ReentrantReadWriteLock.java revision cec4dd4b1d33f78997603d0f89c0d0e56e64dbcd
1/* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/licenses/publicdomain 5 */ 6 7package java.util.concurrent.locks; 8import java.util.concurrent.*; 9import java.util.concurrent.atomic.*; 10import java.util.*; 11 12/** 13 * An implementation of {@link ReadWriteLock} supporting similar 14 * semantics to {@link ReentrantLock}. 15 * <p>This class has the following properties: 16 * 17 * <ul> 18 * <li><b>Acquisition order</b> 19 * 20 * <p> This class does not impose a reader or writer preference 21 * ordering for lock access. However, it does support an optional 22 * <em>fairness</em> policy. 23 * 24 * <dl> 25 * <dt><b><i>Non-fair mode (default)</i></b> 26 * <dd>When constructed as non-fair (the default), the order of entry 27 * to the read and write lock is unspecified, subject to reentrancy 28 * constraints. A nonfair lock that is continuously contended may 29 * indefinitely postpone one or more reader or writer threads, but 30 * will normally have higher throughput than a fair lock. 31 * <p> 32 * 33 * <dt><b><i>Fair mode</i></b> 34 * <dd> When constructed as fair, threads contend for entry using an 35 * approximately arrival-order policy. When the currently held lock 36 * is released either the longest-waiting single writer thread will 37 * be assigned the write lock, or if there is a group of reader threads 38 * waiting longer than all waiting writer threads, that group will be 39 * assigned the read lock. 40 * 41 * <p>A thread that tries to acquire a fair read lock (non-reentrantly) 42 * will block if either the write lock is held, or there is a waiting 43 * writer thread. The thread will not acquire the read lock until 44 * after the oldest currently waiting writer thread has acquired and 45 * released the write lock. Of course, if a waiting writer abandons 46 * its wait, leaving one or more reader threads as the longest waiters 47 * in the queue with the write lock free, then those readers will be 48 * assigned the read lock. 49 * 50 * <p>A thread that tries to acquire a fair write lock (non-reentrantly) 51 * will block unless both the read lock and write lock are free (which 52 * implies there are no waiting threads). (Note that the non-blocking 53 * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods 54 * do not honor this fair setting and will acquire the lock if it is 55 * possible, regardless of waiting threads.) 56 * <p> 57 * </dl> 58 * 59 * <li><b>Reentrancy</b> 60 * 61 * <p>This lock allows both readers and writers to reacquire read or 62 * write locks in the style of a {@link ReentrantLock}. Non-reentrant 63 * readers are not allowed until all write locks held by the writing 64 * thread have been released. 65 * 66 * <p>Additionally, a writer can acquire the read lock, but not 67 * vice-versa. Among other applications, reentrancy can be useful 68 * when write locks are held during calls or callbacks to methods that 69 * perform reads under read locks. If a reader tries to acquire the 70 * write lock it will never succeed. 71 * 72 * <li><b>Lock downgrading</b> 73 * <p>Reentrancy also allows downgrading from the write lock to a read lock, 74 * by acquiring the write lock, then the read lock and then releasing the 75 * write lock. However, upgrading from a read lock to the write lock is 76 * <b>not</b> possible. 77 * 78 * <li><b>Interruption of lock acquisition</b> 79 * <p>The read lock and write lock both support interruption during lock 80 * acquisition. 81 * 82 * <li><b>{@link Condition} support</b> 83 * <p>The write lock provides a {@link Condition} implementation that 84 * behaves in the same way, with respect to the write lock, as the 85 * {@link Condition} implementation provided by 86 * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}. 87 * This {@link Condition} can, of course, only be used with the write lock. 88 * 89 * <p>The read lock does not support a {@link Condition} and 90 * {@code readLock().newCondition()} throws 91 * {@code UnsupportedOperationException}. 92 * 93 * <li><b>Instrumentation</b> 94 * <p>This class supports methods to determine whether locks 95 * are held or contended. These methods are designed for monitoring 96 * system state, not for synchronization control. 97 * </ul> 98 * 99 * <p>Serialization of this class behaves in the same way as built-in 100 * locks: a deserialized lock is in the unlocked state, regardless of 101 * its state when serialized. 102 * 103 * <p><b>Sample usages</b>. Here is a code sketch showing how to perform 104 * lock downgrading after updating a cache (exception handling is 105 * particularly tricky when handling multiple locks in a non-nested 106 * fashion): 107 * 108 * <pre> {@code 109 * class CachedData { 110 * Object data; 111 * volatile boolean cacheValid; 112 * final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); 113 * 114 * void processCachedData() { 115 * rwl.readLock().lock(); 116 * if (!cacheValid) { 117 * // Must release read lock before acquiring write lock 118 * rwl.readLock().unlock(); 119 * rwl.writeLock().lock(); 120 * try { 121 * // Recheck state because another thread might have 122 * // acquired write lock and changed state before we did. 123 * if (!cacheValid) { 124 * data = ... 125 * cacheValid = true; 126 * } 127 * // Downgrade by acquiring read lock before releasing write lock 128 * rwl.readLock().lock(); 129 * } finally { 130 * rwl.writeLock().unlock(); // Unlock write, still hold read 131 * } 132 * } 133 * 134 * try { 135 * use(data); 136 * } finally { 137 * rwl.readLock().unlock(); 138 * } 139 * } 140 * }}</pre> 141 * 142 * ReentrantReadWriteLocks can be used to improve concurrency in some 143 * uses of some kinds of Collections. This is typically worthwhile 144 * only when the collections are expected to be large, accessed by 145 * more reader threads than writer threads, and entail operations with 146 * overhead that outweighs synchronization overhead. For example, here 147 * is a class using a TreeMap that is expected to be large and 148 * concurrently accessed. 