util/concurrent/Exchanger.java

/*
 * Written by Doug Lea, Bill Scherer, and Michael Scott 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.*; // for javadoc (till 6280605 is fixed)
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
import java.util.Random;

/**
 * A synchronization point at which threads can pair and swap elements
 * within pairs.  Each thread presents some object on entry to the
 * {@link #exchange exchange} method, matches with a partner thread,
 * and receives its partner's object on return.
 *
 * <p><b>Sample Usage:</b>
 * Here are the highlights of a class that uses an {@code Exchanger}
 * 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>{@code
 * class FillAndEmpty {
 *   Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>();
 *   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.isFull())
 *             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.isEmpty())
 *             currentBuffer = exchanger.exchange(currentBuffer);
 *         }
 *       } catch (InterruptedException ex) { ... handle ...}
 *     }
 *   }
 *
 *   void start() {
 *     new Thread(new FillingLoop()).start();
 *     new Thread(new EmptyingLoop()).start();
 *   }
 * }
 * }</pre>
 *
 * <p>Memory consistency effects: For each pair of threads that
 * successfully exchange objects via an {@code Exchanger}, actions
 * prior to the {@code exchange()} in each thread
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * those subsequent to a return from the corresponding {@code exchange()}
 * in the other thread.
 *
 * @since 1.5
 * @author Doug Lea and Bill Scherer and Michael Scott
 * @param <V> The type of objects that may be exchanged
 */
public class Exchanger<V> {
    /*
     * The underlying idea is to use a stack to hold nodes containing
     * pairs of items to be exchanged. Except that:
     *
     *  *  Only one element of the pair is known on creation by a
     *     first-arriving thread; the other is a "hole" waiting to be
     *     filled in. This is a degenerate form of the dual stacks
     *     described in "Nonblocking Concurrent Objects with Condition
     *     Synchronization",  by W. N. Scherer III and M. L. Scott.
     *     18th Annual Conf. on  Distributed Computing, Oct. 2004.
     *     It is "degenerate" in that both the items and the holes
     *     are shared in the same nodes.
     *
     *  *  There isn't really a stack here! There can't be -- if two
     *     nodes were both in the stack, they should cancel themselves
     *     out by combining. So that's what we do. The 0th element of
     *     the "arena" array serves only as the top of stack.  The
     *     remainder of the array is a form of the elimination backoff
     *     collision array described in "A Scalable Lock-free Stack
     *     Algorithm", by D. Hendler, N. Shavit, and L. Yerushalmi.
     *     16th ACM Symposium on Parallelism in Algorithms and
     *     Architectures, June 2004. Here, threads spin (using short
     *     timed waits with exponential backoff) looking for each
     *     other.  If they fail to find others waiting, they try the
     *     top spot again.  As shown in that paper, this always
     *     converges.
     *
     * The backoff elimination mechanics never come into play in
     * common usages where only two threads ever meet to exchange
     * items, but they prevent contention bottlenecks when an
     * exchanger is used by a large number of threads.
     *
     * For more details, see the paper "A Scalable Elimination-based
     * Exchange Channel" by William Scherer, Doug Lea, and Michael
     * Scott in Proceedings of SCOOL05 workshop. Available at:
     * http://hdl.handle.net/1802/2104
     */

    /**
     * Size of collision space. Using a size of half the number of
     * CPUs provides enough space for threads to find each other but
     * not so much that it would always require one or more to time
     * out to become unstuck. Note that the arena array holds SIZE+1
     * elements, to include the top-of-stack slot.
     */
    private static final int SIZE =
        (Runtime.getRuntime().availableProcessors() + 1) / 2;

    /**
     * Base unit in nanoseconds for backoffs. Must be a power of two.
     * Should be small because backoffs exponentially increase from
     * base.
     */
    private static final long BACKOFF_BASE = 128L;

