@RequestMapping(method = POST)
@ResponseBody
public ResponseEntity<?> submit(@Valid MyRequestType request, BindingResult result) {
  if (result.hasErrors()) {
    return new ResponseEntity<>(getErrorsInASaneFormat(result), BAD_REQUEST);
  }
  // do stuff
  return new ResponseEntity(CREATED);
}

private Map<String, String> getErrorsInASaneFormat(final BindingResult result) {
  return new HashMap<String, String>() {{
    for (FieldError error : result.getFieldErrors()) {
      put(error.getField(), error.getDefaultMessage());
    }
  }};
}