Source for java.util.concurrent.Exchanger

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