得到一个XMLHttpRequest(对象】

function getXMLHttpRequest(){
  var xmlhttp;
  if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp = new XMLHttpRequest();
  } else {// code for IE6, IE5
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  }
  return xmlhttp;
}


新建一个severlt 用来

package com.itheima.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletDemo1 extends HttpServlet {
 
   public void doGet(HttpServletRequest request, HttpServletResponse response)
         throws ServletException, IOException {
      response.setContentType("text/html;charset=utf-8");
      PrintWriter out = response.getWriter();
      out.write("hello word");
 
   }
 
   public void doPost(HttpServletRequest request, HttpServletResponse response)
         throws ServletException, IOException {
   doGet(request, response);
   }

}

3.实例 ${pageContext.request.contextPath} 是用来获得绝对路径的（只是根目录）   //2.建立一个连接


  req.open("get","ServletDemo1"); 这个我是直接写的

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
 

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<script type="text/javascript" src="${pageContext.request.contextPath}/js/myJS.js"></script>
<script type="text/javascript">
  window.onload = function() {
     var req=getXMLHttpRequest();//1.在myJS里面得到的XMLHttpRequest。通过这个函数调过来的
     req.onreadystatechange=function(){
      //4.处理响应，习惯性的把第四步写到这里
      if (req.readyState==4) {//客户端正常
       //alert(req.status);得到服务器的状态码
      if (req.status==200) {//服务器响应一切正常，200为正常
        alert(req.responseText);
      }
    }
     }
     //2.建立一个连接
     req.open("get","ServletDemo1");
     //3.发送一个请求
     req.send(null);
     
  };
</script>
</head>

<body>
</body>
</html>