Is there a lightweight Java environment for EMACS? I have used JDEE in the past, but it's accumulated a lot of, ahem, features since I used it last, and just trying to set it up has consumed a couple hours.
I would suggest Meghanada. It is light weighted, much easier to setup (compared to JDEE). It is not mature enough, but already supports auto-completion, syntax-checking and runs junit tests. It will also parse maven and gradle projects automatically. The overall experience is smooth enough for small and moderate scale projects. Refactoring support is still in its todo list though.
Ensime also worth a try. It is pretty good for Scala development, also has limited support for Java. But some rough edges are there.
You can try https://github.com/emacs-lsp/lsp-java ,
Eclipse JDT Language Server as backend.
I don't know if you'd consider it "lightweight," but I've had good luck with eclim. It uses eclipse for the heavy lifting, but it also integrates well with eclipse's project management. It has company integration that works well.
There's a bit to learn, but not a whole lot. It works well to reduce the pain of working with Java in Emacs to just the pain of working in Java.