It varies; you'll want to know quite a bit of Java for Scala and Groovy but less so for Clojure, in my opinion. Clojure has a quite distinct flavor and you see

It varies; you'll want to know quite a bit of Java for Scala and Groovy but less so for Clojure, in my opinion. Clojure has a quite distinct flavor and you see things like ring being used instead of servlets directly in compojure, etc. It'll be interesting to see how much of the standard Java toolkit gets reinvented due to distaste for the Java-ness of some of the stack (which isn't necessarily a bad thing.) Practically speaking, I think you'll need to be at least a competent Java programmer for the next few years if you don't plan on reimplementing a lot of bits that are in the extended Java stack.