Java并发同步工具类之Semaphore

Semaphore即信号量，维护一堆许可。初始化时会设置一个计数器值，当调用acquire方法时，如果此时计数器值为0，则阻塞直到计数值大于0；否则计数器值减一，当前线程继续执行。

当release()方法调用时，计数器值会加一。

Semaphore经常被用于限制一定数量的线程来访问某资源。

如下例使用Semaphore来控制对items 池的访问。

class Pool {
     private static final int MAX_AVAILABLE = 100;
     private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
 
     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;
     }
 
   }