149 * 150 * <pre>{@code 151 * class RWDictionary { 152 * private final Map<String, Data> m = new TreeMap<String, Data>(); 153 * private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); 154 * private final Lock r = rwl.readLock(); 155 * private final Lock w = rwl.writeLock(); 156 * 157 * public Data get(String key) { 158 * r.lock(); 159 * try { return m.get(key); } 160 * finally { r.unlock(); } 161 * } 162 * public String[] allKeys() { 163 * r.lock(); 164 * try { return m.keySet().toArray(); } 165 * finally { r.unlock(); } 166 * } 167 * public Data put(String key, Data value) { 168 * w.lock(); 169 * try { return m.put(key, value); } 170 * finally { w.unlock(); } 171 * } 172 * public void clear() { 173 * w.lock(); 174 * try { m.clear(); } 175 * finally { w.unlock(); } 176 * } 177 * }}</pre> 178 * 179 * <h3>Implementation Notes</h3> 180 * 181 * <p>This lock supports a maximum of 65535 recursive write locks 182 * and 65535 read locks. Attempts to exceed these limits result in 183 * {@link Error} throws from locking methods. 184 * 185 * @since 1.5 186 * @author Doug Lea 187 * 188 */ 189public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable { 190 private static final long serialVersionUID = -6992448646407690164L; 191 /** Inner class providing readlock */ 192 private final ReentrantReadWriteLock.ReadLock readerLock; 193 /** Inner class providing writelock */ 194 private final ReentrantReadWriteLock.WriteLock writerLock; 195 /** Performs all synchronization mechanics */ 196 final Sync sync; 197 198 /** 199 * Creates a new {@code ReentrantReadWriteLock} with 200 * default (nonfair) ordering properties. 201 */ 202 public ReentrantReadWriteLock() { 203 this(false); 204 } 205 206 /** 207 * Creates a new {@code ReentrantReadWriteLock} with 208 * the given fairness policy. 209 * 210 * @param fair {@code true} if this lock should use a fair ordering policy 211 */ 212 public ReentrantReadWriteLock(boolean fair) { 213 sync = fair ? new FairSync() : new NonfairSync(); 214 readerLock = new ReadLock(this); 215 writerLock = new WriteLock(this); 216 } 217 218 public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; } 219 public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; } 220 221 /** 222 * Synchronization implementation for ReentrantReadWriteLock. 223 * Subclassed into fair and nonfair versions. 224 */ 225 static abstract class Sync extends AbstractQueuedSynchronizer { 226 private static final long serialVersionUID = 6317671515068378041L; 227 228 /* 229 * Read vs write count extraction constants and functions. 230 * Lock state is logically divided into two unsigned shorts: 231 * The lower one representing the exclusive (writer) lock hold count, 232 * and the upper the shared (reader) hold count. 233 */ 234 235 static final int SHARED_SHIFT = 16; 236 static final int SHARED_UNIT = (1 << SHARED_SHIFT); 237 static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1; 238 static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1; 239 240 /** Returns the number of shared holds represented in count */ 241 static int sharedCount(int c) { return c >>> SHARED_SHIFT; } 242 /** Returns the number of exclusive holds represented in count */ 243 static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; } 244 245 /** 246 * A counter for per-thread read hold counts. 247 * Maintained as a ThreadLocal; cached in cachedHoldCounter 248 */ 249 static final class HoldCounter { 250 int count = 0; 251 // Use id, not reference, to avoid garbage retention 252 final long tid = Thread.currentThread().getId(); 253 } 254 255 /** 256 * ThreadLocal subclass. Easiest to explicitly define for sake 257 * of deserialization mechanics. 258 */ 259 static final class ThreadLocalHoldCounter 260 extends ThreadLocal<HoldCounter> { 261 public HoldCounter initialValue() { 262 return new HoldCounter(); 263 } 264 } 265 266 /** 267 * The number of reentrant read locks held by current thread. 268 * Initialized only in constructor and readObject. 269 * Removed whenever a thread's read hold count drops to 0. 270 */ 271 private transient ThreadLocalHoldCounter readHolds; 272 273 /** 274 * The hold count of the last thread to successfully acquire 275 * readLock. This saves ThreadLocal lookup in the common case 276 * where the next thread to release is the last one to 277 * acquire. This is non-volatile since it is just used 278 * as a heuristic, and would be great for threads to cache. 279 * 280 * <p>Can outlive the Thread for which it is caching the read 281 * hold count, but avoids garbage retention by not retaining a 282 * reference to the Thread. 283 * 284 * <p>Accessed via a benign data race; relies on the memory 285 * model's final field and out-of-thin-air guarantees. 286 */ 287 private transient HoldCounter cachedHoldCounter; 288 289 /** 290 * firstReader is the first thread to have acquired the read lock. 291 * firstReaderHoldCount is firstReader's hold count. 292 * 293 * <p>More precisely, firstReader is the unique thread that last 294 * changed the shared count from 0 to 1, and has not released the 295 * read lock since then; null if there is no such thread. 296 * 297 * <p>Cannot cause garbage retention unless the thread terminated 298 * without relinquishing its read locks, since tryReleaseShared 299 * sets it to null. 300 * 301 * <p>Accessed via a benign data race; relies on the memory 302 * model's out-of-thin-air guarantees for references. 303 * 304 * <p>This allows tracking of read holds for uncontended read 305 * locks to be very cheap. 306 */ 307 private transient Thread firstReader = null; 308 private transient int firstReaderHoldCount; 309 310 Sync() { 311 readHolds = new ThreadLocalHoldCounter(); 312 setState(getState()); // ensures visibility of readHolds 313 } 314 315 /* 316 * Acquires and releases use the same code for fair and 317 * nonfair locks, but differ in whether/how they allow barging 318 * when queues are non-empty. 319 */ 320 321 /** 322 * Returns true if the current thread, when trying to acquire 323 * the read lock, and otherwise eligible to do so, should block 324 * because of policy for overtaking other waiting threads. 325 */ 326 abstract boolean readerShouldBlock(); 327 328 /** 329 * Returns true if the current thread, when trying to acquire 330 * the write lock, and otherwise eligible to do so, should block 331 * because of policy for overtaking other waiting threads. 332 */ 333 abstract boolean writerShouldBlock(); 334 335 /* 336 * Note that tryRelease and tryAcquire can be called by 337 * Conditions. So it is possible that their arguments contain 338 * both read and write holds that are all released during a 339 * condition wait and re-established in tryAcquire. 