code: public static @Nullable @CheckForNull String getCurrentUsername() {
      Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
      if (authentication == null) {
        return null;
      }
      return getUsername(authentication);
    }
docs: Returns the username of the current authentication.
  
  @return username or <tt>null</tt> if 1) the current authentication is null or 2) the currently
  authenticated principal is the system.