How to Handle the response from server after a form submission?

I'm attempting to create ajax functionality to a form that uploads files. I created the isolated iframe and target the form submission to it. The server validates the files and sends back an XML doc in the response body. How would I handle and retrieve the XML doc on the client side, preferably using JavaScript?

This is my handler,

@Controller
@RequestMapping("/upload")
public class UploadController {


@RequestMapping(method = RequestMethod.POST)
public String handleFileUpload(HttpServletResponse response, @RequestParam("file") MultipartFile file) throws IOException{
    response.setContentType("text/xml");
    response.se开发者_运维问答tHeader("Cache-Control", "no-cache");
    response.getWriter().write("<test>hello</test>"); 
    return null;
}
}

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We do something similar but return JSON instead of XML. That JSON is then used as-is by the JavaScript function that triggers the upload ----

If i use the response type as json in the iframe form submit for file upload..i am seeing a download popup asking me to save or open... the application/json response is handled by the browser as a download ... issue occurs in IE and older versions of FF

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There were several suggestions on using a hidden iframe on the net. That's what my final attempt was, I sent an xml doc to the iframe's body and manage the data from there.