/*** @since 1.5* @author Doug Lea*/
public class Semaphore implements java.io.Serializable {private static final long serialVersionUID = -3222578661600680210L;private final Sync sync;public Semaphore(int permits) {// 默认使用非公平锁，设置初始的剩余通行量sync = new NonfairSync(permits);}public Semaphore(int permits, boolean fair) {// 参数决定使用公平还是非公平锁，设置初始的剩余通行量sync = fair ? new FairSync(permits) : new NonfairSync(permits);}abstract static class Sync extends AbstractQueuedSynchronizer {private static final long serialVersionUID = 1192457210091910933L;Sync(int permits) {// 设置初始的剩余通行量
            setState(permits);}final int getPermits() {// 获取剩余通行量return getState();}final int nonfairTryAcquireShared(int acquires) {for (;;) {int available = getState();int remaining = available - acquires;// 设置获取后的剩余通行量// 当剩余量小于0时，则阻塞，否则通行if (remaining < 0 || compareAndSetState(available, remaining))return remaining;}}protected final boolean tryReleaseShared(int releases) {for (;;) {int current = getState();int next = current + releases;// 不允许释放量是负数if (next < current) // overflowthrow new Error("Maximum permit count exceeded");// 设置释放后的剩余通行量，返回释放成功，表示需要通知下一个线程通行if (compareAndSetState(current, next))return true;}}final void reducePermits(int reductions) {for (;;) {int current = getState();int next = current - reductions;// 不允许缩减值是负数if (next > current) // underflowthrow new Error("Permit count underflow");// 设置缩减后的剩余通行量if (compareAndSetState(current, next))return;}}final int drainPermits() {for (;;) {int current = getState();// 直接把剩余通行量设置为0if (current == 0 || compareAndSetState(current, 0))return current;}}}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);}}static final class FairSync extends Sync {private static final long serialVersionUID = 2014338818796000944L;FairSync(int permits) {super(permits);}protected int tryAcquireShared(int acquires) {for (;;) {// 如果当前线程不位于队头，则阻塞if (hasQueuedPredecessors())return -1;int available = getState();int remaining = available - acquires;// 设置获取后的剩余通行量// 当剩余量小0时，则阻塞，否则通行if (remaining < 0 || compareAndSetState(available, remaining))return remaining;}}}public void acquire() throws InterruptedException {// 获取一次通行，可中断sync.acquireSharedInterruptibly(1);}public void acquireUninterruptibly() {// 获取一次通行sync.acquireShared(1);}public boolean tryAcquire() {// 尝试获取一次通行return sync.nonfairTryAcquireShared(1) >= 0;}public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException {// 尝试在指定时间内获取一次通行return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));}public void acquire(int permits) throws InterruptedException {if (permits < 0)throw new IllegalArgumentException();// 获取指定通行次数，可中断
        sync.acquireSharedInterruptibly(permits);}public void acquireUninterruptibly(int permits) {if (permits < 0)throw new IllegalArgumentException();// 获取指定通行次数
        sync.acquireShared(permits);}public boolean tryAcquire(int permits) {if (permits < 0)throw new IllegalArgumentException();// 尝试获取指定通行次数return sync.nonfairTryAcquireShared(permits) >= 0;}public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException {if (permits < 0)throw new IllegalArgumentException();// 尝试在指定时间内获取通行return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));}public void release() {// 释放一次通行sync.releaseShared(1);}public void release(int permits) {if (permits < 0)throw new IllegalArgumentException();// 释放指定的通行次数
        sync.releaseShared(permits);}public int availablePermits() {// 返回剩余的通行量return sync.getPermits();}public int drainPermits() {// 直接把通行量设置为0return sync.drainPermits();}protected void reducePermits(int reduction) {if (reduction < 0)throw new IllegalArgumentException();// 缩减通行量
        sync.reducePermits(reduction);}public boolean isFair() {// 返回是否公平锁（即是否公平分配通行）return sync instanceof FairSync;}public final boolean hasQueuedThreads() {// 返回队列中是否有线程return sync.hasQueuedThreads();}public final int getQueueLength() {// 返回队列的长度return sync.getQueueLength();}protected Collection<Thread> getQueuedThreads() {// 返回队列中的所有线程return sync.getQueuedThreads();}public String toString() {return super.toString() + "[Permits = " + sync.getPermits() + "]";}
}