1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea, Bill Scherer, and Michael Scott with
32 * assistance from members of JCP JSR-166 Expert Group and released to
33 * the public domain, as explained at
34 * http://creativecommons.org/publicdomain/zero/1.0/
35 */
36
37 package java.util.concurrent;
38 import java.util.concurrent.atomic.*;
39 import java.util.concurrent.locks.LockSupport;
40
41 /**
42 * A synchronization point at which threads can pair and swap elements
43 * within pairs. Each thread presents some object on entry to the
44 * {@link #exchange exchange} method, matches with a partner thread,
45 * and receives its partner's object on return. An Exchanger may be
46 * viewed as a bidirectional form of a {@link SynchronousQueue}.
47 * Exchangers may be useful in applications such as genetic algorithms
48 * and pipeline designs.
49 *
50 * <p><b>Sample Usage:</b>
51 * Here are the highlights of a class that uses an {@code Exchanger}
52 * to swap buffers between threads so that the thread filling the
53 * buffer gets a freshly emptied one when it needs it, handing off the
54 * filled one to the thread emptying the buffer.
55 * <pre>{@code
56 * class FillAndEmpty {
57 * Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>();
58 * DataBuffer initialEmptyBuffer = ... a made-up type
59 * DataBuffer initialFullBuffer = ...
60 *
61 * class FillingLoop implements Runnable {
62 * public void run() {
63 * DataBuffer currentBuffer = initialEmptyBuffer;
64 * try {
65 * while (currentBuffer != null) {
66 * addToBuffer(currentBuffer);
67 * if (currentBuffer.isFull())
68 * currentBuffer = exchanger.exchange(currentBuffer);
69 * }
70 * } catch (InterruptedException ex) { ... handle ... }
71 * }
72 * }
73 *
74 * class EmptyingLoop implements Runnable {
75 * public void run() {
76 * DataBuffer currentBuffer = initialFullBuffer;
77 * try {
78 * while (currentBuffer != null) {
79 * takeFromBuffer(currentBuffer);
80 * if (currentBuffer.isEmpty())
81 * currentBuffer = exchanger.exchange(currentBuffer);
82 * }
83 * } catch (InterruptedException ex) { ... handle ...}
84 * }
85 * }
86 *
87 * void start() {
88 * new Thread(new FillingLoop()).start();
89 * new Thread(new EmptyingLoop()).start();
90 * }
91 * }
92 * }</pre>
93 *
94 * <p>Memory consistency effects: For each pair of threads that
95 * successfully exchange objects via an {@code Exchanger}, actions
96 * prior to the {@code exchange()} in each thread
97 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
98 * those subsequent to a return from the corresponding {@code exchange()}
99 * in the other thread.
100 *
101 * @since 1.5
102 * @author Doug Lea and Bill Scherer and Michael Scott
103 * @param <V> The type of objects that may be exchanged
104 */
105 public class Exchanger<V> {
106 /*
107 * Algorithm Description:
108 *
109 * The basic idea is to maintain a "slot", which is a reference to
110 * a Node containing both an Item to offer and a "hole" waiting to
111 * get filled in. If an incoming "occupying" thread sees that the
112 * slot is null, it CAS'es (compareAndSets) a Node there and waits
113 * for another to invoke exchange. That second "fulfilling" thread
114 * sees that the slot is non-null, and so CASes it back to null,
115 * also exchanging items by CASing the hole, plus waking up the
116 * occupying thread if it is blocked. In each case CAS'es may
117 * fail because a slot at first appears non-null but is null upon
118 * CAS, or vice-versa. So threads may need to retry these
119 * actions.
120 *
121 * This simple approach works great when there are only a few
122 * threads using an Exchanger, but performance rapidly
123 * deteriorates due to CAS contention on the single slot when
124 * there are lots of threads using an exchanger. So instead we use
125 * an "arena"; basically a kind of hash table with a dynamically
126 * varying number of slots, any one of which can be used by
127 * threads performing an exchange. Incoming threads pick slots
128 * based on a hash of their Thread ids. If an incoming thread
129 * fails to CAS in its chosen slot, it picks an alternative slot
130 * instead. And similarly from there. If a thread successfully
131 * CASes into a slot but no other thread arrives, it tries
132 * another, heading toward the zero slot, which always exists even
133 * if the table shrinks. The particular mechanics controlling this
134 * are as follows:
135 *
136 * Waiting: Slot zero is special in that it is the only slot that
137 * exists when there is no contention. A thread occupying slot
138 * zero will block if no thread fulfills it after a short spin.
