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
7 package java.util.concurrent;
9 import java.util.concurrent.locks.*;
10 import java.util.concurrent.atomic.*;
13 * A counting semaphore. Conceptually, a semaphore maintains a set of
14 * permits. Each {@link #acquire} blocks if necessary until a permit is
15 * available, and then takes it. Each {@link #release} adds a permit,
16 * potentially releasing a blocking acquirer.
17 * However, no actual permit objects are used; the {@code Semaphore} just
18 * keeps a count of the number available and acts accordingly.
20 * <p>Semaphores are often used to restrict the number of threads than can
21 * access some (physical or logical) resource. For example, here is
22 * a class that uses a semaphore to control access to a pool of items:
25 * private static final int MAX_AVAILABLE = 100;
26 * private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
28 * public Object getItem() throws InterruptedException {
29 * available.acquire();
30 * return getNextAvailableItem();
33 * public void putItem(Object x) {
34 * if (markAsUnused(x))
35 * available.release();
38 * // Not a particularly efficient data structure; just for demo
40 * protected Object[] items = ... whatever kinds of items being managed
41 * protected boolean[] used = new boolean[MAX_AVAILABLE];
43 * protected synchronized Object getNextAvailableItem() {
44 * for (int i = 0; i < MAX_AVAILABLE; ++i) {
50 * return null; // not reached
53 * protected synchronized boolean markAsUnused(Object item) {
54 * for (int i = 0; i < MAX_AVAILABLE; ++i) {
55 * if (item == items[i]) {
69 * <p>Before obtaining an item each thread must acquire a permit from
70 * the semaphore, guaranteeing that an item is available for use. When
71 * the thread has finished with the item it is returned back to the
72 * pool and a permit is returned to the semaphore, allowing another
73 * thread to acquire that item. Note that no synchronization lock is
74 * held when {@link #acquire} is called as that would prevent an item
75 * from being returned to the pool. The semaphore encapsulates the
76 * synchronization needed to restrict access to the pool, separately
77 * from any synchronization needed to maintain the consistency of the
80 * <p>A semaphore initialized to one, and which is used such that it
81 * only has at most one permit available, can serve as a mutual
82 * exclusion lock. This is more commonly known as a <em>binary
83 * semaphore</em>, because it only has two states: one permit
84 * available, or zero permits available. When used in this way, the
85 * binary semaphore has the property (unlike many {@link Lock}
86 * implementations), that the "lock" can be released by a
87 * thread other than the owner (as semaphores have no notion of
88 * ownership). This can be useful in some specialized contexts, such
89 * as deadlock recovery.
91 * <p> The constructor for this class optionally accepts a
92 * <em>fairness</em> parameter. When set false, this class makes no
93 * guarantees about the order in which threads acquire permits. In
94 * particular, <em>barging</em> is permitted, that is, a thread
95 * invoking {@link #acquire} can be allocated a permit ahead of a
96 * thread that has been waiting - logically the new thread places itself at
97 * the head of the queue of waiting threads. When fairness is set true, the
98 * semaphore guarantees that threads invoking any of the {@link
99 * #acquire() acquire} methods are selected to obtain permits in the order in
100 * which their invocation of those methods was processed
101 * (first-in-first-out; FIFO). Note that FIFO ordering necessarily
102 * applies to specific internal points of execution within these
103 * methods. So, it is possible for one thread to invoke
104 * {@code acquire} before another, but reach the ordering point after
105 * the other, and similarly upon return from the method.
106 * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
107 * honor the fairness setting, but will take any permits that are
110 * <p>Generally, semaphores used to control resource access should be
111 * initialized as fair, to ensure that no thread is starved out from
112 * accessing a resource. When using semaphores for other kinds of
113 * synchronization control, the throughput advantages of non-fair
114 * ordering often outweigh fairness considerations.
116 * <p>This class also provides convenience methods to {@link
117 * #acquire(int) acquire} and {@link #release(int) release} multiple
118 * permits at a time. Beware of the increased risk of indefinite
119 * postponement when these methods are used without fairness set true.