340 */ 341 342 protected final boolean tryRelease(int releases) { 343 if (!isHeldExclusively()) 344 throw new IllegalMonitorStateException(); 345 int nextc = getState() - releases; 346 boolean free = exclusiveCount(nextc) == 0; 347 if (free) 348 setExclusiveOwnerThread(null); 349 setState(nextc); 350 return free; 351 } 352 353 protected final boolean tryAcquire(int acquires) { 354 /* 355 * Walkthrough: 356 * 1. If read count nonzero or write count nonzero 357 * and owner is a different thread, fail. 358 * 2. If count would saturate, fail. (This can only 359 * happen if count is already nonzero.) 360 * 3. Otherwise, this thread is eligible for lock if 361 * it is either a reentrant acquire or 362 * queue policy allows it. If so, update state 363 * and set owner. 364 */ 365 Thread current = Thread.currentThread(); 366 int c = getState(); 367 int w = exclusiveCount(c); 368 if (c != 0) { 369 // (Note: if c != 0 and w == 0 then shared count != 0) 370 if (w == 0 || current != getExclusiveOwnerThread()) 371 return false; 372 if (w + exclusiveCount(acquires) > MAX_COUNT) 373 throw new Error("Maximum lock count exceeded"); 374 // Reentrant acquire 375 setState(c + acquires); 376 return true; 377 } 378 if (writerShouldBlock() || 379 !compareAndSetState(c, c + acquires)) 380 return false; 381 setExclusiveOwnerThread(current); 382 return true; 383 } 384 385 protected final boolean tryReleaseShared(int unused) { 386 Thread current = Thread.currentThread(); 387 if (firstReader == current) { 388 // assert firstReaderHoldCount > 0; 389 if (firstReaderHoldCount == 1) 390 firstReader = null; 391 else 392 firstReaderHoldCount--; 393 } else { 394 HoldCounter rh = cachedHoldCounter; 395 if (rh == null || rh.tid != current.getId()) 396 rh = readHolds.get(); 397 int count = rh.count; 398 if (count <= 1) { 399 readHolds.remove(); 400 if (count <= 0) 401 throw unmatchedUnlockException(); 402 } 403 --rh.count; 404 } 405 for (;;) { 406 int c = getState(); 407 int nextc = c - SHARED_UNIT; 408 if (compareAndSetState(c, nextc)) 409 // Releasing the read lock has no effect on readers, 410 // but it may allow waiting writers to proceed if 411 // both read and write locks are now free. 412 return nextc == 0; 413 } 414 } 415 416 private IllegalMonitorStateException unmatchedUnlockException() { 417 return new IllegalMonitorStateException( 418 "attempt to unlock read lock, not locked by current thread"); 419 } 420 421 protected final int tryAcquireShared(int unused) { 422 /* 423 * Walkthrough: 424 * 1. If write lock held by another thread, fail. 425 * 2. Otherwise, this thread is eligible for 426 * lock wrt state, so ask if it should block 427 * because of queue policy. If not, try 428 * to grant by CASing state and updating count. 429 * Note that step does not check for reentrant 430 * acquires, which is postponed to full version 431 * to avoid having to check hold count in 432 * the more typical non-reentrant case. 433 * 3. If step 2 fails either because thread 434 * apparently not eligible or CAS fails or count 435 * saturated, chain to version with full retry loop. 436 */ 437 Thread current = Thread.currentThread(); 438 int c = getState(); 439 if (exclusiveCount(c) != 0 && 440 getExclusiveOwnerThread() != current) 441 return -1; 442 int r = sharedCount(c); 443 if (!readerShouldBlock() && 444 r < MAX_COUNT && 445 compareAndSetState(c, c + SHARED_UNIT)) { 446 if (r == 0) { 447 firstReader = current; 448 firstReaderHoldCount = 1; 449 } else if (firstReader == current) { 450 firstReaderHoldCount++; 451 } else { 452 HoldCounter rh = cachedHoldCounter; 453 if (rh == null || rh.tid != current.getId()) 454 cachedHoldCounter = rh = readHolds.get(); 455 else if (rh.count == 0) 456 readHolds.set(rh); 457 rh.count++; 458 } 459 return 1; 460 } 461 return fullTryAcquireShared(current); 462 } 463 464 /** 465 * Full version of acquire for reads, that handles CAS misses 466 * and reentrant reads not dealt with in tryAcquireShared. 467 */ 468 final int fullTryAcquireShared(Thread current) { 469 /* 470 * This code is in part redundant with that in 471 * tryAcquireShared but is simpler overall by not 472 * complicating tryAcquireShared with interactions between 473 * retries and lazily reading hold counts. 474 */ 475 HoldCounter rh = null; 476 for (;;) { 477 int c = getState(); 478 if (exclusiveCount(c) != 0) { 479 if (getExclusiveOwnerThread() != current) 480 return -1; 481 // else we hold the exclusive lock; blocking here 482 // would cause deadlock. 483 } else if (readerShouldBlock()) { 484 // Make sure we're not acquiring read lock reentrantly 485 if (firstReader == current) { 486 // assert firstReaderHoldCount > 0; 487 } else { 488 if (rh == null) { 489 rh = cachedHoldCounter; 490 if (rh == null || rh.tid != current.getId()) { 491 rh = readHolds.get(); 492 if (rh.count == 0) 493 readHolds.remove(); 494 } 495 } 496 if (rh.count == 0) 497 return -1; 498 } 499 } 500 if (sharedCount(c) == MAX_COUNT) 501 throw new Error("Maximum lock count exceeded"); 502 if (compareAndSetState(c, c + SHARED_UNIT)) { 503 if (sharedCount(c) == 0) { 504 firstReader = current; 505 firstReaderHoldCount = 1; 506 } else if (firstReader == current) { 507 firstReaderHoldCount++; 508 } else { 509 if (rh == null) 510 rh = cachedHoldCounter; 511 if (rh == null || rh.tid != current.getId()) 512 rh = readHolds.get(); 513 else if (rh.count == 0) 514 readHolds.set(rh); 515 rh.count++; 516 cachedHoldCounter = rh; // cache for release 517 } 518 return 1; 519 } 520 } 521 } 522 523 /** 524 * Performs tryLock for write, enabling barging in both modes. 525 * This is identical in effect to tryAcquire except for lack 526 * of calls to writerShouldBlock. 527 */ 528 final boolean tryWriteLock() { 529 Thread current = Thread.currentThread(); 530 int c = getState(); 531 if (c != 0) { 532 int w = exclusiveCount(c); 533 if (w == 0 || current != getExclusiveOwnerThread()) 534 return false; 535 if (w == MAX_COUNT) 536 throw new Error("Maximum lock count exceeded"); 537 } 538 if (!compareAndSetState(c, c + 1)) 539 return false; 540 setExclusiveOwnerThread(current); 541 return true; 542 } 543 544 /** 545 * Performs tryLock for read, enabling barging in both modes. 546 * This is identical in effect to tryAcquireShared except for 547 * lack of calls to readerShouldBlock. 