Semaphores are often used to restrict the number of threads than can + * access some (physical or logical) resource. For example, here is + * a class that uses a semaphore to control access to a pool of items: + *
+ * class Pool {
+ * private static final int MAX_AVAILABLE = 100;
+ * private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
+ *
+ * public Object getItem() throws InterruptedException {
+ * available.acquire();
+ * return getNextAvailableItem();
+ * }
+ *
+ * public void putItem(Object x) {
+ * if (markAsUnused(x))
+ * available.release();
+ * }
+ *
+ * // Not a particularly efficient data structure; just for demo
+ *
+ * protected Object[] items = ... whatever kinds of items being managed
+ * protected boolean[] used = new boolean[MAX_AVAILABLE];
+ *
+ * protected synchronized Object getNextAvailableItem() {
+ * for (int i = 0; i < MAX_AVAILABLE; ++i) {
+ * if (!used[i]) {
+ * used[i] = true;
+ * return items[i];
+ * }
+ * }
+ * return null; // not reached
+ * }
+ *
+ * protected synchronized boolean markAsUnused(Object item) {
+ * for (int i = 0; i < MAX_AVAILABLE; ++i) {
+ * if (item == items[i]) {
+ * if (used[i]) {
+ * used[i] = false;
+ * return true;
+ * } else
+ * return false;
+ * }
+ * }
+ * return false;
+ * }
+ *
+ * }
+ *
+ *
+ * Before obtaining an item each thread must acquire a permit from + * the semaphore, guaranteeing that an item is available for use. When + * the thread has finished with the item it is returned back to the + * pool and a permit is returned to the semaphore, allowing another + * thread to acquire that item. Note that no synchronization lock is + * held when {@link #acquire} is called as that would prevent an item + * from being returned to the pool. The semaphore encapsulates the + * synchronization needed to restrict access to the pool, separately + * from any synchronization needed to maintain the consistency of the + * pool itself. + * + *
A semaphore initialized to one, and which is used such that it + * only has at most one permit available, can serve as a mutual + * exclusion lock. This is more commonly known as a binary + * semaphore, because it only has two states: one permit + * available, or zero permits available. When used in this way, the + * binary semaphore has the property (unlike many {@link Lock} + * implementations), that the "lock" can be released by a + * thread other than the owner (as semaphores have no notion of + * ownership). This can be useful in some specialized contexts, such + * as deadlock recovery. + * + *
The constructor for this class optionally accepts a + * fairness parameter. When set false, this class makes no + * guarantees about the order in which threads acquire permits. In + * particular, barging is permitted, that is, a thread + * invoking {@link #acquire} can be allocated a permit ahead of a + * thread that has been waiting - logically the new thread places itself at + * the head of the queue of waiting threads. When fairness is set true, the + * semaphore guarantees that threads invoking any of the {@link + * #acquire() acquire} methods are selected to obtain permits in the order in + * which their invocation of those methods was processed + * (first-in-first-out; FIFO). Note that FIFO ordering necessarily + * applies to specific internal points of execution within these + * methods. So, it is possible for one thread to invoke + * {@code acquire} before another, but reach the ordering point after + * the other, and similarly upon return from the method. + * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not + * honor the fairness setting, but will take any permits that are + * available. + * + *
Generally, semaphores used to control resource access should be + * initialized as fair, to ensure that no thread is starved out from + * accessing a resource. When using semaphores for other kinds of + * synchronization control, the throughput advantages of non-fair + * ordering often outweigh fairness considerations. + * + *
This class also provides convenience methods to {@link + * #acquire(int) acquire} and {@link #release(int) release} multiple + * permits at a time. Beware of the increased risk of indefinite + * postponement when these methods are used without fairness set true. + * + *
Memory consistency effects: Actions in a thread prior to calling + * a "release" method such as {@code release()} + * happen-before + * actions following a successful "acquire" method such as {@code acquire()} + * in another thread. + * + * @since 1.5 + * @author Doug Lea + * + */ + +public class Semaphore implements java.io.Serializable { + private static final long serialVersionUID = -3222578661600680210L; + /** All mechanics via AbstractQueuedSynchronizer subclass */ + private final Sync sync; + + /** + * Synchronization implementation for semaphore. Uses AQS state + * to represent permits. Subclassed into fair and nonfair + * versions. + */ + abstract static class Sync extends AbstractQueuedSynchronizer { + private static final long serialVersionUID = 1192457210091910933L; + + Sync(int permits) { + setState(permits); + } + + final int getPermits() { + return getState(); + } + + final int