Why `lein uberjar` evaluates variables defined with `def`?

Question

I\'m trying to understand \"Lieningen\" behaviour when creating an uberjar. Following is the minimal example which reproduces the behaviour:

(ns         


        

Answer 1

In order to compile your namespace for the uberjar (if you have AOT turned on), the clojure compiler must load your namespace. This will always invoke all top-level side effects.

The best way to handle this is to never have side effects in top level code (whether inside or outside a def form), and have initialization functions to realize any start-up side effects needed.

A workaround can be to make a small namespace that uses introspection to load the rest of your code at runtime but not while compiling - using a function like this:

(defn -main
  []
  (require 'my.primary.ns)
  ((resolve 'my.primary.ns/start)))

if that namespace is compiled, the jvm can find -main and run it, despite none of your other code being compiled. The runtime require will cause the Clojure compiler to load the rest of your code at runtime only, and resolve is needed so that -main will compile cleanly - it returns the var referenced, which then invokes your function when called.