package semaphore;import java.util.concurrent.Semaphore;public class Ticket {public static void main(String[] args) {Semaphore windows = new Semaphore(5);  // 声明5个窗口for (int i = 0; i < 8; i++) {new Thread() {@Overridepublic void run() {try {windows.acquire();  // 占用窗口System.out.println(Thread.currentThread().getName() + ": 开始买票");sleep(2000);  // 睡2秒，模拟买票流程System.out.println(Thread.currentThread().getName() + ": 购票成功");windows.release();  // 释放窗口} catch (InterruptedException e) {e.printStackTrace();}}}.start();}}
}

Thread-1: 开始买票
Thread-3: 开始买票
Thread-4: 开始买票
Thread-2: 开始买票
Thread-0: 开始买票
Thread-1: 购票成功
Thread-5: 开始买票
Thread-3: 购票成功
Thread-2: 购票成功
Thread-4: 购票成功
Thread-7: 开始买票
Thread-6: 开始买票
Thread-0: 购票成功
Thread-7: 购票成功
Thread-5: 购票成功
Thread-6: 购票成功

View Code

    public final void acquireSharedInterruptibly(int arg)throws InterruptedException {if (Thread.interrupted())throw new InterruptedException();if (tryAcquireShared(arg) < 0)doAcquireSharedInterruptibly(arg);}

        final int nonfairTryAcquireShared(int acquires) {for (;;) {int available = getState();int remaining = available - acquires;if (remaining < 0 ||compareAndSetState(available, remaining))return remaining;}}

    public Semaphore(int permits) {sync = new NonfairSync(permits);}// 内部类NonfairSync(int permits) {super(permits);}// 内部类，NonfairSync的父类Sync(int permits) {setState(permits);}

    /*** The synchronization state.*/private volatile int state;protected final int getState() {return state;}protected final void setState(int newState) {state = newState;}

    /*** 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 GCfailed = false;return;}}if (shouldParkAfterFailedAcquire(p, node) &&parkAndCheckInterrupt())throw new InterruptedException();}} finally {if (failed)cancelAcquire(node);}}

        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;}}