I'm saying this as a massive clojure fanboy: java.util.concurrent is an amazing set of concurrency utilities and is the building block for many of clojure's concurrency features. It's definitely worth knowing when writing performant java AND clojure (you can wrap the little bit you need so you don't have to scatter java interop all over your codebase). Sometimes a (swap! atom fn) doesn't cut it.

Yeah, because some of the best concurrency primitives in the world aren't quite "what teh cool kids are doin'"