548 */ 549 final boolean tryReadLock() { 550 Thread current = Thread.currentThread(); 551 for (;;) { 552 int c = getState(); 553 if (exclusiveCount(c) != 0 && 554 getExclusiveOwnerThread() != current) 555 return false; 556 int r = sharedCount(c); 557 if (r == MAX_COUNT) 558 throw new Error("Maximum lock count exceeded"); 559 if (compareAndSetState(c, c + SHARED_UNIT)) { 560 if (r == 0) { 561 firstReader = current; 562 firstReaderHoldCount = 1; 563 } else if (firstReader == current) { 564 firstReaderHoldCount++; 565 } else { 566 HoldCounter rh = cachedHoldCounter; 567 if (rh == null || rh.tid != current.getId()) 568 cachedHoldCounter = rh = readHolds.get(); 569 else if (rh.count == 0) 570 readHolds.set(rh); 571 rh.count++; 572 } 573 return true; 574 } 575 } 576 } 577 578 protected final boolean isHeldExclusively() { 579 // While we must in general read state before owner, 580 // we don't need to do so to check if current thread is owner 581 return getExclusiveOwnerThread() == Thread.currentThread(); 582 } 583 584 // Methods relayed to outer class 585 586 final ConditionObject newCondition() { 587 return new ConditionObject(); 588 } 589 590 final Thread getOwner() { 591 // Must read state before owner to ensure memory consistency 592 return ((exclusiveCount(getState()) == 0)? 593 null : 594 getExclusiveOwnerThread()); 595 } 596 597 final int getReadLockCount() { 598 return sharedCount(getState()); 599 } 600 601 final boolean isWriteLocked() { 602 return exclusiveCount(getState()) != 0; 603 } 604 605 final int getWriteHoldCount() { 606 return isHeldExclusively() ? exclusiveCount(getState()) : 0; 607 } 608 609 final int getReadHoldCount() { 610 if (getReadLockCount() == 0) 611 return 0; 612 613 Thread current = Thread.currentThread(); 614 if (firstReader == current) 615 return firstReaderHoldCount; 616 617 HoldCounter rh = cachedHoldCounter; 618 if (rh != null && rh.tid == current.getId()) 619 return rh.count; 620 621 int count = readHolds.get().count; 622 if (count == 0) readHolds.remove(); 623 return count; 624 } 625 626 /** 627 * Reconstitute this lock instance from a stream 628 * @param s the stream 629 */ 630 private void readObject(java.io.ObjectInputStream s) 631 throws java.io.IOException, ClassNotFoundException { 632 s.defaultReadObject(); 633 readHolds = new ThreadLocalHoldCounter(); 634 setState(0); // reset to unlocked state 635 } 636 637 final int getCount() { return getState(); } 638 } 639 640 /** 641 * Nonfair version of Sync 642 */ 643 final static class NonfairSync extends Sync { 644 private static final long serialVersionUID = -8159625535654395037L; 645 final boolean writerShouldBlock() { 646 return false; // writers can always barge 647 } 648 final boolean readerShouldBlock() { 649 /* As a heuristic to avoid indefinite writer starvation, 650 * block if the thread that momentarily appears to be head 651 * of queue, if one exists, is a waiting writer. This is 652 * only a probabilistic effect since a new reader will not 653 * block if there is a waiting writer behind other enabled 654 * readers that have not yet drained from the queue. 655 */ 656 return apparentlyFirstQueuedIsExclusive(); 657 } 658 } 659 660 /** 661 * Fair version of Sync 662 */ 663 final static class FairSync extends Sync { 664 private static final long serialVersionUID = -2274990926593161451L; 665 final boolean writerShouldBlock() { 666 return hasQueuedPredecessors(); 667 } 668 final boolean readerShouldBlock() { 669 return hasQueuedPredecessors(); 670 } 671 } 672 673 /** 674 * The lock returned by method {@link ReentrantReadWriteLock#readLock}. 675 */ 676 public static class ReadLock implements Lock, java.io.Serializable { 677 private static final long serialVersionUID = -5992448646407690164L; 678 private final Sync sync; 679 680 /** 681 * Constructor for use by subclasses 682 * 683 * @param lock the outer lock object 684 * @throws NullPointerException if the lock is null 685 */ 686 protected ReadLock(ReentrantReadWriteLock lock) { 687 sync = lock.sync; 688 } 689 690 /** 691 * Acquires the read lock. 692 * 693 * <p>Acquires the read lock if the write lock is not held by 694 * another thread and returns immediately. 695 * 696 * <p>If the write lock is held by another thread then 697 * the current thread becomes disabled for thread scheduling 698 * purposes and lies dormant until the read lock has been acquired. 699 */ 700 public void lock() { 701 sync.acquireShared(1); 702 } 703 704 /** 705 * Acquires the read lock unless the current thread is 706 * {@linkplain Thread#interrupt interrupted}. 707 * 708 * <p>Acquires the read lock if the write lock is not held 709 * by another thread and returns immediately. 710 * 711 * <p>If the write lock is held by another thread then the 712 * current thread becomes disabled for thread scheduling 713 * purposes and lies dormant until one of two things happens: 714 * 715 * <ul> 716 * 717 * <li>The read lock is acquired by the current thread; or 718 * 719 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 720 * the current thread. 721 * 722 * </ul> 723 * 724 * <p>If the current thread: 725 * 726 * <ul> 727 * 728 * <li>has its interrupted status set on entry to this method; or 729 * 730 * <li>is {@linkplain Thread#interrupt interrupted} while 731 * acquiring the read lock, 732 * 733 * </ul> 734 * 735 * then {@link InterruptedException} is thrown and the current 736 * thread's interrupted status is cleared. 737 * 738 * <p>In this implementation, as this method is an explicit 739 * interruption point, preference is given to responding to 740 * the interrupt over normal or reentrant acquisition of the 741 * lock. 742 * 743 * @throws InterruptedException if the current thread is interrupted 744 */ 745 public void lockInterruptibly() throws InterruptedException { 746 sync.acquireSharedInterruptibly(1); 747 } 748 749 /** 750 * Acquires the read lock only if the write lock is not held by 751 * another thread at the time of invocation. 752 * 753 * <p>Acquires the read lock if the write lock is not held by 754 * another thread and returns immediately with the value 755 * {@code true}. Even when this lock has been set to use a 756 * fair ordering policy, a call to {@code tryLock()} 757 * <em>will</em> immediately acquire the read lock if it is 758 * available, whether or not other threads are currently 759 * waiting for the read lock. This "barging" behavior 760 * can be useful in certain circumstances, even though it 761 * breaks fairness. If you want to honor the fairness setting 762 * for this lock, then use {@link #tryLock(long, TimeUnit) 763 * tryLock(0, TimeUnit.SECONDS) } which is almost equivalent 764 * (it also detects interruption). 