How to Call Clojure Functions from Java?

Calling Clojure from Java

Calling Clojure from Java is not as straightforward as merely compiling a jar and invoking its internal methods. Instead, specific techniques are necessary for successful interoperability.

Creating a Clojure Namespace

To make Clojure functions callable from Java, it is essential to define a namespace with the :gen-class directive. This directive specifies the class name and methods to be exposed to Java. For instance, the following snippet defines a namespace with a static method named binomial:

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

Using a Wrapper Function

Java requires a wrapper function to call a Clojure function. This wrapper function should have the same name as the Clojure function but with a leading hyphen (-). For example, the binomial function from the previous snippet can be wrapped into -binomial for Java access:

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

Adding Clojure Jar to Classpath

The Clojure jar must be added to the classpath to make the compiled code accessible to Java. This step ensures that the necessary Clojure classes are available for execution.

Example Java Code

With the Clojure namespace and wrapper function defined, below is a basic Java program that invokes the -binomial function defined in the Clojure 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));
    }
}

Running the Java Program

Ensure the Clojure jar is on the classpath and then run the Java program:

$ java -cp tiny.jar Main
(binomial 5 3): 10.0
(binomial 10042, 111): 4.9068389575068143E263

This output demonstrates that the -binomial function is accessible from Java and its results are available to be used in Java computations.

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