JDK5.0 特性-线程同步装置之Semaphore

来自：http://www.cnblogs.com/taven/archive/2011/12/17/2291474.html

import java.util.ArrayList;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.Semaphore;

import java.util.concurrent.locks.Lock;

import java.util.concurrent.locks.ReentrantLock;

 

/**

 *Java 5.0里新增加了4个协调线程间进程的同步装置,它们分别是:Semaphore,CountDownLatch,CyclicBarrier和Exchanger

 *Semaphore可以控制运行线程的个数

 *Semaphore是一个用来管理资源池的工具,可以看成是个通行证,线程要想从资源池拿到资源必须先拿到通行证,如果线程暂时拿不到通告证,线程就会被阻断,进入等待状态.

 */

/**

 *使用Semaphore的关键技术点如下:

 *1.在构造Semaphore对象时,必须提供通行证的数目,如"newSemaphore(3)"将创建一个具有3个通行证的Semaphore对象,一旦该对象被创建,其通行证数量是不能改变的.

 *2.Semaphore的acquire方法取得一个通行证,如果通行证已经发完了,当前线程将进入等待状态,直到有其他线程释放了通行证.

 *3.Semaphore的release方法释放了资源池.

 */

public class SemaphoreTest {

       /**

        * 模拟资源池的类

        * 只为池发放2个通行证,即同时只允许2个线程获得池中的资源

        */

       public static class Pool{

              ArrayList<String> pool = null;  //保存资源池中的资源

              Semaphore pass = null; //通行证

              Lock lock = new ReentrantLock();

              public Pool(int size){

                     //初始化资源池

                     pool = new ArrayList<String>();

                     for(int i = 0; i < size; i++){

                            pool.add("Resource " + i);

                     }

                     //发送2个通行证

                     pass = new Semaphore(2);

              }

              public String get()throws InterruptedException{

                     //获取通行证,只有得到通行证后才能得到资源

                     System.out.println("Try to get a pass...");

                     pass.acquire();

                     System.out.println("Got a pass");

                     return getResource();

              }

              private String getResource(){

                     lock.lock();

                     String result = pool.remove(0);

                     System.out.println("资源 " + result + "被取走");

                     lock.unlock();

                     return result;

              }

              public void put(String resource){

                     //归还通行证,并那还资源

                     System.out.println("Released a pass");

                     pass.release();

                     releaseResource(resource);

              }

              private void releaseResource(String resource){

                     lock.lock();

                     System.out.println("资源 " + resource + " 被归还");

                     pool.add(resource);

                     lock.unlock();

              }

       }

       public static void testPool(){

              //准备10个资源的资源池

              final Pool aPool = new Pool(10);

              Runnable worker = new Runnable(){

                     @Override

                     public void run() {

                            String resource = null;

                            try{

                                   resource = aPool.get();//取得resource

                                   //用resource做工作

                                   System.out.println("I finished on " + resource);

                                   Thread.sleep(500);

                                   System.out.println("I finished on " + resource);

                            }catch(InterruptedException ex){

                            }

                            aPool.put(resource);

                     }                          

              };

              //启动5个任务

              ExecutorService service = Executors.newCachedThreadPool();

              for(int i = 0; i<5; i++){

                     service.submit(worker);

              }

              service.shutdown();

       }

       public static void main(String... args){

              SemaphoreTest.testPool();

       }

}