139 * In other cases, occupying threads eventually give up and try
140 * another slot. Waiting threads spin for a while (a period that
141 * should be a little less than a typical context-switch time)
142 * before either blocking (if slot zero) or giving up (if other
143 * slots) and restarting. There is no reason for threads to block
144 * unless there are unlikely to be any other threads present.
145 * Occupants are mainly avoiding memory contention so sit there
146 * quietly polling for a shorter period than it would take to
147 * block and then unblock them. Non-slot-zero waits that elapse
148 * because of lack of other threads waste around one extra
149 * context-switch time per try, which is still on average much
150 * faster than alternative approaches.
151 *
152 * Sizing: Usually, using only a few slots suffices to reduce
153 * contention. Especially with small numbers of threads, using
154 * too many slots can lead to just as poor performance as using
155 * too few of them, and there's not much room for error. The
156 * variable "max" maintains the number of slots actually in
157 * use. It is increased when a thread sees too many CAS
158 * failures. (This is analogous to resizing a regular hash table
159 * based on a target load factor, except here, growth steps are
160 * just one-by-one rather than proportional.) Growth requires
161 * contention failures in each of three tried slots. Requiring
162 * multiple failures for expansion copes with the fact that some
163 * failed CASes are not due to contention but instead to simple
164 * races between two threads or thread pre-emptions occurring
165 * between reading and CASing. Also, very transient peak
166 * contention can be much higher than the average sustainable
167 * levels. An attempt to decrease the max limit is usually made
168 * when a non-slot-zero wait elapses without being fulfilled.
169 * Threads experiencing elapsed waits move closer to zero, so
170 * eventually find existing (or future) threads even if the table
171 * has been shrunk due to inactivity. The chosen mechanics and
172 * thresholds for growing and shrinking are intrinsically
173 * entangled with indexing and hashing inside the exchange code,
174 * and can't be nicely abstracted out.
175 *
176 * Hashing: Each thread picks its initial slot to use in accord
177 * with a simple hashcode. The sequence is the same on each
178 * encounter by any given thread, but effectively random across
179 * threads. Using arenas encounters the classic cost vs quality
180 * tradeoffs of all hash tables. Here, we use a one-step FNV-1a
181 * hash code based on the current thread's Thread.getId(), along
182 * with a cheap approximation to a mod operation to select an
183 * index. The downside of optimizing index selection in this way
184 * is that the code is hardwired to use a maximum table size of
185 * 32. But this value more than suffices for known platforms and
186 * applications.
187 *
188 * Probing: On sensed contention of a selected slot, we probe
189 * sequentially through the table, analogously to linear probing
190 * after collision in a hash table. (We move circularly, in
191 * reverse order, to mesh best with table growth and shrinkage
192 * rules.) Except that to minimize the effects of false-alarms
193 * and cache thrashing, we try the first selected slot twice
194 * before moving.
195 *
196 * Padding: Even with contention management, slots are heavily
197 * contended, so use cache-padding to avoid poor memory
198 * performance. Because of this, slots are lazily constructed
199 * only when used, to avoid wasting this space unnecessarily.
200 * While isolation of locations is not much of an issue at first
201 * in an application, as time goes on and garbage-collectors
202 * perform compaction, slots are very likely to be moved adjacent
203 * to each other, which can cause much thrashing of cache lines on
204 * MPs unless padding is employed.
205 *
206 * This is an improvement of the algorithm described in the paper
207 * "A Scalable Elimination-based Exchange Channel" by William
208 * Scherer, Doug Lea, and Michael Scott in Proceedings of SCOOL05
209 * workshop. Available at: http://hdl.handle.net/1802/2104
210 */
211
212 /** The number of CPUs, for sizing and spin control */
213 private static final int NCPU = Runtime.getRuntime().availableProcessors();
214
215 /**
216 * The capacity of the arena. Set to a value that provides more
217 * than enough space to handle contention. On small machines
218 * most slots won't be used, but it is still not wasted because
219 * the extra space provides some machine-level address padding
220 * to minimize interference with heavily CAS'ed Slot locations.
