How to Efficiently Call Clojure Functions from Java Using Modern Techniques?

Calling Clojure from Java with Modern Tools

While some information provided in previous online discussions may be outdated, calling Clojure from Java remains relatively straightforward. Here's a step-by-step guide:

Clojure Jar Preparation:

1. Create a Clojure project using Leiningen: lein new com.domain.tiny

2. Define your Clojure functions in src/com/domain/tiny.clj:

   (ns com.domain.tiny)
   
   (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))))

3. Generate a jar using the Leiningen task: lein jar
4. Copy the Clojure jar and your generated tiny.jar into a convenient location

Java Code:

1. Create a Java class that calls the Clojure function:

   import java.lang.reflect.Method;
   
   import com.domain.tiny;
   
   public class Main {
   
    public static void main(String[] args) {
        try {
            Method binomialMethod = Class.forName("com.domain.tiny").getMethod("binomial", int.class, int.class);
   
            Integer n = 5;
            Integer k = 3;
            Double result = (Double) binomialMethod.invoke(null, n, k);
            System.out.println("(binomial " + n + " " + k + "): " + result);
   
            n = 10042;
            k = 111;
            result = (Double) binomialMethod.invoke(null, n, k);
            System.out.println("(binomial " + n + ", " + k + "): " + result);
        } catch (Exception e) {
            System.err.println("Error calling Clojure function: " + e.getMessage());
            e.printStackTrace();
        }
    }
   }

2. Compile the Java class using javac

3. Create a manifest file (MANIFEST.MF) with the appropriate dependencies:

   Manifest-Version: 1.0
   Class-Path: tiny.jar:clojure-x.y.z.jar
   Main-Class: Main

4. Package the Java class and dependencies into a jar: jar cfm Interop.jar MANIFEST.MF Main.class tiny.jar clojure-x.y.z.jar

Run the Program:

1. Ensure your Clojure jar and tiny.jar are on the classpath
2. Run the Java jar: java -jar Interop.jar

The output should be similar to:

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

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