    /**
     * Sentinel item representing cancellation.  This value is placed
     * in holes on cancellation, and used as a return value from Node
     * methods to indicate failure to set or get hole.
     */
    static final Object FAIL = new Object();

    /**
     * The collision arena. arena[0] is used as the top of the stack.
     * The remainder is used as the collision elimination space.
     */
    private final AtomicReference<Node>[] arena;

    /** Generator for random backoffs and delays. */
    private final Random random = new Random();

    /**
     * Creates a new Exchanger.
     */
    public Exchanger() {
        arena = (AtomicReference<Node>[]) new AtomicReference[SIZE + 1];
        for (int i = 0; i < arena.length; ++i)
            arena[i] = new AtomicReference<Node>();
    }

    /**
     * Main exchange function, handling the different policy variants.
     * Uses Object, not "V" as argument and return value to simplify
     * handling of internal sentinel values. Callers from public
     * methods cast accordingly.
     *
     * @param item the item to exchange
     * @param timed true if the wait is timed
     * @param nanos if timed, the maximum wait time
     * @return the other thread's item
     */
    private Object doExchange(Object item, boolean timed, long nanos)
        throws InterruptedException, TimeoutException {
        Node me = new Node(item);
        long lastTime = timed ? System.nanoTime() : 0;
        int idx = 0;     // start out at slot representing top
        int backoff = 0; // increases on failure to occupy a slot

        for (;;) {
            AtomicReference<Node> slot = arena[idx];

            // If this slot is already occupied, there is a waiting item...
            Node you = slot.get();
            if (you != null) {
                Object v = you.fillHole(item);
                slot.compareAndSet(you, null);
                if (v != FAIL)       // ... unless it was cancelled
                    return v;
            }

            // Try to occupy this slot
            if (slot.compareAndSet(null, me)) {
                // If this is top slot, use regular wait, else backoff-wait
                Object v = ((idx == 0)?
                            me.waitForHole(timed, nanos) :
                            me.waitForHole(true, randomDelay(backoff)));
                slot.compareAndSet(me, null);
                if (v != FAIL)
                    return v;
                if (Thread.interrupted())
                    throw new InterruptedException();
                if (timed) {
                    long now = System.nanoTime();
                    nanos -= now - lastTime;
                    lastTime = now;
                    if (nanos <= 0)
                        throw new TimeoutException();
                }

                me = new Node(item);      // Throw away nodes on failure
                if (backoff < SIZE - 1)   // Increase or stay saturated
                    ++backoff;
                idx = 0;                  // Restart at top
            }

            else // Retry with a random non-top slot <= backoff
                idx = 1 + random.nextInt(backoff + 1);

        }
    }

    /**
     * Returns a random delay less than (base times (2 raised to backoff)).
     */
    private long randomDelay(int backoff) {
        return ((BACKOFF_BASE << backoff) - 1) & random.nextInt();
    }

    /**
     * Nodes hold partially exchanged data. This class
     * opportunistically subclasses AtomicReference to represent the
     * hole. So get() returns hole, and compareAndSet CAS'es value
     * into hole.  Note that this class cannot be parameterized as V
     * because the sentinel value FAIL is only of type Object.
     */
    static final class Node extends AtomicReference<Object> {
        private static final long serialVersionUID = -3221313401284163686L;

        /** The element offered by the Thread creating this node. */
        final Object item;

        /** The Thread creating this node. */
        final Thread waiter;

        /**
         * Creates node with given item and empty hole.
         *
         * @param item the item
         */
        Node(Object item) {
            this.item = item;
            waiter = Thread.currentThread();
        }

        /**
         * Tries to fill in hole. On success, wakes up the waiter.
         *
         * @param val the value to place in hole
         * @return on success, the item; on failure, FAIL
         */
        Object fillHole(Object val) {
            if (compareAndSet(null, val)) {
                LockSupport.unpark(waiter);
                return item;
            }
            return FAIL;
        }

