SpringBoot: Large Streaming File Upload Using Apache Commons FileUpload

Thanks to some very helpful comments by M.Deinum, I managed to solve the problem. I have cleaned up some of my original post and am posting this as a complete answer for future reference.

The first mistake I was making was not disabling the default MultipartResolver that Spring provides. This ended up in the resolver processing the HttpServeletRequest and thus consuming it before my controller could act on it.

The way to disable it, thanks to M. Deinum was as follows:

multipart.enabled=false

However, there was still another hidden pitfall waiting for me after this. As soon as I disabled default multipart resolver, I started getting the following error when trying to make an upload:

Fri Sep 25 20:23:47 IST 2015
There was an unexpected error (type=Method Not Allowed, status=405).
Request method 'POST' not supported

In my security configuration, I had enabled CSRF protection. That necessitated that I send my POST request in the following manner:

<html>
<body>
<form method="POST" enctype="multipart/form-data" action="/upload?${_csrf.parameterName}=${_csrf.token}">
    <input type="file" name="file"><br>
    <input type="submit" value="Upload">
</form>
</body>
</html>

I also modified my controller a bit:

@Controller
public class FileUploadController {
    @RequestMapping(value="/upload", method=RequestMethod.POST)
    public @ResponseBody Response<String> upload(HttpServletRequest request) {
        try {
            boolean isMultipart = ServletFileUpload.isMultipartContent(request);
            if (!isMultipart) {
                // Inform user about invalid request
                Response<String> responseObject = new Response<String>(false, "Not a multipart request.", "");
                return responseObject;
            }

            // Create a new file upload handler
            ServletFileUpload upload = new ServletFileUpload();

            // Parse the request
            FileItemIterator iter = upload.getItemIterator(request);
            while (iter.hasNext()) {
                FileItemStream item = iter.next();
                String name = item.getFieldName();
                InputStream stream = item.openStream();
                if (!item.isFormField()) {
                    String filename = item.getName();
                    // Process the input stream
                    OutputStream out = new FileOutputStream(filename);
                    IOUtils.copy(stream, out);
                    stream.close();
                    out.close();
                }
            }
        } catch (FileUploadException e) {
            return new Response<String>(false, "File upload error", e.toString());
        } catch (IOException e) {
            return new Response<String>(false, "Internal server IO error", e.toString());
        }

        return new Response<String>(true, "Success", "");
    }

    @RequestMapping(value = "/uploader", method = RequestMethod.GET)
    public ModelAndView uploaderPage() {
        ModelAndView model = new ModelAndView();
        model.setViewName("uploader");
        return model;
    }
}

where Response is just a simple generic response type I use:

public class Response<T> {
    /** Boolean indicating if request succeeded **/
    private boolean status;

    /** Message indicating error if any **/
    private String message;

    /** Additional data that is part of this response **/
    private T data;

    public Response(boolean status, String message, T data) {
        this.status = status;
        this.message = message;
        this.data = data;
    }

    // Setters and getters
    ...
}

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