停车场的停车位数量是固定的，Semaphore就像是停车场的管理员控制总的停车数量，如果车位已满，只能出一辆进一辆

Semaphore semaphore=new Semaphore(2);

控制车位数是2，默认不是公平的

Semaphore semaphore=new Semaphore(2,true);

车位数是2，公平锁，先到先停车

package com.vincent.juc; import java.util.Random;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;import java.util.concurrent.TimeUnit; /** * Vincent 创建于 2016/6/1. *  信号灯 */public class SemaphoreDemo {    public static void main(String[] args) {       final Semaphore semaphore=new Semaphore(2,true);         ExecutorService threadPool = Executors.newCachedThreadPool();         final Random random=new Random();         for(int i=0;i<10;i++){            threadPool.execute(new Runnable() {                @Override                public void run() {                    long id = Thread.currentThread().getId();                    try {                        semaphore.acquire();                        System.out.println(id+"获取到锁");                        int t = random.nextInt(5);                        TimeUnit.SECONDS.sleep(t+1);                     } catch (InterruptedException e) {                        e.printStackTrace();                    }finally {                        System.out.println(id+"释放锁");                        semaphore.release();                    }                }            });        }     }}

输出：

12获取到锁13获取到锁12释放锁14获取到锁13释放锁17获取到锁17释放锁15获取到锁14释放锁18获取到锁15释放锁16获取到锁18释放锁19获取到锁19释放锁20获取到锁16释放锁21获取到锁21释放锁20释放锁