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Semaphore是JUC包提供的一个共享锁，一般称之为信号量。
Semaphore通过自定义的同步器维护了一个或多个共享资源，线程通过调用acquire获取共享资源，通过调用release释放。
源代码：
/* * 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 "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. * * <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) // 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}. * * <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 "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}. * * <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 * "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}. * * <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() + "]"; } }
下面我们来详细分下下Semaphore的工作原理。
一、构造函数
public Semaphore(int permits) { sync = new NonfairSync(permits); } public Semaphore(int permits, boolean fair) { sync = fair ? new FairSync(permits) : new NonfairSync(permits); }
初始化Semaphore时需要指定共享资源的个数。Semaphore提供了两种模式：公平模式&非公平模式。如果不指定工作模式的话，默认工作在非公平模式下。后面我们将看到，两种模式的区别在于获取共享资源时的排序策略。Semaphore有三个内部类：Sync&NonfairSync&FairSync。后两个继承自Sync，Sync继承自AQS。除了序列化版本号之外，Semaphore只有一个成员变量sync，公平模式下sync初始化为FairSync，非公平模式下sync初始化为NonfairSync。
二、acquire 响应中断获取资源
Semaphore提供了两种获取资源的方式：响应中断&不响应中断。我们先来看一下响应中断的获取。
public void acquire() throws InterruptedException { sync.acquireSharedInterruptibly(1); }
acquire方法由同步器sync调用上层AQS提供的acquireSharedInterruptibly方法获取：
public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); }
acquireSharedInterruptibly方法先检测中断。然后调用tryAcquireShared方法试图获取共享资源。这时公平模式和非公平模式的代码执行路径发生分叉，FairSync和NonfairSync各自重写了tryAcquireShared方法。
我们先来看下非公平模式下的tryAcquireShared方法：
protected int tryAcquireShared(int acquires) { return nonfairTryAcquireShared(acquires); }
它直接代用了父类Sync提供的nonfairTryAcquireShared方法：
final int nonfairTryAcquireShared(int acquires) { for (;;) { int available = getState(); int remaining = available - acquires; if (remaining < 0 || compareAndSetState(available, remaining)) return remaining; } }
注意，这里是一个CAS自旋。因为Semaphore是一个共享锁，可能有多个线程同时申请共享资源，因此CAS操作可能失败。直到成功获取返回剩余资源数目，或者发现没有剩余资源返回负值代表申请失败。有一个问题，为什么我们不在CAS操作失败后就直接返回失败呢？因为这样做虽然不会导致错误，但会降低效率：在还有剩余资源的情况下，一个线程因为竞争导致CAS失败后被放入等待序列尾，一定在队列头部有一个线程被唤醒去试图获取资源，这比直接自旋继续获取多了操作等待队列的开销。
这里“非公平”的语义体现在：如果一个线程通过nonfairTryAcquireShared成功获取了共享资源，对于此时正在等待队列中的线程来说，可能是不公平的：队列中线程先到，却没能先获取资源。
如果tryAcquireShared没能成功获取，acquireSharedInterruptibly方法调用doAcquireSharedInterruptibly方法将当前线程放入等待队列并开始自旋检测获取资源：
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException { final Node node = addWaiter(Node.SHARED); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } }
我们注意到，doAcquireSharedInterruptibly中，当一个线程从parkAndCheckInterrupt方法中被中断唤醒之后，直接抛出了中断异常。还记得我们分析AQS时的doAcquireShared方法吗，它在这里的处理方式是用一个局部变量interrupted记录下这个异常但不立即处理，而是等到成功获取资源之后返回这个中断标志，并在上层调用selfInterrupt方法补上中断。这正是两个方法的关键区别：是否及时响应中断。
我们再来看公平模式下的tryAcquireShared方法：
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; } }
相比较非公平模式的nonfairTryAcquireShared方法，公平模式下的tryAcquireShared方法在试图获取之前做了一个判断，如果发现等对队列中有线程在等待获取资源，就直接返回-1表示获取失败。当前线程会被上层的acquireSharedInterruptibly方法调用doAcquireShared方法放入等待队列中。这正是“公平”模式的语义：如果有线程先于我进入等待队列且正在等待，就直接进入等待队列，效果便是各个线程按照申请的顺序获得共享资源，具有公平性。
三、acquireUnInterruptibly 不响应中断获取资源
public void acquireUninterruptibly() { sync.acquireShared(1); }
acquireUnInterruptibly方法调用AQS提供的acquireShared方法：
public final void acquireShared(int arg) { if (tryAcquireShared(arg) < 0) doAcquireShared(arg); }
acquireShared方法首先试图获取资源，这与acquireSharedInterruptibly方法相比，没有先检测中断的这一步。紧接着调用doAcquireShared方法，由于这个方法我在另一篇博文AQS源码学习笔记中已经详细分析过，这里我们只关注它与doAcquireSharedInterruptibly方法的区别：
private void doAcquireShared(int arg) { final Node node = addWaiter(Node.SHARED); boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { setHeadAndPropagate(node, r); p.next = null; // help GC if (interrupted) selfInterrupt(); failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) interrupted = true; } } finally { if (failed) cancelAcquire(node); } }
正如刚刚说过的，区别只在线程从parkAndCheckInterrupt方法中因中断而返回时的处理：在这里它没有抛出异常，而是用一个局部变量interrupted记录下这个异常但不立即处理，而是等到成功获取资源之后返回这个中断标志，并在上层调用selfInterrupt方法补上中断。
四、acquire(int) & acquireUninterruptibly(int) 指定申请的资源数目的获取
