<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>ajax</title>
    <script src="/ajax/js/jquery-3.3.1.min.js"></script>
  </head>
  <body>
  <form action="http://www.ubfgho.com/" method="post" enctype="application/x-www-form-urlencoded">
    姓名：<input type="text" name="name"><span></span><br>
    密码：<input type="password" name="psw"><br>
    <input type="submit" value="提交"><br>
  </form>
  </body>
  <script>
      //js实现post方式的异步请求
      $(':text').blur(function () {
          var name1=$(this).val().trim()
          //1、创建发送异步请求的对象req
          var req=XMLHttpRequest();
          //2、准备发送
          req.open("POST","ajax/login",true)
          //3、设置请求头
          req.setRequestHeader("Content-type","application/x-www-form-urlencoded")
          //4、发送
          req.send("name"+name1)
          
          //回调参数
          req.onreadystatechange=function () {
              if (req.readyState==4&&req.status==200){
                  $("span").html(req.responseText).css("color","red")
              }
          }
      })




    //1、失去焦点事件--判断姓名是否为空
    $(':text').blur(function () {
        var v=$(this).val()
        if (v.trim()==""){
            $("span").html("姓名为空").css("color","red")
        }else{
            $("span").html("")//清空span
        }
    })
    //2、提交事件---点击提交按钮触发submit
    $("form").submit(function () {
        //选中text文本
        var v=$(':text').val().trim();
        if (v==""){
        //如果姓名为空。就返回false，阻止提交，
            return false
        }
    })


  </script>
</html>

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

    String name=request.getParameter("name");
    boolean b=dao.selectName(name);
    String msg=b?"用户名已被占用":"用户名正常";
    response.setContentType("text/html;charset=UTF-8");
    response.getWriter().write(msg);
}