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 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>
 *
 * <p> Memory visibility effects: Actions in a thread prior to calling
 * <tt>exchange()</tt> <a
 * href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * those subsequent to the matching <tt>exchange()</tt> in another 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.
     */

    /**
     * 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.
     * Each slot holds an AtomicReference<Node>, but this cannot be
     * expressed for arrays, so elements are casted on each use.
     */
    private final AtomicReference[] arena;

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

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

    /**
     * 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 = (AtomicReference<Node>)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))
                    compareAndSet(null, 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
     * 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 (V)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 (V)doExchange(x, true, unit.toNanos(timeout));
    }
}
Revision:	1.22
Committed:	Fri Sep 2 01:03:08 2005 UTC (18 years, 9 months ago) by brian
Branch:	MAIN
Changes since 1.21:	+5 -0 lines
Log Message:	Happens-before markup
#	User	Rev	Content
1	dl	1.2	/*
2	dl	1.16	* 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	dl	1.14	* http://creativecommons.org/licenses/publicdomain
6	dl	1.2	*/
7
8	tim	1.1	package java.util.concurrent;
9	jsr166	1.21	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
10	dl	1.4	import java.util.concurrent.locks.*;
11	dl	1.16	import java.util.concurrent.atomic.*;
12			import java.util.Random;
13	tim	1.1
14			/**
15	dl	1.12	* A synchronization point at which two threads can exchange objects.
16			* Each thread presents some object on entry to the {@link #exchange
17			* exchange} method, and receives the object presented by the other
18			* thread on return.
19	tim	1.1	*
20			* <p><b>Sample Usage:</b>
21			* Here are the highlights of a class that uses an <tt>Exchanger</tt> to
22			* swap buffers between threads so that the thread filling the
23			* buffer gets a freshly
24			* emptied one when it needs it, handing off the filled one to
25			* the thread emptying the buffer.
26			* <pre>
27			* class FillAndEmpty {
28	dl	1.9	* Exchanger<DataBuffer> exchanger = new Exchanger();
29			* DataBuffer initialEmptyBuffer = ... a made-up type
30			* DataBuffer initialFullBuffer = ...
31	tim	1.1	*
32			* class FillingLoop implements Runnable {
33			* public void run() {
34	dl	1.9	* DataBuffer currentBuffer = initialEmptyBuffer;
35	tim	1.1	* try {
36			* while (currentBuffer != null) {
37			* addToBuffer(currentBuffer);
38			* if (currentBuffer.full())
39			* currentBuffer = exchanger.exchange(currentBuffer);
40			* }
41	tim	1.7	* } catch (InterruptedException ex) { ... handle ... }
42	tim	1.1	* }
43			* }
44			*
45			* class EmptyingLoop implements Runnable {
46			* public void run() {
47	dl	1.9	* DataBuffer currentBuffer = initialFullBuffer;
48	tim	1.1	* try {
49			* while (currentBuffer != null) {
50			* takeFromBuffer(currentBuffer);
51			* if (currentBuffer.empty())
52			* currentBuffer = exchanger.exchange(currentBuffer);
53			* }
54	tim	1.7	* } catch (InterruptedException ex) { ... handle ...}
55	tim	1.1	* }
56			* }
57			*
58			* void start() {
59			* new Thread(new FillingLoop()).start();
60			* new Thread(new EmptyingLoop()).start();
61			* }
62			* }
63			* </pre>
64			*
65	brian	1.22	* <p> Memory visibility effects: Actions in a thread prior to calling
66			* <tt>exchange()</tt> <a
67			* href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
68			* those subsequent to the matching <tt>exchange()</tt> in another thread.
