Question

Most of the top google hits for “calling clojure from java” are outdated and recommend using clojure.lang.RT to compile the source code. Could you help with a clear explanation of how to call Clojure from Java assuming you have already built a jar from the Clojure project and included it in the classpath?

Answer 1

Update: Since this answer was posted, some of the tools available have changed. After the original answer, there is an update including information on how to build the example with current tools.

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
  (:gen-class
    :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)
        c
        (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.

Update: This answer has been re-tested using the following tools:

• Clojure 1.5.1
• Leiningen 2.1.3
• JDK 1.7.0 Update 25

The Clojure Part

First create a project and associated directory structure using Leiningen:

C:\projects>lein new com.domain.tiny

Now, change to the project directory.

C:\projects>cd com.domain.tiny

In the project directory, open the project.clj file and edit it such that the contents are as shown below.

(defproject com.domain.tiny "0.1.0-SNAPSHOT"
  :description "An example of stand alone Clojure-Java interop"
  :url "http://clarkonium.net/2013/06/java-clojure-interop-an-update/"
  :license {:name "Eclipse Public License"
  :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.5.1"]]
  :aot :all
  :main com.domain.tiny)

Now, make sure all of the dependencies (Clojure) are available.

C:\projects\com.domain.tiny>lein deps

You may see a message about downloading the Clojure jar at this point.

Now edit the Clojure file C:\projects\com.domain.tiny\src\com\domain\tiny.clj such that it contains the Clojure program shown in the original answer. (This file was created when Leiningen created the project.)

Much of the magic here is in the namespace declaration. The :gen-class tells the system to create a class named com.domain.tiny with a single static method called binomial, a function taking two integer arguments and returning a double. There are two similarly named functions binomial, a traditional Clojure function, and -binomial and wrapper accessible from Java. Note the hyphen in the function name -binomial. The default prefix is a hyphen, but it can be changed to something else if desired. The -main function just makes a couple of calls to the binomial function to assure that we are getting the correct results. To do that, compile the class and run the program.

C:\projects\com.domain.tiny>lein run

You should see output shown in the original answer.

Now package it up in a jar and put it someplace convenient. Copy the Clojure jar there too.

C:\projects\com.domain.tiny>lein jar
Created C:\projects\com.domain.tiny\target\com.domain.tiny-0.1.0-SNAPSHOT.jar
C:\projects\com.domain.tiny>mkdir \target\lib

C:\projects\com.domain.tiny>copy target\com.domain.tiny-0.1.0-SNAPSHOT.jar target\lib\
        1 file(s) copied.

C:\projects\com.domain.tiny>copy "C:<path to clojure jar>\clojure-1.5.1.jar" target\lib\
        1 file(s) copied.

The Java Part

Leiningen has a built-in task, lein-javac, that should be able to help with the Java compilation. Unfortunately, it seems to be broken in version 2.1.3. It can’t find the installed JDK and it can’t find the Maven repository. The paths to both have embedded spaces on my system. I assume that is the problem. Any Java IDE could handle the compilation and packaging too. But for this post, we’re going old school and doing it at the command line.

First create the file Main.java with the contents shown in the original answer.

To compile java part

javac -g -cp target\com.domain.tiny-0.1.0-SNAPSHOT.jar -d target\src\com\domain\Main.java

Now create a file with some meta-information to add to the jar we want to build. In Manifest.txt, add the following text

Class-Path: lib\com.domain.tiny-0.1.0-SNAPSHOT.jar lib\clojure-1.5.1.jar
Main-Class: Main

Now package it all up into one big jar file, including our Clojure program and the Clojure jar.

C:\projects\com.domain.tiny\target>jar cfm Interop.jar Manifest.txt Main.class lib\com.domain.tiny-0.1.0-SNAPSHOT.jar lib\clojure-1.5.1.jar

To run the program:

C:\projects\com.domain.tiny\target>java -jar Interop.jar
(binomial 5 3): 10.0
(binomial 10042, 111): 4.9068389575068143E263

The output is essentially identical to that produced by Clojure alone, but the result has been converted to a Java double.

As mentioned, a Java IDE will probably take care of the messy compilation arguments and the packaging.