registerpost.jsp  这里注意与GET方式不同的地方


<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
  <title>POST方式，检查用户名是否 可以注册</title>
</head>
<body>
  <input id="usernameID" type="text" name="username">
  <span id="tipID"></span>
    
<script type="text/javascript">
  document.getElementById("usernameID").onblur = function(){
    var username = this.value;
    //创建ajax对象
    var ajax = new XMLHttpRequest();
    var method = "POST";
    var url = "${pageContext.request.contextPath}/AjaxRegisterServlet";
    ajax.open(method,url);
    var content = "username="+username;
    //POST方式提交的一定要这行，会将请求体的汉子进行UTF-8编码
    ajax.setRequestHeader("content-type","application/x-www-form-urlencoded");
    ajax.send(content);
    //------------------数据已发送
    
    ajax.onreadystatechange = function(){
      if(ajax.readyState == 4){
        if(ajax.status == 200){
          var tip = ajax.responseText;
          document.getElementById("tipID").innerHTML = tip;
        }
      }
    }
  }
</script>
</body>
</html>




服务端 

protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
  request.setCharacterEncoding("UTF-8");
  response.setContentType("text/html;charset=UTF-8");
  String username = request.getParameter("username");
  System.out.println(username);
  String tip = "";
  if("杰克".equals(username)){
    tip = "用户名已经存在";
  }else{
    tip = "可以注册";
  }
  PrintWriter pw = response.getWriter();
  pw.write(tip);
  pw.flush();
  pw.close();
}