Spring HandlerMethodArgumentResolver not executing

OK I worked out that Spring was already resolving the Principal object in my above example and so my argument resolver was not kicking in. I had been lazy and added the @CurrentUser annotation to an existing parameter.

So I changed my example

@RequestMapping(method = RequestMethod.POST, value = "/update")
public ModelAndView update(@RequestParam MultipartFile background, @CurrentUser Principal principal) {
  ...
}

to use my User model class

@RequestMapping(method = RequestMethod.POST, value = "/update")
public ModelAndView update(@RequestParam MultipartFile background, @CurrentUser User user) {
  ...
}

and now it works!

If anybody ever wants to prioritize custom handlers over default handlers added by spring, here's a snippet that does it for me, I do this in a @Configuration file

private @Inject RequestMappingHandlerAdapter adapter;

@PostConstruct
public void prioritizeCustomArgumentMethodHandlers () {
  List<HandlerMethodArgumentResolver> argumentResolvers = 
      new ArrayList<> (adapter.getArgumentResolvers ());
  List<HandlerMethodArgumentResolver> customResolvers = 
      adapter.getCustomArgumentResolvers ();
  argumentResolvers.removeAll (customResolvers);
  argumentResolvers.addAll (0, customResolvers);
  adapter.setArgumentResolvers (argumentResolvers);
}