1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent.locks;
37
38 import java.util.Collection;
39 import java.util.concurrent.TimeUnit;
40 import jdk.internal.vm.annotation.ReservedStackAccess;
41
42 /**
43 * An implementation of {@link ReadWriteLock} supporting similar
44 * semantics to {@link ReentrantLock}.
45 * <p>This class has the following properties:
46 *
47 * <ul>
48 * <li><b>Acquisition order</b>
49 *
50 * <p>This class does not impose a reader or writer preference
51 * ordering for lock access. However, it does support an optional
52 * <em>fairness</em> policy.
53 *
54 * <dl>
55 * <dt><b><i>Non-fair mode (default)</i></b>
56 * <dd>When constructed as non-fair (the default), the order of entry
57 * to the read and write lock is unspecified, subject to reentrancy
58 * constraints. A nonfair lock that is continuously contended may
59 * indefinitely postpone one or more reader or writer threads, but
60 * will normally have higher throughput than a fair lock.
61 *
62 * <dt><b><i>Fair mode</i></b>
63 * <dd>When constructed as fair, threads contend for entry using an
64 * approximately arrival-order policy. When the currently held lock
65 * is released, either the longest-waiting single writer thread will
66 * be assigned the write lock, or if there is a group of reader threads
67 * waiting longer than all waiting writer threads, that group will be
68 * assigned the read lock.
69 *
70 * <p>A thread that tries to acquire a fair read lock (non-reentrantly)
71 * will block if either the write lock is held, or there is a waiting
72 * writer thread. The thread will not acquire the read lock until
73 * after the oldest currently waiting writer thread has acquired and
74 * released the write lock. Of course, if a waiting writer abandons
75 * its wait, leaving one or more reader threads as the longest waiters
76 * in the queue with the write lock free, then those readers will be
77 * assigned the read lock.
78 *
79 * <p>A thread that tries to acquire a fair write lock (non-reentrantly)
80 * will block unless both the read lock and write lock are free (which
81 * implies there are no waiting threads). (Note that the non-blocking
82 * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods
83 * do not honor this fair setting and will immediately acquire the lock
84 * if it is possible, regardless of waiting threads.)
85 * </dl>
86 *
87 * <li><b>Reentrancy</b>
88 *
89 * <p>This lock allows both readers and writers to reacquire read or
90 * write locks in the style of a {@link ReentrantLock}. Non-reentrant
91 * readers are not allowed until all write locks held by the writing
92 * thread have been released.
93 *
94 * <p>Additionally, a writer can acquire the read lock, but not
95 * vice-versa. Among other applications, reentrancy can be useful
96 * when write locks are held during calls or callbacks to methods that
97 * perform reads under read locks. If a reader tries to acquire the
98 * write lock it will never succeed.
99 *
100 * <li><b>Lock downgrading</b>
101 * <p>Reentrancy also allows downgrading from the write lock to a read lock,
102 * by acquiring the write lock, then the read lock and then releasing the
103 * write lock. However, upgrading from a read lock to the write lock is
104 * <b>not</b> possible.
105 *
106 * <li><b>Interruption of lock acquisition</b>
107 * <p>The read lock and write lock both support interruption during lock
108 * acquisition.
109 *
110 * <li><b>{@link Condition} support</b>
111 * <p>The write lock provides a {@link Condition} implementation that
112 * behaves in the same way, with respect to the write lock, as the
113 * {@link Condition} implementation provided by
114 * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.
115 * This {@link Condition} can, of course, only be used with the write lock.
116 *
117 * <p>The read lock does not support a {@link Condition} and
118 * {@code readLock().newCondition()} throws
119 * {@code UnsupportedOperationException}.
120 *
121 * <li><b>Instrumentation</b>
122 * <p>This class supports methods to determine whether locks
123 * are held or contended. These methods are designed for monitoring
124 * system state, not for synchronization control.
125 * </ul>
126 *
127 * <p>Serialization of this class behaves in the same way as built-in
128 * locks: a deserialized lock is in the unlocked state, regardless of
129 * its state when serialized.
130 *
131 * <p><b>Sample usages</b>. Here is a code sketch showing how to perform
132 * lock downgrading after updating a cache (exception handling is
133 * particularly tricky when handling multiple locks in a non-nested
134 * fashion):
135 *
136 * <pre> {@code
137 * class CachedData {
138 * Object data;
139 * boolean cacheValid;
140 * final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
141 *
142 * void processCachedData() {
143 * rwl.readLock().lock();
144 * if (!cacheValid) {
145 * // Must release read lock before acquiring write lock
146 * rwl.readLock().unlock();
147 * rwl.writeLock().lock();
148 * try {
149 * // Recheck state because another thread might have
150 * // acquired write lock and changed state before we did.
151 * if (!cacheValid) {
152 * data = ...
153 * cacheValid = true;
154 * }
155 * // Downgrade by acquiring read lock before releasing write lock
156 * rwl.readLock().lock();
157 * } finally {
158 * rwl.writeLock().unlock(); // Unlock write, still hold read
159 * }
160 * }
161 *
162 * try {
163 * use(data);
164 * } finally {
165 * rwl.readLock().unlock();
166 * }
167 * }
168 * }}</pre>
169 *
170 * ReentrantReadWriteLocks can be used to improve concurrency in some
171 * uses of some kinds of Collections. This is typically worthwhile
172 * only when the collections are expected to be large, accessed by
173 * more reader threads than writer threads, and entail operations with
174 * overhead that outweighs synchronization overhead. For example, here
175 * is a class using a TreeMap that is expected to be large and
176 * concurrently accessed.
177 *
178 * <pre> {@code
179 * class RWDictionary {
180 * private final Map<String, Data> m = new TreeMap<>();
181 * private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
182 * private final Lock r = rwl.readLock();
183 * private final Lock w = rwl.writeLock();
184 *
185 * public Data get(String key) {
186 * r.lock();
187 * try { return m.get(key); }
188 * finally { r.unlock(); }
189 * }
190 * public List<String> allKeys() {
191 * r.lock();
192 * try { return new ArrayList<>(m.keySet()); }
193 * finally { r.unlock(); }
194 * }
195 * public Data put(String key, Data value) {
196 * w.lock();
197 * try { return m.put(key, value); }
198 * finally { w.unlock(); }
199 * }
200 * public void clear() {
201 * w.lock();
202 * try { m.clear(); }
203 * finally { w.unlock(); }
204 * }
205 * }}</pre>
206 *
207 * <h3>Implementation Notes</h3>
208 *
209 * <p>This lock supports a maximum of 65535 recursive write locks
210 * and 65535 read locks. Attempts to exceed these limits result in
211 * {@link Error} throws from locking methods.
