java Semaphore的介绍和使用

import java.util.concurrent.*;

/**
 * by lv xiao long
 *
 */
public class App 
{
    public static void main( String[] args )
    {
        // 线程池
        ExecutorService exec = Executors.newCachedThreadPool();
        // 模拟只能5个客户端同时访问
        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);
                        Thread.sleep((long) (Math.random() * 10000));
                        // 访问完后，释放 ，如果屏蔽下面的语句，则在控制台只能打印5条记录，之后线程一直阻塞
                        semp.release();
                    } catch (InterruptedException e) {
                    }
                }
            };
            exec.execute(run);
        }
        // 退出线程池
        exec.shutdown();
    }
}