69			*
70	tim	1.1	* @since 1.5
71	dl	1.16	* @author Doug Lea and Bill Scherer and Michael Scott
72	dl	1.11	* @param <V> The type of objects that may be exchanged
73	tim	1.1	*/
74			public class Exchanger<V> {
75	dl	1.16	/*
76			* The underlying idea is to use a stack to hold nodes containing
77			* pairs of items to be exchanged. Except that:
78			*
79			* * Only one element of the pair is known on creation by a
80			* first-arriving thread; the other is a "hole" waiting to be
81			* filled in. This is a degenerate form of the dual stacks
82			* described in "Nonblocking Concurrent Objects with Condition
83			* Synchronization", by W. N. Scherer III and M. L. Scott.
84			* 18th Annual Conf. on Distributed Computing, Oct. 2004.
85			* It is "degenerate" in that both the items and the holes
86			* are shared in the same nodes.
87			*
88			* * There isn't really a stack here! There can't be -- if two
89			* nodes were both in the stack, they should cancel themselves
90			* out by combining. So that's what we do. The 0th element of
91			* the "arena" array serves only as the top of stack. The
92			* remainder of the array is a form of the elimination backoff
93			* collision array described in "A Scalable Lock-free Stack
94			* Algorithm", by D. Hendler, N. Shavit, and L. Yerushalmi.
95			* 16th ACM Symposium on Parallelism in Algorithms and
96			* Architectures, June 2004. Here, threads spin (using short
97			* timed waits with exponential backoff) looking for each
98			* other. If they fail to find others waiting, they try the
99			* top spot again. As shown in that paper, this always
100			* converges.
101			*
102			* The backoff elimination mechanics never come into play in
103			* common usages where only two threads ever meet to exchange
104			* items, but they prevent contention bottlenecks when an
105			* exchanger is used by a large number of threads.
106			*/
107	dl	1.2
108	jsr166	1.17	/**
109	dl	1.16	* Size of collision space. Using a size of half the number of
110			* CPUs provides enough space for threads to find each other but
111			* not so much that it would always require one or more to time
112			* out to become unstuck. Note that the arena array holds SIZE+1
113			* elements, to include the top-of-stack slot.
114			*/
115	jsr166	1.17	private static final int SIZE =
116	dl	1.16	(Runtime.getRuntime().availableProcessors() + 1) / 2;
117	jsr166	1.15
118	dl	1.2	/**
119	dl	1.16	* Base unit in nanoseconds for backoffs. Must be a power of two.
120			* Should be small because backoffs exponentially increase from
121			* base.
122	dl	1.2	*/
123	dl	1.16	private static final long BACKOFF_BASE = 128L;
124	dl	1.2
125	jsr166	1.17	/**
126	dl	1.16	* Sentinel item representing cancellation. This value is placed
127			* in holes on cancellation, and used as a return value from Node
128			* methods to indicate failure to set or get hole.
129	jsr166	1.17	*/
130	dl	1.16	static final Object FAIL = new Object();
131
132	jsr166	1.17	/**
133	dl	1.16	* The collision arena. arena[0] is used as the top of the stack.
134			* The remainder is used as the collision elimination space.
135	jsr166	1.17	* Each slot holds an AtomicReference<Node>, but this cannot be
136	dl	1.16	* expressed for arrays, so elements are casted on each use.
137	dl	1.3	*/
138	dl	1.16	private final AtomicReference[] arena;
139	dl	1.5
140	dl	1.16	/** Generator for random backoffs and delays. */
141			private final Random random = new Random();
142	dl	1.5
143	dl	1.16	/**
144			* Creates a new Exchanger.
145			*/
146			public Exchanger() {
147			arena = new AtomicReference[SIZE + 1];
148			for (int i = 0; i < arena.length; ++i)
149			arena[i] = new AtomicReference();
150			}
151	dl	1.2
152	dl	1.16	/**
153			* Main exchange function, handling the different policy variants.
154			* Uses Object, not "V" as argument and return value to simplify
155			* handling of internal sentinel values. Callers from public
156			* methods cast accordingly.
157			* @param item the item to exchange.
158			* @param timed true if the wait is timed.
159			* @param nanos if timed, the maximum wait time.
160			* @return the other thread's item.