212 *
213 * @since 1.5
214 * @author Doug Lea
215 */
216 public class ReentrantReadWriteLock
217 implements ReadWriteLock, java.io.Serializable {
218 private static final long serialVersionUID = -6992448646407690164L;
219 /** Inner class providing readlock */
220 private final ReentrantReadWriteLock.ReadLock readerLock;
221 /** Inner class providing writelock */
222 private final ReentrantReadWriteLock.WriteLock writerLock;
223 /** Performs all synchronization mechanics */
224 final Sync sync;
225
226 /**
227 * Creates a new {@code ReentrantReadWriteLock} with
228 * default (nonfair) ordering properties.
229 */
230 public ReentrantReadWriteLock() {
231 this(false);
232 }
233
234 /**
235 * Creates a new {@code ReentrantReadWriteLock} with
236 * the given fairness policy.
237 *
238 * @param fair {@code true} if this lock should use a fair ordering policy
239 */
240 public ReentrantReadWriteLock(boolean fair) {
241 sync = fair ? new FairSync() : new NonfairSync();
242 readerLock = new ReadLock(this);
243 writerLock = new WriteLock(this);
244 }
245
246 public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
247 public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; }
248
249 /**
250 * Synchronization implementation for ReentrantReadWriteLock.
251 * Subclassed into fair and nonfair versions.
252 */
253 abstract static class Sync extends AbstractQueuedSynchronizer {
254 private static final long serialVersionUID = 6317671515068378041L;
255
256 /*
257 * Read vs write count extraction constants and functions.
258 * Lock state is logically divided into two unsigned shorts:
259 * The lower one representing the exclusive (writer) lock hold count,
260 * and the upper the shared (reader) hold count.
261 */
262
263 static final int SHARED_SHIFT = 16;
264 static final int SHARED_UNIT = (1 << SHARED_SHIFT);
265 static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
266 static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
267
268 /** Returns the number of shared holds represented in count. */
269 static int sharedCount(int c) { return c >>> SHARED_SHIFT; }
270 /** Returns the number of exclusive holds represented in count. */
271 static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
272
273 /**
274 * A counter for per-thread read hold counts.
275 * Maintained as a ThreadLocal; cached in cachedHoldCounter.
276 */
277 static final class HoldCounter {
278 int count; // initially 0
279 // Use id, not reference, to avoid garbage retention
280 final long tid = LockSupport.getThreadId(Thread.currentThread());
281 }
282
283 /**
284 * ThreadLocal subclass. Easiest to explicitly define for sake
285 * of deserialization mechanics.
286 */
287 static final class ThreadLocalHoldCounter
288 extends ThreadLocal<HoldCounter> {
289 public HoldCounter initialValue() {
290 return new HoldCounter();
291 }
292 }
293
294 /**
295 * The number of reentrant read locks held by current thread.
296 * Initialized only in constructor and readObject.
297 * Removed whenever a thread's read hold count drops to 0.
298 */
299 private transient ThreadLocalHoldCounter readHolds;
300
301 /**
302 * The hold count of the last thread to successfully acquire
303 * readLock. This saves ThreadLocal lookup in the common case
304 * where the next thread to release is the last one to
305 * acquire. This is non-volatile since it is just used
306 * as a heuristic, and would be great for threads to cache.
307 *
308 * <p>Can outlive the Thread for which it is caching the read
309 * hold count, but avoids garbage retention by not retaining a
310 * reference to the Thread.
311 *
312 * <p>Accessed via a benign data race; relies on the memory
313 * model's final field and out-of-thin-air guarantees.
314 */
315 private transient HoldCounter cachedHoldCounter;
316
317 /**
318 * firstReader is the first thread to have acquired the read lock.
319 * firstReaderHoldCount is firstReader's hold count.
320 *
321 * <p>More precisely, firstReader is the unique thread that last
322 * changed the shared count from 0 to 1, and has not released the
323 * read lock since then; null if there is no such thread.
324 *
325 * <p>Cannot cause garbage retention unless the thread terminated
326 * without relinquishing its read locks, since tryReleaseShared
327 * sets it to null.
328 *
329 * <p>Accessed via a benign data race; relies on the memory
330 * model's out-of-thin-air guarantees for references.
331 *
332 * <p>This allows tracking of read holds for uncontended read
333 * locks to be very cheap.
334 */
335 private transient Thread firstReader;
336 private transient int firstReaderHoldCount;
337
338 Sync() {
339 readHolds = new ThreadLocalHoldCounter();
340 setState(getState()); // ensures visibility of readHolds
341 }
342
343 /*
344 * Acquires and releases use the same code for fair and
345 * nonfair locks, but differ in whether/how they allow barging
346 * when queues are non-empty.
347 */
348
349 /**
350 * Returns true if the current thread, when trying to acquire
351 * the read lock, and otherwise eligible to do so, should block
352 * because of policy for overtaking other waiting threads.
353 */
354 abstract boolean readerShouldBlock();
355
356 /**
357 * Returns true if the current thread, when trying to acquire
358 * the write lock, and otherwise eligible to do so, should block
359 * because of policy for overtaking other waiting threads.
360 */
361 abstract boolean writerShouldBlock();
362
363 /*
364 * Note that tryRelease and tryAcquire can be called by
365 * Conditions. So it is possible that their arguments contain
366 * both read and write holds that are all released during a
367 * condition wait and re-established in tryAcquire.
368 */
369 @ReservedStackAccess
370 protected final boolean tryRelease(int releases) {
371 if (!isHeldExclusively())
372 throw new IllegalMonitorStateException();
373 int nextc = getState() - releases;
374 boolean free = exclusiveCount(nextc) == 0;
375 if (free)
376 setExclusiveOwnerThread(null);
377 setState(nextc);
378 return free;
379 }
380
381 @ReservedStackAccess
382 protected final boolean tryAcquire(int acquires) {
383 /*
384 * Walkthrough:
385 * 1. If read count nonzero or write count nonzero
386 * and owner is a different thread, fail.
