// 创建http_request对象
var http_request;
function getXMLHttp()
{
   var http_request=false;
    if(window.XMLHttpRequest)//针对FF,Netscape,Opera等浏览器
    {
      http_request=new XMLHttpRequest();
 
      if(http_request.overrideMimeType)
      {
　　　   http_request.overrideMimeType("text/xml");
 　　  }
    }
    else //针对IE等浏览器
    {
       try{
         http_request=new ActiveXObject("Msxml2.XMLHTTP");
       }
       catch(e)
       {
          try{
           http_request=new ActiveXObject("Microsoft.XMLHTTP");
           }
           catch(E){ }
        }
    }
    if(!http_request)
    {
        alert("不能创建XMLHttpRequest实例!");
        return false;
    }
    return http_request;
}

//使用http_request异步传送函数
function ajax(method,url,postData,callback)
{
  http_request=getXMLHttp();
  http_request.onreadystatechange=callback;
  //alert(url);
  
  if (method.toUpperCase()=="POST")
  {
    http_request.open(method,url,true);
    http_request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
  }
  else if(method.toUpperCase()=="GET")
  {
     http_request.open(method,url,true);
  }
  http_request.send(postData);
   
 return ;
}