J.U.C并发框架源码阅读（五）Semaphore
基于版本jdk1.7.0_80
java.util.concurrent.Semaphore
代码如下
/* * 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() + "]"; } }
0. Semaphore简介
Semaphore也是一个线程同步的辅助类，可以维护当前访问自身的线程个数，并提供了同步机制。使用Semaphore可以控制同时访问资源的线程个数，例如，实现一个文件允许的并发访问数。
Semaphore的主要方法摘要：
void acquire():从此信号量获取一个许可，在提供一个许可前一直将线程阻塞，否则线程被中断。
void release():释放一个许可，将其返回给信号量。
int availablePermits():返回此信号量中当前可用的许可数。
boolean hasQueuedThreads():查询是否有线程正在等待获取。
1. Semaphore原理概述
Semaphore利用AQS的state变量维护了信号量的计数，一旦发现acquire操作会导致state小于0，就阻塞当前线程，直到前驱线程释放了足够的信号量，才会唤醒当前线程继续执行。
release操作则会更新state变量，表示释放信号量的语义，同时还会去AQS的等待队列里检查，尝试唤醒后续的等待线程。
Semaphore提供了公平与非公平的实现方式。
2. Semaphore.acquire方法的执行轨迹
默认情况下，Semaphore采用的是非公平实现。
Semaphore.acquire public void acquire() throws InterruptedException { sync.acquireSharedInterruptibly(1); } AbstractQueuedSynchronizer.acquireSharedInterruptibly public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); } Semaphore.NonfairSync.tryAcquireShared protected int tryAcquireShared(int acquires) { return nonfairTryAcquireShared(acquires); } Semaphore.Sync.nonfairTryAcquireShared final int nonfairTryAcquireShared(int acquires) { for (;;) { int available = getState(); int remaining = available - acquires; if (remaining < 0 || compareAndSetState(available, remaining))//尝试插队抢占信号量 return remaining; } } AbstractQueuedSynchronizer.doAcquireSharedInterruptibly /** * Acquires in shared interruptible mode. * @param arg the acquire argument */ 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); } }
很经典的逻辑，除了自定义的tryAcquireShared方法以外，与上一篇提到的CountDownLatch.await方法的调用轨迹并无区别，在此不赘述其逻辑了
非公平的语义体现在Semaphore.Sync.nonfairTryAcquireShared方法中，会尝试直接用cas操作更新AQS维护的state变量，这是一个插队的行为。
3. Semaphore.release方法的执行轨迹
Semaphore.release public void release() { sync.releaseShared(1); } AbstractQueuedSynchronizer.releaseShared /** * Releases in shared mode. Implemented by unblocking one or more * threads if {@link #tryReleaseShared} returns true. * * @param arg the release argument. This value is conveyed to * {@link #tryReleaseShared} but is otherwise uninterpreted * and can represent anything you like. * @return the value returned from {@link #tryReleaseShared} */ public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } Semaphore.Sync.tryReleaseShared protected final boolean tryReleaseShared(int releases) { for (;;) {//在无限循环中用cas操作更新state变量 int current = getState(); int next = current + releases; if (next < current) // overflow throw new Error("Maximum permit count exceeded"); if (compareAndSetState(current, next)) return true; } } AbstractQueuedSynchronizer.doReleaseShared /** * Release action for shared mode -- signal successor and ensure * propagation. (Note: For exclusive mode, release just amounts * to calling unparkSuccessor of head if it needs signal.) */ private void doReleaseShared() { /* * Ensure that a release propagates, even if there are other * in-progress acquires/releases. This proceeds in the usual * way of trying to unparkSuccessor of head if it needs * signal. But if it does not, status is set to PROPAGATE to * ensure that upon release, propagation continues. * Additionally, we must loop in case a new node is added * while we are doing this. Also, unlike other uses of * unparkSuccessor, we need to know if CAS to reset status * fails, if so rechecking. */ for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) { if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } if (h == head) // loop if head changed break; } }
除了自定义的Semaphore.Sync.tryReleaseShared方法以外，与上一篇提到的CountDownLatch.countDown方法的调用轨迹并无区别，在此不赘述其逻辑了
4. Semaphore的公平语义
Semaphore.FairSync /** * 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())//如果AQS维护的等待队列中有线程在排队，则拒绝执行tryAcquire操作，直接走后续的线程排队等待流程 return -1; int available = getState();//用cas操作占领信号量的操作，只在AQS维护的等待队列为空时才会进行，这样就实现了公平语义 int remaining = available - acquires; if (remaining < 0 || compareAndSetState(available, remaining)) return remaining; } } }
从Semaphore.FairSync的源码可以看出，在调用tryAcquireShared方法时，如果AQS维护的等待队列中有线程在排队，则拒绝执行tryAcquire操作，直接走后续的线程排队等待流程。如果等待队列为空，也就是无竞争的情况下，才会尝试用cas操作去更新state变量。
这样就实现了公平语义。
