Ganesh Puli
...(please give the clarity with the full pledged example)...
this might take a couple of weeks, especially given that we know nothing other than "everything is custom"...
Apart from that I'd say: Extract all the unit-testable code out of your portlet, so that the portlet remains a bit of the UI-layer that's largely wiring and can be reviewed. Unit-Test the heck out of the extracted code. Have your business code behind the service layer, again: Extract the testable code and behavior, to make it as independent of the persistence layer as possible. Then test the extracted code.
Reduce the code that you currently intend to mock. Turn it down to wiring the components to the APIs used. Unit testing a portlet is inherently hard, as it's on the UI layer - and unit testing UI is hard. I've not seen a lot of cases where mocking helped a great deal other than approving the implementation didn't change (but not any correctness).