nonfairTryAcquireShared(int acquires) { + for (;;) { + int available = getState(); + int remaining = available - acquires; + if (remaining < 0 || + compareAndSetState(available, remaining)) + return remaining; + } + } + + protected final boolean tryReleaseShared(int releases) { + for (;;) { + int current = getState(); + int next = current + releases; + if (next < current) // overflow + throw new Error("Maximum permit count exceeded"); + if (compareAndSetState(current, next)) + return true; + } + } + + final void reducePermits(int reductions) { + for (;;) { + int current = getState(); + int next = current - reductions; + if (next > current) // underflow + throw new Error("Permit count underflow"); + if (compareAndSetState(current, next)) + return; + } + } + + final int drainPermits() { + for (;;) { + int current = getState(); + if (current == 0 || compareAndSetState(current, 0)) + return current; + } + } + } + + /** + * NonFair version + */ + static final class NonfairSync extends Sync { + private static final long serialVersionUID = -2694183684443567898L; + + NonfairSync(int permits) { + super(permits); + } + + protected int tryAcquireShared(int acquires) { + return nonfairTryAcquireShared(acquires); + } + } + + /** + * Fair version + */ + static final class FairSync extends Sync { + private static final long serialVersionUID = 2014338818796000944L; + + FairSync(int permits) { + super(permits); + } + + protected int tryAcquireShared(int acquires) { + for (;;) { + if (hasQueuedPredecessors()) + return -1; + int available = getState(); + int remaining = available - acquires; + if (remaining < 0 || + compareAndSetState(available, remaining)) + return remaining; + } + } + } + + /** + * Creates a {@code Semaphore} with the given number of + * permits and nonfair fairness setting. + * + * @param permits the initial number of permits available. + * This value may be negative, in which case releases + * must occur before any acquires will be granted. + */ + public Semaphore(int permits) { + sync = new NonfairSync(permits); + } + + /** + * Creates a {@code Semaphore} with the given number of + * permits and the given fairness setting. + * + * @param permits the initial number of permits available. + * This value may be negative, in which case releases + * must occur before any acquires will be granted. + * @param fair {@code true} if this semaphore will guarantee + * first-in first-out granting of permits under contention, + * else {@code false} + */ + public Semaphore(int permits, boolean fair) { + sync = fair ? new FairSync(permits) : new NonfairSync(permits); + } + + /** + * Acquires a permit from this semaphore, blocking until one is + * available, or the thread is {@linkplain Thread#interrupt interrupted}. + * + *
Acquires a permit, if one is available and returns immediately, + * reducing the number of available permits by one. + * + *
If no permit is available then the current thread becomes + * disabled for thread scheduling purposes and lies dormant until + * one of two things happens: + *
If the current thread: + *
Acquires a permit, if one is available and returns immediately, + * reducing the number of available permits by one. + * + *
If no permit is available then the current thread becomes + * disabled for thread scheduling purposes and lies dormant until + * some other thread invokes the {@link #release} method for this + * semaphore and the current thread is next to be assigned a permit. + * + *
If the current thread is {@linkplain Thread#interrupt interrupted} + * while waiting for a permit then it will continue to wait, but the + * time at which the thread is assigned a permit may change compared to + * the time it would have received the permit had no interruption + * occurred. When the thread does return from this method its interrupt + * status will be set. + */ + public void acquireUninterruptibly() { + sync.acquireShared(1); + } + + /** + * Acquires a permit from this semaphore, only if one is available at the + * time of invocation. + * + *
Acquires a permit, if one is available and returns immediately, + * with the value {@code true}, + * reducing the number of available permits by one. + * + *
If no permit is available then this method will return + * immediately with the value {@code false}. + * + *
Even when this semaphore has been set to use a + * fair ordering policy, a call to {@code tryAcquire()} will + * immediately acquire a permit if one is available, whether or not + * other threads are currently waiting. + * This "barging" behavior can be useful in certain + * circumstances, even though it breaks fairness. If you want to honor + * the fairness setting, then use + * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) } + * which is almost equivalent (it also detects interruption). + * + * @return {@code true} if a permit was acquired and {@code false} + * otherwise + */ + public boolean tryAcquire() { + return sync.nonfairTryAcquireShared(1) >= 0; + } + + /** + * Acquires a permit from this semaphore, if one becomes available + * within the given waiting time and the current thread has not + * been {@linkplain Thread#interrupt interrupted}. + * + *
