code: public void setSupportedUserNames(Collection<String> supportedUserNames) {
          if (supportedUserNames == null) {
              this.supportedUserNames = null;
          } else {
              this.supportedUserNames = ImmutableSet.copyOf(supportedUserNames);
          }
      }
docs: If no <code>supportedUserNames</code> {@link Collection} is configured all user-names are
  supported otherwise exact String equality matching is done to determine supported userNames.
  The property defaults to null (all user names)
  
  @param supportedUserNames the supportedUserNames to set