1.先导包，然后添加进制浏览器缓存代码

index.jsp

<%--
  Created by IntelliJ IDEA.
  User: 49841
  Date: 2020/9/4
  Time: 10:59
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>index</title>
  </head>
  <body>
      <h2>欢迎登录</h2>
      <h2>呱？！，当前登录用户：${user}</h2>
      <img src="${pageContext.request.contextPath}/img">
  </body>
</html>

ImageServlet

package servlet;

import cn.dsna.util.images.ValidateCode;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * zt
 * 2020/9/5
 * 17:53
 */
@WebServlet(name = "ImageServlet",value = "/img")
public class ImageServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setHeader("Cache-Control", "no-cache");
        response.setDateHeader("Expires", 0);
        response.setHeader("Pragma", "no-cacahe");

        ValidateCode validateCode = new ValidateCode(100, 30, 4, 30);
        String code = validateCode.getCode();
        request.getSession().setAttribute("code", code);
        validateCode.write(response.getOutputStream());
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

运行效果图

此时按回车可以刷新验证码

解决ie浏览器回车不刷新验证码问题

1.先导包，然后添加进制浏览器缓存代码

index.jsp

ImageServlet

运行效果图