        /**
         * Waits for and gets the hole filled in by another thread.
         * Fails if timed out or interrupted before hole filled.
         *
         * @param timed true if the wait is timed
         * @param nanos if timed, the maximum wait time
         * @return on success, the hole; on failure, FAIL
         */
        Object waitForHole(boolean timed, long nanos) {
            long lastTime = timed ? System.nanoTime() : 0;
            Object h;
            while ((h = get()) == null) {
                // If interrupted or timed out, try to cancel by
                // CASing FAIL as hole value.
                if (Thread.currentThread().isInterrupted() ||
                    (timed && nanos <= 0)) {
                    if (compareAndSet(null, FAIL))
                        return FAIL;
                }
                else if (!timed)
                    LockSupport.park();
                else {
                    LockSupport.parkNanos(nanos);
                    long now = System.nanoTime();
                    nanos -= now - lastTime;
                    lastTime = now;
                }
            }
            return h;
        }
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * the current thread 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 the current thread was
     *         interrupted while waiting
     */
    public V exchange(V x) throws InterruptedException {
        try {
            return (V)doExchange(x, false, 0);
        } catch (TimeoutException cannotHappen) {
            throw new Error(cannotHappen);
        }
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * the current thread 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 the 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 (V)doExchange(x, true, unit.toNanos(timeout));
    }
}
Revision:	1.31
Committed:	Sun Nov 6 21:34:52 2005 UTC (18 years, 7 months ago) by jsr166
Branch:	MAIN
Changes since 1.30:	+11 -11 lines
Log Message:	doc fixes
#	Content
1	/*
2	* Written by Doug Lea, Bill Scherer, and Michael Scott with
3	* assistance from members of JCP JSR-166 Expert Group and released to
4	* the public domain, as explained at
5	* http://creativecommons.org/licenses/publicdomain
6	*/
7
8	package java.util.concurrent;
9	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
10	import java.util.concurrent.locks.*;
11	import java.util.concurrent.atomic.*;
12	import java.util.Random;
13
14	/**
15	* A synchronization point at which threads can pair and swap elements
16	* within pairs. Each thread presents some object on entry to the
17	* {@link #exchange exchange} method, matches with a partner thread,
18	* and receives its partner's object on return.
19	*
20	* <p><b>Sample Usage:</b>
21	* Here are the highlights of a class that uses an {@code Exchanger}
22	* to swap buffers between threads so that the thread filling the
23	* buffer gets a freshly emptied one when it needs it, handing off the
24	* filled one to the thread emptying the buffer.
25	* <pre>{@code
26	* class FillAndEmpty {
27	* Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>();
28	* DataBuffer initialEmptyBuffer = ... a made-up type
29	* DataBuffer initialFullBuffer = ...
30	*
31	* class FillingLoop implements Runnable {
32	* public void run() {
33	* DataBuffer currentBuffer = initialEmptyBuffer;
34	* try {
35	* while (currentBuffer != null) {
36	* addToBuffer(currentBuffer);
37	* if (currentBuffer.isFull())
38	* currentBuffer = exchanger.exchange(currentBuffer);
39	* }
40	* } catch (InterruptedException ex) { ... handle ... }
41	* }
42	* }
43	*
44	* class EmptyingLoop implements Runnable {
45	* public void run() {
46	* DataBuffer currentBuffer = initialFullBuffer;
47	* try {
48	* while (currentBuffer != null) {
49	* takeFromBuffer(currentBuffer);
50	* if (currentBuffer.isEmpty())
51	* currentBuffer = exchanger.exchange(currentBuffer);
52	* }
53	* } catch (InterruptedException ex) { ... handle ...}
54	* }