121 * <p>Memory consistency effects: Actions in a thread prior to calling
122 * a "release" method such as {@code release()}
123 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
124 * actions following a successful "acquire" method such as {@code acquire()}
132 public class Semaphore implements java.io.Serializable {
133 private static final long serialVersionUID = -3222578661600680210L;
134 /** All mechanics via AbstractQueuedSynchronizer subclass */
135 private final Sync sync;
138 * Synchronization implementation for semaphore. Uses AQS state
139 * to represent permits. Subclassed into fair and nonfair
142 abstract static class Sync extends AbstractQueuedSynchronizer {
143 private static final long serialVersionUID = 1192457210091910933L;
149 final int getPermits() {
153 final int nonfairTryAcquireShared(int acquires) {
155 int available = getState();
156 int remaining = available - acquires;
158 compareAndSetState(available, remaining))
163 protected final boolean tryReleaseShared(int releases) {
166 if (compareAndSetState(p, p + releases))
171 final void reducePermits(int reductions) {
173 int current = getState();
174 int next = current - reductions;
175 if (compareAndSetState(current, next))
180 final int drainPermits() {
182 int current = getState();
183 if (current == 0 || compareAndSetState(current, 0))
192 final static class NonfairSync extends Sync {
193 private static final long serialVersionUID = -2694183684443567898L;
195 NonfairSync(int permits) {
199 protected int tryAcquireShared(int acquires) {
200 return nonfairTryAcquireShared(acquires);
207 final static class FairSync extends Sync {
208 private static final long serialVersionUID = 2014338818796000944L;
210 FairSync(int permits) {
214 protected int tryAcquireShared(int acquires) {
215 Thread current = Thread.currentThread();
217 Thread first = getFirstQueuedThread();
218 if (first != null && first != current)
220 int available = getState();
221 int remaining = available - acquires;
223 compareAndSetState(available, remaining))
230 * Creates a {@code Semaphore} with the given number of
231 * permits and nonfair fairness setting.
233 * @param permits the initial number of permits available.
234 * This value may be negative, in which case releases
235 * must occur before any acquires will be granted.
237 public Semaphore(int permits) {
238 sync = new NonfairSync(permits);
242 * Creates a {@code Semaphore} with the given number of
243 * permits and the given fairness setting.
245 * @param permits the initial number of permits available.
246 * This value may be negative, in which case releases
247 * must occur before any acquires will be granted.
248 * @param fair {@code true} if this semaphore will guarantee
249 * first-in first-out granting of permits under contention,
252 public Semaphore(int permits, boolean fair) {
253 sync = (fair)? new FairSync(permits) : new NonfairSync(permits);
257 * Acquires a permit from this semaphore, blocking until one is
258 * available, or the thread is {@linkplain Thread#interrupt interrupted}.
260 * <p>Acquires a permit, if one is available and returns immediately,
261 * reducing the number of available permits by one.
263 * <p>If no permit is available then the current thread becomes
264 * disabled for thread scheduling purposes and lies dormant until
265 * one of two things happens:
267 * <li>Some other thread invokes the {@link #release} method for this
268 * semaphore and the current thread is next to be assigned a permit; or
269 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
270 * the current thread.
273 * <p>If the current thread:
275 * <li>has its interrupted status set on entry to this method; or
276 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
279 * then {@link InterruptedException} is thrown and the current thread's
280 * interrupted status is cleared.
282 * @throws InterruptedException if the current thread is interrupted
284 public void acquire() throws InterruptedException {
285 sync.acquireSharedInterruptibly(1);
289 * Acquires a permit from this semaphore, blocking until one is
292 * <p>Acquires a permit, if one is available and returns immediately,
293 * reducing the number of available permits by one.
295 * <p>If no permit is available then the current thread becomes
296 * disabled for thread scheduling purposes and lies dormant until
297 * some other thread invokes the {@link #release} method for this
298 * semaphore and the current thread is next to be assigned a permit.
300 * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
301 * while waiting for a permit then it will continue to wait, but the
302 * time at which the thread is assigned a permit may change compared to
303 * the time it would have received the permit had no interruption
304 * occurred. When the thread does return from this method its interrupt
305 * status will be set.