Acquires a permit, if one is available and returns immediately, + * with the value {@code true}, + * reducing the number of available permits by one. + * + *
If no permit is available then the current thread becomes + * disabled for thread scheduling purposes and lies dormant until + * one of three things happens: + *
If a permit is acquired then the value {@code true} is returned. + * + *
If the current thread: + *
If the specified waiting time elapses then the value {@code false} + * is returned. If the time is less than or equal to zero, the method + * will not wait at all. + * + * @param timeout the maximum time to wait for a permit + * @param unit the time unit of the {@code timeout} argument + * @return {@code true} if a permit was acquired and {@code false} + * if the waiting time elapsed before a permit was acquired + * @throws InterruptedException if the current thread is interrupted + */ + public boolean tryAcquire(long timeout, TimeUnit unit) + throws InterruptedException { + return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); + } + + /** + * Releases a permit, returning it to the semaphore. + * + *
Releases a permit, increasing the number of available permits by + * one. If any threads are trying to acquire a permit, then one is + * selected and given the permit that was just released. That thread + * is (re)enabled for thread scheduling purposes. + * + *
There is no requirement that a thread that releases a permit must + * have acquired that permit by calling {@link #acquire}. + * Correct usage of a semaphore is established by programming convention + * in the application. + */ + public void release() { + sync.releaseShared(1); + } + + /** + * Acquires the given number of permits from this semaphore, + * blocking until all are available, + * or the thread is {@linkplain Thread#interrupt interrupted}. + * + *
Acquires the given number of permits, if they are available, + * and returns immediately, reducing the number of available permits + * by the given amount. + * + *
If insufficient permits are available then the current thread becomes + * disabled for thread scheduling purposes and lies dormant until + * one of two things happens: + *
If the current thread: + *
Acquires the given number of permits, if they are available, + * and returns immediately, reducing the number of available permits + * by the given amount. + * + *
If insufficient permits are available then the current thread becomes + * disabled for thread scheduling purposes and lies dormant until + * some other thread invokes one of the {@link #release() release} + * methods for this semaphore, the current thread is next to be assigned + * permits and the number of available permits satisfies this request. + * + *
If the current thread is {@linkplain Thread#interrupt interrupted} + * while waiting for permits then it will continue to wait and its + * position in the queue is not affected. When the thread does return + * from this method its interrupt status will be set. + * + * @param permits the number of permits to acquire + * @throws IllegalArgumentException if {@code permits} is negative + * + */ + public void acquireUninterruptibly(int permits) { + if (permits < 0) throw new IllegalArgumentException(); + sync.acquireShared(permits); + } + + /** + * Acquires the given number of permits from this semaphore, only + * if all are available at the time of invocation. + * + *
Acquires the given number of permits, if they are available, and + * returns immediately, with the value {@code true}, + * reducing the number of available permits by the given amount. + * + *
If insufficient permits are available then this method will return + * immediately with the value {@code false} and the number of available + * permits is unchanged. + * + *
Even when this semaphore has been set to use a fair ordering + * policy, a call to {@code tryAcquire} will + * immediately acquire a permit if one is available, whether or + * not other threads are currently waiting. This + * "barging" behavior can be useful in certain + * circumstances, even though it breaks fairness. If you want to + * honor the fairness setting, then use {@link #tryAcquire(int, + * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) } + * which is almost equivalent (it also detects interruption). + * + * @param permits the number of permits to acquire + * @return {@code true} if the permits were acquired and + * {@code false} otherwise + * @throws IllegalArgumentException if {@code permits} is negative + */ + public boolean tryAcquire(int permits) { + if (permits < 0) throw new IllegalArgumentException(); + return sync.nonfairTryAcquireShared(permits) >= 0; + } + + /** + * Acquires the given number of permits from this semaphore, if all + * become available within the given waiting time and the current + * thread has not been {@linkplain Thread#interrupt interrupted}. + * + *
Acquires the given number of permits, if they are available and + * returns immediately, with the value {@code true}, + * reducing the number of available permits by the given amount. + * + *
If insufficient permits are available then + * the current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of three things happens: + *