161			*/
162			private Object doExchange(Object item, boolean timed, long nanos)
163			throws InterruptedException, TimeoutException {
164			Node me = new Node(item);
165			long lastTime = (timed)? System.nanoTime() : 0;
166			int idx = 0; // start out at slot representing top
167			int backoff = 0; // increases on failure to occupy a slot
168
169			for (;;) {
170			AtomicReference<Node> slot = (AtomicReference<Node>)arena[idx];
171
172			// If this slot is already occupied, there is a waiting item...
173			Node you = slot.get();
174			if (you != null) {
175			Object v = you.fillHole(item);
176			slot.compareAndSet(you, null);
177			if (v != FAIL) // ... unless it was cancelled
178			return v;
179	dl	1.2	}
180
181	dl	1.16	// Try to occupy this slot
182			if (slot.compareAndSet(null, me)) {
183			// If this is top slot, use regular wait, else backoff-wait
184			Object v = ((idx == 0)?
185			me.waitForHole(timed, nanos) :
186			me.waitForHole(true, randomDelay(backoff)));
187			slot.compareAndSet(me, null);
188			if (v != FAIL)
189			return v;
190			if (Thread.interrupted())
191			throw new InterruptedException();
192			if (timed) {
193			long now = System.nanoTime();
194			nanos -= now - lastTime;
195			lastTime = now;
196			if (nanos <= 0)
197			throw new TimeoutException();
198			}
199	dl	1.2
200	dl	1.16	me = new Node(item); // Throw away nodes on failure
201			if (backoff < SIZE - 1) // Increase or stay saturated
202			++backoff;
203			idx = 0; // Restart at top
204	dl	1.2	}
205
206	dl	1.16	else // Retry with a random non-top slot <= backoff
207			idx = 1 + random.nextInt(backoff + 1);
208	dl	1.2
209			}
210			}
211	tim	1.1
212			/**
213	dl	1.16	* Returns a random delay less than (base times (2 raised to backoff))
214			*/
215			private long randomDelay(int backoff) {
216			return ((BACKOFF_BASE << backoff) - 1) & random.nextInt();
217			}
218
219			/**
220			* Nodes hold partially exchanged data. This class
221			* opportunistically subclasses AtomicReference to represent the
222			* hole. So get() returns hole, and compareAndSet CAS'es value
223			* into hole. Note that this class cannot be parameterized as V
224			* because the sentinel value FAIL is only of type Object.
225	jsr166	1.15	*/
226	dl	1.16	static final class Node extends AtomicReference<Object> {
227	dl	1.20	private static final long serialVersionUID = -3221313401284163686L;
228	jsr166	1.21
229	dl	1.16	/** The element offered by the Thread creating this node. */
230			final Object item;
231			/** The Thread creating this node. */
232			final Thread waiter;
233
234			/**
235			* Creates node with given item and empty hole.
236			* @param item the item.
237			*/
238			Node(Object item) {
239			this.item = item;
240			waiter = Thread.currentThread();
241			}
242
243			/**
244			* Tries to fill in hole. On success, wakes up the waiter.
245			* @param val the value to place in hole.
246			* @return on success, the item; on failure, FAIL.
247			*/
248			Object fillHole(Object val) {
249			if (compareAndSet(null, val)) {
250			LockSupport.unpark(waiter);
251			return item;
252			}
253			return FAIL;
254			}
255
256			/**
257	jsr166	1.17	* Waits for and gets the hole filled in by another thread.
258			* Fails if timed out or interrupted before hole filled.
259	dl	1.16	* @param timed true if the wait is timed.
260			* @param nanos if timed, the maximum wait time.
261			* @return on success, the hole; on failure, FAIL.
262			*/
263			Object waitForHole(boolean timed, long nanos) {
264			long lastTime = (timed)? System.nanoTime() : 0;
265	dl	1.18	Object h;
266			while ((h = get()) == null) {
267			// If interrupted or timed out, try to cancel by
268			// CASing FAIL as hole value.