765 * 766 * <p>If the write lock is held by another thread then 767 * this method will return immediately with the value 768 * {@code false}. 769 * 770 * @return {@code true} if the read lock was acquired 771 */ 772 public boolean tryLock() { 773 return sync.tryReadLock(); 774 } 775 776 /** 777 * Acquires the read lock if the write lock is not held by 778 * another thread within the given waiting time and the 779 * current thread has not been {@linkplain Thread#interrupt 780 * interrupted}. 781 * 782 * <p>Acquires the read lock if the write lock is not held by 783 * another thread and returns immediately with the value 784 * {@code true}. If this lock has been set to use a fair 785 * ordering policy then an available lock <em>will not</em> be 786 * acquired if any other threads are waiting for the 787 * lock. This is in contrast to the {@link #tryLock()} 788 * method. If you want a timed {@code tryLock} that does 789 * permit barging on a fair lock then combine the timed and 790 * un-timed forms together: 791 * 792 * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } 793 * </pre> 794 * 795 * <p>If the write lock is held by another thread then the 796 * current thread becomes disabled for thread scheduling 797 * purposes and lies dormant until one of three things happens: 798 * 799 * <ul> 800 * 801 * <li>The read lock is acquired by the current thread; or 802 * 803 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 804 * the current thread; or 805 * 806 * <li>The specified waiting time elapses. 807 * 808 * </ul> 809 * 810 * <p>If the read lock is acquired then the value {@code true} is 811 * returned. 812 * 813 * <p>If the current thread: 814 * 815 * <ul> 816 * 817 * <li>has its interrupted status set on entry to this method; or 818 * 819 * <li>is {@linkplain Thread#interrupt interrupted} while 820 * acquiring the read lock, 821 * 822 * </ul> then {@link InterruptedException} is thrown and the 823 * current thread's interrupted status is cleared. 824 * 825 * <p>If the specified waiting time elapses then the value 826 * {@code false} is returned. If the time is less than or 827 * equal to zero, the method will not wait at all. 828 * 829 * <p>In this implementation, as this method is an explicit 830 * interruption point, preference is given to responding to 831 * the interrupt over normal or reentrant acquisition of the 832 * lock, and over reporting the elapse of the waiting time. 833 * 834 * @param timeout the time to wait for the read lock 835 * @param unit the time unit of the timeout argument 836 * @return {@code true} if the read lock was acquired 837 * @throws InterruptedException if the current thread is interrupted 838 * @throws NullPointerException if the time unit is null 839 * 840 */ 841 public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { 842 return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); 843 } 844 845 /** 846 * Attempts to release this lock. 847 * 848 * <p> If the number of readers is now zero then the lock 849 * is made available for write lock attempts. 850 */ 851 public void unlock() { 852 sync.releaseShared(1); 853 } 854 855 /** 856 * Throws {@code UnsupportedOperationException} because 857 * {@code ReadLocks} do not support conditions. 858 * 859 * @throws UnsupportedOperationException always 860 */ 861 public Condition newCondition() { 862 throw new UnsupportedOperationException(); 863 } 864 865 /** 866 * Returns a string identifying this lock, as well as its lock state. 867 * The state, in brackets, includes the String {@code "Read locks ="} 868 * followed by the number of held read locks. 869 * 870 * @return a string identifying this lock, as well as its lock state 871 */ 872 public String toString() { 873 int r = sync.getReadLockCount(); 874 return super.toString() + 875 "[Read locks = " + r + "]"; 876 } 877 } 878 879 /** 880 * The lock returned by method {@link ReentrantReadWriteLock#writeLock}. 881 */ 882 public static class WriteLock implements Lock, java.io.Serializable { 883 private static final long serialVersionUID = -4992448646407690164L; 884 private final Sync sync; 885 886 /** 887 * Constructor for use by subclasses 888 * 889 * @param lock the outer lock object 890 * @throws NullPointerException if the lock is null 891 */ 892 protected WriteLock(ReentrantReadWriteLock lock) { 893 sync = lock.sync; 894 } 895 896 /** 897 * Acquires the write lock. 898 * 899 * <p>Acquires the write lock if neither the read nor write lock 900 * are held by another thread 901 * and returns immediately, setting the write lock hold count to 902 * one. 903 * 904 * <p>If the current thread already holds the write lock then the 905 * hold count is incremented by one and the method returns 906 * immediately. 907 * 908 * <p>If the lock is held by another thread then the current 909 * thread becomes disabled for thread scheduling purposes and 910 * lies dormant until the write lock has been acquired, at which 911 * time the write lock hold count is set to one. 912 */ 913 public void lock() { 914 sync.acquire(1); 915 } 916 917 /** 918 * Acquires the write lock unless the current thread is 919 * {@linkplain Thread#interrupt interrupted}. 920 * 921 * <p>Acquires the write lock if neither the read nor write lock 922 * are held by another thread 923 * and returns immediately, setting the write lock hold count to 924 * one. 925 * 926 * <p>If the current thread already holds this lock then the 927 * hold count is incremented by one and the method returns 928 * immediately. 929 * 930 * <p>If the lock is held by another thread then the current 931 * thread becomes disabled for thread scheduling purposes and 932 * lies dormant until one of two things happens: 933 * 934 * <ul> 935 * 936 * <li>The write lock is acquired by the current thread; or 937 * 938 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 939 * the current thread. 940 * 941 * </ul> 942 * 943 * <p>If the write lock is acquired by the current thread then the 944 * lock hold count is set to one. 945 * 946 * <p>If the current thread: 947 * 948 * <ul> 949 * 950 * <li>has its interrupted status set on entry to this method; 951 * or 952 * 953 * <li>is {@linkplain Thread#interrupt interrupted} while 954 * acquiring the write lock, 955 * 956 * </ul> 957 * 958 * then {@link InterruptedException} is thrown and the current 959 * thread's interrupted status is cleared. 