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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.

Source Layout

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 :source-paths and :java-source-path options in the project definition:

(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. src and src/java) can cause obscure problems.

Java Source Compilation

To compile Java sources, you can explicitly run

lein javac

However, it is usually not necessary because tasks that need to run project code (lein test, lein run, etc.) will trigger compilation automatically. Manually running lein javac may be necessary when using lein do, lein with-profiles or lein repl actively while also actively changing Java sources in the project.

Running

lein clean

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.

Other Languages

Java is not the only language you can mix with Leiningen, but it's the only one supported out of the box. Plugins exist for Scala and Groovy as well.

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