怎么让用户名错误提示不在一开始就显示

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<%@ 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;
    }
}