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 ()
        {
            if (window.XMLHttpRequest)
            {
                var 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
                    {
                        var 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, arg, data, callback)
        {
            var objXMLHttp = this._getInstance();
            with(objXMLHttp)
            {
                try
                {
                    // 加随机数防止缓存
                    if (arg)
                    {
                        url += "?"+encodeURI(arg)+"&randnum=" + Math.random();
                    }
                    else
                    {
                        url += "?randnum=" + Math.random();
                    }
                    open(method, url, true);
                    // 设定请求编码方式
                    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(responseText);
                        }
                    }
                }
                catch(e)
                {
                    alert(e);
                }
            }
        }
    };