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();

        }

 }