Using private jars in Clojure

One of the many things that makes clojure an extremely practical tool is that it can be used to explore other java libraries - no compiling needed - a faster feed back loop! However, we need to ensure a few things before loading private jar files into clojure. In this post, I am documenting what works for me for a test project packaged as com.acme.soup.

Let’s say that we have the following:

package com.acme.soup;

public class Soup
{
    public void make()
    {
        System.out.println( "Making soup!" );
    }
}

The objective is to call make on an instance of Soup inside Clojure.

For Java projects, I am using maven and for Clojure I am using leiningen.

Create the jars from the maven project

The jars can be created using:

mvn package

However, this doesn’t create the check sums (which is needed by leiningen in order to load the jar files). So, we can call install with the createChecksum switch:

mvn install -DcreateChecksum=true

Considering that the project is packaged as com.acme.soup, all relevant files will be available at ~/.m2/repository/com/acme/soup

Using the jars in the leiningen project

Create a directory structure that acts as a local repository

mkdir localrepo

The local repository can’t just contain plain jar files, the structure needs to match the package structure of the jars being imported. Once you have copied the files, it should like like this:

tree localrepo -L 5

localrepo
└── com
    └── acme
        └── soup
            └── soup
                ├── 1.0-SNAPSHOT
                ├── maven-metadata-local.xml
                ├── maven-metadata-local.xml.md5
                └── maven-metadata-local.xml.sha1

Configure leiningen

The local repository needs to be added to the project so that leiningen knows where to load the dependencies from, in case they are not found in any public maven repository or clojars. The following needs to be added to the project.clj file:

  :repositories {"project" "file:repo"}

Continue to add the dependencies you would normally do:

  :dependencies [
    [com.acme.soup/soup "1.0-SNAPSHOT"]
  ]

Loading the dependency in clojure code

The dependency needs to be imported, e.g. if the namespace is com.acme.soup and the class is Soup, then the following needs to be added to the ns expression

(ns soup.core
  (:import [com.acme.soup Soup]))

A new instance of Soup is created as follows:

(.make (Soup.))

or something that I prefer:

(-> (Soup.)  ;; Create a instance of soup
    (.make)) ;; Call native functions on it

That makes me very happy happy joy joy! :D