/* * @(#)Semaphore.java 1.8 04/07/12 * * Copyright 2004 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ 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 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 * 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. * * @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 { 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 p = getState(); if (compareAndSetState(p, p + releases)) return true; } } final void reducePermits(int reductions) { for (;;) { int current = getState(); int next = current - reductions; if (compareAndSetState(current, next)) return; } } final int drainPermits() { for (;;) { int current = getState(); if (current == 0 || compareAndSetState(current, 0)) return current; } } } /** * NonFair version */ final static class NonfairSync extends Sync { NonfairSync(int permits) { super(permits); } protected int tryAcquireShared(int acquires) { return nonfairTryAcquireShared(acquires); } } /** * Fair version */ final static class FairSync extends Sync { FairSync(int permits) { super(permits); } protected int tryAcquireShared(int acquires) { Thread current = Thread.currentThread(); for (;;) { Thread first = getFirstQueuedThread(); if (first != null && first != current) return -1; int available = getState(); int remaining = available - acquires; if (remaining < 0 || compareAndSetState(available, remaining)) return remaining; } } } /** * Creates a 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 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 = (fair)? new FairSync(permits) : new NonfairSync(permits); } /** * 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.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 true, * reducing the number of available permits by one. * *
If no permit is available then this method will return * immediately with the value false. * *
Even when this semaphore has been set to use a * fair ordering policy, a call to 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 true if a permit was acquired and 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 {@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.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 {@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.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 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. * *
Even when this semaphore has been set to use a fair ordering * policy, a call to 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 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.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 {@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 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 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.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 permits less than zero. */ 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();
}
/**
* Acquire and return all permits that are immediately available.
* @return the number of permits
*/
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 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.reducePermits(reduction);
}
/**
* Returns true if this semaphore has fairness set true.
* @return 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 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 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