How to know when @ResponseBody ends

Question

I have a method annotated with @ResponseBody that return xls file and works fine if is normla post request.

Now I need to show on the view a loading effect. So first show loading ajax popup, second I call method with ajax post but i don't know how to handle response on success to download or open the file and close the popup.

Can you recommend me another strategy?

Now the code is like this.

    //Controller
    @RequestMapping(value = "/generateXls", method = RequestMethod.POST)
    @ResponseBody
    public void generateXls(@ModelAttribute Filter filter, HttpServletRequest request, HttpSession session,  HttpServletResponse response) 
                throws Exception 
    {
         /*Code for generate xls*/
    }


//View
//function called on form submit.
function callGenerateXls() {
    /* SHOW LOADING POPUP */
    $.ajax({
    type: 'POST',
    url: "report/generateXls",
    data: $("#filter").serialize(),
    success: function(result) {
       /* HOW TO HANDLE RESULT ????? */
       /* HIDE LOADING POPUP */
    }
    });
}