960 * 961 * <p>In this implementation, as this method is an explicit 962 * interruption point, preference is given to responding to 963 * the interrupt over normal or reentrant acquisition of the 964 * lock. 965 * 966 * @throws InterruptedException if the current thread is interrupted 967 */ 968 public void lockInterruptibly() throws InterruptedException { 969 sync.acquireInterruptibly(1); 970 } 971 972 /** 973 * Acquires the write lock only if it is not held by another thread 974 * at the time of invocation. 975 * 976 * <p>Acquires the write lock if neither the read nor write lock 977 * are held by another thread 978 * and returns immediately with the value {@code true}, 979 * setting the write lock hold count to one. Even when this lock has 980 * been set to use a fair ordering policy, a call to 981 * {@code tryLock()} <em>will</em> immediately acquire the 982 * lock if it is available, whether or not other threads are 983 * currently waiting for the write lock. This "barging" 984 * behavior can be useful in certain circumstances, even 985 * though it breaks fairness. If you want to honor the 986 * fairness setting for this lock, then use {@link 987 * #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) } 988 * which is almost equivalent (it also detects interruption). 989 * 990 * <p> If the current thread already holds this lock then the 991 * hold count is incremented by one and the method returns 992 * {@code true}. 993 * 994 * <p>If the lock is held by another thread then this method 995 * will return immediately with the value {@code false}. 996 * 997 * @return {@code true} if the lock was free and was acquired 998 * by the current thread, or the write lock was already held 999 * by the current thread; and {@code false} otherwise. 1000 */ 1001 public boolean tryLock( ) { 1002 return sync.tryWriteLock(); 1003 } 1004 1005 /** 1006 * Acquires the write lock if it is not held by another thread 1007 * within the given waiting time and the current thread has 1008 * not been {@linkplain Thread#interrupt interrupted}. 1009 * 1010 * <p>Acquires the write lock if neither the read nor write lock 1011 * are held by another thread 1012 * and returns immediately with the value {@code true}, 1013 * setting the write lock hold count to one. If this lock has been 1014 * set to use a fair ordering policy then an available lock 1015 * <em>will not</em> be acquired if any other threads are 1016 * waiting for the write lock. This is in contrast to the {@link 1017 * #tryLock()} method. If you want a timed {@code tryLock} 1018 * that does permit barging on a fair lock then combine the 1019 * timed and un-timed forms together: 1020 * 1021 * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } 1022 * </pre> 1023 * 1024 * <p>If the current thread already holds this lock then the 1025 * hold count is incremented by one and the method returns 1026 * {@code true}. 1027 * 1028 * <p>If the lock is held by another thread then the current 1029 * thread becomes disabled for thread scheduling purposes and 1030 * lies dormant until one of three things happens: 1031 * 1032 * <ul> 1033 * 1034 * <li>The write lock is acquired by the current thread; or 1035 * 1036 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 1037 * the current thread; or 1038 * 1039 * <li>The specified waiting time elapses 1040 * 1041 * </ul> 1042 * 1043 * <p>If the write lock is acquired then the value {@code true} is 1044 * returned and the write lock hold count is set to one. 1045 * 1046 * <p>If the current thread: 1047 * 1048 * <ul> 1049 * 1050 * <li>has its interrupted status set on entry to this method; 1051 * or 1052 * 1053 * <li>is {@linkplain Thread#interrupt interrupted} while 1054 * acquiring the write lock, 1055 * 1056 * </ul> 1057 * 1058 * then {@link InterruptedException} is thrown and the current 1059 * thread's interrupted status is cleared. 1060 * 1061 * <p>If the specified waiting time elapses then the value 1062 * {@code false} is returned. If the time is less than or 1063 * equal to zero, the method will not wait at all. 1064 * 1065 * <p>In this implementation, as this method is an explicit 1066 * interruption point, preference is given to responding to 1067 * the interrupt over normal or reentrant acquisition of the 1068 * lock, and over reporting the elapse of the waiting time. 1069 * 1070 * @param timeout the time to wait for the write lock 1071 * @param unit the time unit of the timeout argument 1072 * 1073 * @return {@code true} if the lock was free and was acquired 1074 * by the current thread, or the write lock was already held by the 1075 * current thread; and {@code false} if the waiting time 1076 * elapsed before the lock could be acquired. 1077 * 1078 * @throws InterruptedException if the current thread is interrupted 1079 * @throws NullPointerException if the time unit is null 1080 * 1081 */ 1082 public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { 1083 return sync.tryAcquireNanos(1, unit.toNanos(timeout)); 1084 } 1085 1086 /** 1087 * Attempts to release this lock. 1088 * 1089 * <p>If the current thread is the holder of this lock then 1090 * the hold count is decremented. If the hold count is now 1091 * zero then the lock is released. If the current thread is 1092 * not the holder of this lock then {@link 1093 * IllegalMonitorStateException} is thrown. 1094 * 1095 * @throws IllegalMonitorStateException if the current thread does not 1096 * hold this lock. 1097 */ 1098 public void unlock() { 1099 sync.release(1); 1100 } 1101 1102 /** 1103 * Returns a {@link Condition} instance for use with this 1104 * {@link Lock} instance. 1105 * <p>The returned {@link Condition} instance supports the same 1106 * usages as do the {@link Object} monitor methods ({@link 1107 * Object#wait() wait}, {@link Object#notify notify}, and {@link 1108 * Object#notifyAll notifyAll}) when used with the built-in 1109 * monitor lock. 1110 * 1111 * <ul> 1112 * 1113 * <li>If this write lock is not held when any {@link 1114 * Condition} method is called then an {@link 1115 * IllegalMonitorStateException} is thrown. (Read locks are 1116 * held independently of write locks, so are not checked or 1117 * affected. However it is essentially always an error to 1118 * invoke a condition waiting method when the current thread 1119 * has also acquired read locks, since other threads that 1120 * could unblock it will not be able to acquire the write 1121 * lock.) 1122 * 1123 * <li>When the condition {@linkplain Condition#await() waiting} 1124 * methods are called the write lock is released and, before 1125 * they return, the write lock is reacquired and the lock hold 1126 * count restored to what it was when the method was called. 