221 * And on very large machines, performance eventually becomes
222 * bounded by memory bandwidth, not numbers of threads/CPUs.
223 * This constant cannot be changed without also modifying
224 * indexing and hashing algorithms.
225 */
226 private static final int CAPACITY = 32;
227
228 /**
229 * The value of "max" that will hold all threads without
230 * contention. When this value is less than CAPACITY, some
231 * otherwise wasted expansion can be avoided.
232 */
233 private static final int FULL =
234 Math.max(0, Math.min(CAPACITY, NCPU / 2) - 1);
235
236 /**
237 * The number of times to spin (doing nothing except polling a
238 * memory location) before blocking or giving up while waiting to
239 * be fulfilled. Should be zero on uniprocessors. On
240 * multiprocessors, this value should be large enough so that two
241 * threads exchanging items as fast as possible block only when
242 * one of them is stalled (due to GC or preemption), but not much
243 * longer, to avoid wasting CPU resources. Seen differently, this
244 * value is a little over half the number of cycles of an average
245 * context switch time on most systems. The value here is
246 * approximately the average of those across a range of tested
247 * systems.
248 */
249 private static final int SPINS = (NCPU == 1) ? 0 : 2000;
250
251 /**
252 * The number of times to spin before blocking in timed waits.
253 * Timed waits spin more slowly because checking the time takes
254 * time. The best value relies mainly on the relative rate of
255 * System.nanoTime vs memory accesses. The value is empirically
256 * derived to work well across a variety of systems.
257 */
258 private static final int TIMED_SPINS = SPINS / 20;
259
260 /**
261 * Sentinel item representing cancellation of a wait due to
262 * interruption, timeout, or elapsed spin-waits. This value is
263 * placed in holes on cancellation, and used as a return value
264 * from waiting methods to indicate failure to set or get hole.
265 */
266 private static final Object CANCEL = new Object();
267
268 /**
269 * Value representing null arguments/returns from public
270 * methods. This disambiguates from internal requirement that
271 * holes start out as null to mean they are not yet set.
272 */
273 private static final Object NULL_ITEM = new Object();
274
275 /**
276 * Nodes hold partially exchanged data. This class
277 * opportunistically subclasses AtomicReference to represent the
278 * hole. So get() returns hole, and compareAndSet CAS'es value
279 * into hole. This class cannot be parameterized as "V" because
280 * of the use of non-V CANCEL sentinels.
281 */
282 private static final class Node extends AtomicReference<Object> {
283 /** The element offered by the Thread creating this node. */
284 public final Object item;
285
286 /** The Thread waiting to be signalled; null until waiting. */
287 public volatile Thread waiter;
288
289 /**
290 * Creates node with given item and empty hole.
291 * @param item the item
292 */
293 public Node(Object item) {
294 this.item = item;
295 }
296 }
297
298 /**
299 * A Slot is an AtomicReference with heuristic padding to lessen
300 * cache effects of this heavily CAS'ed location. While the
301 * padding adds noticeable space, all slots are created only on
302 * demand, and there will be more than one of them only when it
303 * would improve throughput more than enough to outweigh using
304 * extra space.
305 */
306 private static final class Slot extends AtomicReference<Object> {
307 // Improve likelihood of isolation on <= 64 byte cache lines
308 long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe;
309 }
310
311 /**
312 * Slot array. Elements are lazily initialized when needed.
313 * Declared volatile to enable double-checked lazy construction.
314 */
315 private volatile Slot[] arena = new Slot[CAPACITY];
316
317 /**
318 * The maximum slot index being used. The value sometimes
319 * increases when a thread experiences too many CAS contentions,
320 * and sometimes decreases when a spin-wait elapses. Changes
321 * are performed only via compareAndSet, to avoid stale values
322 * when a thread happens to stall right before setting.
323 */
324 private final AtomicInteger max = new AtomicInteger();
325
326 /**
327 * Main exchange function, handling the different policy variants.
328 * Uses Object, not "V" as argument and return value to simplify
329 * handling of sentinel values. Callers from public methods decode
330 * and cast accordingly.
