import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * 信号量 * * @author Vincent Zhao * @version 1.0.0 * @Time 2015/4/20 15:57 */public class SemaphoreDemo {    public static void main(String[] args) {        // 线程池        ExecutorService exec = Executors.newCachedThreadPool();        // 只能5个线程同时访问        final Semaphore semp = new Semaphore(5);        // 模拟20个客户端访问        for (int index = 0; index < 50; index++) {            final int NO = index;            Runnable run = new Runnable() {                public void run() {                    try {                        // 获取许可                        semp.acquire();                        System.out.println("Accessing: " + NO);                        Thread.sleep((long) (Math.random() * 10000));                        // 访问完后，释放                        semp.release();                        //availablePermits()指的是当前信号灯库中有多少个可以被使用                        System.out.println("-----------------" + semp.availablePermits());                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                }            };            exec.execute(run);        }        // 退出线程池        exec.shutdown();    }}