Calling Clojure from Java

java

prompt-engineering

functions

best-practices

byNikita Barsukov·Jan 31, 2025

You can call Clojure functions from Java using the RT and Var classes from clojure.lang. Here's a quick demo:

import clojure.lang.RT;
import clojure.lang.Var;

public class Main {
    public static void main(String[] args) throws Exception {
        RT.loadResourceScript("script.clj");       // Loading the brain of the operation
        Var fn = RT.var("namespace", "function");  // Finding the "ninja" in shadow
        System.out.println(fn.invoke(1, 2));       // Calling for the ninja's duty
    }
}

Don't forget to swap script.clj, namespace, and function with your own Clojure script path, namespace, and function name.

Proper setup and precautions

Before we blend Java and Clojure, let's talk about some considerations:

Including the Clojure jar

Add Clojure jar to your classpath for gaining access to the Clojure libraries. You can handle this using Maven or Gradle.

A well-ordered kitchen with Leiningen

Leiningen is your lovable kitchen organizer. It Assists with your project structure, tidies up your code, and manages your dependencies. Just like how you can't reach for any spice without toppling over two others, you can't call Clojure functions without a bit of structure!

The magic of `:gen-class`

In your Clojure code, :gen-class is your magic spell. It helps create Java classes with static methods. If you need to start your function calls from Clojure, define a main function.

Ahead-Of-Time (AOT) compilation

To keep the runtime smooth, Leiningen provides AOT compilation via the :aot directive in project.clj. This creates Java bytecode from Clojure code, think of it as having the cooking instructions ready before starting!

Referencing Clojure the Java way

To call Clojure functions, use clojure.java.api.Clojure and Clojure.var. It's similar to asking someone their name before calling them!

Smooth sailing with wrappers and conversion

Sometimes, Java and Clojure don't play nice due to type differences. In that cases, build wrapper functions in Clojure that smoothen out such issues. It's like translating between two people who don't speak the same language.

Give Clojure a heads up with initialization

Similar to how you switch on the TV before changing channels, initialize the Clojure runtime before invoking any functions.

Execute Clojure on the fly

In the cases when you wish to execute Clojure code represented as strings within Java, use clojure.lang.Compiler. It's like having a personal chef who cooks anything you ask!

Packaging your masterpiece

When packaging, it's traditional to bundle your Clojure program and jar into a single executable. A manifest file drops in some metadata, like a note to say who the chef was!

Mindful of errors

Keep an eye out for troublesome errors, such as namespace loading issues or function invocation errors. Exception handling in your Java code should take care of those!

Setting an example

Real-world examples are always great for understanding abstract concepts. Provide a few examples that showcase Java applications tapping into Clojure code power.

Maximize Clojure's functional charm in Java

Functional charm with IFn

Pass Clojure IFn objects to Java to share Clojure's functional paradigms in Java's territory. It's like infusing an exotic flavor into a traditional dish.

Knowledge Hunting in Docs

For an even greater understanding, check out the Clojure documentation for the Java API at http://clojure.github.io/clojure/javadoc/. It's like an exclusive cooking class from a master chef!

Github: A cookbook for programmers

GitHub repositories are a treasure trove of practical examples. They are the cookbooks of the programming world that illustrate how to cook up some functional goodness in your Java applications.

Mirror Clojure's structure in Java

Try to reflect Clojure's code structure in Java where possible. It's similar to learning how flavors combine by following a master chef's recipe closely.

Maintainability and scalability— the salt and pepper of good project structure— are crucial for long-term project success.

explain-codes / Java / Calling Clojure from Java

Linked

Convert Existing Eclipse Project to Maven Project



How can I create an executable/runnable JAR with dependencies using Maven?



Mac OS X and multiple Java versions



How to add local .jar file dependency to build.gradle file?



Correct way to add external jars (lib/*.jar) to an IntelliJ IDEA project