Semaphore(信号量),用于做限流处理,比如说同时只允许5五个人访问,超过五个人访问就需要等待,类似这样的需求,下面的案例可以看出执行是五个五个的执行,等上一个五个执行完了,才会执行下一个

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class UseSemaphore {
    public static void main(String[] args) {
        // 线程池  
        ExecutorService exec = Executors.newCachedThreadPool();
        // 只能5个线程同时访问  
        final Semaphore semp = new Semaphore(5);
        // 模拟20个客户端访问  
        for (int index = 0; index < 20; index++) {
            final int NO = index;
            Runnable run = new Runnable() {
                public void run() {
                    try {
                        // 获取许可  
                        semp.acquire();
                        System.out.println("Accessing: " + NO);
                        //模拟实际业务逻辑
                        Thread.sleep((long) (Math.random() * 10000));
                        // 访问完后,释放  
                        semp.release();
                    } catch (InterruptedException e) {
                    }
                }
            };
            exec.execute(run);
        }
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        //System.out.println(semp.getQueueLength());
        // 退出线程池
        exec.shutdown();
    }
}


https://blog.csdn.net/u010316188/article/details/86562395