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: For each pair of threads that
 * successfully exchange objects via an <tt>Exchanger</tt>, 
 * actions prior to to the <tt>exchange()</tt> 
 * in each thread <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 
 * those subsequent to the corresponding <tt>exchange()</tt> 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.
     */

    /**
     * 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:	1.23
Committed:	Fri Sep 2 03:03:17 2005 UTC (18 years, 9 months ago) by brian
Branch:	MAIN
Changes since 1.22:	+7 -4 lines
Log Message:	More tweaks to HB 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	1.23
66			* <p> Memory visibility effects: For each pair of threads that
67			* successfully exchange objects via an <tt>Exchanger</tt>,
68			* actions prior to to the <tt>exchange()</tt>
69			* in each thread <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
70			* those subsequent to the corresponding <tt>exchange()</tt> in the other
71			* thread.
72	brian	1.22	*
73	tim	1.1	* @since 1.5
74	dl	1.16	* @author Doug Lea and Bill Scherer and Michael Scott
75	dl	1.11	* @param <V> The type of objects that may be exchanged
76	tim	1.1	*/
77			public class Exchanger<V> {
78	dl	1.16	/*
79			* The underlying idea is to use a stack to hold nodes containing
80			* pairs of items to be exchanged. Except that:
81			*
82			* * Only one element of the pair is known on creation by a
83			* first-arriving thread; the other is a "hole" waiting to be
84			* filled in. This is a degenerate form of the dual stacks
85			* described in "Nonblocking Concurrent Objects with Condition
86			* Synchronization", by W. N. Scherer III and M. L. Scott.
87			* 18th Annual Conf. on Distributed Computing, Oct. 2004.
88			* It is "degenerate" in that both the items and the holes
89			* are shared in the same nodes.
90			*
91			* * There isn't really a stack here! There can't be -- if two
92			* nodes were both in the stack, they should cancel themselves
93			* out by combining. So that's what we do. The 0th element of
94			* the "arena" array serves only as the top of stack. The
95			* remainder of the array is a form of the elimination backoff
96			* collision array described in "A Scalable Lock-free Stack
97			* Algorithm", by D. Hendler, N. Shavit, and L. Yerushalmi.
98			* 16th ACM Symposium on Parallelism in Algorithms and
99			* Architectures, June 2004. Here, threads spin (using short
100			* timed waits with exponential backoff) looking for each
101			* other. If they fail to find others waiting, they try the
102			* top spot again. As shown in that paper, this always
103			* converges.
104			*
105			* The backoff elimination mechanics never come into play in
106			* common usages where only two threads ever meet to exchange
107			* items, but they prevent contention bottlenecks when an
108			* exchanger is used by a large number of threads.
109			*/
110	dl	1.2
111	jsr166	1.17	/**
112	dl	1.16	* Size of collision space. Using a size of half the number of
113			* CPUs provides enough space for threads to find each other but
114			* not so much that it would always require one or more to time
115			* out to become unstuck. Note that the arena array holds SIZE+1
116			* elements, to include the top-of-stack slot.
117			*/
118	jsr166	1.17	private static final int SIZE =
119	dl	1.16	(Runtime.getRuntime().availableProcessors() + 1) / 2;
120	jsr166	1.15
121	dl	1.2	/**
122	dl	1.16	* Base unit in nanoseconds for backoffs. Must be a power of two.
123			* Should be small because backoffs exponentially increase from
124			* base.
125	dl	1.2	*/
126	dl	1.16	private static final long BACKOFF_BASE = 128L;
127	dl	1.2
128	jsr166	1.17	/**
129	dl	1.16	* Sentinel item representing cancellation. This value is placed
130			* in holes on cancellation, and used as a return value from Node
131			* methods to indicate failure to set or get hole.
132	jsr166	1.17	*/
133	dl	1.16	static final Object FAIL = new Object();
134
135	jsr166	1.17	/**
136	dl	1.16	* The collision arena. arena[0] is used as the top of the stack.
137			* The remainder is used as the collision elimination space.
138	jsr166	1.17	* Each slot holds an AtomicReference<Node>, but this cannot be
139	dl	1.16	* expressed for arrays, so elements are casted on each use.
140	dl	1.3	*/
141	dl	1.16	private final AtomicReference[] arena;
142	dl	1.5
143	dl	1.16	/** Generator for random backoffs and delays. */
144			private final Random random = new Random();
145	dl	1.5
146	dl	1.16	/**
147			* Creates a new Exchanger.