387 * 2. If count would saturate, fail. (This can only
388 * happen if count is already nonzero.)
389 * 3. Otherwise, this thread is eligible for lock if
390 * it is either a reentrant acquire or
391 * queue policy allows it. If so, update state
392 * and set owner.
393 */
394 Thread current = Thread.currentThread();
395 int c = getState();
396 int w = exclusiveCount(c);
397 if (c != 0) {
398 // (Note: if c != 0 and w == 0 then shared count != 0)
399 if (w == 0 || current != getExclusiveOwnerThread())
400 return false;
401 if (w + exclusiveCount(acquires) > MAX_COUNT)
402 throw new Error("Maximum lock count exceeded");
403 // Reentrant acquire
404 setState(c + acquires);
405 return true;
406 }
407 if (writerShouldBlock() ||
408 !compareAndSetState(c, c + acquires))
409 return false;
410 setExclusiveOwnerThread(current);
411 return true;
412 }
413
414 @ReservedStackAccess
415 protected final boolean tryReleaseShared(int unused) {
416 Thread current = Thread.currentThread();
417 if (firstReader == current) {
418 // assert firstReaderHoldCount > 0;
419 if (firstReaderHoldCount == 1)
420 firstReader = null;
421 else
422 firstReaderHoldCount--;
423 } else {
424 HoldCounter rh = cachedHoldCounter;
425 if (rh == null ||
426 rh.tid != LockSupport.getThreadId(current))
427 rh = readHolds.get();
428 int count = rh.count;
429 if (count <= 1) {
430 readHolds.remove();
431 if (count <= 0)
432 throw unmatchedUnlockException();
433 }
434 --rh.count;
435 }
436 for (;;) {
437 int c = getState();
438 int nextc = c - SHARED_UNIT;
439 if (compareAndSetState(c, nextc))
440 // Releasing the read lock has no effect on readers,
441 // but it may allow waiting writers to proceed if
442 // both read and write locks are now free.
443 return nextc == 0;
444 }
445 }
446
447 private static IllegalMonitorStateException unmatchedUnlockException() {
448 return new IllegalMonitorStateException(
449 "attempt to unlock read lock, not locked by current thread");
450 }
451
452 @ReservedStackAccess
453 protected final int tryAcquireShared(int unused) {
454 /*
455 * Walkthrough:
456 * 1. If write lock held by another thread, fail.
457 * 2. Otherwise, this thread is eligible for
458 * lock wrt state, so ask if it should block
459 * because of queue policy. If not, try
460 * to grant by CASing state and updating count.
461 * Note that step does not check for reentrant
462 * acquires, which is postponed to full version
463 * to avoid having to check hold count in
464 * the more typical non-reentrant case.
465 * 3. If step 2 fails either because thread
466 * apparently not eligible or CAS fails or count
467 * saturated, chain to version with full retry loop.
468 */
469 Thread current = Thread.currentThread();
470 int c = getState();
471 if (exclusiveCount(c) != 0 &&
472 getExclusiveOwnerThread() != current)
473 return -1;
474 int r = sharedCount(c);
475 if (!readerShouldBlock() &&
476 r < MAX_COUNT &&
477 compareAndSetState(c, c + SHARED_UNIT)) {
478 if (r == 0) {
479 firstReader = current;
480 firstReaderHoldCount = 1;
481 } else if (firstReader == current) {
482 firstReaderHoldCount++;
483 } else {
484 HoldCounter rh = cachedHoldCounter;
485 if (rh == null ||
486 rh.tid != LockSupport.getThreadId(current))
487 cachedHoldCounter = rh = readHolds.get();
488 else if (rh.count == 0)
489 readHolds.set(rh);
490 rh.count++;
491 }
492 return 1;
493 }
494 return fullTryAcquireShared(current);
495 }
496
497 /**
498 * Full version of acquire for reads, that handles CAS misses
499 * and reentrant reads not dealt with in tryAcquireShared.
500 */
501 final int fullTryAcquireShared(Thread current) {
502 /*
503 * This code is in part redundant with that in
504 * tryAcquireShared but is simpler overall by not
505 * complicating tryAcquireShared with interactions between
506 * retries and lazily reading hold counts.
507 */
508 HoldCounter rh = null;
509 for (;;) {
510 int c = getState();
511 if (exclusiveCount(c) != 0) {
512 if (getExclusiveOwnerThread() != current)
513 return -1;
514 // else we hold the exclusive lock; blocking here
515 // would cause deadlock.
516 } else if (readerShouldBlock()) {
517 // Make sure we're not acquiring read lock reentrantly
518 if (firstReader == current) {
519 // assert firstReaderHoldCount > 0;
520 } else {
521 if (rh == null) {
522 rh = cachedHoldCounter;
523 if (rh == null ||
524 rh.tid != LockSupport.getThreadId(current)) {
525 rh = readHolds.get();
526 if (rh.count == 0)
527 readHolds.remove();
528 }
529 }
530 if (rh.count == 0)
531 return -1;
532 }
533 }
534 if (sharedCount(c) == MAX_COUNT)
535 throw new Error("Maximum lock count exceeded");
536 if (compareAndSetState(c, c + SHARED_UNIT)) {
537 if (sharedCount(c) == 0) {
538 firstReader = current;
539 firstReaderHoldCount = 1;
540 } else if (firstReader == current) {
541 firstReaderHoldCount++;
542 } else {
543 if (rh == null)
544 rh = cachedHoldCounter;
545 if (rh == null ||
546 rh.tid != LockSupport.getThreadId(current))
547 rh = readHolds.get();
548 else if (rh.count == 0)
549 readHolds.set(rh);
550 rh.count++;
551 cachedHoldCounter = rh; // cache for release
552 }
553 return 1;
554 }
555 }
556 }
557
558 /**
559 * Performs tryLock for write, enabling barging in both modes.
560 * This is identical in effect to tryAcquire except for lack
561 * of calls to writerShouldBlock.
