UserController.java 
package cn.itcast.controller;

import cn.itcast.domain.User;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
@RequestMapping("/user")
public class UserController {

    @RequestMapping("/testString")
    public String testString(Model model) {
        System.out.println("testString方法执行了...");
        //模拟从数据库中取出User对象
        User user = new User();
        user.setUsername("美美");
       user.setPassword("123");
        user.setAge(30);
        //model对象
        model.addAttribute("user",user);
        return "success";
    }
}
success.jsp 
<%--
  Created by IntelliJ IDEA.
  User: hp
  Date: 2019/11/3
  Time: 21:16
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" isELIgnored="false" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <h3>执行成功</h3>

    ${user.username}
    ${user.password}
</body>
</html>