Is there a way to get the logged in user using API? (Any class that provides sup

In one of my servlet plugin I am using
ServiceContext sc = ServiceContextFactory.getServiceContext("userName");
this servlet is invoked from external system. So I want to get the context of the logged in user rather than passing the userName! If this is not the better approach please suggest the best way to get the UserContext ?

In one of my servlet plugin I am using
ServiceContext sc = ServiceContextFactory.getServiceContext("userName");
this servlet is invoked from external system. So I want to get the context of the logged in user rather than passing the userName! If this is not the better approach please suggest the best way to get the UserContext ?