HTTP Status 500 - Servlet execution threw an exception

问题遇到的现象和发生背景

用maven环境java ，实现用户注册

用代码块功能插入代码，请勿粘贴截图
package com.wyl.web;

import com.wyl.pojo.User;
import com.wyl.service.impl.UserService;
import com.wyl.service.impl.UserServiceImpl;
import org.junit.jupiter.api.Test;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;

public class RegistServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         UserService userService = new UserServiceImpl();
// 1、获取请求的参数
            String username =request.getParameter("username");
            String password = request.getParameter("password");
            String email = request.getParameter("email");
            String code = request.getParameter("code");
// 2、检查 验证码是否正确 === 写死,要求验证码为:abcde
            if ("abcde".equalsIgnoreCase(code)) {
//3、检查 用户名是否可用
                if (userService.existUsername(username)) {
                    System.out.println("用户名[" + username + "]已存在!");
// 跳回注册页面
                    request.getRequestDispatcher("/pages/user/regist.html").forward(request, response);} else {
//可用
// 调用 Sservice 保存到数据库
                    userService.registUser(new User(null, username, password, email));
//跳到注册成功页面 regist_success.html
                    request.getRequestDispatcher("/pages/user/regist_success.html").forward(request, response);
                }
            } else {
                System.out.println("验证码[" + code + "]错误");
                request.getRequestDispatcher("/pages/user/regist.html").forward(request, response);
            }
        }

    }


action="regist" method="post">


    <servlet>
        <servlet-name>RegistServletservlet-name>
        <servlet-class>com.wyl.web.RegistServletservlet-class>
    servlet>
    <servlet-mapping>
        <servlet-name>RegistServletservlet-name>
        <url-pattern>/registurl-pattern>
    servlet-mapping>
运行结果及报错内容

img

看一下UserServiceImpl这个类第8行，可以粘一下代码看看，这一行报错了

img