可以实现流量控制，同时访问文件的用户数

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

public class SemaphoreTest {
    public static void main(String[] args) {
        ExecutorService service = Executors.newCachedThreadPool();
        final  Semaphore sp = new Semaphore(3);//创建Semaphore信号量，初始化许可大小为3
        Runnable runnable = new Runnable(){
            public void run(){
            try {
                sp.acquire();//请求获得许可，如果有可获得的许可则继续往下执行，许可数减1。否则进入阻塞状态
            } catch (InterruptedException e1) {
                e1.printStackTrace();
            }
            System.out.println("线程" + Thread.currentThread().getName() + 
                    "进入，当前已有" + (3-sp.availablePermits()) + "个并发");
            try {
                Thread.sleep((long)(Math.random()*10000));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("线程" + Thread.currentThread().getName() + 
                    "即将离开");                    
            sp.release();//释放许可，许可数加1
            //下面代码有时候执行不准确，因为其没有和上面的代码合成原子单元
            System.out.println("线程" + Thread.currentThread().getName() + 
                    "已离开，当前已有" + (3-sp.availablePermits()) + "个并发");                    
        }
    };
        for(int i=0;i<10;i++){
            service.execute(runnable); //提交十个任务           
        }
    }                                                                                                                   

}