Java多线程之Semephore

Semaphore 可以维护当前访问自身的线程个数，并提供了同步机制。使用Semaphore可以控制同时访问资源的线程个数，例如，实现一个文件允许的并发访问数。

单个信号量的Semaphore对象可以实现互斥锁的功能，并且可以是由一个线程获得了“锁”，再由另一个线程释放“锁”，这可应用于死锁恢复的一些场合。

package com.javase.thread;import java.util.Random;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;import java.util.concurrent.TimeUnit;/** * 10个线程并发访问牌数为3的信号灯 * @author Alex Zhuang * */public class SemaphoreDemo {public static void main(String[] args) {final Semaphore semaphore = new Semaphore(3);ExecutorService threadPool = Executors.newCachedThreadPool();for(int i=0;i<10;i++){Runnable command =new Runnable(){public void run() {System.out.println(Thread.currentThread().getName()+" is waiting now");try {// Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted.semaphore.acquire();} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName()+" is running now");System.out.println("There are "+ (3-semaphore.availablePermits()) + " thread got the permit of semaphore now");try {//Java SE5 引入的更加显示的sleep()版本，这个方法允许指定sleep()延迟的时间单元，因此有更好的可读性TimeUnit.MILLISECONDS.sleep(new Random().nextInt(10000));} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}// Releases a permit, returning it to the semaphore.semaphore.release();System.out.println(Thread.currentThread().getName()+" has been end now");System.out.println("There are "+ (3-semaphore.availablePermits()) + " thread got the permit of semaphore now");}};threadPool.execute(command);}threadPool.shutdown();}}