You cannot get them. They are not connected.
Saving or retrieving data (especially user roles) happens at many points in Liferay, some of whom are not connected to a particular page (i.e. the web services, liferay authentication, administration via the control panel, etc.).
You are trying to bind the retrieve of data to a page id (probably you have some sort of weird idea that you'll return a different set of roles if a user is on a page), but in effect you are going to break your liferay as a result.
Instead of trying your own messed up idea, why not try asking how you would accomplish a particular task and let experienced people chime in on how to do it?
