Regarding my issue above, I figured out that Java code is compiled when I remove :source-paths from my dev profile. That breaks some other things (env settings are missing). I tried various combinations of adding :source-paths and :java-source-paths to base/dev profiles, without success. DEBUG=true does output only very little before the ClassNotFound exception is thrown.

Leiningen's classpath: :/usr/local/Cellar/leiningen/2.9.8/libexec/leiningen-2.9.8-standalone.jar
Applying task javac to []
Running javac with [@/var/folders/3s/hdt8zl1928s72b__95mnxjw40000gn/T/.leiningen-cmdline13060072444507920830.tmp]

You can look at that tmp file. Could be insightful.

It may clean quickly from tmp though so I'd watch that directory during the build