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;
37
38 import java.util.Collection;
39 import java.util.concurrent.locks.AbstractQueuedSynchronizer;
40
41 /**
42 * A counting semaphore. Conceptually, a semaphore maintains a set of
43 * permits. Each {@link #acquire} blocks if necessary until a permit is
44 * available, and then takes it. Each {@link #release} adds a permit,
45 * potentially releasing a blocking acquirer.
46 * However, no actual permit objects are used; the {@code Semaphore} just
47 * keeps a count of the number available and acts accordingly.
48 *
49 * <p>Semaphores are often used to restrict the number of threads than can
50 * access some (physical or logical) resource. For example, here is
51 * a class that uses a semaphore to control access to a pool of items:
52 * <pre> {@code
53 * class Pool {
54 * private static final int MAX_AVAILABLE = 100;
55 * private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
56 *
57 * public Object getItem() throws InterruptedException {
58 * available.acquire();
59 * return getNextAvailableItem();
60 * }
61 *
62 * public void putItem(Object x) {
63 * if (markAsUnused(x))
64 * available.release();
65 * }
66 *
67 * // Not a particularly efficient data structure; just for demo
68 *
69 * protected Object[] items = ... whatever kinds of items being managed
70 * protected boolean[] used = new boolean[MAX_AVAILABLE];
71 *
72 * protected synchronized Object getNextAvailableItem() {
73 * for (int i = 0; i < MAX_AVAILABLE; ++i) {
74 * if (!used[i]) {
75 * used[i] = true;
76 * return items[i];
77 * }
78 * }
79 * return null; // not reached
80 * }
81 *
82 * protected synchronized boolean markAsUnused(Object item) {
83 * for (int i = 0; i < MAX_AVAILABLE; ++i) {
84 * if (item == items[i]) {
85 * if (used[i]) {
86 * used[i] = false;
87 * return true;
88 * } else
89 * return false;
90 * }
91 * }
92 * return false;
93 * }
94 * }}</pre>
95 *
96 * <p>Before obtaining an item each thread must acquire a permit from
97 * the semaphore, guaranteeing that an item is available for use. When
98 * the thread has finished with the item it is returned back to the
99 * pool and a permit is returned to the semaphore, allowing another
100 * thread to acquire that item. Note that no synchronization lock is
101 * held when {@link #acquire} is called as that would prevent an item
102 * from being returned to the pool. The semaphore encapsulates the
103 * synchronization needed to restrict access to the pool, separately
104 * from any synchronization needed to maintain the consistency of the
105 * pool itself.
106 *
107 * <p>A semaphore initialized to one, and which is used such that it
108 * only has at most one permit available, can serve as a mutual
109 * exclusion lock. This is more commonly known as a <em>binary
110 * semaphore</em>, because it only has two states: one permit
111 * available, or zero permits available. When used in this way, the
112 * binary semaphore has the property (unlike many {@link java.util.concurrent.locks.Lock}
113 * implementations), that the "lock" can be released by a
114 * thread other than the owner (as semaphores have no notion of
115 * ownership). This can be useful in some specialized contexts, such
116 * as deadlock recovery.
117 *
118 * <p>The constructor for this class optionally accepts a
119 * <em>fairness</em> parameter. When set false, this class makes no
120 * guarantees about the order in which threads acquire permits. In
121 * particular, <em>barging</em> is permitted, that is, a thread
122 * invoking {@link #acquire} can be allocated a permit ahead of a
123 * thread that has been waiting - logically the new thread places itself at
124 * the head of the queue of waiting threads. When fairness is set true, the
125 * semaphore guarantees that threads invoking any of the {@link
126 * #acquire() acquire} methods are selected to obtain permits in the order in
127 * which their invocation of those methods was processed
128 * (first-in-first-out; FIFO). Note that FIFO ordering necessarily
129 * applies to specific internal points of execution within these
130 * methods. So, it is possible for one thread to invoke
131 * {@code acquire} before another, but reach the ordering point after
132 * the other, and similarly upon return from the method.
133 * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
134 * honor the fairness setting, but will take any permits that are
135 * available.
