Is there any open source Java EE application for online banking and processing that you guys are aware of?
Say for example I want to implement the whole online banking stack, from front-end, middleware, and back-end. Has anybody open-sourced this before?
You may find some projects that could help to build small parts of a banking stack but nothing covering the front, middle and back office activities (if I understand your question properly). Such a stack or ERP is actually the core of a bank (i.e. specific at some point) and simply represents too much money (to give you an idea, a securities back office costs several hundreds millions euros). I don't know any bank that would give such an investment to a potential concurrent for free.
Just in case, some links you might want to look at: