/** ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.*********************//******* 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/publicdomain/zero/1.0/*/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 {@code Semaphore} just* keeps a count of the number available and acts accordingly.** <p>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:* <pre>* 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;*   }** }* </pre>** <p>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.** <p>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 <em>binary* semaphore</em>, 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 &quot;lock&quot; 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.** <p> The constructor for this class optionally accepts a* <em>fairness</em> parameter. When set false, this class makes no* guarantees about the order in which threads acquire permits. In* particular, <em>barging</em> 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.** <p>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.** <p>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.** <p>Memory consistency effects: Actions in a thread prior to calling* a "release" method such as {@code release()}* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>* 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) // overflowthrow 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) // underflowthrow 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}.** <p>Acquires a permit, if one is available and returns immediately,* reducing the number of available permits by one.** <p>If no permit is available then the current thread becomes* disabled for thread scheduling purposes and lies dormant until* one of two things happens:* <ul>* <li>Some other thread invokes the {@link #release} method for this* semaphore and the current thread is next to be assigned a permit; or* <li>Some other thread {@linkplain Thread#interrupt interrupts}* the current thread.* </ul>** <p>If the current thread:* <ul>* <li>has its interrupted status set on entry to this method; or* <li>is {@linkplain Thread#interrupt interrupted} while waiting* for a permit,* </ul>* then {@link InterruptedException} is thrown and the current thread's* interrupted status is cleared.** @throws InterruptedException if the current thread is interrupted*/public void acquire() throws InterruptedException {sync.acquireSharedInterruptibly(1);}/*** Acquires a permit from this semaphore, blocking until one is* available.** <p>Acquires a permit, if one is available and returns immediately,* reducing the number of available permits by one.** <p>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.** <p>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.** <p>Acquires a permit, if one is available and returns immediately,* with the value {@code true},* reducing the number of available permits by one.** <p>If no permit is available then this method will return* immediately with the value {@code false}.** <p>Even when this semaphore has been set to use a* fair ordering policy, a call to {@code tryAcquire()} <em>will</em>* immediately acquire a permit if one is available, whether or not* other threads are currently waiting.* This &quot;barging&quot; 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}.** <p>Acquires a permit, if one is available and returns immediately,* with the value {@code true},* reducing the number of available permits by one.** <p>If no permit is available then the current thread becomes* disabled for thread scheduling purposes and lies dormant until* one of three things happens:* <ul>* <li>Some other thread invokes the {@link #release} method for this* semaphore and the current thread is next to be assigned a permit; or* <li>Some other thread {@linkplain Thread#interrupt interrupts}* the current thread; or* <li>The specified waiting time elapses.* </ul>** <p>If a permit is acquired then the value {@code true} is returned.** <p>If the current thread:* <ul>* <li>has its interrupted status set on entry to this method; or* <li>is {@linkplain Thread#interrupt interrupted} while waiting* to acquire a permit,* </ul>* then {@link InterruptedException} is thrown and the current thread's* interrupted status is cleared.** <p>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.** <p>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.** <p>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}.** <p>Acquires the given number of permits, if they are available,* and returns immediately, reducing the number of available permits* by the given amount.** <p>If insufficient permits are available then the current thread becomes* disabled for thread scheduling purposes and lies dormant until* one of two things happens:* <ul>* <li>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; or* <li>Some other thread {@linkplain Thread#interrupt interrupts}* the current thread.* </ul>** <p>If the current thread:* <ul>* <li>has its interrupted status set on entry to this method; or* <li>is {@linkplain Thread#interrupt interrupted} while waiting* for a permit,* </ul>* then {@link InterruptedException} is thrown and the current thread's* interrupted status is cleared.* Any permits that were to be assigned to this thread are instead* assigned to other threads trying to acquire permits, as if* permits had been made available by a call to {@link #release()}.** @param permits the number of permits to acquire* @throws InterruptedException if the current thread is interrupted* @throws IllegalArgumentException if {@code permits} is negative*/public void acquire(int permits) throws InterruptedException {if (permits < 0) throw new IllegalArgumentException();sync.acquireSharedInterruptibly(permits);}/*** Acquires the given number of permits from this semaphore,* blocking until all are available.** <p>Acquires the given number of permits, if they are available,* and returns immediately, reducing the number of available permits* by the given amount.** <p>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.** <p>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.** <p>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.** <p>If insufficient permits are available then this method will return* immediately with the value {@code false} and the number of available* permits is unchanged.** <p>Even when this semaphore has been set to use a fair ordering* policy, a call to {@code tryAcquire} <em>will</em>* immediately acquire a permit if one is available, whether or* not other threads are currently waiting.  This* &quot;barging&quot; 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}.** <p>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.** <p>If insufficient permits are available then* the current thread becomes disabled for thread scheduling* purposes and lies dormant until one of three things happens:* <ul>* <li>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; or* <li>Some other thread {@linkplain Thread#interrupt interrupts}* the current thread; or* <li>The specified waiting time elapses.* </ul>** <p>If the permits are acquired then the value {@code true} is returned.** <p>If the current thread:* <ul>* <li>has its interrupted status set on entry to this method; or* <li>is {@linkplain Thread#interrupt interrupted} while waiting* to acquire the permits,* </ul>* then {@link InterruptedException} is thrown and the current thread's* interrupted status is cleared.* 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()}.** <p>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.** <p>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.** <p>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.** <p>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<Thread> getQueuedThreads() {return sync.getQueuedThreads();}/*** Returns a string identifying this semaphore, as well as its state.* The state, in brackets, includes the String {@code "Permits ="}* followed by the number of permits.** @return a string identifying this semaphore, as well as its state*/public String toString() {return super.toString() + "[Permits = " + sync.getPermits() + "]";}
}