136 *
137 * <p>Generally, semaphores used to control resource access should be
138 * initialized as fair, to ensure that no thread is starved out from
139 * accessing a resource. When using semaphores for other kinds of
140 * synchronization control, the throughput advantages of non-fair
141 * ordering often outweigh fairness considerations.
142 *
143 * <p>This class also provides convenience methods to {@link
144 * #acquire(int) acquire} and {@link #release(int) release} multiple
145 * permits at a time. These methods are generally more efficient and
146 * effective than loops. However, they do not establish any preference
147 * order. For example, if thread A invokes {@code s.acquire(3}) and
148 * thread B invokes {@code s.acquire(2)}, and two permits become
149 * available, then there is no guarantee that thread B will obtain
150 * them unless its acquire came first and Semaphore {@code s} is in
151 * fair mode.
152 *
153 * <p>Memory consistency effects: Actions in a thread prior to calling
154 * a "release" method such as {@code release()}
155 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
156 * actions following a successful "acquire" method such as {@code acquire()}
157 * in another thread.
158 *
159 * @since 1.5
160 * @author Doug Lea
161 */
162 public class Semaphore implements java.io.Serializable {
163 private static final long serialVersionUID = -3222578661600680210L;
164 /** All mechanics via AbstractQueuedSynchronizer subclass */
165 private final Sync sync;
166
167 /**
168 * Synchronization implementation for semaphore. Uses AQS state
169 * to represent permits. Subclassed into fair and nonfair
170 * versions.
171 */
172 abstract static class Sync extends AbstractQueuedSynchronizer {
173 private static final long serialVersionUID = 1192457210091910933L;
174
175 Sync(int permits) {
176 setState(permits);
177 }
178
179 final int getPermits() {
180 return getState();
181 }
182
183 final int nonfairTryAcquireShared(int acquires) {
184 for (;;) {
185 int available = getState();
186 int remaining = available - acquires;
187 if (remaining < 0 ||
188 compareAndSetState(available, remaining))
189 return remaining;
190 }
191 }
192
193 protected final boolean tryReleaseShared(int releases) {
194 for (;;) {
195 int current = getState();
196 int next = current + releases;
197 if (next < current) // overflow
198 throw new Error("Maximum permit count exceeded");
199 if (compareAndSetState(current, next))
200 return true;
201 }
202 }
203
204 final void reducePermits(int reductions) {
205 for (;;) {
206 int current = getState();
207 int next = current - reductions;
208 if (next > current) // underflow
209 throw new Error("Permit count underflow");
210 if (compareAndSetState(current, next))
211 return;
212 }
213 }
214
215 final int drainPermits() {
216 for (;;) {
217 int current = getState();
218 if (current == 0 || compareAndSetState(current, 0))
219 return current;
220 }
221 }
222 }
223
224 /**
225 * NonFair version
226 */
227 static final class NonfairSync extends Sync {
228 private static final long serialVersionUID = -2694183684443567898L;
229
230 NonfairSync(int permits) {
231 super(permits);
232 }
233
234 protected int tryAcquireShared(int acquires) {
235 return nonfairTryAcquireShared(acquires);
236 }
237 }
238
239 /**
240 * Fair version
241 */
242 static final class FairSync extends Sync {
243 private static final long serialVersionUID = 2014338818796000944L;
244
245 FairSync(int permits) {
246 super(permits);
247 }
248
249 protected int tryAcquireShared(int acquires) {
250 for (;;) {
251 if (hasQueuedPredecessors())
252 return -1;
253 int available = getState();
254 int remaining = available - acquires;
255 if (remaining < 0 ||
256 compareAndSetState(available, remaining))
257 return remaining;
258 }
259 }
260 }
261
262 /**
263 * Creates a {@code Semaphore} with the given number of
264 * permits and nonfair fairness setting.
265 *
266 * @param permits the initial number of permits available.
267 * This value may be negative, in which case releases
268 * must occur before any acquires will be granted.
