简单概括：
Semphore是一把共享锁（即读锁），即实现了AQS的tryAcquireShared&&tryReleaseShared函数
Semphore的逻辑是这样：
创建semphore的时候会初始化一个锁容量即permits，即最多同时允许permits个线程获取读锁资源。AQS的state在semphore中表示锁资源的剩余容量。Semphore.tryAcquireShared就是如果锁资源剩余容量大于0则表示可以成功获取锁，然后锁资源容量减一，Semphore的tryReleaseShared就是锁资源容量+1。
笔记：
semphore没有锁所有者的概念，只有资源剩余量的概念，也就是说不管你是谁，只要资源还有剩余，那么就允许访问，也就是说一个线程可以获取多次资源
import java.util.Collection;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;public class Semaphore implements java.io.Serializable {private static final long serialVersionUID = -3222578661600680210L;private final Sync sync;abstract static class Sync extends AbstractQueuedSynchronizer {private static final long serialVersionUID = 1192457210091910933L;Sync(int permits) {//state就是表示锁资源剩余量setState(permits);}final int getPermits() {return getState();}//尝试获取共享锁资源。while cas方式扣减，准确说是：do{}while(!CAS)//获取锁资源的逻辑就是：如果锁资源剩余容量大于0就允许立即获得锁而无需入aqs队列排队//反之则获取失败，返回false//semphore没有锁所有者的概念，只有资源剩余量的概念//也就是说不管你是谁，只要资源还有剩余，那么就允许访问//也就是说一个线程可以获取多次资源final int nonfairTryAcquireShared(int acquires) {for (;;) {//笔记：getState是非并发安全的，但是没关系，//因为getState的返回值只是用来快速判断是否有资源剩余//真正决定能否成功获得锁的还是cas(available,remainging)这个操作//也就是说这里是一个乐观的做法：先扣减，再写回，如果冲突了就重试//通过getState获取资源剩余量int available = getState();//先计算要扣减的资源量int remaining = available - acquires;//如果remaining小于0表示资源剩余量小于0则，此时无法成功获取锁，所以返回负数//如果compareAndSetState(available, remaining)成功则表明资源剩余量大于0//并且资源扣减成功，此时remaing大于等于0，表示获取锁成功，所以返回非负数if (remaining < 0 ||compareAndSetState(available, remaining))return remaining;//走到这里则进入下一轮循环重试}}//尝试释放锁资源，也是do{}while(!cas)方式增加锁资源//因为是共享锁，且没有所有者的概念，所以可以一个线程多次释放，//每次释放都会锁资源+1，甚至能无线多次释放，然后就能无限获取了，也就是卡bug了//如下所示：信号量最初资源容量限制为2，但是因为没有锁所有者的概念以及上限检测//所以直接release(100)即增加100个令牌，这样就能获取102个令牌而不会阻塞// Semaphore sem=new java.util.concurrent.Semaphore(2);// sem.release(100);// for(int i=0;i<102;i++){// sem.acquire();// }protected final boolean tryReleaseShared(int releases) {for (;;) {//读取锁资源状态int current = getState();//计算更新后的锁资源量int next = current + releases;if (next < current) throw new Error("Maximum permit count exceeded");//cas方式更新锁资源状态if (compareAndSetState(current, next))return true;//走到这里则进入下一轮循环重试}}//扣减锁资源，也是通过do{}while(!CAS)方式更新final void reducePermits(int reductions) {for (;;) {int current = getState();int next = current - reductions;if (next > current) throw new Error("Permit count underflow");if (compareAndSetState(current, next))return;}}//清空锁资源。就是把state设置为0，也是通过do{}while(!CAS)方式更新final int drainPermits() {for (;;) {int current = getState();if (current == 0 || compareAndSetState(current, 0))return current;}}}static final class NonfairSync extends Sync {private static final long serialVersionUID = -2694183684443567898L;NonfairSync(int permits) {super(permits);}//非公平方式获取锁资源：就是如果有锁资源剩余容量大于0就允许立即获得锁而无需入aqs队列排队protected int tryAcquireShared(int acquires) {return nonfairTryAcquireShared(acquires);}}static final class FairSync extends Sync {private static final long serialVersionUID = 2014338818796000944L;FairSync(int permits) {super(permits);}//公平方式获取锁资源：先来先服务的原则。//也就是说只要aqs队列不为空则说明在本线程之前有其他线程已经在排队了//公平的原则就是先来先服务，所以这里就直接返回false表示获取锁失败//一旦tryAcquireShared返回失败，则aqs会把该节点丢到aqs list的最末尾protected int tryAcquireShared(int acquires) {for (;;) {//判断是有在此之前有其他线程等待获取锁资源（即信号量）if (hasQueuedPredecessors())//如果有则返回false表获取锁资源失败return -1;//如果在此之前没有其他线程等待获取锁资源（即信号量）//则通过do{}while(!CAS)扣减锁资源int available = getState();int remaining = available - acquires;if (remaining < 0 ||compareAndSetState(available, remaining))return remaining;}}}public Semaphore(int permits) {sync = new NonfairSync(permits);}public Semaphore(int permits, boolean fair) {sync = fair ? new FairSync(permits) : new NonfairSync(permits);}//下面的函数都输对sync的一个简单封装，所以下面的函数就没注释了，一眼就能看明白public void acquire() throws InterruptedException {sync.acquireSharedInterruptibly(1);}public void acquireUninterruptibly() {sync.acquireShared(1);}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 void release() {sync.releaseShared(1);}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);}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));}public void release(int permits) {if (permits < 0) throw new IllegalArgumentException();sync.releaseShared(permits);}public int availablePermits() {return sync.getPermits();}public int drainPermits() {return sync.drainPermits();}protected void reducePermits(int reduction) {if (reduction < 0) throw new IllegalArgumentException();sync.reducePermits(reduction);}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() + "]";}
}
