In general knowing more than one platform serves you much better than becoming stuck with one of these web-dev frameworks that may not be around in a few years.
Java is MASSIVE, if you ever want to be a serious technologist in a big corporation, Java is a requirement, and don't let anyone tell you that is going to change anytime soon.
Java is a very mature and versatile platform with a host of advanced technologies built on top of it (check out apache.org). There is a reason the likes of Google build their platform on top of Java not Ruby/PHP/Python or any of the other more talked about platforms.
If nothing else Java can be a good way to get some problem solving exercise. Unlike some of the other web-dev frame works that abstract a lot of stuff from the developer, in Java you'll have to deal with more low level concerns which can be good for your own training as a developer.
PS: I am the founder/developer of a Javascript APP platform that you as a student may find interesting: www.appynote.com. It is built on top of Java:)