xmlHttp池解决多ajax请求问题

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, 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();
                }
                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(objXMLHttp);
                    }
                }
            }
            catch(e)
            {
                alert(e);
            }
        }
    }
};

通过建立xmlHttp池来有效避免多个ajax同时请求冲突的问题，可以复用空闲的xmlHttp对象避免为每个ajax请求生成新xmlHttp对象的问题

对以上代码可以简化一下，缩短长度

 1var XMLHttp = {
 2    _objPool: [],
 3    _getInstance: function ()
 4    {
 5        for (var i = 0; i < this._objPool.length; i ++)
 6        {
 7            if (this._objPool[i].readyState == 0 || this._objPool[i].readyState == 4)
 8            {
 9                return this._objPool[i];
            }
        }
        this._objPool[this._objPool.length] = (window.ActiveXObject) ? new ActiveXObject("Microsoft.XMLHTTP") : new XMLHttpRequest();
        return this._objPool[this._objPool.length - 1];
    },
    sendReq: function (method, url, data, callback)
    {
        var objXMLHttp = this._getInstance();
        with(objXMLHttp)
        {
            url += ((url.indexOf("?")>0) ? "&" : "?") + "r="+Math.random();
            open(method, url, true);
            send(data);
            onreadystatechange = function ()
            {                   
                if (objXMLHttp.readyState == 4 && objXMLHttp.status == 200)
                    callback(objXMLHttp);
            }
        }
    }
};