One of my most-upvoted answers on Stackoverflow is a simple example of how to call Clojure functions from Java. It doesn’t require calling through the Clojure run-time as so many responses do. But there is more to writing programs than calling static functions, as in my answer. You also might need to call methods of objects and on objects across the Clojure/Java divide.

Here is the original answer I provided.

It isn’t quite as simple as compiling to a jar and calling the internal methods. There do seem to be a few tricks to make it all work though. Here’s an example of a simple Clojure file that can be compiled to a jar:

(ns com.domain.tiny
    :name com.domain.tiny
    :methods [#^{:static true} [binomial [int int] double]]))

(defn binomial
  "Calculate the binomial coefficient."
  [n k]
  (let [a (inc n)]
    (loop [b 1
           c 1]
      (if (> b k)
        (recur (inc b) (* (/ (- a b) b) c))))))

(defn -binomial
  "A Java-callable wrapper around the 'binomial' function."
  [n k]
  (binomial n k))

(defn -main []
  (println (str "(binomial 5 3): " (binomial 5 3)))
  (println (str "(binomial 10042 111): " (binomial 10042 111))))

If you run it, you should see something like:

(binomial 5 3): 10
(binomial 10042 111): 49068389575068144946633777...

And here’s a Java program that calls the -binomial function in the tiny.jar.

import com.domain.tiny;

public class Main {

    public static void main(String[] args) {
        System.out.println("(binomial 5 3): " + tiny.binomial(5, 3));
        System.out.println("(binomial 10042, 111): " + tiny.binomial(10042, 111));

It’s output is:

(binomial 5 3): 10.0
(binomial 10042, 111): 4.9068389575068143E263

The first piece of magic is using the :methods keyword in the gen-class statement. That seems to be required to let you access the Clojure function something like static methods in Java.

The second thing is to create a wrapper function that can be called by Java. Notice that the second version of -binomial has a dash in front of it.

And of course the Clojure jar itself must be on the class path. This example used the Clojure-1.1.0 jar.

That’s all well and good as far as it goes.

But the example was only intended to call static functions in Clojure code, such as I often do when writing statistics or mathematical programs. When you want to call classes in Clojure or into particular namespaces, it gets a little more complicated (but not much).

This answer shows how to create a class object in Clojure and call a method in that class. It is similar to calling a static function but you have to include a “this” pointer in the argument list to the clojure function. The “this” pointer is usually passed invisibly in other object-oriented languages like Java or C++.