18.线程池

频繁创建线程就会大大降低系统的效率，因为频繁创建线程和销毁线程需要时间，那么有没有一种办法使得线程可以复用，就是执行完一个任务，并不被销毁，而是可以继续执行其他的任务？在Java中可以通过线程池来达到这样的效果。

http://www.cnblogs.com/dolphin0520/p/3932921.html

https://blog.csdn.net/weixin_28760063/article/details/81266152

https://blog.csdn.net/xiaojin21cen/article/details/81810534

https://www.cnblogs.com/jmsjh/p/7762034.html

https://www.cnblogs.com/dolphin0520/p/3949310.html

package duoxiancheng;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Demo4 {
    public static void main(String[] args) {   
        ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 10, 200, TimeUnit.MILLISECONDS,
                new ArrayBlockingQueue<Runnable>(5));
         
        for(int i=0;i<15;i++){
            MyTask myTask = new MyTask(i);
            executor.execute(myTask);
            System.out.println("线程池中线程数目："+executor.getPoolSize()+"，队列中等待执行的任务数目："+
            executor.getQueue().size()+"，已执行完毕的任务数目："+executor.getCompletedTaskCount());
        }
        executor.shutdown();
    }
}

class MyTask implements Runnable {
   private int taskNum;
    
   public MyTask(int num) {
       this.taskNum = num;
   }
    
   @Override
   public void run() {
       System.out.println("正在执行task "+taskNum);
       try {
           Thread.currentThread().sleep(4000);
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
       System.out.println("task "+taskNum+"执行完毕");
   }
}