269 */
270 public Semaphore(int permits) {
271 sync = new NonfairSync(permits);
272 }
273
274 /**
275 * Creates a {@code Semaphore} with the given number of
276 * permits and the given fairness setting.
277 *
278 * @param permits the initial number of permits available.
279 * This value may be negative, in which case releases
280 * must occur before any acquires will be granted.
281 * @param fair {@code true} if this semaphore will guarantee
282 * first-in first-out granting of permits under contention,
283 * else {@code false}
284 */
285 public Semaphore(int permits, boolean fair) {
286 sync = fair ? new FairSync(permits) : new NonfairSync(permits);
287 }
288
289 /**
290 * Acquires a permit from this semaphore, blocking until one is
291 * available, or the thread is {@linkplain Thread#interrupt interrupted}.
292 *
293 * <p>Acquires a permit, if one is available and returns immediately,
294 * reducing the number of available permits by one.
295 *
296 * <p>If no permit is available then the current thread becomes
297 * disabled for thread scheduling purposes and lies dormant until
298 * one of two things happens:
299 * <ul>
300 * <li>Some other thread invokes the {@link #release} method for this
301 * semaphore and the current thread is next to be assigned a permit; or
302 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
303 * the current thread.
304 * </ul>
305 *
306 * <p>If the current thread:
307 * <ul>
308 * <li>has its interrupted status set on entry to this method; or
309 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
310 * for a permit,
311 * </ul>
312 * then {@link InterruptedException} is thrown and the current thread's
313 * interrupted status is cleared.
314 *
315 * @throws InterruptedException if the current thread is interrupted
316 */
317 public void acquire() throws InterruptedException {
318 sync.acquireSharedInterruptibly(1);
319 }
320
321 /**
322 * Acquires a permit from this semaphore, blocking until one is
323 * available.
324 *
325 * <p>Acquires a permit, if one is available and returns immediately,
326 * reducing the number of available permits by one.
327 *
328 * <p>If no permit is available then the current thread becomes
329 * disabled for thread scheduling purposes and lies dormant until
330 * some other thread invokes the {@link #release} method for this
331 * semaphore and the current thread is next to be assigned a permit.
332 *
333 * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
334 * while waiting for a permit then it will continue to wait, but the
335 * time at which the thread is assigned a permit may change compared to
336 * the time it would have received the permit had no interruption
337 * occurred. When the thread does return from this method its interrupt
338 * status will be set.
339 */
340 public void acquireUninterruptibly() {
341 sync.acquireShared(1);
342 }
343
344 /**
345 * Acquires a permit from this semaphore, only if one is available at the
346 * time of invocation.
347 *
348 * <p>Acquires a permit, if one is available and returns immediately,
349 * with the value {@code true},
350 * reducing the number of available permits by one.
351 *
352 * <p>If no permit is available then this method will return
353 * immediately with the value {@code false}.
354 *
355 * <p>Even when this semaphore has been set to use a
356 * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
357 * immediately acquire a permit if one is available, whether or not
358 * other threads are currently waiting.
359 * This "barging" behavior can be useful in certain
360 * circumstances, even though it breaks fairness. If you want to honor
361 * the fairness setting, then use
362 * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS)}
363 * which is almost equivalent (it also detects interruption).
364 *
365 * @return {@code true} if a permit was acquired and {@code false}
366 * otherwise
367 */
368 public boolean tryAcquire() {
369 return sync.nonfairTryAcquireShared(1) >= 0;
370 }
371
372 /**
373 * Acquires a permit from this semaphore, if one becomes available
374 * within the given waiting time and the current thread has not
375 * been {@linkplain Thread#interrupt interrupted}.
376 *
377 * <p>Acquires a permit, if one is available and returns immediately,
378 * with the value {@code true},
379 * reducing the number of available permits by one.
380 *
381 * <p>If no permit is available then the current thread becomes
382 * disabled for thread scheduling purposes and lies dormant until
383 * one of three things happens:
384 * <ul>
385 * <li>Some other thread invokes the {@link #release} method for this
386 * semaphore and the current thread is next to be assigned a permit; or
387 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
388 * the current thread; or
389 * <li>The specified waiting time elapses.
