code: protected String getMessageFromParent(String code, Object[] args, Locale locale) {
          MessageSource parent = getParentMessageSource();
          if (parent != null) {
              if (parent instanceof AbstractMessageSource) {
                  // Call internal method to avoid getting the default code back
                  // in case of "useCodeAsDefaultMessage" being activated.
                  return ((AbstractMessageSource) parent).getMessageInternal(code, args, locale);
              } else {
                  // Check parent MessageSource, returning null if not found there.
                  return parent.getMessage(code, args, null, locale);
              }
          }
          // Not found in parent either.
          return null;
      }
docs: Try to retrieve the given message from the parent MessageSource, if any.
  
  @param code the code to lookup up, such as 'calculator.noRateSet'
  @param args array of arguments that will be filled in for params within the message
  @param locale the Locale in which to do the lookup
  @return the resolved message, or {@code null} if not found
  @see #getParentMessageSource() #getParentMessageSource()