Sharing Clojure code with Java developers

Here are the five easy steps to deliver a function written in Clojure so that Java programmers can call it as if were a function written in Java and included in a .jar file:

1. Create a Java-like project with Leiningen
2. Write a (very thin) wrapper function for your function
3. Use the ns form at the top of your source file to export your function
4. Edit your project.clj file to use AOT compilation
5. Build an uberjar file

Our example

To illustrate these steps we assume that you work at the mythical Example company and promise your Java colleagues to provide a function called plus in a package called com.example.cl4j.

1 - Create the project

Use Leiningen to create a project designed to play nicely with Java:

lein new com.example.cl4j

Note that Leiningen created your project in a directory called com.example.cl4j and your “main” source file is a little deeper in the directory tree than usually.

2 - Write a wrapper for your function

Open your “main” source file (com.example.cl4j/src/com/example/cl4j.clj) an write your plus function and a second one called -plus which acts as kind of a wrapper.

(ns com.example.cl4j)

(defn plus
  "calculate a + b"
      [a b]
          (+ a b))

(defn -plus
  "a Java-callable wrapper around the `plus` function"
    [a b]
      (plus a b))

3 - Export your function

Now edit the (ns ...) form at the top of the file to generate .class files and export your function:

(ns com.example.cl4j
  (:gen-class
   :name com.example.cl4j
   :methods  [#^{:static true} [plus [int int] int]]))

(defn plus
  "calculate a + b"
    [a b]
    (+ a b))

(defn -plus
  "a Java-callable wrapper around the `plus` function"
  [a b]
  (plus a b))

Your source file is now ready to go.

4 - Configure the project

The configuration is just adding a line (:aot :all) to your project.clj file:

(defproject com.example.cl4j "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url "http://example.com/FIXME"
  :license {:name "EPL-2.0 OR GPL-2.0-or-later WITH Classpath-exception-2.0"
            :url "https://www.eclipse.org/legal/epl-2.0/"}
  :dependencies [[org.clojure/clojure "1.10.1"]]
  :aot :all
  :repl-options {:init-ns com.example.cl4j})

5 - Build an uberjar file

Use Leiningen to create a jar file which contains everything your Java colleagues will need:

lein uberjar

That’s it!

You’re done! Java programmers now just have to add the jar file (called something like com.example.cl4j/target/com.example.cl4j-0.1.0-SNAPSHOT-standalone.jar) when building and running their program.

Calling your plus function in a Java program is pretty conventional:

package com.example.something;

import com.example.cl4j;

public class Client {
    public static void main(String [] args) {
        System.out.println(cl4j.plus(4, 4));
    }
}

Credits: this page is based on the answer provided by StackOverflow user clartaq