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                   }    }                                                                                                                   }

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