怎么让用户名错误提示不在一开始就显示

<%@ page language="java" contentType="text/html;charset=utf-8" pageEncoding="utf-8" %>
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>登录</title>
        <link rel="stylesheet" href="css/login.css">
    </head>
    <body>
        <div class="login">
            <div class="header">
                <h1>
                    <a href="/login.do">登录</a>${msg}
                </h1>
                <button></button>
            </div>
            <form action="${pageContext.request.contextPath}/LoginServlet" method="post">
                <div class="name">
                    <input type="text" id="name" name="username">
                    <p></p>
                </div>
                <div class="pwd">
                    <input type="password" id="pwd" name="password">
                    <p></p>
                </div>
                <div class="code">
                    <input type="text" id="code" name="verifyCode" style="width: 150px">
                    &nbsp;&nbsp;&nbsp;&nbsp;
                    <img src="code.jpg" style="width: 150px;height: 42px;vertical-align: middle;">
                    <p></p>
                </div>
                <div class="btn-red">
                    <input type="submit" value="登录" id="login-btn">
                </div>
            </form>
        </div>
    </body>
</html>

package com.book.web.servlet;

import java.io.IOException;
import java.util.List;

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 com.book.web.domain.User;
import com.book.web.service.UserServiceImpl;


@WebServlet("/LoginServlet")
public class LoginServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//接收数据:
		String username=request.getParameter("username");
		String password=request.getParameter("password");
		//封装数据:
		User user=new User(username,password);
		//处理数据:
		UserServiceImpl userServiceImpl=new UserServiceImpl();
		//获得用户列表的集合:
		List<User> userList=(List)getServletContext().getAttribute("userList");
		User existUser=userServiceImpl.login(userList,user);
		//显示结果:
		if(existUser==null) {
			//登录失败
			request.setAttribute("msg", "用户名或密码错误!");
			request.getRequestDispatcher("/login.jsp").forward(request, response);
		}else {
			//登录成功
			request.getSession().setAttribute("existUser", existUser);
			response.sendRedirect(request.getContextPath()+"/bookList.jsp");
		}
	}

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

}

package com.book.web.listener;

import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import com.book.web.domain.User;


@WebListener
public class InitListener implements ServletContextListener {

   
    public void contextDestroyed(ServletContextEvent sce)  { 
         
    }

	
    public void contextInitialized(ServletContextEvent sce)  { 
         //创建一个用于保存用户信息的集合
    	List<User> userList=new ArrayList();
    	userList.add(new User("admin1","123456"));
    	userList.add(new User("admin2","123123"));
    	userList.add(new User("admin3","111111"));
    	//将List集合存入ServletContext域中
    	sce.getServletContext().setAttribute("userList", userList);
    }
	
}

package com.book.web.domain;

public class User {
	private String username;//用户名
	private String password;//密码
	public User() {
		
	}
	public User(String username, String password) {
		super();
		this.username = username;
		this.password = password;
	}

	public String getUsername() {
		return username;
	}

	public void setUsername(String username) {
		this.username = username;
	}

	public String getPassword() {
		return password;
	}

	public void setPassword(String password) {
		this.password = password;
	}

	@Override
	public String toString() {
		return "User [username=" + username + ", password=" + password + "]";
	}
}

package com.book.web.service;

import java.util.List;

import com.book.web.domain.User;

public class UserServiceImpl {
	public User login(List<User> userList,User user) {
		for(User existUser:userList) {
			if(existUser.getUsername().equals(user.getUsername())&&existUser.getPassword().equals(user.getPassword())) {
				return existUser;
			}
		}
		return null;
	}
}

打开登录页面时，还没提交表单就已显示登录错误的提示，请问怎么样才能不在一开始就出现呢

慕UI5285863 2019-09-09

源自：项目：登录注册功能 6-2 项目作业