55	* }
56	*
57	* void start() {
58	* new Thread(new FillingLoop()).start();
59	* new Thread(new EmptyingLoop()).start();
60	* }
61	* }
62	* }</pre>
63	*
64	* <p>Memory consistency effects: For each pair of threads that
65	* successfully exchange objects via an {@code Exchanger}, actions
66	* prior to the {@code exchange()} in each thread
67	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
68	* those subsequent to a return from the corresponding {@code exchange()}
69	* in the other thread.
70	*
71	* @since 1.5
72	* @author Doug Lea and Bill Scherer and Michael Scott
73	* @param <V> The type of objects that may be exchanged
74	*/
75	public class Exchanger<V> {
76	/*
77	* The underlying idea is to use a stack to hold nodes containing
78	* pairs of items to be exchanged. Except that:
79	*
80	* * Only one element of the pair is known on creation by a
81	* first-arriving thread; the other is a "hole" waiting to be
82	* filled in. This is a degenerate form of the dual stacks
83	* described in "Nonblocking Concurrent Objects with Condition
84	* Synchronization", by W. N. Scherer III and M. L. Scott.
85	* 18th Annual Conf. on Distributed Computing, Oct. 2004.
86	* It is "degenerate" in that both the items and the holes
87	* are shared in the same nodes.
88	*
89	* * There isn't really a stack here! There can't be -- if two
90	* nodes were both in the stack, they should cancel themselves
91	* out by combining. So that's what we do. The 0th element of
92	* the "arena" array serves only as the top of stack. The
93	* remainder of the array is a form of the elimination backoff
94	* collision array described in "A Scalable Lock-free Stack
95	* Algorithm", by D. Hendler, N. Shavit, and L. Yerushalmi.
96	* 16th ACM Symposium on Parallelism in Algorithms and
97	* Architectures, June 2004. Here, threads spin (using short
98	* timed waits with exponential backoff) looking for each
99	* other. If they fail to find others waiting, they try the
100	* top spot again. As shown in that paper, this always
101	* converges.
102	*
103	* The backoff elimination mechanics never come into play in
104	* common usages where only two threads ever meet to exchange
105	* items, but they prevent contention bottlenecks when an
106	* exchanger is used by a large number of threads.
107	*
108	* For more details, see the paper "A Scalable Elimination-based
109	* Exchange Channel" by William Scherer, Doug Lea, and Michael
110	* Scott in Proceedings of SCOOL05 workshop. Available at:
111	* http://hdl.handle.net/1802/2104
112	*/
113
114	/**
115	* Size of collision space. Using a size of half the number of
116	* CPUs provides enough space for threads to find each other but
117	* not so much that it would always require one or more to time
118	* out to become unstuck. Note that the arena array holds SIZE+1
119	* elements, to include the top-of-stack slot.
120	*/
121	private static final int SIZE =
122	(Runtime.getRuntime().availableProcessors() + 1) / 2;
123
124	/**
125	* Base unit in nanoseconds for backoffs. Must be a power of two.
126	* Should be small because backoffs exponentially increase from
127	* base.
128	*/
129	private static final long BACKOFF_BASE = 128L;
130
131	/**
132	* Sentinel item representing cancellation. This value is placed
133	* in holes on cancellation, and used as a return value from Node
134	* methods to indicate failure to set or get hole.
135	*/
136	static final Object FAIL = new Object();
137
138	/**
139	* The collision arena. arena[0] is used as the top of the stack.
140	* The remainder is used as the collision elimination space.
141	*/
142	private final AtomicReference<Node>[] arena;
143
144	/** Generator for random backoffs and delays. */
145	private final Random random = new Random();
146
147	/**
148	* Creates a new Exchanger.
149	*/
150	public Exchanger() {
151	arena = (AtomicReference<Node>[]) new AtomicReference[SIZE + 1];
152	for (int i = 0; i < arena.length; ++i)
153	arena[i] = new AtomicReference<Node>();
