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

Is there any reason why the out of the box loggedinuser() function can't be used in this case?

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 ?

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

ServiceContext sc = WebServiceContextFactory.getServiceContext(req);

As you can see you can build a service context from the HttpServletRequest

Thanks Eduardo!!