在ajax应用中，通常一个页面要同时发送多个请求，如果只有一个XMLHttpRequest对象，前面
的请求还未完成，后面的就会把前面的覆盖掉，如果每次都创建一个新的XMLHttpRequest对象，
也会造成浪费。解决的办法就是创建一个XMLHttpRequset的对象池，如果池里有空闲的对象，
则使用此对象，否则将创建一个新的对象。

下面是我最近写的一个简单的类：

/**
 * XMLHttpRequest Object Pool
 *
 * @author    legend <legendsky@hotmail.com>
 * @link      http://www.ugia.cn/?p=85
 * @Copyright www.ugia.cn
 */
   
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);
            }
        }
    }
}; 

示例：

<script type="text/javascript" src="xmlhttp.js"></script>
<script type="text/javascript">
function test(obj)
{
    alert(obj.statusText);
}
   
XMLHttp.sendReq('GET', 'http://www.ugia.cn/wp-data/test.htm', '', test);
XMLHttp.sendReq('GET', 'http://www.ugia.cn/wp-data/test.htm', '', test);
XMLHttp.sendReq('GET', 'http://www.ugia.cn/wp-data/test.htm', '', test);
XMLHttp.sendReq('GET', 'http://www.ugia.cn/wp-data/test.htm', '', test);
   
alert('Pool length:' + XMLHttp._objPool.length);
</script>