154	}
155
156	/**
157	* Main exchange function, handling the different policy variants.
158	* Uses Object, not "V" as argument and return value to simplify
159	* handling of internal sentinel values. Callers from public
160	* methods cast accordingly.
161	*
162	* @param item the item to exchange
163	* @param timed true if the wait is timed
164	* @param nanos if timed, the maximum wait time
165	* @return the other thread's item
166	*/
167	private Object doExchange(Object item, boolean timed, long nanos)
168	throws InterruptedException, TimeoutException {
169	Node me = new Node(item);
170	long lastTime = timed ? System.nanoTime() : 0;
171	int idx = 0; // start out at slot representing top
172	int backoff = 0; // increases on failure to occupy a slot
173
174	for (;;) {
175	AtomicReference<Node> slot = arena[idx];
176
177	// If this slot is already occupied, there is a waiting item...
178	Node you = slot.get();
179	if (you != null) {
180	Object v = you.fillHole(item);
181	slot.compareAndSet(you, null);
182	if (v != FAIL) // ... unless it was cancelled
183	return v;
184	}
185
186	// Try to occupy this slot
187	if (slot.compareAndSet(null, me)) {
188	// If this is top slot, use regular wait, else backoff-wait
189	Object v = ((idx == 0)?
190	me.waitForHole(timed, nanos) :
191	me.waitForHole(true, randomDelay(backoff)));
192	slot.compareAndSet(me, null);
193	if (v != FAIL)
194	return v;
195	if (Thread.interrupted())
196	throw new InterruptedException();
197	if (timed) {
198	long now = System.nanoTime();
199	nanos -= now - lastTime;
200	lastTime = now;
201	if (nanos <= 0)
202	throw new TimeoutException();
203	}
204
205	me = new Node(item); // Throw away nodes on failure
206	if (backoff < SIZE - 1) // Increase or stay saturated
207	++backoff;
208	idx = 0; // Restart at top
209	}
210
211	else // Retry with a random non-top slot <= backoff
212	idx = 1 + random.nextInt(backoff + 1);
213
214	}
215	}
216
217	/**
218	* Returns a random delay less than (base times (2 raised to backoff)).
219	*/
220	private long randomDelay(int backoff) {
221	return ((BACKOFF_BASE << backoff) - 1) & random.nextInt();
222	}
223
224	/**
225	* Nodes hold partially exchanged data. This class
226	* opportunistically subclasses AtomicReference to represent the
227	* hole. So get() returns hole, and compareAndSet CAS'es value
228	* into hole. Note that this class cannot be parameterized as V
229	* because the sentinel value FAIL is only of type Object.
230	*/
231	static final class Node extends AtomicReference<Object> {
232	private static final long serialVersionUID = -3221313401284163686L;
233
234	/** The element offered by the Thread creating this node. */
235	final Object item;
236
237	/** The Thread creating this node. */
238	final Thread waiter;
239
240	/**
241	* Creates node with given item and empty hole.
242	*
243	* @param item the item
244	*/
245	Node(Object item) {
246	this.item = item;
247	waiter = Thread.currentThread();
248	}
249
250	/**
251	* Tries to fill in hole. On success, wakes up the waiter.
252	*
253	* @param val the value to place in hole
254	* @return on success, the item; on failure, FAIL
255	*/
256	Object fillHole(Object val) {
257	if (compareAndSet(null, val)) {
258	LockSupport.unpark(waiter);
259	return item;
260	}
261	return FAIL;
262	}
263
264	/**
265	* Waits for and gets the hole filled in by another thread.
266	* Fails if timed out or interrupted before hole filled.
267	*
268	* @param timed true if the wait is timed
269	* @param nanos if timed, the maximum wait time
270	* @return on success, the hole; on failure, FAIL
271	*/
272	Object waitForHole(boolean timed, long nanos) {
273	long lastTime = timed ? System.nanoTime() : 0;
274	Object h;
275	while ((h = get()) == null) {
276	// If interrupted or timed out, try to cancel by
277	// CASing FAIL as hole value.
278	if (Thread.currentThread().isInterrupted() \|\|
279	(timed && nanos <= 0)) {
280	if (compareAndSet(null, FAIL))
281	return FAIL;