148			*/
149			public Exchanger() {
150			arena = new AtomicReference[SIZE + 1];
151			for (int i = 0; i < arena.length; ++i)
152			arena[i] = new AtomicReference();
153			}
154	dl	1.2
155	dl	1.16	/**
156			* Main exchange function, handling the different policy variants.
157			* Uses Object, not "V" as argument and return value to simplify
158			* handling of internal sentinel values. Callers from public
159			* methods cast accordingly.
160			* @param item the item to exchange.
161			* @param timed true if the wait is timed.
162			* @param nanos if timed, the maximum wait time.
163			* @return the other thread's item.
164			*/
165			private Object doExchange(Object item, boolean timed, long nanos)
166			throws InterruptedException, TimeoutException {
167			Node me = new Node(item);
168			long lastTime = (timed)? System.nanoTime() : 0;
169			int idx = 0; // start out at slot representing top
170			int backoff = 0; // increases on failure to occupy a slot
171
172			for (;;) {
173			AtomicReference<Node> slot = (AtomicReference<Node>)arena[idx];
174
175			// If this slot is already occupied, there is a waiting item...
176			Node you = slot.get();
177			if (you != null) {
178			Object v = you.fillHole(item);
179			slot.compareAndSet(you, null);
180			if (v != FAIL) // ... unless it was cancelled
181			return v;
182	dl	1.2	}
183
184	dl	1.16	// Try to occupy this slot
185			if (slot.compareAndSet(null, me)) {
186			// If this is top slot, use regular wait, else backoff-wait
187			Object v = ((idx == 0)?
188			me.waitForHole(timed, nanos) :
189			me.waitForHole(true, randomDelay(backoff)));
190			slot.compareAndSet(me, null);
191			if (v != FAIL)
192			return v;
193			if (Thread.interrupted())
194			throw new InterruptedException();
195			if (timed) {
196			long now = System.nanoTime();
197			nanos -= now - lastTime;
198			lastTime = now;
199			if (nanos <= 0)
200			throw new TimeoutException();
201			}
202	dl	1.2
203	dl	1.16	me = new Node(item); // Throw away nodes on failure
204			if (backoff < SIZE - 1) // Increase or stay saturated
205			++backoff;
206			idx = 0; // Restart at top
207	dl	1.2	}
208
209	dl	1.16	else // Retry with a random non-top slot <= backoff
210			idx = 1 + random.nextInt(backoff + 1);
211	dl	1.2
212			}
213			}
214	tim	1.1
215			/**
216	dl	1.16	* Returns a random delay less than (base times (2 raised to backoff))
217			*/
218			private long randomDelay(int backoff) {
219			return ((BACKOFF_BASE << backoff) - 1) & random.nextInt();
220			}
221
222			/**
223			* Nodes hold partially exchanged data. This class
224			* opportunistically subclasses AtomicReference to represent the
225			* hole. So get() returns hole, and compareAndSet CAS'es value
226			* into hole. Note that this class cannot be parameterized as V
227			* because the sentinel value FAIL is only of type Object.
228	jsr166	1.15	*/
229	dl	1.16	static final class Node extends AtomicReference<Object> {
230	dl	1.20	private static final long serialVersionUID = -3221313401284163686L;
231	jsr166	1.21
232	dl	1.16	/** The element offered by the Thread creating this node. */
233			final Object item;
234			/** The Thread creating this node. */
235			final Thread waiter;
236
237			/**
238			* Creates node with given item and empty hole.
239			* @param item the item.
240			*/
241			Node(Object item) {
242			this.item = item;
243			waiter = Thread.currentThread();
244			}
245
246			/**
247			* Tries to fill in hole. On success, wakes up the waiter.
248			* @param val the value to place in hole.
249			* @return on success, the item; on failure, FAIL.
250			*/
251			Object fillHole(Object val) {
252			if (compareAndSet(null, val)) {
253			LockSupport.unpark(waiter);
254			return item;
255			}
256			return FAIL;
257			}
258
259			/**
260	jsr166	1.17	* Waits for and gets the hole filled in by another thread.
261			* Fails if timed out or interrupted before hole filled.
262	dl	1.16	* @param timed true if the wait is timed.
263			* @param nanos if timed, the maximum wait time.