390 * </ul>
391 *
392 * <p>If a permit is acquired then the value {@code true} is returned.
393 *
394 * <p>If the current thread:
395 * <ul>
396 * <li>has its interrupted status set on entry to this method; or
397 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
398 * to acquire a permit,
399 * </ul>
400 * then {@link InterruptedException} is thrown and the current thread's
401 * interrupted status is cleared.
402 *
403 * <p>If the specified waiting time elapses then the value {@code false}
404 * is returned. If the time is less than or equal to zero, the method
405 * will not wait at all.
406 *
407 * @param timeout the maximum time to wait for a permit
408 * @param unit the time unit of the {@code timeout} argument
409 * @return {@code true} if a permit was acquired and {@code false}
410 * if the waiting time elapsed before a permit was acquired
411 * @throws InterruptedException if the current thread is interrupted
412 */
413 public boolean tryAcquire(long timeout, TimeUnit unit)
414 throws InterruptedException {
415 return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
416 }
417
418 /**
419 * Releases a permit, returning it to the semaphore.
420 *
421 * <p>Releases a permit, increasing the number of available permits by
422 * one. If any threads are trying to acquire a permit, then one is
423 * selected and given the permit that was just released. That thread
424 * is (re)enabled for thread scheduling purposes.
425 *
426 * <p>There is no requirement that a thread that releases a permit must
427 * have acquired that permit by calling {@link #acquire}.
428 * Correct usage of a semaphore is established by programming convention
429 * in the application.
430 */
431 public void release() {
432 sync.releaseShared(1);
433 }
434
435 /**
436 * Acquires the given number of permits from this semaphore,
437 * blocking until all are available,
438 * or the thread is {@linkplain Thread#interrupt interrupted}.
439 *
440 * <p>Acquires the given number of permits, if they are available,
441 * and returns immediately, reducing the number of available permits
442 * by the given amount. This method has the same effect as the
443 * loop {@code for (int i = 0; i < permits; ++i) acquire();} except
444 * that it atomically acquires the permits all at once:
445 *
446 * <p>If insufficient permits are available then the current thread becomes
447 * disabled for thread scheduling purposes and lies dormant until
448 * one of two things happens:
449 * <ul>
450 * <li>Some other thread invokes one of the {@link #release() release}
451 * methods for this semaphore and the current thread is next to be assigned
452 * permits and the number of available permits satisfies this request; or
453 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
454 * the current thread.
455 * </ul>
456 *
457 * <p>If the current thread:
458 * <ul>
459 * <li>has its interrupted status set on entry to this method; or
460 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
461 * for a permit,
462 * </ul>
463 * then {@link InterruptedException} is thrown and the current thread's
464 * interrupted status is cleared.
465 * Any permits that were to be assigned to this thread are instead
466 * assigned to other threads trying to acquire permits, as if
467 * permits had been made available by a call to {@link #release()}.
468 *
469 * @param permits the number of permits to acquire
470 * @throws InterruptedException if the current thread is interrupted
471 * @throws IllegalArgumentException if {@code permits} is negative
472 */
473 public void acquire(int permits) throws InterruptedException {
474 if (permits < 0) throw new IllegalArgumentException();
475 sync.acquireSharedInterruptibly(permits);
476 }
477
478 /**
479 * Acquires the given number of permits from this semaphore,
480 * blocking until all are available.
481 *
482 * <p>Acquires the given number of permits, if they are available,
483 * and returns immediately, reducing the number of available permits
484 * by the given amount. This method has the same effect as the
485 * loop {@code for (int i = 0; i < permits; ++i) acquireUninterruptibly();}
486 * except that it atomically acquires the permits all at once:
487 *
488 * <p>If insufficient permits are available then the current thread becomes
489 * disabled for thread scheduling purposes and lies dormant until
490 * some other thread invokes one of the {@link #release() release}
491 * methods for this semaphore and the current thread is next to be assigned
492 * permits and the number of available permits satisfies this request.
