package com.lbl.msg;

public class Msg {
  String msg;
  int code;

  public Msg() {
  }

  public Msg(String msg, int code) {
    this.msg = msg;
    this.code = code;
  }

  public String getMsg() {
    return msg;
  }

  public void setMsg(String msg) {
    this.msg = msg;
  }

  public int getCode() {
    return code;
  }

  public void setCode(int code) {
    this.code = code;
  }
}

package com.lbl.servlet;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.lbl.msg.Msg;

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;

@WebServlet("/register")
public class RegisterServlet extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doGet(request, response);
  }

  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //1:获取请求发送的数据
    String username = request.getParameter("username");
    response.setContentType("text/html;charset=UTF-8");
    //2:判断用户名是否注册
    if("abc123".equals(username)){
      Msg msg = new Msg("用户名已经注册",0);
      //用户名已经注册 {"flag":false,"info":"用户名已经注册"};
//      String jsonStr = "{\"flag\":false,\"info\":\"用户名已经注册\"}";
      //响应回浏览器
      response.getWriter().write(new ObjectMapper().writeValueAsString(msg));
    }else{
      Msg msg = new Msg("可以注册",1);
      //用户名没有注册
//      String jsonStr = "{\"flag\":true,\"info\":\"可以注册\"}";
      //响应回浏览器
      response.getWriter().write(new ObjectMapper().writeValueAsString(msg));
    }

  }
}

!DOCTYPE html>
html lang="en">
head>
  meta charset="UTF-8">
  title>Title/title>
  script src="js/jquery-3.3.1.js">/script>
  script type="application/javascript">
    $(function () {
      //1: 给用户名输入框绑定失去焦点事件(onfocus onblur)
      $("#username").on("blur",function () {
        //获取用户名输入框数据
        var username = $("#username").val();
        //2:向服务器发出异步请求，让服务器去查询用户名是否存在
        $.post(
          "register", //表示服务器的servlet路径
          "username="+username, //表示向服务器发送的数据
          function (data) { // msg:用户名已经注册 code:0
            if(data.code==0){
              // alert(data.info);
              $("#spanId").html(data.msg).css("color","green");
            }else {
              $("#spanId").html(data.msg).css("color","red");
            }

          },
          "json"
        );
      });
    });

  /script>
/head>
body>
div>
  font>会员注册/font>USER REGISTER
  form class="form-horizontal" style="margin-top: 5px;">
    table>
      tr>
        td>用户名/td>
        td>
          input type="text" id="username" name="username" placeholder="请输入用户名">
          span id="spanId">/span>
        /td>
      /tr>
      tr>
        td>密码/td>
        td>
          input type="password" placeholder="请输入密码">
        /td>
      /tr>
    /table>
    input type="submit" value="注册"/>
  /form>
/div>

/body>
/html>