Weblogic domain - environment variables

I would like to know how does Weblogic gets system Environemt Variables(USERNAME, USERDNSDOMAIN, USERDOMAIN)? I need to change some of Environemt Variables so my SPNEGO web app can work properly.

In AdminServer.out I can see Environemt Variables are set by Nodemanager(I assume NodeManager gets those while starting). There are few scripts that sets Environment Variables, like setDomainEnv, startWeblogic, setStartuoEnv, ... but I can't find system Environment Variables in any of those scripts! I have searched Registry too, but I can't find those Environemt Variables there.

Java code I am using to check Environment Variables:

String username = (String) System.getenv("USERNAME");