331 *
332 * @param item the (non-null) item to exchange
333 * @param timed true if the wait is timed
334 * @param nanos if timed, the maximum wait time
335 * @return the other thread's item, or CANCEL if interrupted or timed out
336 */
337 private Object doExchange(Object item, boolean timed, long nanos) {
338 Node me = new Node(item); // Create in case occupying
339 int index = hashIndex(); // Index of current slot
340 int fails = 0; // Number of CAS failures
341
342 for (;;) {
343 Object y; // Contents of current slot
344 Slot slot = arena[index];
345 if (slot == null) // Lazily initialize slots
346 createSlot(index); // Continue loop to reread
347 else if ((y = slot.get()) != null && // Try to fulfill
348 slot.compareAndSet(y, null)) {
349 Node you = (Node)y; // Transfer item
350 if (you.compareAndSet(null, item)) {
351 LockSupport.unpark(you.waiter);
352 return you.item;
353 } // Else cancelled; continue
354 }
355 else if (y == null && // Try to occupy
356 slot.compareAndSet(null, me)) {
357 if (index == 0) // Blocking wait for slot 0
358 return timed ?
359 awaitNanos(me, slot, nanos) :
360 await(me, slot);
361 Object v = spinWait(me, slot); // Spin wait for non-0
362 if (v != CANCEL)
363 return v;
364 me = new Node(item); // Throw away cancelled node
365 int m = max.get();
366 if (m > (index >>>= 1)) // Decrease index
367 max.compareAndSet(m, m - 1); // Maybe shrink table
368 }
369 else if (++fails > 1) { // Allow 2 fails on 1st slot
370 int m = max.get();
371 if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1))
372 index = m + 1; // Grow on 3rd failed slot
373 else if (--index < 0)
374 index = m; // Circularly traverse
375 }
376 }
377 }
378
379 /**
380 * Returns a hash index for the current thread. Uses a one-step
381 * FNV-1a hash code (http://www.isthe.com/chongo/tech/comp/fnv/)
382 * based on the current thread's Thread.getId(). These hash codes
383 * have more uniform distribution properties with respect to small
384 * moduli (here 1-31) than do other simple hashing functions.
385 *
386 * <p>To return an index between 0 and max, we use a cheap
387 * approximation to a mod operation, that also corrects for bias
388 * due to non-power-of-2 remaindering (see {@link
389 * java.util.Random#nextInt}). Bits of the hashcode are masked
390 * with "nbits", the ceiling power of two of table size (looked up
391 * in a table packed into three ints). If too large, this is
392 * retried after rotating the hash by nbits bits, while forcing new
393 * top bit to 0, which guarantees eventual termination (although
394 * with a non-random-bias). This requires an average of less than
395 * 2 tries for all table sizes, and has a maximum 2% difference
396 * from perfectly uniform slot probabilities when applied to all
397 * possible hash codes for sizes less than 32.
398 *
399 * @return a per-thread-random index, 0 <= index < max
400 */
401 private final int hashIndex() {
402 long id = Thread.currentThread().getId();
403 int hash = (((int)(id ^ (id >>> 32))) ^ 0x811c9dc5) * 0x01000193;
404
405 int m = max.get();
406 int nbits = (((0xfffffc00 >> m) & 4) | // Compute ceil(log2(m+1))
407 ((0x000001f8 >>> m) & 2) | // The constants hold
408 ((0xffff00f2 >>> m) & 1)); // a lookup table
409 int index;
410 while ((index = hash & ((1 << nbits) - 1)) > m) // May retry on
411 hash = (hash >>> nbits) | (hash << (33 - nbits)); // non-power-2 m
412 return index;
413 }
414
415 /**
416 * Creates a new slot at given index. Called only when the slot
417 * appears to be null. Relies on double-check using builtin
418 * locks, since they rarely contend. This in turn relies on the
419 * arena array being declared volatile.
420 *
421 * @param index the index to add slot at
422 */
423 private void createSlot(int index) {
424 // Create slot outside of lock to narrow sync region
425 Slot newSlot = new Slot();
426 Slot[] a = arena;
427 synchronized (a) {
428 if (a[index] == null)
429 a[index] = newSlot;
430 }
431 }
432
433 /**
434 * Tries to cancel a wait for the given node waiting in the given
435 * slot, if so, helping clear the node from its slot to avoid
436 * garbage retention.
