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                      @Override136 137                      public void run() {138 139                             String resource = null;140 141                             try{142 143                                    resource = aPool.get();//取得resource144 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 }