docs: @CheckForNull
      public UserDetails getCached(String idOrFullName) throws UsernameNotFoundException {
          Boolean exists = existenceCache.getIfPresent(idOrFullName);
          if (exists != null && !exists) {
              throw new UserMayOrMayNotExistException(String.format("\"%s\" does not exist", idOrFullName));
          } else {
              return detailsCache.getIfPresent(idOrFullName);
          }
      }