562 */
563 @ReservedStackAccess
564 final boolean tryWriteLock() {
565 Thread current = Thread.currentThread();
566 int c = getState();
567 if (c != 0) {
568 int w = exclusiveCount(c);
569 if (w == 0 || current != getExclusiveOwnerThread())
570 return false;
571 if (w == MAX_COUNT)
572 throw new Error("Maximum lock count exceeded");
573 }
574 if (!compareAndSetState(c, c + 1))
575 return false;
576 setExclusiveOwnerThread(current);
577 return true;
578 }
579
580 /**
581 * Performs tryLock for read, enabling barging in both modes.
582 * This is identical in effect to tryAcquireShared except for
583 * lack of calls to readerShouldBlock.
584 */
585 @ReservedStackAccess
586 final boolean tryReadLock() {
587 Thread current = Thread.currentThread();
588 for (;;) {
589 int c = getState();
590 if (exclusiveCount(c) != 0 &&
591 getExclusiveOwnerThread() != current)
592 return false;
593 int r = sharedCount(c);
594 if (r == MAX_COUNT)
595 throw new Error("Maximum lock count exceeded");
596 if (compareAndSetState(c, c + SHARED_UNIT)) {
597 if (r == 0) {
598 firstReader = current;
599 firstReaderHoldCount = 1;
600 } else if (firstReader == current) {
601 firstReaderHoldCount++;
602 } else {
603 HoldCounter rh = cachedHoldCounter;
604 if (rh == null ||
605 rh.tid != LockSupport.getThreadId(current))
606 cachedHoldCounter = rh = readHolds.get();
607 else if (rh.count == 0)
608 readHolds.set(rh);
609 rh.count++;
610 }
611 return true;
612 }
613 }
614 }
615
616 protected final boolean isHeldExclusively() {
617 // While we must in general read state before owner,
618 // we don't need to do so to check if current thread is owner
619 return getExclusiveOwnerThread() == Thread.currentThread();
620 }
621
622 // Methods relayed to outer class
623
624 final ConditionObject newCondition() {
625 return new ConditionObject();
626 }
627
628 final Thread getOwner() {
629 // Must read state before owner to ensure memory consistency
630 return ((exclusiveCount(getState()) == 0) ?
631 null :
632 getExclusiveOwnerThread());
633 }
634
635 final int getReadLockCount() {
636 return sharedCount(getState());
637 }
638
639 final boolean isWriteLocked() {
640 return exclusiveCount(getState()) != 0;
641 }
642
643 final int getWriteHoldCount() {
644 return isHeldExclusively() ? exclusiveCount(getState()) : 0;
645 }
646
647 final int getReadHoldCount() {
648 if (getReadLockCount() == 0)
649 return 0;
650
651 Thread current = Thread.currentThread();
652 if (firstReader == current)
653 return firstReaderHoldCount;
654
655 HoldCounter rh = cachedHoldCounter;
656 if (rh != null && rh.tid == LockSupport.getThreadId(current))
657 return rh.count;
658
659 int count = readHolds.get().count;
660 if (count == 0) readHolds.remove();
661 return count;
662 }
663
664 /**
665 * Reconstitutes the instance from a stream (that is, deserializes it).
666 */
667 private void readObject(java.io.ObjectInputStream s)
668 throws java.io.IOException, ClassNotFoundException {
669 s.defaultReadObject();
670 readHolds = new ThreadLocalHoldCounter();
671 setState(0); // reset to unlocked state
672 }
673
674 final int getCount() { return getState(); }
675 }
676
677 /**
678 * Nonfair version of Sync
679 */
680 static final class NonfairSync extends Sync {
681 private static final long serialVersionUID = -8159625535654395037L;
682 final boolean writerShouldBlock() {
683 return false; // writers can always barge
684 }
685 final boolean readerShouldBlock() {
686 /* As a heuristic to avoid indefinite writer starvation,
687 * block if the thread that momentarily appears to be head
688 * of queue, if one exists, is a waiting writer. This is
689 * only a probabilistic effect since a new reader will not
690 * block if there is a waiting writer behind other enabled
691 * readers that have not yet drained from the queue.
692 */
693 return apparentlyFirstQueuedIsExclusive();
694 }
695 }
696
697 /**
698 * Fair version of Sync
699 */
700 static final class FairSync extends Sync {
701 private static final long serialVersionUID = -2274990926593161451L;
702 final boolean writerShouldBlock() {
703 return hasQueuedPredecessors();
704 }
705 final boolean readerShouldBlock() {
706 return hasQueuedPredecessors();
707 }
708 }
709
710 /**
711 * The lock returned by method {@link ReentrantReadWriteLock#readLock}.
712 */
713 public static class ReadLock implements Lock, java.io.Serializable {
714 private static final long serialVersionUID = -5992448646407690164L;
715 private final Sync sync;
716
717 /**
718 * Constructor for use by subclasses.
719 *
720 * @param lock the outer lock object
721 * @throws NullPointerException if the lock is null
722 */
723 protected ReadLock(ReentrantReadWriteLock lock) {
724 sync = lock.sync;
725 }
726
727 /**
728 * Acquires the read lock.
729 *
730 * <p>Acquires the read lock if the write lock is not held by
731 * another thread and returns immediately.
732 *
733 * <p>If the write lock is held by another thread then
734 * the current thread becomes disabled for thread scheduling
735 * purposes and lies dormant until the read lock has been acquired.
736 */
737 public void lock() {
738 sync.acquireShared(1);
739 }
740
741 /**
742 * Acquires the read lock unless the current thread is
743 * {@linkplain Thread#interrupt interrupted}.
744 *
745 * <p>Acquires the read lock if the write lock is not held
746 * by another thread and returns immediately.
747 *
748 * <p>If the write lock is held by another thread then the
749 * current thread becomes disabled for thread scheduling
750 * purposes and lies dormant until one of two things happens:
751 *
752 * <ul>
753 *
754 * <li>The read lock is acquired by the current thread; or
755 *
756 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
757 * the current thread.
758 *
759 * </ul>
760 *
761 * <p>If the current thread:
762 *
763 * <ul>
764 *
765 * <li>has its interrupted status set on entry to this method; or
766 *
767 * <li>is {@linkplain Thread#interrupt interrupted} while
768 * acquiring the read lock,
769 *
770 * </ul>
771 *
772 * then {@link InterruptedException} is thrown and the current
773 * thread's interrupted status is cleared.
