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
  1 import java.util.ArrayList;
  2 
  3 import java.util.concurrent.ExecutorService;
  4 
  5 import java.util.concurrent.Executors;
  6 
  7 import java.util.concurrent.Semaphore;
  8 
  9 import java.util.concurrent.locks.Lock;
 10 
 11 import java.util.concurrent.locks.ReentrantLock;
 12 
 13  
 14 
 15 /**
 16 
 17  *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
 18 
 19  *Semaphore可以控制运行线程的个数
 20 
 21  *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.
 22 
 23  */
 24 
 25 /**
 26 
 27  *使用Semaphore的关键技术点如下:
 28 
 29  *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.
 30 
 31  *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.
 32 
 33  *3.Semaphore的release方法释放了资源池.
 34 
 35  */
 36 
 37 public class SemaphoreTest {
 38 
 39        /**
 40 
 41         * 模拟资源池的类
 42 
 43         * 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
 44 
 45         */
 46 
 47        public static class Pool{
 48 
 49               ArrayList<String> pool = null;  //保存资源池中的资源
 50 
 51               Semaphore pass = null; //通行证
 52 
 53               Lock lock = new ReentrantLock();
 54 
 55               public Pool(int size){
 56 
 57                      //初始化资源池
 58 
 59                      pool = new ArrayList<String>();
 60 
 61                      for(int i = 0; i < size; i++){
 62 
 63                             pool.add("Resource " + i);
 64 
 65                      }
 66 
 67                      //发送2个通行证
 68 
 69                      pass = new Semaphore(2);
 70 
 71               }
 72 
 73               public String get()throws InterruptedException{
 74 
 75                      //获取通行证,只有得到通行证后才能得到资源
 76 
 77                      System.out.println("Try to get a pass...");
 78 
 79                      pass.acquire();
 80 
 81                      System.out.println("Got a pass");
 82 
 83                      return getResource();
 84 
 85               }
 86 
 87               private String getResource(){
 88 
 89                      lock.lock();
 90 
 91                      String result = pool.remove(0);
 92 
 93                      System.out.println("资源 " + result + "被取走");
 94 
 95                      lock.unlock();
 96 
 97                      return result;
 98 
 99               }
100 
101               public void put(String resource){
102 
103                      //归还通行证,并那还资源
104 
105                      System.out.println("Released a pass");
106 
107                      pass.release();
108 
109                      releaseResource(resource);
110 
111               }
112 
113               private void releaseResource(String resource){
114 
115                      lock.lock();
116 
117                      System.out.println("资源 " + resource + " 被归还");
118 
119                      pool.add(resource);
120 
121                      lock.unlock();
122 
123               }
124 
125        }
126 
127        public static void testPool(){
128 
129               //准备10个资源的资源池
130 
131               final Pool aPool = new Pool(10);
132 
133               Runnable worker = new Runnable(){
134 
135                      @Override
136 
137                      public void run() {
138 
139                             String resource = null;
140 
141                             try{
142 
143                                    resource = aPool.get();//取得resource
144 
145                                    //用resource做工作
146 
147                                    System.out.println("I finished on " + resource);
148 
149                                    Thread.sleep(500);
150 
151                                    System.out.println("I finished on " + resource);
152 
153                             }catch(InterruptedException ex){
154 
155                             }
156 
157                             aPool.put(resource);
158 
159                      }                          
160 
161               };
162 
163               //启动5个任务
164 
165               ExecutorService service = Executors.newCachedThreadPool();
166 
167               for(int i = 0; i<5; i++){
168 
169                      service.submit(worker);
170 
171               }
172 
173               service.shutdown();
174 
175        }
176 
177        public static void main(String... args){
178 
179               SemaphoreTest.testPool();
180 
181        }
182 
183 }