264			* @return on success, the hole; on failure, FAIL.
265			*/
266			Object waitForHole(boolean timed, long nanos) {
267			long lastTime = (timed)? System.nanoTime() : 0;
268	dl	1.18	Object h;
269			while ((h = get()) == null) {
270			// If interrupted or timed out, try to cancel by
271			// CASing FAIL as hole value.
272			if (Thread.currentThread().isInterrupted() \|\|
273	jsr166	1.19	(timed && nanos <= 0))
274	dl	1.18	compareAndSet(null, FAIL);
275			else if (!timed)
276	dl	1.16	LockSupport.park();
277			else {
278			LockSupport.parkNanos(nanos);
279			long now = System.nanoTime();
280			nanos -= now - lastTime;
281			lastTime = now;
282			}
283			}
284			return h;
285			}
286	tim	1.1	}
287
288			/**
289			* Waits for another thread to arrive at this exchange point (unless
290			* it is {@link Thread#interrupt interrupted}),
291			* and then transfers the given object to it, receiving its object
292			* in return.
293	jsr166	1.17	*
294	tim	1.1	* <p>If another thread is already waiting at the exchange point then
295			* it is resumed for thread scheduling purposes and receives the object
296			* passed in by the current thread. The current thread returns immediately,
297			* receiving the object passed to the exchange by that other thread.
298	jsr166	1.17	*
299	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
300	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
301			* dormant until one of two things happens:
302			* <ul>
303			* <li>Some other thread enters the exchange; or
304			* <li>Some other thread {@link Thread#interrupt interrupts} the current
305			* thread.
306			* </ul>
307			* <p>If the current thread:
308			* <ul>
309	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
310	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
311	jsr166	1.15	* for the exchange,
312	tim	1.1	* </ul>
313	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
314			* interrupted status is cleared.
315	tim	1.1	*
316			* @param x the object to exchange
317			* @return the object provided by the other thread.
318	jsr166	1.15	* @throws InterruptedException if current thread was interrupted
319	dl	1.3	* while waiting
320	jsr166	1.15	*/
321	tim	1.1	public V exchange(V x) throws InterruptedException {
322	dl	1.2	try {
323	dl	1.16	return (V)doExchange(x, false, 0);
324	jsr166	1.15	} catch (TimeoutException cannotHappen) {
325	dl	1.2	throw new Error(cannotHappen);
326			}
327	tim	1.1	}
328
329			/**
330			* Waits for another thread to arrive at this exchange point (unless
331			* it is {@link Thread#interrupt interrupted}, or the specified waiting
332			* time elapses),
333			* and then transfers the given object to it, receiving its object
334			* in return.
335			*
336			* <p>If another thread is already waiting at the exchange point then
337			* it is resumed for thread scheduling purposes and receives the object
338			* passed in by the current thread. The current thread returns immediately,
339			* receiving the object passed to the exchange by that other thread.
340			*
341	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
342	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
343			* dormant until one of three things happens:
344			* <ul>
345			* <li>Some other thread enters the exchange; or
346			* <li>Some other thread {@link Thread#interrupt interrupts} the current
347			* thread; or
348			* <li>The specified waiting time elapses.
349			* </ul>
350			* <p>If the current thread:
351			* <ul>
352	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
353	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
354	jsr166	1.15	* for the exchange,
355	tim	1.1	* </ul>
356	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
357			* interrupted status is cleared.
358	tim	1.1	*
359			* <p>If the specified waiting time elapses then {@link TimeoutException}
360			* is thrown.
361	jsr166	1.15	* If the time is
362	tim	1.1	* less than or equal to zero, the method will not wait at all.
363			*
364			* @param x the object to exchange
365			* @param timeout the maximum time to wait
366	dl	1.2	* @param unit the time unit of the <tt>timeout</tt> argument.
367	tim	1.1	* @return the object provided by the other thread.
368	dl	1.3	* @throws InterruptedException if current thread was interrupted
369			* while waiting
370	tim	1.1	* @throws TimeoutException if the specified waiting time elapses before
371			* another thread enters the exchange.
372	jsr166	1.15	*/
373			public V exchange(V x, long timeout, TimeUnit unit)
374	tim	1.1	throws InterruptedException, TimeoutException {
375	dl	1.16	return (V)doExchange(x, true, unit.toNanos(timeout));
376	tim	1.1	}
377			}