774 *
775 * <p>In this implementation, as this method is an explicit
776 * interruption point, preference is given to responding to
777 * the interrupt over normal or reentrant acquisition of the
778 * lock.
779 *
780 * @throws InterruptedException if the current thread is interrupted
781 */
782 public void lockInterruptibly() throws InterruptedException {
783 sync.acquireSharedInterruptibly(1);
784 }
785
786 /**
787 * Acquires the read lock only if the write lock is not held by
788 * another thread at the time of invocation.
789 *
790 * <p>Acquires the read lock if the write lock is not held by
791 * another thread and returns immediately with the value
792 * {@code true}. Even when this lock has been set to use a
793 * fair ordering policy, a call to {@code tryLock()}
794 * <em>will</em> immediately acquire the read lock if it is
795 * available, whether or not other threads are currently
796 * waiting for the read lock. This "barging" behavior
797 * can be useful in certain circumstances, even though it
798 * breaks fairness. If you want to honor the fairness setting
799 * for this lock, then use {@link #tryLock(long, TimeUnit)
800 * tryLock(0, TimeUnit.SECONDS)} which is almost equivalent
801 * (it also detects interruption).
802 *
803 * <p>If the write lock is held by another thread then
804 * this method will return immediately with the value
805 * {@code false}.
806 *
807 * @return {@code true} if the read lock was acquired
808 */
809 public boolean tryLock() {
810 return sync.tryReadLock();
811 }
812
813 /**
814 * Acquires the read lock if the write lock is not held by
815 * another thread within the given waiting time and the
816 * current thread has not been {@linkplain Thread#interrupt
817 * interrupted}.
818 *
819 * <p>Acquires the read lock if the write lock is not held by
820 * another thread and returns immediately with the value
821 * {@code true}. If this lock has been set to use a fair
822 * ordering policy then an available lock <em>will not</em> be
823 * acquired if any other threads are waiting for the
824 * lock. This is in contrast to the {@link #tryLock()}
825 * method. If you want a timed {@code tryLock} that does
826 * permit barging on a fair lock then combine the timed and
827 * un-timed forms together:
828 *
829 * <pre> {@code
830 * if (lock.tryLock() ||
831 * lock.tryLock(timeout, unit)) {
832 * ...
833 * }}</pre>
834 *
835 * <p>If the write lock is held by another thread then the
836 * current thread becomes disabled for thread scheduling
837 * purposes and lies dormant until one of three things happens:
838 *
839 * <ul>
840 *
841 * <li>The read lock is acquired by the current thread; or
842 *
843 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
844 * the current thread; or
845 *
846 * <li>The specified waiting time elapses.
847 *
848 * </ul>
849 *
850 * <p>If the read lock is acquired then the value {@code true} is
851 * returned.
852 *
853 * <p>If the current thread:
854 *
855 * <ul>
856 *
857 * <li>has its interrupted status set on entry to this method; or
858 *
859 * <li>is {@linkplain Thread#interrupt interrupted} while
860 * acquiring the read lock,
861 *
862 * </ul> then {@link InterruptedException} is thrown and the
863 * current thread's interrupted status is cleared.
864 *
865 * <p>If the specified waiting time elapses then the value
866 * {@code false} is returned. If the time is less than or
867 * equal to zero, the method will not wait at all.
868 *
869 * <p>In this implementation, as this method is an explicit
870 * interruption point, preference is given to responding to
871 * the interrupt over normal or reentrant acquisition of the
872 * lock, and over reporting the elapse of the waiting time.
873 *
874 * @param timeout the time to wait for the read lock
875 * @param unit the time unit of the timeout argument
876 * @return {@code true} if the read lock was acquired
877 * @throws InterruptedException if the current thread is interrupted
878 * @throws NullPointerException if the time unit is null
879 */
880 public boolean tryLock(long timeout, TimeUnit unit)
881 throws InterruptedException {
882 return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
883 }
884
885 /**
886 * Attempts to release this lock.
887 *
888 * <p>If the number of readers is now zero then the lock
889 * is made available for write lock attempts. If the current
890 * thread does not hold this lock then {@link
891 * IllegalMonitorStateException} is thrown.
892 *
893 * @throws IllegalMonitorStateException if the current thread
894 * does not hold this lock
895 */
896 public void unlock() {
897 sync.releaseShared(1);
898 }
899
900 /**
901 * Throws {@code UnsupportedOperationException} because
902 * {@code ReadLocks} do not support conditions.
903 *
904 * @throws UnsupportedOperationException always
905 */
906 public Condition newCondition() {
907 throw new UnsupportedOperationException();
908 }
909
910 /**
911 * Returns a string identifying this lock, as well as its lock state.
912 * The state, in brackets, includes the String {@code "Read locks ="}
913 * followed by the number of held read locks.
914 *
915 * @return a string identifying this lock, as well as its lock state
916 */
917 public String toString() {
918 int r = sync.getReadLockCount();
919 return super.toString() +
920 "[Read locks = " + r + "]";
921 }
922 }
923
924 /**
925 * The lock returned by method {@link ReentrantReadWriteLock#writeLock}.
926 */
927 public static class WriteLock implements Lock, java.io.Serializable {
928 private static final long serialVersionUID = -4992448646407690164L;
929 private final Sync sync;
930
931 /**
932 * Constructor for use by subclasses.
933 *
934 * @param lock the outer lock object
935 * @throws NullPointerException if the lock is null
936 */
937 protected WriteLock(ReentrantReadWriteLock lock) {
938 sync = lock.sync;
939 }
940
941 /**
942 * Acquires the write lock.
943 *
944 * <p>Acquires the write lock if neither the read nor write lock
945 * are held by another thread
946 * and returns immediately, setting the write lock hold count to
947 * one.
948 *
949 * <p>If the current thread already holds the write lock then the
950 * hold count is incremented by one and the method returns
951 * immediately.
952 *
953 * <p>If the lock is held by another thread then the current
954 * thread becomes disabled for thread scheduling purposes and
955 * lies dormant until the write lock has been acquired, at which
956 * time the write lock hold count is set to one.
957 */
958 public void lock() {
959 sync.acquire(1);
960 }
961
962 /**
963 * Acquires the write lock unless the current thread is
964 * {@linkplain Thread#interrupt interrupted}.
965 *
966 * <p>Acquires the write lock if neither the read nor write lock
967 * are held by another thread
968 * and returns immediately, setting the write lock hold count to
969 * one.
