code: @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);
          }
      }
docs: Gets the cached UserDetails for the given username.
  Similar to {@link #loadUserByUsername(String)} except it doesn't perform the actual lookup if there is a cache miss.
  
  @param idOrFullName the username
  
  @return {@code null} if the cache doesn't contain any data for the key or the user details cached for the key.
  @throws UsernameNotFoundException if a previous lookup resulted in the same