I'm working with : Liferay 6.0.6 with JBoss 5.1 and Struts2.
My question is, how to get the current user in Liferay once logged in, using a Java code.
In your doView/processAction method do following
User user = (User) request.getAttribute(WebKeys.USER);
or use the ThemeDisplay object. It contains another information like companyId, groupId, ...
ThemeDisplay td =(ThemeDisplay)request.getAttribute(WebKeys.THEME_DISPLAY);
User user = td.getUser();
Classes ThemeDisplay, User nad WebKeys are part of portal-service.jar.
If you need just some id to identify current user you can also use
String userId = request.getRemoteUser();
This solution is not Liferay specific and should be portable among jsr-286 portals.