var XMLHttp =
{
   _objPool : [],

   _getInstance : function ()
   {
      for (var i = 0; i < this._objPool.length; i ++ )
      {
         if (this._objPool[i].readyState == 0 || this._objPool[i].readyState == 4)
         {
            return this._objPool[i];
         }
      }

      // IE5中不支持push方法
      this._objPool[this._objPool.length] = this._createObj();

      return this._objPool[this._objPool.length - 1];
   }
   ,

   _createObj : function ()
   {
      var objXMLHttp=null;
      if (window.XMLHttpRequest)
      {
         objXMLHttp = new XMLHttpRequest();
      }
      else
      {
         var MSXML = ["MSXML2.XMLHTTP.5.0", "MSXML2.XMLHTTP.4.0", "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP","Microsoft.XMLHTTP"];
         for(var n = 0; n < MSXML.length; n ++ )
         {
            try
            {
               objXMLHttp = new ActiveXObject(MSXML[n]);
               break;
            }
            catch(e)
            {
            }
         }
      }

      // mozilla某些版本没有readyState属性
      if (objXMLHttp.readyState == null)
      {
         objXMLHttp.readyState = 0;

         objXMLHttp.addEventListener("load", function ()
         {
            objXMLHttp.readyState = 4;

            if (typeof objXMLHttp.onreadystatechange == "function")
            {
               objXMLHttp.onreadystatechange();
            }
         }
         , false);
      }

      return objXMLHttp;
   }
   ,

   // 发送请求(方法[post, get], 地址, 数据, 回调函数)
   sendReq : function (method, url, data, callback)
   {
      var objXMLHttp = this._getInstance();

      with(objXMLHttp)
      {
         try
         {
            // 加随机数防止缓存
            if (url.indexOf("?") > 0)
            {
               url += "&randnum=" + Math.random()+new Date().valueOf();
            }
            else
            {
               url += "?randnum=" + Math.random()+new Date().valueOf();
            }

            open(method, url, true);

            // 设定请求编码方式
            setRequestHeader("Content-Length",data.length);    
            setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
            send(data);
            onreadystatechange = function ()
            {
               if (objXMLHttp.readyState == 4 && (objXMLHttp.status == 200 || objXMLHttp.status == 304))
               {
                  callback(objXMLHttp);
               }
            }
         }
         catch(e)
         {
            alert(e);
         }
      }
   }
}