269			if (Thread.currentThread().isInterrupted() \|\|
270	jsr166	1.19	(timed && nanos <= 0))
271	dl	1.18	compareAndSet(null, FAIL);
272			else if (!timed)
273	dl	1.16	LockSupport.park();
274			else {
275			LockSupport.parkNanos(nanos);
276			long now = System.nanoTime();
277			nanos -= now - lastTime;
278			lastTime = now;
279			}
280			}
281			return h;
282			}
283	tim	1.1	}
284
285			/**
286			* Waits for another thread to arrive at this exchange point (unless
287			* it is {@link Thread#interrupt interrupted}),
288			* and then transfers the given object to it, receiving its object
289			* in return.
290	jsr166	1.17	*
291	tim	1.1	* <p>If another thread is already waiting at the exchange point then
292			* it is resumed for thread scheduling purposes and receives the object
293			* passed in by the current thread. The current thread returns immediately,
294			* receiving the object passed to the exchange by that other thread.
295	jsr166	1.17	*
296	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
297	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
298			* dormant until one of two things happens:
299			* <ul>
300			* <li>Some other thread enters the exchange; or
301			* <li>Some other thread {@link Thread#interrupt interrupts} the current
302			* thread.
303			* </ul>
304			* <p>If the current thread:
305			* <ul>
306	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
307	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
308	jsr166	1.15	* for the exchange,
309	tim	1.1	* </ul>
310	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
311			* interrupted status is cleared.
312	tim	1.1	*
313			* @param x the object to exchange
314			* @return the object provided by the other thread.
315	jsr166	1.15	* @throws InterruptedException if current thread was interrupted
316	dl	1.3	* while waiting
317	jsr166	1.15	*/
318	tim	1.1	public V exchange(V x) throws InterruptedException {
319	dl	1.2	try {
320	dl	1.16	return (V)doExchange(x, false, 0);
321	jsr166	1.15	} catch (TimeoutException cannotHappen) {
322	dl	1.2	throw new Error(cannotHappen);
323			}
324	tim	1.1	}
325
326			/**
327			* Waits for another thread to arrive at this exchange point (unless
328			* it is {@link Thread#interrupt interrupted}, or the specified waiting
329			* time elapses),
330			* and then transfers the given object to it, receiving its object
331			* in return.
332			*
333			* <p>If another thread is already waiting at the exchange point then
334			* it is resumed for thread scheduling purposes and receives the object
335			* passed in by the current thread. The current thread returns immediately,
336			* receiving the object passed to the exchange by that other thread.
337			*
338	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
339	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
340			* dormant until one of three things happens:
341			* <ul>
342			* <li>Some other thread enters the exchange; or
343			* <li>Some other thread {@link Thread#interrupt interrupts} the current
344			* thread; or
345			* <li>The specified waiting time elapses.
346			* </ul>
347			* <p>If the current thread:
348			* <ul>
349	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
350	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
351	jsr166	1.15	* for the exchange,
352	tim	1.1	* </ul>
353	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
354			* interrupted status is cleared.
355	tim	1.1	*
356			* <p>If the specified waiting time elapses then {@link TimeoutException}
357			* is thrown.
358	jsr166	1.15	* If the time is
359	tim	1.1	* less than or equal to zero, the method will not wait at all.
360			*
361			* @param x the object to exchange
362			* @param timeout the maximum time to wait
363	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
364	tim	1.1	* @return the object provided by the other thread.
365	dl	1.3	* @throws InterruptedException if current thread was interrupted
366			* while waiting
367	tim	1.1	* @throws TimeoutException if the specified waiting time elapses before
368			* another thread enters the exchange.
369	jsr166	1.15	*/
370			public V exchange(V x, long timeout, TimeUnit unit)
371	tim	1.1	throws InterruptedException, TimeoutException {
372	dl	1.16	return (V)doExchange(x, true, unit.toNanos(timeout));
373	tim	1.1	}
374			}