970 *
971 * <p>If the current thread already holds this lock then the
972 * hold count is incremented by one and the method returns
973 * immediately.
974 *
975 * <p>If the lock is held by another thread then the current
976 * thread becomes disabled for thread scheduling purposes and
977 * lies dormant until one of two things happens:
978 *
979 * <ul>
980 *
981 * <li>The write lock is acquired by the current thread; or
982 *
983 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
984 * the current thread.
985 *
986 * </ul>
987 *
988 * <p>If the write lock is acquired by the current thread then the
989 * lock hold count is set to one.
990 *
991 * <p>If the current thread:
992 *
993 * <ul>
994 *
995 * <li>has its interrupted status set on entry to this method;
996 * or
997 *
998 * <li>is {@linkplain Thread#interrupt interrupted} while
999 * acquiring the write lock,
1000 *
1001 * </ul>
1002 *
1003 * then {@link InterruptedException} is thrown and the current
1004 * thread's interrupted status is cleared.
1005 *
1006 * <p>In this implementation, as this method is an explicit
1007 * interruption point, preference is given to responding to
1008 * the interrupt over normal or reentrant acquisition of the
1009 * lock.
1010 *
1011 * @throws InterruptedException if the current thread is interrupted
1012 */
1013 public void lockInterruptibly() throws InterruptedException {
1014 sync.acquireInterruptibly(1);
1015 }
1016
1017 /**
1018 * Acquires the write lock only if it is not held by another thread
1019 * at the time of invocation.
1020 *
1021 * <p>Acquires the write lock if neither the read nor write lock
1022 * are held by another thread
1023 * and returns immediately with the value {@code true},
1024 * setting the write lock hold count to one. Even when this lock has
1025 * been set to use a fair ordering policy, a call to
1026 * {@code tryLock()} <em>will</em> immediately acquire the
1027 * lock if it is available, whether or not other threads are
1028 * currently waiting for the write lock. This "barging"
1029 * behavior can be useful in certain circumstances, even
1030 * though it breaks fairness. If you want to honor the
1031 * fairness setting for this lock, then use {@link
1032 * #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS)}
1033 * which is almost equivalent (it also detects interruption).
1034 *
1035 * <p>If the current thread already holds this lock then the
1036 * hold count is incremented by one and the method returns
1037 * {@code true}.
1038 *
1039 * <p>If the lock is held by another thread then this method
1040 * will return immediately with the value {@code false}.
1041 *
1042 * @return {@code true} if the lock was free and was acquired
1043 * by the current thread, or the write lock was already held
1044 * by the current thread; and {@code false} otherwise.
1045 */
1046 public boolean tryLock() {
1047 return sync.tryWriteLock();
1048 }
1049
1050 /**
1051 * Acquires the write lock if it is not held by another thread
1052 * within the given waiting time and the current thread has
1053 * not been {@linkplain Thread#interrupt interrupted}.
1054 *
1055 * <p>Acquires the write lock if neither the read nor write lock
1056 * are held by another thread
1057 * and returns immediately with the value {@code true},
1058 * setting the write lock hold count to one. If this lock has been
1059 * set to use a fair ordering policy then an available lock
1060 * <em>will not</em> be acquired if any other threads are
1061 * waiting for the write lock. This is in contrast to the {@link
1062 * #tryLock()} method. If you want a timed {@code tryLock}
1063 * that does permit barging on a fair lock then combine the
1064 * timed and un-timed forms together:
1065 *
1066 * <pre> {@code
1067 * if (lock.tryLock() ||
1068 * lock.tryLock(timeout, unit)) {
1069 * ...
1070 * }}</pre>
1071 *
1072 * <p>If the current thread already holds this lock then the
1073 * hold count is incremented by one and the method returns
1074 * {@code true}.
1075 *
1076 * <p>If the lock is held by another thread then the current
1077 * thread becomes disabled for thread scheduling purposes and
1078 * lies dormant until one of three things happens:
1079 *
1080 * <ul>
1081 *
1082 * <li>The write lock is acquired by the current thread; or
1083 *
1084 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
1085 * the current thread; or
1086 *
1087 * <li>The specified waiting time elapses
1088 *
1089 * </ul>
1090 *
1091 * <p>If the write lock is acquired then the value {@code true} is
1092 * returned and the write lock hold count is set to one.
1093 *
1094 * <p>If the current thread:
1095 *
1096 * <ul>
1097 *
1098 * <li>has its interrupted status set on entry to this method;
1099 * or
1100 *
1101 * <li>is {@linkplain Thread#interrupt interrupted} while
1102 * acquiring the write lock,
1103 *
1104 * </ul>
1105 *
1106 * then {@link InterruptedException} is thrown and the current
1107 * thread's interrupted status is cleared.
1108 *
1109 * <p>If the specified waiting time elapses then the value
1110 * {@code false} is returned. If the time is less than or
1111 * equal to zero, the method will not wait at all.
1112 *
1113 * <p>In this implementation, as this method is an explicit
1114 * interruption point, preference is given to responding to
1115 * the interrupt over normal or reentrant acquisition of the
1116 * lock, and over reporting the elapse of the waiting time.
1117 *
1118 * @param timeout the time to wait for the write lock
1119 * @param unit the time unit of the timeout argument
1120 *
1121 * @return {@code true} if the lock was free and was acquired
1122 * by the current thread, or the write lock was already held by the
1123 * current thread; and {@code false} if the waiting time
1124 * elapsed before the lock could be acquired.
1125 *
1126 * @throws InterruptedException if the current thread is interrupted
1127 * @throws NullPointerException if the time unit is null
1128 */
1129 public boolean tryLock(long timeout, TimeUnit unit)
1130 throws InterruptedException {
1131 return sync.tryAcquireNanos(1, unit.toNanos(timeout));
1132 }
1133
1134 /**
1135 * Attempts to release this lock.
1136 *
1137 * <p>If the current thread is the holder of this lock then
1138 * the hold count is decremented. If the hold count is now
1139 * zero then the lock is released. If the current thread is
1140 * not the holder of this lock then {@link
1141 * IllegalMonitorStateException} is thrown.
