/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/licenses/publicdomain */ package java.util.concurrent; import java.util.*; import java.util.concurrent.locks.*; import java.util.concurrent.atomic.*; /** * A counting semaphore. Conceptually, a semaphore maintains a set of * permits. Each {@link #acquire} blocks if necessary until a permit is * available, and then takes it. Each {@link #release} adds a permit, * potentially releasing a blocking acquirer. * However, no actual permit objects are used; the Semaphore just * keeps a count of the number available and acts accordingly. * *
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 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 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. When * fairness is set true, the semaphore guarantees that threads * invoking any of the {@link #acquire() acquire} methods are * allocated 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 acquire before another, but reach the ordering * point after the other, and similarly upon return from the method. * *
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. * * @since 1.5 * @author Doug Lea * */ public class Semaphore implements java.io.Serializable { private static final long serialVersionUID = -3222578661600680210L; /** Sync mechanics via AbstractQueuedSynchronizer subclass */ private final Sync sync; private final static class Sync extends AbstractQueuedSynchronizer { final boolean fair; Sync(int permits, boolean fair) { this.fair = fair; getState().set(permits); } public final int acquireSharedState(boolean isQueued, int acquires, Thread current) { final AtomicInteger perms = getState(); if (!isQueued && fair && hasWaiters()) return -1; for (;;) { int available = perms.get(); int remaining = available - acquires; if (remaining < 0 || perms.compareAndSet(available, remaining)) return remaining; } } public final boolean releaseSharedState(int releases) { final AtomicInteger perms = getState(); for (;;) { int p = perms.get(); if (perms.compareAndSet(p, p + releases)) return true; } } public final int acquireExclusiveState(boolean isQueued, int acquires, Thread current) { throw new UnsupportedOperationException(); } public final boolean releaseExclusiveState(int releases) { throw new UnsupportedOperationException(); } public final void checkConditionAccess(Thread thread, boolean waiting) { throw new UnsupportedOperationException(); } } /** * Construct a 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 true if this semaphore will guarantee first-in * first-out granting of permits under contention, else false. */ public Semaphore(int permits, boolean fair) { sync = new Sync(permits, fair); } /** * Acquires a permit from this semaphore, blocking until one is * available, or the thread is {@link 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 {@link 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.acquireSharedUninterruptibly(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 true, * reducing the number of available permits by one. * *
If no permit is available then this method will return * immediately with the value false. * * @return true if a permit was acquired and false * otherwise. */ public boolean tryAcquire() { return sync.acquireSharedState(false, 1, null) >= 0; } /** * Acquires a permit from this semaphore, if one becomes available * within the given waiting time and the * current thread has not been {@link Thread#interrupt interrupted}. *
Acquires a permit, if one is available and returns immediately, * with the value 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 true is returned. *
If the current thread: *
If the specified waiting time elapses then the value 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 timeout argument. * @return true if a permit was acquired and false * if the waiting time elapsed before a permit was acquired. * * @throws InterruptedException if the current thread is interrupted * * @see Thread#interrupt * */ public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException { return sync.acquireSharedTimed(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 blocking 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 {@link 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 {@link 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 permits less than zero. * */ public void acquireUninterruptibly(int permits) { if (permits < 0) throw new IllegalArgumentException(); sync.acquireSharedUninterruptibly(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 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 false and the number of available * permits is unchanged. * * @param permits the number of permits to acquire * * @return true if the permits were acquired and false * otherwise. * @throws IllegalArgumentException if permits less than zero. */ public boolean tryAcquire(int permits) { if (permits < 0) throw new IllegalArgumentException(); return sync.acquireSharedState(false, permits, null) >= 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 {@link Thread#interrupt interrupted}. *
Acquires the given number of permits, if they are available and * returns immediately, with the value 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 true is returned. *
If the current thread: *
If the specified waiting time elapses then the value 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 the next waiting thread(s), as if * they 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 timeout argument. * @return true if all permits were acquired and false * if the waiting time elapsed before all permits were acquired. * * @throws InterruptedException if the current thread is interrupted * @throws IllegalArgumentException if permits less than zero. * * @see Thread#interrupt * */ public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException { if (permits < 0) throw new IllegalArgumentException(); return sync.acquireSharedTimed(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 blocking trying to acquire permits, then the * one that has been waiting the longest * 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 continues to wait. If there are still permits available * after the first thread's request has been satisfied, then those permits * are assigned to the next waiting thread. If it is satisfied then it is * re-enabled for thread scheduling purposes. This continues until there * are insufficient permits to satisfy the next waiting thread, or there * are no more waiting threads. * *
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 permits less than zero. */ public void release(int permits) { if (permits < 0) throw new IllegalArgumentException(); sync.releaseShared(permits); } /** * Return 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.getState().get();
}
/**
* Shrink the number of available permits by the indicated
* reduction. This method can be useful in subclasses that
* use semaphores to track available resources that become
* unavailable. This method differs from acquire
* in that it does not block waiting for permits to become
* available.
* @param reduction the number of permits to remove
* @throws IllegalArgumentException if reduction is negative
*/
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
sync.getState().getAndAdd(-reduction);
}
/**
* Return true if this semaphore has fairness set true.
* @return true if this semaphore has fairness set true.
*/
public boolean isFair() {
return sync.fair;
}
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations may occur at any time, a 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 true if there may be other threads waiting to acquire
* the lock.
*/
public final boolean hasWaiters() {
return sync.hasWaiters();
}
/**
* 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