// 调用方法 
 //var _ajax = new Ajax()              
 //_ajax.method = "post";   //是get还是post
 //_ajax.url = "http://aaa.com/a.asp";     //请求的地址
 //_ajax.responsetype = "text";   //处理返回内容的类型text或xml
 //_ajax.content = "id=1";   //发送的内容
 //_ajax.isasynchronous = true;   //是否是异步请求,true为异步请求,false为同步请求
 //_ajax.getExecObj(function(str){document.getElementById("aaa").innerHTML = str});//对返回值处理  

function Ajax(){
 this.method;
 this.url;
 this.responsetype;
 this.content;
 this.isasynchronous=true;
 var http_request = false;
 this.getExecObj = function(reValue){ 
  if(window.XMLHttpRequest) {
   http_request = new XMLHttpRequest();
   if (http_request.overrideMimeType) {
       http_request.overrideMimeType("text/xml");
   }
  }
  else if (window.ActiveXObject) {
   try {
       http_request = new ActiveXObject("Msxml2.XMLHTTP");
   } catch (e) {
       try {
        http_request = new ActiveXObject("Microsoft.XMLHTTP");
       } catch (e) {}
   }
  }
  if (!http_request) {
   window.alert("创建XMLHttpRequest对象实例失败.");
   return false;
  }
  
  if(this.method.toLowerCase()=="get") {
   http_request.open(this.method, this.url, this.isasynchronous);
  }
  else if(this.method.toLowerCase()=="post") {
   http_request.open(this.method, this.url, this.isasynchronous);
   http_request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
  }
  else {                           
   window.alert("http请求类别参数错误。");
   return false;
  }
  http_request.setRequestHeader("If-Modified-Since","0");
  http_request.send(this.content);  
  
  var reTextResponse = function() {
   if (http_request.readyState == 4) {
    if (http_request.status == 200) {   
      reValue(http_request.responseText);       
    } else {
//        alert("页面有异常。");
    }
   }
  }  
  var reXMLResponse = function() {
   if (http_request.readyState == 4) {
    if (http_request.status == 200) {   
      reValue(http_request.responseXML);       
    } else {
//        alert("页面有异常。");
    }
   }
  }
   
  if(this.responsetype.toLowerCase()=="text") {    
   http_request.onreadystatechange = reTextResponse;
  }
  else if(this.responsetype.toLowerCase()=="xml") {
   http_request.onreadystatechange = reXMLResponse;
  }
  else {
   window.alert("参数错误。");
   return false;
  }
 }  
}