How to pass username from SAP authentication to SAPUI5?

Hello everybody,

I managed to secure my SAPUI5 application (which is deployed on Netweaver 7.31 Java AS) via SAP Form like this:

Now, how can I pass the username from this Form into my SAPUI5 application?

Many threads recommend UI2 approach (/sap/bc/ui2/start_up), but I can't use that, because that's ABAP. I use Java AS.
In Hana Cloud Platform you can use userapi which is defined in neo-app.json file. I'm not using Hana Cloud Platform.

Maybe I can use a Jersey REST entity to get the user via "IUser user = UMFactory.getAuthenticator().getLoggedInUser(request, response)"? I haven't done REST in NWDS before, only EJB webservices.