Reg - Unit Testing and Technology Knowledge

I'm new to the unit testing and I have mostly worked on automating functional Test (front end using - Selenium and Jmeter).

I would like to know if I need to unit test Java based web application by looking into the code, then should a I have a clear understanding on underlying technologies like: servlets, jsp, ejb, and design patterns; based on which the application is built? Or is it sufficient to have knowledge on Core Java alone to look look into the code and understand them and write unit test in junit?

Please provide your suggestions. Thank You.