437 *
438 * @param node the waiting node
439 * @param the slot it is waiting in
440 * @return true if successfully cancelled
441 */
442 private static boolean tryCancel(Node node, Slot slot) {
443 if (!node.compareAndSet(null, CANCEL))
444 return false;
445 if (slot.get() == node) // pre-check to minimize contention
446 slot.compareAndSet(node, null);
447 return true;
448 }
449
450 // Three forms of waiting. Each just different enough not to merge
451 // code with others.
452
453 /**
454 * Spin-waits for hole for a non-0 slot. Fails if spin elapses
455 * before hole filled. Does not check interrupt, relying on check
456 * in public exchange method to abort if interrupted on entry.
457 *
458 * @param node the waiting node
459 * @return on success, the hole; on failure, CANCEL
460 */
461 private static Object spinWait(Node node, Slot slot) {
462 int spins = SPINS;
463 for (;;) {
464 Object v = node.get();
465 if (v != null)
466 return v;
467 else if (spins > 0)
468 --spins;
469 else
470 tryCancel(node, slot);
471 }
472 }
473
474 /**
475 * Waits for (by spinning and/or blocking) and gets the hole
476 * filled in by another thread. Fails if interrupted before
477 * hole filled.
478 *
479 * When a node/thread is about to block, it sets its waiter field
480 * and then rechecks state at least one more time before actually
481 * parking, thus covering race vs fulfiller noticing that waiter
482 * is non-null so should be woken.
483 *
484 * Thread interruption status is checked only surrounding calls to
485 * park. The caller is assumed to have checked interrupt status
486 * on entry.
487 *
488 * @param node the waiting node
489 * @return on success, the hole; on failure, CANCEL
490 */
491 private static Object await(Node node, Slot slot) {
492 Thread w = Thread.currentThread();
493 int spins = SPINS;
494 for (;;) {
495 Object v = node.get();
496 if (v != null)
497 return v;
498 else if (spins > 0) // Spin-wait phase
499 --spins;
500 else if (node.waiter == null) // Set up to block next
501 node.waiter = w;
502 else if (w.isInterrupted()) // Abort on interrupt
503 tryCancel(node, slot);
504 else // Block
505 LockSupport.park(node);
506 }
507 }
508
509 /**
510 * Waits for (at index 0) and gets the hole filled in by another
511 * thread. Fails if timed out or interrupted before hole filled.
512 * Same basic logic as untimed version, but a bit messier.
513 *
514 * @param node the waiting node
515 * @param nanos the wait time
516 * @return on success, the hole; on failure, CANCEL
517 */
518 private Object awaitNanos(Node node, Slot slot, long nanos) {
519 int spins = TIMED_SPINS;
520 long lastTime = 0;
521 Thread w = null;
522 for (;;) {
523 Object v = node.get();
524 if (v != null)
525 return v;
526 long now = System.nanoTime();
527 if (w == null)
528 w = Thread.currentThread();
529 else
530 nanos -= now - lastTime;
531 lastTime = now;
532 if (nanos > 0) {
533 if (spins > 0)
534 --spins;
535 else if (node.waiter == null)
536 node.waiter = w;
537 else if (w.isInterrupted())
538 tryCancel(node, slot);
539 else
540 LockSupport.parkNanos(node, nanos);
541 }
542 else if (tryCancel(node, slot) && !w.isInterrupted())
543 return scanOnTimeout(node);
544 }
545 }
546
547 /**
548 * Sweeps through arena checking for any waiting threads. Called
549 * only upon return from timeout while waiting in slot 0. When a
550 * thread gives up on a timed wait, it is possible that a
551 * previously-entered thread is still waiting in some other
552 * slot. So we scan to check for any. This is almost always
553 * overkill, but decreases the likelihood of timeouts when there
554 * are other threads present to far less than that in lock-based
555 * exchangers in which earlier-arriving threads may still be
556 * waiting on entry locks.
