protected String getConfiguredUserNameAttribute() {
        if (unmappedUsernameAttribute != null) {
            return unmappedUsernameAttribute;
        }
        val usernameAttributeProvider = getUsernameAttributeProvider();
        return usernameAttributeProvider.getUsernameAttribute();
    }
Indicates which attribute found by the subclass should be taken as the
'username' attribute.  (E.g. 'uid' or 'sAMAccountName')  NOTE:  Any two
instances if BasePersonImpl with the same username are considered
equal.  Since {@link #getUsernameAttributeProvider()} should never return
null, this method should never return null either.

@return The name of the attribute corresponding to the  user's username.
core/cas-server-core-authentication-api/src/main/java/org/apereo/cas/authentication/attribute/AbstractQueryPersonAttributeDao.java