public Set<String> getUserIdentifiers(UserData userData) {

        // Each user can only see themselves
        return Collections.singleton(userData.getUsername());

    }
Returns the identifiers of all users readable by the user whose data is
given by the provided UserData object. As users of the
JSONAuthenticationProvider can only see themselves, this will always
simply be a set of the user's own username.

@param userData
    All data associated with the user whose accessible user identifiers
    are being retrieved.

@return
    A set containing the identifiers of all users readable by the user
    whose data is given by the provided UserData object.
extensions/guacamole-auth-json/src/main/java/org/apache/guacamole/auth/json/user/UserDataService.java