exchange(V x, long timeout, TimeUnit unit)
241	tim	1.1	throws InterruptedException, TimeoutException {
242	dl	1.2	return doExchange(x, true, unit.toNanos(timeout));
243	tim	1.1	}
244
245			}