If the permits are acquired then the value {@code true} is returned. + * + *
If the current thread: + *
If the specified waiting time elapses then the value {@code false} + * is returned. If the time is less than or equal to zero, the method + * will not wait at all. Any permits that were to be assigned to this + * thread, are instead assigned to other threads trying to acquire + * permits, as if the permits had been made available by a call to + * {@link #release()}. + * + * @param permits the number of permits to acquire + * @param timeout the maximum time to wait for the permits + * @param unit the time unit of the {@code timeout} argument + * @return {@code true} if all permits were acquired and {@code false} + * if the waiting time elapsed before all permits were acquired + * @throws InterruptedException if the current thread is interrupted + * @throws IllegalArgumentException if {@code permits} is negative + */ + public boolean tryAcquire(int permits, long timeout, TimeUnit unit) + throws InterruptedException { + if (permits < 0) throw new IllegalArgumentException(); + return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); + } + + /** + * Releases the given number of permits, returning them to the semaphore. + * + *
Releases the given number of permits, increasing the number of + * available permits by that amount. + * If any threads are trying to acquire permits, then one + * is selected and given the permits that were just released. + * If the number of available permits satisfies that thread's request + * then that thread is (re)enabled for thread scheduling purposes; + * otherwise the thread will wait until sufficient permits are available. + * If there are still permits available + * after this thread's request has been satisfied, then those permits + * are assigned in turn to other threads trying to acquire permits. + * + *
There is no requirement that a thread that releases a permit must + * have acquired that permit by calling {@link Semaphore#acquire acquire}. + * Correct usage of a semaphore is established by programming convention + * in the application. + * + * @param permits the number of permits to release + * @throws IllegalArgumentException if {@code permits} is negative + */ + public void release(int permits) { + if (permits < 0) throw new IllegalArgumentException(); + sync.releaseShared(permits); + } + + /** + * Returns the current number of permits available in this semaphore. + * + *
This method is typically used for debugging and testing purposes.
+ *
+ * @return the number of permits available in this semaphore
+ */
+ public int availablePermits() {
+ return sync.getPermits();
+ }
+
+ /**
+ * Acquires and returns all permits that are immediately available.
+ *
+ * @return the number of permits acquired
+ */
+ public int drainPermits() {
+ return sync.drainPermits();
+ }
+
+ /**
+ * Shrinks the number of available permits by the indicated
+ * reduction. This method can be useful in subclasses that use
+ * semaphores to track resources that become unavailable. This
+ * method differs from {@code acquire} in that it does not block
+ * waiting for permits to become available.
+ *
+ * @param reduction the number of permits to remove
+ * @throws IllegalArgumentException if {@code reduction} is negative
+ */
+ protected void reducePermits(int reduction) {
+ if (reduction < 0) throw new IllegalArgumentException();
+ sync.reducePermits(reduction);
+ }
+
+ /**
+ * Returns {@code true} if this semaphore has fairness set true.
+ *
+ * @return {@code true} if this semaphore has fairness set true
+ */
+ public boolean isFair() {
+ return sync instanceof FairSync;
+ }
+
+ /**
+ * Queries whether any threads are waiting to acquire. Note that
+ * because cancellations may occur at any time, a {@code true}
+ * return does not guarantee that any other thread will ever
+ * acquire. This method is designed primarily for use in
+ * monitoring of the system state.
+ *
+ * @return {@code true} if there may be other threads waiting to
+ * acquire the lock
+ */
+ public final boolean hasQueuedThreads() {
+ return sync.hasQueuedThreads();
+ }
+
+ /**
+ * Returns an estimate of the number of threads waiting to acquire.
+ * The value is only an estimate because the number of threads may
+ * change dynamically while this method traverses internal data
+ * structures. This method is designed for use in monitoring of the
+ * system state, not for synchronization control.
+ *
+ * @return the estimated number of threads waiting for this lock
+ */
+ public final int getQueueLength() {
+ return sync.getQueueLength();
+ }
+
+ /**
+ * Returns a collection containing threads that may be waiting to acquire.
+ * Because the actual set of threads may change dynamically while
+ * constructing this result, the returned collection is only a best-effort
+ * estimate. The elements of the returned collection are in no particular
+ * order. This method is designed to facilitate construction of
+ * subclasses that provide more extensive monitoring facilities.
+ *
+ * @return the collection of threads
+ */
+ protected Collection