为什么根本停不下来！！！

package com.imooc.play;

//军队线程，模拟作战双方的行为
public class Army implements Runnable {
 volatile boolean keepRunning = true;// volatile保证线程可以正确读取其他线程写入的该值

 public void run() {
  while (keepRunning = true) {
   for (int i = 0; i < 5; i++) {
    System.out.println(Thread.currentThread().getName() + "第"
      + (i + 1) + "次进攻对方");
    Thread.yield();// 让出处理器时间，下次该谁进攻还不一定
   }
  }
  System.out.println(Thread.currentThread().getName()+"战斗结束了！！");
 }
}

package com.imooc.play;

/**
 * 隋唐演义的大戏舞台
 *
 * @author cc
 *
 */

 public class Stage extends Thread {
 public void run() {
  Army army1 = new Army();
  Army army2 = new Army();
  // 使用runnable接口创建线程
  Thread army1Thread = new Thread(army1, "隋军");
  Thread army2Thread = new Thread(army2, "农民起义军");
  army1Thread.start();
  army2Thread.start();

  try {
   Thread.sleep(50);// 舞台线程休眠
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
  army1.keepRunning = false;
  army2.keepRunning = false;
  try {
   army1Thread.join();
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
 }

 public static void main(String[] args) {
  new Stage().start();

 }

}