Polyglot (Clojure, Java) Projects With Leiningen
Clojure is a hosted language that encourages interoperability with its platform. It is not uncommon to find some amount of Java code in Clojure projects managed by Leiningen.
This guide explains how you can control source code layout with Leiningen, compile Java sources and other topics related to polyglot codebases.
By default, Leiningen assumes your project only has Clojure source code under
src. When using both Clojure and Java in the same codebase, however, it is
necessary to tell Leiningen where to find Java sources.
To do so, use
:java-source-path options in the project
(defproject megacorp/superservice "1.0.0-SNAPSHOT" :description "A Clojure project with a little bit of Java sprinkled here and there" :min-lein-version "2.0.0" :source-paths ["src/clojure"] :java-source-paths ["src/java"])
Having one source root contain another (e.g.
cause obscure problems.
Java Source Compilation
To compile Java sources, you can explicitly run
However, it is usually not necessary because tasks that need to run
project code (
lein run, etc.) will trigger compilation
automatically. Manually running
lein javac may be necessary when
lein with-profiles or
lein repl actively while
also actively changing Java sources in the project.
will remove all compilation artifacts.
Setting Java Compiler Options With Leiningen
When compiling Java sources, it may be necessary to pass extra arguments to the compiler. For example, it is very important to target the JVM version you are going to deploy your project to.
This is done via the
;javac-options which takes a vector of arguments as you
would pass them to
javac on the command line. In this case we say that Java
sources use features up to JDK 6 and target JVM is also version 6:
(defproject megacorp/superservice "1.0.0-SNAPSHOT" :description "A Clojure project with a little bit of Java sprinkled here and there" :min-lein-version "2.0.0" :source-paths ["src/clojure"] :java-source-paths ["src/java"] :javac-options ["-target" "1.6" "-source" "1.6"])
Leiningen 2 and later versions uses the JDK compiler API to compile Java sources.
Failing to specify the target version will lead JDK compiler to target whatever JDK Leiningen is running on. It is a good practice to explicitly specify target JVM version in mixed Clojure/Java projects.