jsp代码

<%--
  Created by IntelliJ IDEA.
  User: 23369
  Date: 2019/8/26
  Time: 14:06
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" pageEncoding="UTF-8" %>
<html>
<head>
    <title>Title</title>
    <script type="text/javascript" src="jQuery/jquery-3.4.1.js"></script>
    <script type="text/javascript">
       $(function () {
            $(":input[name='username']").change(function () {
                var val = $(this).val();//获取值

                val = $.trim(val);

                if (val != ""){
                    var url = "${pageContext.request.contextPath}/SelectUserNameServlet";
                    var args = {"username":val,"time":new Date()};

                    $.post(url,args,function (data) {
                        $("#message").html(data);
                    });
                }
            });
        });

    </script>
</head>
<body>
<form action="${pageContext.request.contextPath}" method="post">
    用户名<input id="username" type="text" name="username"><div id="message"> </div><br>
    密码<input type="password" name="userpass"><div id="messagePassword"></div><br>
    <input type="submit" name="submitInput" value="登录"><br>
</form>
</body>
</html>

java代码

package com.hern;

import javax.jws.WebService;
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;
import java.util.Arrays;
import java.util.List;

@WebServlet(name = "/SelectUserNameServlet")
public class SelectUserNameServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        response.setCharacterEncoding("UTF-8");

        List<String> userNames = Arrays.asList("AAA","BBB","CCC");
        String userName = request.getParameter("username");
        String result = null;
        if (userNames.contains(userName)){
            result = "<font color='red'>该用户名已被使用</font>";
        }else {
            result = "<font color='red'>该用户名可以使用</font>";
        }


        response.getWriter().print(result);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}