Java Semaphore用法

操作系统的信号量是个很重要的概念，在进程控制方面都有应用。Java 并发库 的Semaphore 可以很轻松完成信号量控制，Semaphore可以控制某个资源可被同时访问的个数，acquire()获取一个许可，如果没有就等待，而 release()释放一个许可。比如在Windows下可以设置共享文件的最大客户端访问个数。 

Semaphore维护了当前访问的个数，提供同步机制，控制同时访问的个数。在数据结构中链表可以保存“无限”的节点，用Semaphore可以实现有限大小的链表。另外重入锁ReentrantLock也可以实现该功能，但实现上要负责些，代码也要复杂些。 

下面的Demo中申明了一个只有5个许可的Semaphore，而有20个线程要访问这个资源，通过acquire()和release()获取和释放访问许可。

package com.cisco.learning;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

public class TestSemaphore {

 public static void main(String[] args) {
  // 线程池
  ExecutorService exec = Executors.newCachedThreadPool();

  // 只能个线程同时访问
  final Semaphore semp = new Semaphore(5);

  // 模拟个客户端访问
  for (int index = 1; index < 21; index++) {
   final int NO = index;
   Runnable run = new Runnable() {
    public void run() {
     try {
      // 获取许可
      semp.acquire();
      System.out.println("Accessing: " + NO);
      
      Thread.sleep((long) (Math.random() * 1000));

      // 访问完后，释放
      System.out.println("Release: " + NO);
      semp.release();
     } catch (InterruptedException e) {

     }
    }

   };
   exec.execute(run);
  }

  // 退出线程池
  exec.shutdown();
 }
}

Accessing: 1
Accessing: 3
Accessing: 2
Accessing: 4
Accessing: 5
Release: 5
Accessing: 6
Release: 1
Accessing: 7
Release: 4
Accessing: 9
Release: 3
Accessing: 8
Release: 9
Accessing: 10
Release: 6
Accessing: 11
Release: 7
Accessing: 12
Release: 12
Accessing: 13
Release: 13
Accessing: 14
Release: 2
Accessing: 15
Release: 10
Accessing: 16
Release: 15
Accessing: 17
Release: 8
Accessing: 18
Release: 18
Accessing: 19
Release: 17
Accessing: 20
Release: 16
Release: 11
Release: 14
Release: 19
Release: 20