307 public void acquireUninterruptibly() {
308 sync.acquireShared(1);
312 * Acquires a permit from this semaphore, only if one is available at the
313 * time of invocation.
315 * <p>Acquires a permit, if one is available and returns immediately,
316 * with the value {@code true},
317 * reducing the number of available permits by one.
319 * <p>If no permit is available then this method will return
320 * immediately with the value {@code false}.
322 * <p>Even when this semaphore has been set to use a
323 * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
324 * immediately acquire a permit if one is available, whether or not
325 * other threads are currently waiting.
326 * This "barging" behavior can be useful in certain
327 * circumstances, even though it breaks fairness. If you want to honor
328 * the fairness setting, then use
329 * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
330 * which is almost equivalent (it also detects interruption).
332 * @return {@code true} if a permit was acquired and {@code false}
335 public boolean tryAcquire() {
336 return sync.nonfairTryAcquireShared(1) >= 0;
340 * Acquires a permit from this semaphore, if one becomes available
341 * within the given waiting time and the current thread has not
342 * been {@linkplain Thread#interrupt interrupted}.
344 * <p>Acquires a permit, if one is available and returns immediately,
345 * with the value {@code true},
346 * reducing the number of available permits by one.
348 * <p>If no permit is available then the current thread becomes
349 * disabled for thread scheduling purposes and lies dormant until
350 * one of three things happens:
352 * <li>Some other thread invokes the {@link #release} method for this
353 * semaphore and the current thread is next to be assigned a permit; or
354 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
355 * the current thread; or
356 * <li>The specified waiting time elapses.
359 * <p>If a permit is acquired then the value {@code true} is returned.
361 * <p>If the current thread:
363 * <li>has its interrupted status set on entry to this method; or
364 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
365 * to acquire a permit,
367 * then {@link InterruptedException} is thrown and the current thread's
368 * interrupted status is cleared.
370 * <p>If the specified waiting time elapses then the value {@code false}
371 * is returned. If the time is less than or equal to zero, the method
372 * will not wait at all.
374 * @param timeout the maximum time to wait for a permit
375 * @param unit the time unit of the {@code timeout} argument
376 * @return {@code true} if a permit was acquired and {@code false}
377 * if the waiting time elapsed before a permit was acquired
378 * @throws InterruptedException if the current thread is interrupted
380 public boolean tryAcquire(long timeout, TimeUnit unit)
381 throws InterruptedException {
382 return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
386 * Releases a permit, returning it to the semaphore.
388 * <p>Releases a permit, increasing the number of available permits by
389 * one. If any threads are trying to acquire a permit, then one is
390 * selected and given the permit that was just released. That thread
391 * is (re)enabled for thread scheduling purposes.
393 * <p>There is no requirement that a thread that releases a permit must
394 * have acquired that permit by calling {@link #acquire}.
395 * Correct usage of a semaphore is established by programming convention
396 * in the application.
398 public void release() {
399 sync.releaseShared(1);
403 * Acquires the given number of permits from this semaphore,
404 * blocking until all are available,
405 * or the thread is {@linkplain Thread#interrupt interrupted}.
407 * <p>Acquires the given number of permits, if they are available,
408 * and returns immediately, reducing the number of available permits
409 * by the given amount.
411 * <p>If insufficient permits are available then the current thread becomes
412 * disabled for thread scheduling purposes and lies dormant until
413 * one of two things happens:
415 * <li>Some other thread invokes one of the {@link #release() release}
416 * methods for this semaphore, the current thread is next to be assigned
417 * permits and the number of available permits satisfies this request; or
418 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
419 * the current thread.
422 * <p>If the current thread:
424 * <li>has its interrupted status set on entry to this method; or
425 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
428 * then {@link InterruptedException} is thrown and the current thread's
429 * interrupted status is cleared.
430 * Any permits that were to be assigned to this thread are instead
431 * assigned to other threads trying to acquire permits, as if
432 * permits had been made available by a call to {@link #release()}.