493 *
494 * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
495 * while waiting for permits then it will continue to wait and its
496 * position in the queue is not affected. When the thread does return
497 * from this method its interrupt status will be set.
498 *
499 * @param permits the number of permits to acquire
500 * @throws IllegalArgumentException if {@code permits} is negative
501 */
502 public void acquireUninterruptibly(int permits) {
503 if (permits < 0) throw new IllegalArgumentException();
504 sync.acquireShared(permits);
505 }
506
507 /**
508 * Acquires the given number of permits from this semaphore, only
509 * if all are available at the time of invocation.
510 *
511 * <p>Acquires the given number of permits, if they are available, and
512 * returns immediately, with the value {@code true},
513 * reducing the number of available permits by the given amount.
514 *
515 * <p>If insufficient permits are available then this method will return
516 * immediately with the value {@code false} and the number of available
517 * permits is unchanged.
518 *
519 * <p>Even when this semaphore has been set to use a fair ordering
520 * policy, a call to {@code tryAcquire} <em>will</em>
521 * immediately acquire a permit if one is available, whether or
522 * not other threads are currently waiting. This
523 * "barging" behavior can be useful in certain
524 * circumstances, even though it breaks fairness. If you want to
525 * honor the fairness setting, then use {@link #tryAcquire(int,
526 * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS)}
527 * which is almost equivalent (it also detects interruption).
528 *
529 * @param permits the number of permits to acquire
530 * @return {@code true} if the permits were acquired and
531 * {@code false} otherwise
532 * @throws IllegalArgumentException if {@code permits} is negative
533 */
534 public boolean tryAcquire(int permits) {
535 if (permits < 0) throw new IllegalArgumentException();
536 return sync.nonfairTryAcquireShared(permits) >= 0;
537 }
538
539 /**
540 * Acquires the given number of permits from this semaphore, if all
541 * become available within the given waiting time and the current
542 * thread has not been {@linkplain Thread#interrupt interrupted}.
543 *
544 * <p>Acquires the given number of permits, if they are available and
545 * returns immediately, with the value {@code true},
546 * reducing the number of available permits by the given amount.
547 *
548 * <p>If insufficient permits are available then
549 * the current thread becomes disabled for thread scheduling
550 * purposes and lies dormant until one of three things happens:
551 * <ul>
552 * <li>Some other thread invokes one of the {@link #release() release}
553 * methods for this semaphore and the current thread is next to be assigned
554 * permits and the number of available permits satisfies this request; or
555 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
556 * the current thread; or
557 * <li>The specified waiting time elapses.
558 * </ul>
559 *
560 * <p>If the permits are acquired then the value {@code true} is returned.
561 *
562 * <p>If the current thread:
563 * <ul>
564 * <li>has its interrupted status set on entry to this method; or
565 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
566 * to acquire the permits,
567 * </ul>
568 * then {@link InterruptedException} is thrown and the current thread's
569 * interrupted status is cleared.
570 * Any permits that were to be assigned to this thread, are instead
571 * assigned to other threads trying to acquire permits, as if
572 * the permits had been made available by a call to {@link #release()}.
573 *
574 * <p>If the specified waiting time elapses then the value {@code false}
575 * is returned. If the time is less than or equal to zero, the method
576 * will not wait at all. Any permits that were to be assigned to this
577 * thread, are instead assigned to other threads trying to acquire
578 * permits, as if the permits had been made available by a call to
579 * {@link #release()}.
580 *
581 * @param permits the number of permits to acquire
582 * @param timeout the maximum time to wait for the permits
583 * @param unit the time unit of the {@code timeout} argument
584 * @return {@code true} if all permits were acquired and {@code false}
585 * if the waiting time elapsed before all permits were acquired
586 * @throws InterruptedException if the current thread is interrupted
587 * @throws IllegalArgumentException if {@code permits} is negative
588 */
589 public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
590 throws InterruptedException {
591 if (permits < 0) throw new IllegalArgumentException();
592 return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
593 }
594
595 /**
596 * Releases the given number of permits, returning them to the semaphore.
597 *
598 * <p>Releases the given number of permits, increasing the number of
599 * available permits by that amount.