1142 *
1143 * @throws IllegalMonitorStateException if the current thread does not
1144 * hold this lock
1145 */
1146 public void unlock() {
1147 sync.release(1);
1148 }
1149
1150 /**
1151 * Returns a {@link Condition} instance for use with this
1152 * {@link Lock} instance.
1153 * <p>The returned {@link Condition} instance supports the same
1154 * usages as do the {@link Object} monitor methods ({@link
1155 * Object#wait() wait}, {@link Object#notify notify}, and {@link
1156 * Object#notifyAll notifyAll}) when used with the built-in
1157 * monitor lock.
1158 *
1159 * <ul>
1160 *
1161 * <li>If this write lock is not held when any {@link
1162 * Condition} method is called then an {@link
1163 * IllegalMonitorStateException} is thrown. (Read locks are
1164 * held independently of write locks, so are not checked or
1165 * affected. However it is essentially always an error to
1166 * invoke a condition waiting method when the current thread
1167 * has also acquired read locks, since other threads that
1168 * could unblock it will not be able to acquire the write
1169 * lock.)
1170 *
1171 * <li>When the condition {@linkplain Condition#await() waiting}
1172 * methods are called the write lock is released and, before
1173 * they return, the write lock is reacquired and the lock hold
1174 * count restored to what it was when the method was called.
1175 *
1176 * <li>If a thread is {@linkplain Thread#interrupt interrupted} while
1177 * waiting then the wait will terminate, an {@link
1178 * InterruptedException} will be thrown, and the thread's
1179 * interrupted status will be cleared.
1180 *
1181 * <li>Waiting threads are signalled in FIFO order.
1182 *
1183 * <li>The ordering of lock reacquisition for threads returning
1184 * from waiting methods is the same as for threads initially
1185 * acquiring the lock, which is in the default case not specified,
1186 * but for <em>fair</em> locks favors those threads that have been
1187 * waiting the longest.
1188 *
1189 * </ul>
1190 *
1191 * @return the Condition object
1192 */
1193 public Condition newCondition() {
1194 return sync.newCondition();
1195 }
1196
1197 /**
1198 * Returns a string identifying this lock, as well as its lock
1199 * state. The state, in brackets includes either the String
1200 * {@code "Unlocked"} or the String {@code "Locked by"}
1201 * followed by the {@linkplain Thread#getName name} of the owning thread.
1202 *
1203 * @return a string identifying this lock, as well as its lock state
1204 */
1205 public String toString() {
1206 Thread o = sync.getOwner();
1207 return super.toString() + ((o == null) ?
1208 "[Unlocked]" :
1209 "[Locked by thread " + o.getName() + "]");
1210 }
1211
1212 /**
1213 * Queries if this write lock is held by the current thread.
1214 * Identical in effect to {@link
1215 * ReentrantReadWriteLock#isWriteLockedByCurrentThread}.
1216 *
1217 * @return {@code true} if the current thread holds this lock and
1218 * {@code false} otherwise
1219 * @since 1.6
1220 */
1221 public boolean isHeldByCurrentThread() {
1222 return sync.isHeldExclusively();
1223 }
1224
1225 /**
1226 * Queries the number of holds on this write lock by the current
1227 * thread. A thread has a hold on a lock for each lock action
1228 * that is not matched by an unlock action. Identical in effect
1229 * to {@link ReentrantReadWriteLock#getWriteHoldCount}.
1230 *
1231 * @return the number of holds on this lock by the current thread,
1232 * or zero if this lock is not held by the current thread
1233 * @since 1.6
1234 */
1235 public int getHoldCount() {
1236 return sync.getWriteHoldCount();
1237 }
1238 }
1239
1240 // Instrumentation and status
1241
1242 /**
1243 * Returns {@code true} if this lock has fairness set true.
1244 *
1245 * @return {@code true} if this lock has fairness set true
1246 */
1247 public final boolean isFair() {
1248 return sync instanceof FairSync;
1249 }
1250
1251 /**
1252 * Returns the thread that currently owns the write lock, or
1253 * {@code null} if not owned. When this method is called by a
1254 * thread that is not the owner, the return value reflects a
1255 * best-effort approximation of current lock status. For example,
1256 * the owner may be momentarily {@code null} even if there are
1257 * threads trying to acquire the lock but have not yet done so.
1258 * This method is designed to facilitate construction of
1259 * subclasses that provide more extensive lock monitoring
1260 * facilities.
1261 *
1262 * @return the owner, or {@code null} if not owned
1263 */
1264 protected Thread getOwner() {
1265 return sync.getOwner();
1266 }
1267
1268 /**
1269 * Queries the number of read locks held for this lock. This
1270 * method is designed for use in monitoring system state, not for
1271 * synchronization control.
1272 * @return the number of read locks held
1273 */
1274 public int getReadLockCount() {
1275 return sync.getReadLockCount();
1276 }
1277
1278 /**
1279 * Queries if the write lock is held by any thread. This method is
1280 * designed for use in monitoring system state, not for
1281 * synchronization control.
1282 *
1283 * @return {@code true} if any thread holds the write lock and
1284 * {@code false} otherwise
1285 */
1286 public boolean isWriteLocked() {
1287 return sync.isWriteLocked();
1288 }
1289
1290 /**
1291 * Queries if the write lock is held by the current thread.
1292 *
1293 * @return {@code true} if the current thread holds the write lock and
1294 * {@code false} otherwise
1295 */
1296 public boolean isWriteLockedByCurrentThread() {
1297 return sync.isHeldExclusively();
1298 }
1299
1300 /**
1301 * Queries the number of reentrant write holds on this lock by the
1302 * current thread. A writer thread has a hold on a lock for
1303 * each lock action that is not matched by an unlock action.
1304 *
1305 * @return the number of holds on the write lock by the current thread,
1306 * or zero if the write lock is not held by the current thread
1307 */
1308 public int getWriteHoldCount() {
1309 return sync.getWriteHoldCount();
1310 }
1311
1312 /**
1313 * Queries the number of reentrant read holds on this lock by the
1314 * current thread. A reader thread has a hold on a lock for
1315 * each lock action that is not matched by an unlock action.