557 *
558 * @param node the waiting node
559 * @return another thread's item, or CANCEL
560 */
561 private Object scanOnTimeout(Node node) {
562 Object y;
563 for (int j = arena.length - 1; j >= 0; --j) {
564 Slot slot = arena[j];
565 if (slot != null) {
566 while ((y = slot.get()) != null) {
567 if (slot.compareAndSet(y, null)) {
568 Node you = (Node)y;
569 if (you.compareAndSet(null, node.item)) {
570 LockSupport.unpark(you.waiter);
571 return you.item;
572 }
573 }
574 }
575 }
576 }
577 return CANCEL;
578 }
579
580 /**
581 * Creates a new Exchanger.
582 */
583 public Exchanger() {
584 }
585
586 /**
587 * Waits for another thread to arrive at this exchange point (unless
588 * the current thread is {@linkplain Thread#interrupt interrupted}),
589 * and then transfers the given object to it, receiving its object
590 * in return.
591 *
592 * <p>If another thread is already waiting at the exchange point then
593 * it is resumed for thread scheduling purposes and receives the object
594 * passed in by the current thread. The current thread returns immediately,
595 * receiving the object passed to the exchange by that other thread.
596 *
597 * <p>If no other thread is already waiting at the exchange then the
598 * current thread is disabled for thread scheduling purposes and lies
599 * dormant until one of two things happens:
600 * <ul>
601 * <li>Some other thread enters the exchange; or
602 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
603 * the current thread.
604 * </ul>
605 * <p>If the current thread:
606 * <ul>
607 * <li>has its interrupted status set on entry to this method; or
608 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
609 * for the exchange,
610 * </ul>
611 * then {@link InterruptedException} is thrown and the current thread's
612 * interrupted status is cleared.
613 *
614 * @param x the object to exchange
615 * @return the object provided by the other thread
616 * @throws InterruptedException if the current thread was
617 * interrupted while waiting
618 */
619 public V exchange(V x) throws InterruptedException {
620 if (!Thread.interrupted()) {
621 Object v = doExchange((x == null) ? NULL_ITEM : x, false, 0);
622 if (v == NULL_ITEM)
623 return null;
624 if (v != CANCEL)
625 return (V)v;
626 Thread.interrupted(); // Clear interrupt status on IE throw
627 }
628 throw new InterruptedException();
629 }
630
631 /**
632 * Waits for another thread to arrive at this exchange point (unless
633 * the current thread is {@linkplain Thread#interrupt interrupted} or
634 * the specified waiting time elapses), and then transfers the given
635 * object to it, receiving its object in return.
636 *
637 * <p>If another thread is already waiting at the exchange point then
638 * it is resumed for thread scheduling purposes and receives the object
639 * passed in by the current thread. The current thread returns immediately,
640 * receiving the object passed to the exchange by that other thread.
641 *
642 * <p>If no other thread is already waiting at the exchange then the
643 * current thread is disabled for thread scheduling purposes and lies
644 * dormant until one of three things happens:
645 * <ul>
646 * <li>Some other thread enters the exchange; or
647 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
648 * the current thread; or
649 * <li>The specified waiting time elapses.
650 * </ul>
651 * <p>If the current thread:
652 * <ul>
653 * <li>has its interrupted status set on entry to this method; or
654 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
655 * for the exchange,
656 * </ul>
657 * then {@link InterruptedException} is thrown and the current thread's
658 * interrupted status is cleared.
659 *
660 * <p>If the specified waiting time elapses then {@link
661 * TimeoutException} is thrown. If the time is less than or equal
662 * to zero, the method will not wait at all.
663 *
664 * @param x the object to exchange
665 * @param timeout the maximum time to wait
666 * @param unit the time unit of the <tt>timeout</tt> argument
667 * @return the object provided by the other thread
668 * @throws InterruptedException if the current thread was
669 * interrupted while waiting
670 * @throws TimeoutException if the specified waiting time elapses
671 * before another thread enters the exchange
672 */
673 public V exchange(V x, long timeout, TimeUnit unit)
674 throws InterruptedException, TimeoutException {
675 if (!Thread.interrupted()) {
676 Object v = doExchange((x == null) ? NULL_ITEM : x,
677 true, unit.toNanos(timeout));
678 if (v == NULL_ITEM)
679 return null;
680 if (v != CANCEL)
681 return (V)v;
682 if (!Thread.interrupted())
683 throw new TimeoutException();
684 }
685 throw new InterruptedException();
686 }
687 }