600 * If any threads are trying to acquire permits, then one thread
601 * is selected and given the permits that were just released.
602 * If the number of available permits satisfies that thread's request
603 * then that thread is (re)enabled for thread scheduling purposes;
604 * otherwise the thread will wait until sufficient permits are available.
605 * If there are still permits available
606 * after this thread's request has been satisfied, then those permits
607 * are assigned in turn to other threads trying to acquire permits.
608 *
609 * <p>There is no requirement that a thread that releases a permit must
610 * have acquired that permit by calling {@link Semaphore#acquire acquire}.
611 * Correct usage of a semaphore is established by programming convention
612 * in the application.
613 *
614 * @param permits the number of permits to release
615 * @throws IllegalArgumentException if {@code permits} is negative
616 */
617 public void release(int permits) {
618 if (permits < 0) throw new IllegalArgumentException();
619 sync.releaseShared(permits);
620 }
621
622 /**
623 * Returns the current number of permits available in this semaphore.
624 *
625 * <p>This method is typically used for debugging and testing purposes.
626 *
627 * @return the number of permits available in this semaphore
628 */
629 public int availablePermits() {
630 return sync.getPermits();
631 }
632
633 /**
634 * Acquires and returns all permits that are immediately
635 * available, or if negative permits are available, releases them.
636 * Upon return, zero permits are available.
637 *
638 * @return the number of permits acquired or, if negative, the
639 * number released
640 */
641 public int drainPermits() {
642 return sync.drainPermits();
643 }
644
645 /**
646 * Shrinks the number of available permits by the indicated
647 * reduction. This method can be useful in subclasses that use
648 * semaphores to track resources that become unavailable. This
649 * method differs from {@code acquire} in that it does not block
650 * waiting for permits to become available.
651 *
652 * @param reduction the number of permits to remove
653 * @throws IllegalArgumentException if {@code reduction} is negative
654 */
655 protected void reducePermits(int reduction) {
656 if (reduction < 0) throw new IllegalArgumentException();
657 sync.reducePermits(reduction);
658 }
659
660 /**
661 * Returns {@code true} if this semaphore has fairness set true.
662 *
663 * @return {@code true} if this semaphore has fairness set true
664 */
665 public boolean isFair() {
666 return sync instanceof FairSync;
667 }
668
669 /**
670 * Queries whether any threads are waiting to acquire. Note that
671 * because cancellations may occur at any time, a {@code true}
672 * return does not guarantee that any other thread will ever
673 * acquire. This method is designed primarily for use in
674 * monitoring of the system state.
675 *
676 * @return {@code true} if there may be other threads waiting to
677 * acquire the lock
678 */
679 public final boolean hasQueuedThreads() {
680 return sync.hasQueuedThreads();
681 }
682
683 /**
684 * Returns an estimate of the number of threads waiting to acquire.
685 * The value is only an estimate because the number of threads may
686 * change dynamically while this method traverses internal data
687 * structures. This method is designed for use in monitoring
688 * system state, not for synchronization control.
689 *
690 * @return the estimated number of threads waiting for this lock
691 */
692 public final int getQueueLength() {
693 return sync.getQueueLength();
694 }
695
696 /**
697 * Returns a collection containing threads that may be waiting to acquire.
698 * Because the actual set of threads may change dynamically while
699 * constructing this result, the returned collection is only a best-effort
700 * estimate. The elements of the returned collection are in no particular
701 * order. This method is designed to facilitate construction of
702 * subclasses that provide more extensive monitoring facilities.
703 *
704 * @return the collection of threads
705 */
706 protected Collection<Thread> getQueuedThreads() {
707 return sync.getQueuedThreads();
708 }
709
710 /**
711 * Returns a string identifying this semaphore, as well as its state.
712 * The state, in brackets, includes the String {@code "Permits ="}
713 * followed by the number of permits.
714 *
715 * @return a string identifying this semaphore, as well as its state
716 */
717 public String toString() {
718 return super.toString() + "[Permits = " + sync.getPermits() + "]";
719 }
720 }
721