1127 * 1128 * <li>If a thread is {@linkplain Thread#interrupt interrupted} while 1129 * waiting then the wait will terminate, an {@link 1130 * InterruptedException} will be thrown, and the thread's 1131 * interrupted status will be cleared. 1132 * 1133 * <li> Waiting threads are signalled in FIFO order. 1134 * 1135 * <li>The ordering of lock reacquisition for threads returning 1136 * from waiting methods is the same as for threads initially 1137 * acquiring the lock, which is in the default case not specified, 1138 * but for <em>fair</em> locks favors those threads that have been 1139 * waiting the longest. 1140 * 1141 * </ul> 1142 * 1143 * @return the Condition object 1144 */ 1145 public Condition newCondition() { 1146 return sync.newCondition(); 1147 } 1148 1149 /** 1150 * Returns a string identifying this lock, as well as its lock 1151 * state. The state, in brackets includes either the String 1152 * {@code "Unlocked"} or the String {@code "Locked by"} 1153 * followed by the {@linkplain Thread#getName name} of the owning thread. 1154 * 1155 * @return a string identifying this lock, as well as its lock state 1156 */ 1157 public String toString() { 1158 Thread o = sync.getOwner(); 1159 return super.toString() + ((o == null) ? 1160 "[Unlocked]" : 1161 "[Locked by thread " + o.getName() + "]"); 1162 } 1163 1164 /** 1165 * Queries if this write lock is held by the current thread. 1166 * Identical in effect to {@link 1167 * ReentrantReadWriteLock#isWriteLockedByCurrentThread}. 1168 * 1169 * @return {@code true} if the current thread holds this lock and 1170 * {@code false} otherwise 1171 * @since 1.6 1172 */ 1173 public boolean isHeldByCurrentThread() { 1174 return sync.isHeldExclusively(); 1175 } 1176 1177 /** 1178 * Queries the number of holds on this write lock by the current 1179 * thread. A thread has a hold on a lock for each lock action 1180 * that is not matched by an unlock action. Identical in effect 1181 * to {@link ReentrantReadWriteLock#getWriteHoldCount}. 1182 * 1183 * @return the number of holds on this lock by the current thread, 1184 * or zero if this lock is not held by the current thread 1185 * @since 1.6 1186 */ 1187 public int getHoldCount() { 1188 return sync.getWriteHoldCount(); 1189 } 1190 } 1191 1192 // Instrumentation and status 1193 1194 /** 1195 * Returns {@code true} if this lock has fairness set true. 1196 * 1197 * @return {@code true} if this lock has fairness set true 1198 */ 1199 public final boolean isFair() { 1200 return sync instanceof FairSync; 1201 } 1202 1203 /** 1204 * Returns the thread that currently owns the write lock, or 1205 * {@code null} if not owned. When this method is called by a 1206 * thread that is not the owner, the return value reflects a 1207 * best-effort approximation of current lock status. For example, 1208 * the owner may be momentarily {@code null} even if there are 1209 * threads trying to acquire the lock but have not yet done so. 1210 * This method is designed to facilitate construction of 1211 * subclasses that provide more extensive lock monitoring 1212 * facilities. 1213 * 1214 * @return the owner, or {@code null} if not owned 1215 */ 1216 protected Thread getOwner() { 1217 return sync.getOwner(); 1218 } 1219 1220 /** 1221 * Queries the number of read locks held for this lock. This 1222 * method is designed for use in monitoring system state, not for 1223 * synchronization control. 1224 * @return the number of read locks held. 1225 */ 1226 public int getReadLockCount() { 1227 return sync.getReadLockCount(); 1228 } 1229 1230 /** 1231 * Queries if the write lock is held by any thread. This method is 1232 * designed for use in monitoring system state, not for 1233 * synchronization control. 1234 * 1235 * @return {@code true} if any thread holds the write lock and 1236 * {@code false} otherwise 1237 */ 1238 public boolean isWriteLocked() { 1239 return sync.isWriteLocked(); 1240 } 1241 1242 /** 1243 * Queries if the write lock is held by the current thread. 1244 * 1245 * @return {@code true} if the current thread holds the write lock and 1246 * {@code false} otherwise 1247 */ 1248 public boolean isWriteLockedByCurrentThread() { 1249 return sync.isHeldExclusively(); 1250 } 1251 1252 /** 1253 * Queries the number of reentrant write holds on this lock by the 1254 * current thread. A writer thread has a hold on a lock for 1255 * each lock action that is not matched by an unlock action. 1256 * 1257 * @return the number of holds on the write lock by the current thread, 1258 * or zero if the write lock is not held by the current thread 1259 */ 1260 public int getWriteHoldCount() { 1261 return sync.getWriteHoldCount(); 1262 } 1263 1264 /** 1265 * Queries the number of reentrant read holds on this lock by the 1266 * current thread. A reader thread has a hold on a lock for 1267 * each lock action that is not matched by an unlock action. 1268 * 1269 * @return the number of holds on the read lock by the current thread, 1270 * or zero if the read lock is not held by the current thread 1271 * @since 1.6 1272 */ 1273 public int getReadHoldCount() { 1274 return sync.getReadHoldCount(); 1275 } 1276 1277 /** 1278 * Returns a collection containing threads that may be waiting to 1279 * acquire the write lock. Because the actual set of threads may 1280 * change dynamically while constructing this result, the returned 1281 * collection is only a best-effort estimate. The elements of the 1282 * returned collection are in no particular order. This method is 1283 * designed to facilitate construction of subclasses that provide 1284 * more extensive lock monitoring facilities. 1285 * 1286 * @return the collection of threads 1287 */ 1288 protected Collection<Thread> getQueuedWriterThreads() { 1289 return sync.getExclusiveQueuedThreads(); 1290 } 1291 1292 /** 1293 * Returns a collection containing threads that may be waiting to 1294 * acquire the read lock. Because the actual set of threads may 1295 * change dynamically while constructing this result, the returned 1296 * collection is only a best-effort estimate. The elements of the 1297 * returned collection are in no particular order. This method is 1298 * designed to facilitate construction of subclasses that provide 1299 * more extensive lock monitoring facilities. 