282	}
283	else if (!timed)
284	LockSupport.park();
285	else {
286	LockSupport.parkNanos(nanos);
287	long now = System.nanoTime();
288	nanos -= now - lastTime;
289	lastTime = now;
290	}
291	}
292	return h;
293	}
294	}
295
296	/**
297	* Waits for another thread to arrive at this exchange point (unless
298	* the current thread is {@link Thread#interrupt interrupted}),
299	* and then transfers the given object to it, receiving its object
300	* in return.
301	*
302	* <p>If another thread is already waiting at the exchange point then
303	* it is resumed for thread scheduling purposes and receives the object
304	* passed in by the current thread. The current thread returns immediately,
305	* receiving the object passed to the exchange by that other thread.
306	*
307	* <p>If no other thread is already waiting at the exchange then the
308	* current thread is disabled for thread scheduling purposes and lies
309	* dormant until one of two things happens:
310	* <ul>
311	* <li>Some other thread enters the exchange; or
312	* <li>Some other thread {@link Thread#interrupt interrupts} the current
313	* thread.
314	* </ul>
315	* <p>If the current thread:
316	* <ul>
317	* <li>has its interrupted status set on entry to this method; or
318	* <li>is {@link Thread#interrupt interrupted} while waiting
319	* for the exchange,
320	* </ul>
321	* then {@link InterruptedException} is thrown and the current thread's
322	* interrupted status is cleared.
323	*
324	* @param x the object to exchange
325	* @return the object provided by the other thread
326	* @throws InterruptedException if the current thread was
327	* interrupted while waiting
328	*/
329	public V exchange(V x) throws InterruptedException {
330	try {
331	return (V)doExchange(x, false, 0);
332	} catch (TimeoutException cannotHappen) {
333	throw new Error(cannotHappen);
334	}
335	}
336
337	/**
338	* Waits for another thread to arrive at this exchange point (unless
339	* the current thread is {@link Thread#interrupt interrupted} or
340	* the specified waiting time elapses), and then transfers the given
341	* object to it, receiving its object in return.
342	*
343	* <p>If another thread is already waiting at the exchange point then
344	* it is resumed for thread scheduling purposes and receives the object
345	* passed in by the current thread. The current thread returns immediately,
346	* receiving the object passed to the exchange by that other thread.
347	*
348	* <p>If no other thread is already waiting at the exchange then the
349	* current thread is disabled for thread scheduling purposes and lies
350	* dormant until one of three things happens:
351	* <ul>
352	* <li>Some other thread enters the exchange; or
353	* <li>Some other thread {@link Thread#interrupt interrupts} the current
354	* thread; or
355	* <li>The specified waiting time elapses.
356	* </ul>
357	* <p>If the current thread:
358	* <ul>
359	* <li>has its interrupted status set on entry to this method; or
360	* <li>is {@link Thread#interrupt interrupted} while waiting
361	* for the exchange,
362	* </ul>
363	* then {@link InterruptedException} is thrown and the current thread's
364	* interrupted status is cleared.
365	*
366	* <p>If the specified waiting time elapses then {@link TimeoutException}
367	* is thrown.
368	* If the time is
369	* less than or equal to zero, the method will not wait at all.
370	*
371	* @param x the object to exchange
372	* @param timeout the maximum time to wait
373	* @param unit the time unit of the <tt>timeout</tt> argument
374	* @return the object provided by the other thread
375	* @throws InterruptedException if the current thread was
376	* interrupted while waiting
377	* @throws TimeoutException if the specified waiting time elapses
378	* before another thread enters the exchange
379	*/
380	public V exchange(V x, long timeout, TimeUnit unit)
381	throws InterruptedException, TimeoutException {
382	return (V)doExchange(x, true, unit.toNanos(timeout));
383	}
384	}