how to locate a redirect url while the request server response a 302 redirect code?

hi, guys

I am unable to locate a redirect url while the request server response a 302 redirect code.

var url:String = "http://a.com/"; (response a 302 code and redirect to http://b.vom/)

var req:URLRequest = new URLRequest(url);

var dispatcher:URLLoader = new URLLoader(req);

dispatcher.addEventListener(Event.COMPLETE, handlefunction);

dispatcher.addEventListener(HTTPStatusEvent.HTTP_STATUS, httpStatusHandler);

function handlefunction(event:Event):void

{

     trace(req.url);

     // the console outputs  "http://a.com/"

     // it should be "http://b.com/"

     // but the event.currentTarget.data is ok .    e.g.  ok.htm

}

function httpStatusHandler(event:HTTPStatusEvent):void

{

     trace("httpStatusHandler: " + event);

     //the console outputs:  httpStatusHandler: [HTTPStatusEvent type="httpStatus" bubbles=false cancelable=false eventPhase=2 status=200 responseURL=null]

     //responseURL is null

}

there some relative links in the data ,so I must get the redirect url .

I  got  404 from http://a.com/ok.htm , but i want http://b.com/ok.htm

thanks.