1300 * 1301 * @return the collection of threads 1302 */ 1303 protected Collection<Thread> getQueuedReaderThreads() { 1304 return sync.getSharedQueuedThreads(); 1305 } 1306 1307 /** 1308 * Queries whether any threads are waiting to acquire the read or 1309 * write lock. Note that because cancellations may occur at any 1310 * time, a {@code true} return does not guarantee that any other 1311 * thread will ever acquire a lock. This method is designed 1312 * primarily for use in monitoring of the system state. 1313 * 1314 * @return {@code true} if there may be other threads waiting to 1315 * acquire the lock 1316 */ 1317 public final boolean hasQueuedThreads() { 1318 return sync.hasQueuedThreads(); 1319 } 1320 1321 /** 1322 * Queries whether the given thread is waiting to acquire either 1323 * the read or write lock. Note that because cancellations may 1324 * occur at any time, a {@code true} return does not guarantee 1325 * that this thread will ever acquire a lock. This method is 1326 * designed primarily for use in monitoring of the system state. 1327 * 1328 * @param thread the thread 1329 * @return {@code true} if the given thread is queued waiting for this lock 1330 * @throws NullPointerException if the thread is null 1331 */ 1332 public final boolean hasQueuedThread(Thread thread) { 1333 return sync.isQueued(thread); 1334 } 1335 1336 /** 1337 * Returns an estimate of the number of threads waiting to acquire 1338 * either the read or write lock. The value is only an estimate 1339 * because the number of threads may change dynamically while this 1340 * method traverses internal data structures. This method is 1341 * designed for use in monitoring of the system state, not for 1342 * synchronization control. 1343 * 1344 * @return the estimated number of threads waiting for this lock 1345 */ 1346 public final int getQueueLength() { 1347 return sync.getQueueLength(); 1348 } 1349 1350 /** 1351 * Returns a collection containing threads that may be waiting to 1352 * acquire either the read or write lock. Because the actual set 1353 * of threads may change dynamically while constructing this 1354 * result, the returned collection is only a best-effort estimate. 1355 * The elements of the returned collection are in no particular 1356 * order. This method is designed to facilitate construction of 1357 * subclasses that provide more extensive monitoring facilities. 1358 * 1359 * @return the collection of threads 1360 */ 1361 protected Collection<Thread> getQueuedThreads() { 1362 return sync.getQueuedThreads(); 1363 } 1364 1365 /** 1366 * Queries whether any threads are waiting on the given condition 1367 * associated with the write lock. Note that because timeouts and 1368 * interrupts may occur at any time, a {@code true} return does 1369 * not guarantee that a future {@code signal} will awaken any 1370 * threads. This method is designed primarily for use in 1371 * monitoring of the system state. 1372 * 1373 * @param condition the condition 1374 * @return {@code true} if there are any waiting threads 1375 * @throws IllegalMonitorStateException if this lock is not held 1376 * @throws IllegalArgumentException if the given condition is 1377 * not associated with this lock 1378 * @throws NullPointerException if the condition is null 1379 */ 1380 public boolean hasWaiters(Condition condition) { 1381 if (condition == null) 1382 throw new NullPointerException(); 1383 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) 1384 throw new IllegalArgumentException("not owner"); 1385 return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition); 1386 } 1387 1388 /** 1389 * Returns an estimate of the number of threads waiting on the 1390 * given condition associated with the write lock. Note that because 1391 * timeouts and interrupts may occur at any time, the estimate 1392 * serves only as an upper bound on the actual number of waiters. 1393 * This method is designed for use in monitoring of the system 1394 * state, not for synchronization control. 1395 * 1396 * @param condition the condition 1397 * @return the estimated number of waiting threads 1398 * @throws IllegalMonitorStateException if this lock is not held 1399 * @throws IllegalArgumentException if the given condition is 1400 * not associated with this lock 1401 * @throws NullPointerException if the condition is null 1402 */ 1403 public int getWaitQueueLength(Condition condition) { 1404 if (condition == null) 1405 throw new NullPointerException(); 1406 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) 1407 throw new IllegalArgumentException("not owner"); 1408 return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition); 1409 } 1410 1411 /** 1412 * Returns a collection containing those threads that may be 1413 * waiting on the given condition associated with the write lock. 1414 * Because the actual set of threads may change dynamically while 1415 * constructing this result, the returned collection is only a 1416 * best-effort estimate. The elements of the returned collection 1417 * are in no particular order. This method is designed to 1418 * facilitate construction of subclasses that provide more 1419 * extensive condition monitoring facilities. 1420 * 1421 * @param condition the condition 1422 * @return the collection of threads 1423 * @throws IllegalMonitorStateException if this lock is not held 1424 * @throws IllegalArgumentException if the given condition is 1425 * not associated with this lock 1426 * @throws NullPointerException if the condition is null 1427 */ 1428 protected Collection<Thread> getWaitingThreads(Condition condition) { 1429 if (condition == null) 1430 throw new NullPointerException(); 1431 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) 1432 throw new IllegalArgumentException("not owner"); 1433 return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition); 1434 } 1435 1436 /** 1437 * Returns a string identifying this lock, as well as its lock state. 1438 * The state, in brackets, includes the String {@code "Write locks ="} 1439 * followed by the number of reentrantly held write locks, and the 1440 * String {@code "Read locks ="} followed by the number of held 1441 * read locks. 1442 * 1443 * @return a string identifying this lock, as well as its lock state 1444 */ 1445 public String toString() { 1446 int c = sync.getCount(); 1447 int w = Sync.exclusiveCount(c); 1448 int r = Sync.sharedCount(c); 1449 1450 return super.toString() + 1451 "[Write locks = " + w + ", Read locks = " + r + "]"; 1452 } 1453 1454} 1455