434 * @param permits the number of permits to acquire
435 * @throws InterruptedException if the current thread is interrupted
436 * @throws IllegalArgumentException if {@code permits} is negative
438 public void acquire(int permits) throws InterruptedException {
439 if (permits < 0) throw new IllegalArgumentException();
440 sync.acquireSharedInterruptibly(permits);
444 * Acquires the given number of permits from this semaphore,
445 * blocking until all are available.
447 * <p>Acquires the given number of permits, if they are available,
448 * and returns immediately, reducing the number of available permits
449 * by the given amount.
451 * <p>If insufficient permits are available then the current thread becomes
452 * disabled for thread scheduling purposes and lies dormant until
453 * some other thread invokes one of the {@link #release() release}
454 * methods for this semaphore, the current thread is next to be assigned
455 * permits and the number of available permits satisfies this request.
457 * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
458 * while waiting for permits then it will continue to wait and its
459 * position in the queue is not affected. When the thread does return
460 * from this method its interrupt status will be set.
462 * @param permits the number of permits to acquire
463 * @throws IllegalArgumentException if {@code permits} is negative
466 public void acquireUninterruptibly(int permits) {
467 if (permits < 0) throw new IllegalArgumentException();
468 sync.acquireShared(permits);
472 * Acquires the given number of permits from this semaphore, only
473 * if all are available at the time of invocation.
475 * <p>Acquires the given number of permits, if they are available, and
476 * returns immediately, with the value {@code true},
477 * reducing the number of available permits by the given amount.
479 * <p>If insufficient permits are available then this method will return
480 * immediately with the value {@code false} and the number of available
481 * permits is unchanged.
483 * <p>Even when this semaphore has been set to use a fair ordering
484 * policy, a call to {@code tryAcquire} <em>will</em>
485 * immediately acquire a permit if one is available, whether or
486 * not other threads are currently waiting. This
487 * "barging" behavior can be useful in certain
488 * circumstances, even though it breaks fairness. If you want to
489 * honor the fairness setting, then use {@link #tryAcquire(int,
490 * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
491 * which is almost equivalent (it also detects interruption).
493 * @param permits the number of permits to acquire
494 * @return {@code true} if the permits were acquired and
495 * {@code false} otherwise
496 * @throws IllegalArgumentException if {@code permits} is negative
498 public boolean tryAcquire(int permits) {
499 if (permits < 0) throw new IllegalArgumentException();
500 return sync.nonfairTryAcquireShared(permits) >= 0;
504 * Acquires the given number of permits from this semaphore, if all
505 * become available within the given waiting time and the current
506 * thread has not been {@linkplain Thread#interrupt interrupted}.
508 * <p>Acquires the given number of permits, if they are available and
509 * returns immediately, with the value {@code true},
510 * reducing the number of available permits by the given amount.
512 * <p>If insufficient permits are available then
513 * the current thread becomes disabled for thread scheduling
514 * purposes and lies dormant until one of three things happens:
516 * <li>Some other thread invokes one of the {@link #release() release}
517 * methods for this semaphore, the current thread is next to be assigned
518 * permits and the number of available permits satisfies this request; or
519 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
520 * the current thread; or
521 * <li>The specified waiting time elapses.
524 * <p>If the permits are acquired then the value {@code true} is returned.
526 * <p>If the current thread:
528 * <li>has its interrupted status set on entry to this method; or
529 * <li>is {@linkplain Thread#interrupt interrupted} while waiting
530 * to acquire the permits,
532 * then {@link InterruptedException} is thrown and the current thread's
533 * interrupted status is cleared.
534 * Any permits that were to be assigned to this thread, are instead
535 * assigned to other threads trying to acquire permits, as if
536 * the permits had been made available by a call to {@link #release()}.
538 * <p>If the specified waiting time elapses then the value {@code false}
539 * is returned. If the time is less than or equal to zero, the method
540 * will not wait at all. Any permits that were to be assigned to this
541 * thread, are instead assigned to other threads trying to acquire
542 * permits, as if the permits had been made available by a call to
543 * {@link #release()}.
