在ajax應(yīng)用中，通常一個(gè)頁面要同時(shí)發(fā)送多個(gè)請(qǐng)求，如果只有一個(gè)XMLHttpRequest對(duì)象，前面的請(qǐng)求還未完成，后面的就會(huì)把前面的覆蓋掉，如果每次都創(chuàng)建一個(gè)新的XMLHttpRequest對(duì)象，也會(huì)造成浪費(fèi)。解決的辦法就是創(chuàng)建一個(gè)XMLHttpRequset的對(duì)象池，如果池里有空閑的對(duì)象，則使用此對(duì)象，否則將創(chuàng)建一個(gè)新的對(duì)象。
下面是我最近寫的一個(gè)簡(jiǎn)單的類：

/**
 * 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某些版本沒有readyState屬性
        if (objXMLHttp.readyState == null)
        {
            objXMLHttp.readyState = 0;
    
            objXMLHttp.addEventListener("load", function ()
                {
                    objXMLHttp.readyState = 4;
    
                    if (typeof objXMLHttp.onreadystatechange == "function")
                    {
                        objXMLHttp.onreadystatechange();
                    }
                },  false);
        }
    
        return objXMLHttp;
    },
    
    // 發(fā)送請(qǐng)求(方法[post,get], 地址, 數(shù)據(jù), 回調(diào)函數(shù))
    sendReq: function (method, url, data, callback)
    {
        var objXMLHttp = this._getInstance();
    
        with(objXMLHttp)
        {
            try
            {
                // 加隨機(jī)數(shù)防止緩存
                if (url.indexOf("?") > 0)
                {
                    url += "&randnum=" + Math.random();
                }
                else
                {
                    url += "?randnum=" + Math.random();
                }
    
                open(method, url, true);
    
                // 設(shè)定請(qǐng)求編碼方式
                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>