1316 *
1317 * @return the number of holds on the read lock by the current thread,
1318 * or zero if the read lock is not held by the current thread
1319 * @since 1.6
1320 */
1321 public int getReadHoldCount() {
1322 return sync.getReadHoldCount();
1323 }
1324
1325 /**
1326 * Returns a collection containing threads that may be waiting to
1327 * acquire the write lock. Because the actual set of threads may
1328 * change dynamically while constructing this result, the returned
1329 * collection is only a best-effort estimate. The elements of the
1330 * returned collection are in no particular order. This method is
1331 * designed to facilitate construction of subclasses that provide
1332 * more extensive lock monitoring facilities.
1333 *
1334 * @return the collection of threads
1335 */
1336 protected Collection<Thread> getQueuedWriterThreads() {
1337 return sync.getExclusiveQueuedThreads();
1338 }
1339
1340 /**
1341 * Returns a collection containing threads that may be waiting to
1342 * acquire the read lock. Because the actual set of threads may
1343 * change dynamically while constructing this result, the returned
1344 * collection is only a best-effort estimate. The elements of the
1345 * returned collection are in no particular order. This method is
1346 * designed to facilitate construction of subclasses that provide
1347 * more extensive lock monitoring facilities.
1348 *
1349 * @return the collection of threads
1350 */
1351 protected Collection<Thread> getQueuedReaderThreads() {
1352 return sync.getSharedQueuedThreads();
1353 }
1354
1355 /**
1356 * Queries whether any threads are waiting to acquire the read or
1357 * write lock. Note that because cancellations may occur at any
1358 * time, a {@code true} return does not guarantee that any other
1359 * thread will ever acquire a lock. This method is designed
1360 * primarily for use in monitoring of the system state.
1361 *
1362 * @return {@code true} if there may be other threads waiting to
1363 * acquire the lock
1364 */
1365 public final boolean hasQueuedThreads() {
1366 return sync.hasQueuedThreads();
1367 }
1368
1369 /**
1370 * Queries whether the given thread is waiting to acquire either
1371 * the read or write lock. Note that because cancellations may
1372 * occur at any time, a {@code true} return does not guarantee
1373 * that this thread will ever acquire a lock. This method is
1374 * designed primarily for use in monitoring of the system state.
1375 *
1376 * @param thread the thread
1377 * @return {@code true} if the given thread is queued waiting for this lock
1378 * @throws NullPointerException if the thread is null
1379 */
1380 public final boolean hasQueuedThread(Thread thread) {
1381 return sync.isQueued(thread);
1382 }
1383
1384 /**
1385 * Returns an estimate of the number of threads waiting to acquire
1386 * either the read or write lock. The value is only an estimate
1387 * because the number of threads may change dynamically while this
1388 * method traverses internal data structures. This method is
1389 * designed for use in monitoring system state, not for
1390 * synchronization control.
1391 *
1392 * @return the estimated number of threads waiting for this lock
1393 */
1394 public final int getQueueLength() {
1395 return sync.getQueueLength();
1396 }
1397
1398 /**
1399 * Returns a collection containing threads that may be waiting to
1400 * acquire either the read or write lock. Because the actual set
1401 * of threads may change dynamically while constructing this
1402 * result, the returned collection is only a best-effort estimate.
1403 * The elements of the returned collection are in no particular
1404 * order. This method is designed to facilitate construction of
1405 * subclasses that provide more extensive monitoring facilities.
1406 *
1407 * @return the collection of threads
1408 */
1409 protected Collection<Thread> getQueuedThreads() {
1410 return sync.getQueuedThreads();
1411 }
1412
1413 /**
1414 * Queries whether any threads are waiting on the given condition
1415 * associated with the write lock. Note that because timeouts and
1416 * interrupts may occur at any time, a {@code true} return does
1417 * not guarantee that a future {@code signal} will awaken any
1418 * threads. This method is designed primarily for use in
1419 * monitoring of the system state.
1420 *
1421 * @param condition the condition
1422 * @return {@code true} if there are any waiting 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 public boolean hasWaiters(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.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
1434 }
1435
1436 /**
1437 * Returns an estimate of the number of threads waiting on the
1438 * given condition associated with the write lock. Note that because
1439 * timeouts and interrupts may occur at any time, the estimate
1440 * serves only as an upper bound on the actual number of waiters.
1441 * This method is designed for use in monitoring of the system
1442 * state, not for synchronization control.
1443 *
1444 * @param condition the condition
1445 * @return the estimated number of waiting threads
1446 * @throws IllegalMonitorStateException if this lock is not held
1447 * @throws IllegalArgumentException if the given condition is
1448 * not associated with this lock
1449 * @throws NullPointerException if the condition is null
1450 */
1451 public int getWaitQueueLength(Condition condition) {
1452 if (condition == null)
1453 throw new NullPointerException();
1454 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
1455 throw new IllegalArgumentException("not owner");
1456 return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
1457 }
1458
1459 /**
1460 * Returns a collection containing those threads that may be
1461 * waiting on the given condition associated with the write lock.
1462 * Because the actual set of threads may change dynamically while
1463 * constructing this result, the returned collection is only a
1464 * best-effort estimate. The elements of the returned collection
1465 * are in no particular order. This method is designed to
1466 * facilitate construction of subclasses that provide more
1467 * extensive condition monitoring facilities.
1468 *
1469 * @param condition the condition
1470 * @return the collection of threads
1471 * @throws IllegalMonitorStateException if this lock is not held
1472 * @throws IllegalArgumentException if the given condition is
1473 * not associated with this lock
1474 * @throws NullPointerException if the condition is null
1475 */
1476 protected Collection<Thread> getWaitingThreads(Condition condition) {
1477 if (condition == null)
1478 throw new NullPointerException();
1479 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
1480 throw new IllegalArgumentException("not owner");
1481 return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
1482 }
1483
1484 /**
1485 * Returns a string identifying this lock, as well as its lock state.
1486 * The state, in brackets, includes the String {@code "Write locks ="}
1487 * followed by the number of reentrantly held write locks, and the
1488 * String {@code "Read locks ="} followed by the number of held
1489 * read locks.
1490 *
1491 * @return a string identifying this lock, as well as its lock state
1492 */
1493 public String toString() {
1494 int c = sync.getCount();
1495 int w = Sync.exclusiveCount(c);
1496 int r = Sync.sharedCount(c);
1497
1498 return super.toString() +
1499 "[Write locks = " + w + ", Read locks = " + r + "]";
1500 }
1501
1502 }
1503