public void acquire(int permits) throws InterruptedException { if (permits < 0) throw new IllegalArgumentException(); sync.acquireSharedInterruptibly(permits); } public void acquireUninterruptibly(int permits) { if (permits < 0) throw new IllegalArgumentException(); sync.acquireShared(permits); }
可以看到，与不指定数目时的获取的区别仅在参数值，不再赘述。
五、release 释放资源
公平模式和非公平模式的释放资源操作是一样的：
public void release() { sync.releaseShared(1); } public void release(int permits) { if (permits < 0) throw new IllegalArgumentException(); sync.releaseShared(permits); }
调用AQS提供的releaseShared方法：
public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; }
releaseShared方法首先调用我们重写的tryReleaseShared方法试图释放资源。然后调用doReleaseShared方法唤醒队列之后的等待线程。由于在我的另一篇博文AQS源码学习笔记中已经详细分析了doReleaseShared方法，因此不再赘述。我们主要关注tryReleaseShared方法：
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; } }
这个方法也是一个CAS自旋，原因是应为Semaphore是一个共享锁，可能有多个线程同时释放资源，因此CAS操作可能失败。最后方法总会成功释放并返回true（如果不出错的话）。
六、tryAcquire & tryAcquire(timeout) 方法
public boolean tryAcquire() { return sync.nonfairTryAcquireShared(1) >= 0; } public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); } public boolean tryAcquire(int permits) { if (permits < 0) throw new IllegalArgumentException(); return sync.nonfairTryAcquireShared(permits) >= 0; } public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException { if (permits < 0) throw new IllegalArgumentException(); return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); }
没有指定等待时间的tryAcquire调用的是nonfairTryAcquireShared方法，我们已经分析过，不再赘述。我们重点关注指定等待时长的方法。限时等待是通过调用AQS提供的tryAcquireSharedNanos方法实现的：
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout); }
注意：限时等待默认都是及时响应中断的。方法开始先检测中断，然后调用tryAcquireShared方法试图获取资源，如果成功的话直接返回true，不成功则调用doAcquireSharedNanos方法：
private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { if (nanosTimeout <= 0L) return false; final long deadline = System.nanoTime() + nanosTimeout; final Node node = addWaiter(Node.SHARED); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return true; } } nanosTimeout = deadline - System.nanoTime(); if (nanosTimeout <= 0L) return false; if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if (Thread.interrupted()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } }
方法在自旋之前先计算了一个结束等待的时间节点deadline，然后便开始自旋，每次自旋都要计算一下剩余等待时间nanosTimeout，如果nanosTimeout小于等于0，说明已经到达deadline，直接返回false表示超时。
有一点值得注意，spinForTimeoutThreshold这个值规定了一个阈值，当剩余等待时间小于这个值的时候，线程将不再被park，而是一直在自旋试图获取资源。关于这个值的作用Doug Lea是这样注释的：
/** * The number of nanoseconds for which it is faster to spin * rather than to use timed park. A rough estimate suffices * to improve responsiveness with very short timeouts. */
我的理解是，park和unpark操作需要一定的开销，当nanosTimeout很小的时候，这个开销就相对很大了。这个阈值的设置可以让短时等待的线程一直保持自旋，可以提高短时等待的反应效率，而由于nanosTimeout很小，自旋又不会有过多的开销。
除此之外，doAcquireSharedNanos方法与不限时等待的doAcquireShared方法还有两点重要区别：①由于有等待时限，所以线程从park方法返回时我们不能确定返回的原因是中断还是超时，因此需要调用interrupted方法检测一下中断标志；②doAcquireSharedNanos方法是及时响应中断的，而doAcquireShared方法延迟处理中断。
七、drainPermits & reducePermits 修改剩余共享资源数量
Semaphore提供了“耗尽”所有剩余共享资源的操作：
public int drainPermits() { return sync.drainPermits(); }
drainPermits调用了自定义同步器Sync的同名方法：
final int drainPermits() { for (;;) { int current = getState(); if (current == 0 || compareAndSetState(current, 0)) return current; } }
用CAS自旋将剩余资源清空。
我们再来看看“缩减”剩余共享资源的操作：
protected void reducePermits(int reduction) { if (reduction < 0) throw new IllegalArgumentException(); sync.reducePermits(reduction); }
首先，缩减必须是单向的，即只能减少不能增加，然后调用Sync的同名方法：
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; } }
用CAS自旋在剩余共享资源上做缩减。
上述两个对共享资源数量的修改操作有两点需要注意：①是不可逆的②是对剩余资源的操作而不是全部资源，当剩余资源数目不足或已经为0时，方法就返回，正咋被占用的资源不参与。
八、其他
public int availablePermits() { return sync.getPermits(); }public boolean isFair() { return sync instanceof FairSync; } public final boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } public final int getQueueLength() { return sync.getQueueLength(); } protected Collection<Thread> getQueuedThreads() { return sync.getQueuedThreads(); } public String toString() { return super.toString() + "[Permits = " + sync.getPermits() + "]"; }
这些方法比较简单，不再赘述。
总结：
Semaphore是JUC包提供的一个典型的共享锁，它通过自定义两种不同的同步器（FairSync&NonfairSync）提供了公平&非公平两种工作模式，两种模式下分别提供了限时/不限时、响应中断/不响应中断的获取资源的方法（限时获取总是及时响应中断的），而所有的释放资源的release操作是统一的。