How Can I Efficiently Call Clojure Functions from Java?

Calling Clojure from Java

Introduction:

Interfacing between Clojure and Java is a common requirement in many projects. However, older approaches using clojure.lang.RT are now outdated. In this post, we'll explain a simplified method for performing this integration, assuming a pre-built Clojure jar and its inclusion in the classpath.

Step-by-Step Instructions:

1. Create a Clojure Namespace:

   Define a Clojure namespace with the :gen-class keyword to specify a Java class and methods accessible from Java. For instance:

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

2. Define a Static Method Wrapper:

   Wrap the Clojure function you want to call from Java with a Java-callable wrapper, prefixed with a hyphen (e.g., -binomial). This allows Java to invoke the Clojure function.

3. Build and Include the Clojure Jar:

   Compile the Clojure namespace to a jar file and include it in the Java project's classpath. Ensure that the Clojure jar is also present in the classpath.

4. Call Clojure from Java:

   In your Java program, import the Clojure class and call the static methods as if they were Java methods. For example:

   import com.domain.tiny;
   
   public class Main {
       public static void main(String[] args) {
           System.out.println("(binomial 5 3): " + tiny.binomial(5, 3));
       }
   }

5. Set Compilation Parameters:

   When compiling the Java part, specify the classpath to include the Clojure jar and package the resulting class and manifest into a JAR file.

Updated Example (Using Modern Tools):

Using Clojure 1.5.1, Leiningen 2.1.3, and JDK 1.7.0 Update 25:

Clojure Part:

• Create a Leiningen project and update project.clj with the following:

  ; Same as the original example code from the introduction

• Ensure dependencies are resolved using lein deps.
• Package the Clojure code into a jar using lein jar.
• Place the Clojure jar and the built Clojure jar in a convenient location.

Java Part:

• Create a Java class (e.g., Main.java) as in the original example.

• Compile the Java class:

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

• Create a manifest file (Manifest.txt):

  ; Same as the original example code from the introduction

• Package the Java class along with the Clojure jars into a JAR file:

  jar cfm Interop.jar Manifest.txt Main.class lib\com.domain.tiny-0.1.0-SNAPSHOT.jar lib\clojure-1.5.1.jar

• Run the Java program:

  java -jar Interop.jar

This updated example demonstrates the interoperability between Clojure and Java using contemporary tools.

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