Ajax读取xml出错,ajax读取xml文件xmlhttp.responseXML.documentElement为空,该如何处理

JScript codefunction getXmlHttpPrefix() {

if (getXmlHttpPrefix.prefix)

return getXmlHttpPrefix.prefix;

var prefixes = ["MSXML2", "Microsoft", "MSXML", "MSXML3"];

var o;

for (var i = 0; i < prefixes.length; i++) {

try {

// try to create the objects

o = new ActiveXObject(prefixes[i] + ".XmlHttp");

return getXmlHttpPrefix.prefix = prefixes[i];

}

catch (ex) {};

}

throw new Error("Could not find an installed XML parser");

}

//

// Start the Real stuff //

//

// XmlHttp factory

function XmlHttp() {}

XmlHttp.create = function () {

try {

if (window.ActiveXObject) {

return new ActiveXObject(getXmlHttpPrefix() + ".XmlHttp");

}

else

if (window.XMLHttpRequest) {

var req = new XMLHttpRequest();

//some versions of Moz do not support the readyState property

//and the onreadystate event so we patch it!

if (req.readyState == null) {

req.readyState = 1;

req.addEventListener("load", function () {

req.readyState = 4;

if (typeof req.onreadystatechange == "function")

req.onreadystatechange();

}, false);

}

return req;

}

}

catch (ex) {

throw new Error("Your browser does not support XmlHttp objects");

}

}

function _startLoadXmlTree(url) {

var xmlHttp = XmlHttp.create();

if (xmlHttp.overrideMimeType){

xmlHttp.overrideMimeType("text/xml; charset=UTF-8");

}

xmlHttp.open("GET", sSrc, true); // async

xmlHttp.onreadystatechange = function () {

if (xmlHttp.readyState == 4) {

alert(xmlHttp.responseText); //可以取得xml的内容?????????????

alert(xmlhttp.responseXML.documentElement); //为空???????????

}

};

// call in new thread to allow ui to update

window.setTimeout(function () {

xmlHttp.send(null);

}, 10);

}