545 * @param permits the number of permits to acquire
546 * @param timeout the maximum time to wait for the permits
547 * @param unit the time unit of the {@code timeout} argument
548 * @return {@code true} if all permits were acquired and {@code false}
549 * if the waiting time elapsed before all permits were acquired
550 * @throws InterruptedException if the current thread is interrupted
551 * @throws IllegalArgumentException if {@code permits} is negative
553 public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
554 throws InterruptedException {
555 if (permits < 0) throw new IllegalArgumentException();
556 return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
560 * Releases the given number of permits, returning them to the semaphore.
562 * <p>Releases the given number of permits, increasing the number of
563 * available permits by that amount.
564 * If any threads are trying to acquire permits, then one
565 * is selected and given the permits that were just released.
566 * If the number of available permits satisfies that thread's request
567 * then that thread is (re)enabled for thread scheduling purposes;
568 * otherwise the thread will wait until sufficient permits are available.
569 * If there are still permits available
570 * after this thread's request has been satisfied, then those permits
571 * are assigned in turn to other threads trying to acquire permits.
573 * <p>There is no requirement that a thread that releases a permit must
574 * have acquired that permit by calling {@link Semaphore#acquire acquire}.
575 * Correct usage of a semaphore is established by programming convention
576 * in the application.
578 * @param permits the number of permits to release
579 * @throws IllegalArgumentException if {@code permits} is negative
581 public void release(int permits) {
582 if (permits < 0) throw new IllegalArgumentException();
583 sync.releaseShared(permits);
587 * Returns the current number of permits available in this semaphore.
589 * <p>This method is typically used for debugging and testing purposes.
591 * @return the number of permits available in this semaphore
593 public int availablePermits() {
594 return sync.getPermits();
598 * Acquires and returns all permits that are immediately available.
600 * @return the number of permits acquired
602 public int drainPermits() {
603 return sync.drainPermits();
607 * Shrinks the number of available permits by the indicated
608 * reduction. This method can be useful in subclasses that use
609 * semaphores to track resources that become unavailable. This
610 * method differs from {@code acquire} in that it does not block
611 * waiting for permits to become available.
613 * @param reduction the number of permits to remove
614 * @throws IllegalArgumentException if {@code reduction} is negative
616 protected void reducePermits(int reduction) {
617 if (reduction < 0) throw new IllegalArgumentException();
618 sync.reducePermits(reduction);
622 * Returns {@code true} if this semaphore has fairness set true.
624 * @return {@code true} if this semaphore has fairness set true
626 public boolean isFair() {
627 return sync instanceof FairSync;
631 * Queries whether any threads are waiting to acquire. Note that
632 * because cancellations may occur at any time, a {@code true}
633 * return does not guarantee that any other thread will ever
634 * acquire. This method is designed primarily for use in
635 * monitoring of the system state.
637 * @return {@code true} if there may be other threads waiting to
640 public final boolean hasQueuedThreads() {
641 return sync.hasQueuedThreads();
645 * Returns an estimate of the number of threads waiting to acquire.
646 * The value is only an estimate because the number of threads may
647 * change dynamically while this method traverses internal data
648 * structures. This method is designed for use in monitoring of the
649 * system state, not for synchronization control.
651 * @return the estimated number of threads waiting for this lock
653 public final int getQueueLength() {
654 return sync.getQueueLength();
658 * Returns a collection containing threads that may be waiting to acquire.
659 * Because the actual set of threads may change dynamically while
660 * constructing this result, the returned collection is only a best-effort
661 * estimate. The elements of the returned collection are in no particular
662 * order. This method is designed to facilitate construction of
663 * subclasses that provide more extensive monitoring facilities.
665 * @return the collection of threads
667 protected Collection<Thread> getQueuedThreads() {
668 return sync.getQueuedThreads();
672 * Returns a string identifying this semaphore, as well as its state.
673 * The state, in brackets, includes the String {@code "Permits ="}
674 * followed by the number of permits.
676 * @return a string identifying this semaphore, as well as its state
678 public String toString() {
679 return super.toString() + "[Permits = " + sync.getPermits() + "]";