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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: }
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