Semaphore介绍

java.util.concurrent.Semaphore

?

介绍：

? ??

? ??Semaphore是一个信号灯模型，简单理解为N进N出的模型。每次调用acquire时，如果已经有N个线程进入管道，则会阻塞。每次线程离开管道时，通过release释放掉。当Semaphore的count为1时，它的语义就差不多是个互斥锁了，一进一出。

? ??

//JDK中的 例子，通过一个Semphore数组来控制线程池数量。class Pool {   private static final int MAX_AVAILABLE = 100;   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);//第二参数标识公平锁还是非公平锁   public Object getItem() throws InterruptedException {     available.acquire();     return getNextAvailableItem();   }   public void putItem(Object x) {     if (markAsUnused(x))       available.release();   }   // Not a particularly efficient data structure; just for demo   protected Object[] items = ... whatever kinds of items being managed   protected boolean[] used = new boolean[MAX_AVAILABLE];   protected synchronized Object getNextAvailableItem() {     for (int i = 0; i < MAX_AVAILABLE; ++i) {       if (!used[i]) {          used[i] = true;          return items[i];       }     }     return null; // not reached   }   protected synchronized boolean markAsUnused(Object item) {     for (int i = 0; i < MAX_AVAILABLE; ++i) {       if (item == items[i]) {          if (used[i]) {            used[i] = false;            return true;          } else            return false;       }     }     return false;   } }

?

? ?