Invoking Clojure code from Java

I often see people trying to call Clojure code from Java and having some trouble doing this. So I decided to write a quick utility to do this (inspired by Rich Hickey’s Lightening Talk), and provide a simple example.

The idea is that we use clojure.lang.RT to get access to the key functions that we need to call compile / Clojure code. We do this by setting up some static final variables in Java of type clojure.lang.Var, which we can then invoke to get the required behaviour from Clojure. Setting up these vars looks like this:

import clojure.lang.RT;
import clojure.lang.Symbol;
import clojure.lang.Var;
...
public class Clojure {
 ...
 public static final Var REQUIRE=RT.var("clojure.core", "require");
 ...
}

Once we have the REQUIRE var available, we can invoke it to load any Clojure namespace we need, using code like below:

public static Object require(String nsName) {
  return REQUIRE.invoke(Symbol.intern(nsName));
}

static {
  require("my.special.namespace");
}

I’ve wrapped all this functionality up into a little utility class mikera.cljutils.Clojure, so you can write quick and easy Java apps that call into Clojure as below:

import mikera.cljutils.Clojure;

public class DemoApp {
  public static void main(String [] args) {
    String s = "(+ 1 2)";
    System.out.println("Evaluating Clojure code: "+s);
    Object result=Clojure.eval(s);
    System.out.println("=> "+ result);
  }
}

The utility is available here:

– GitHub site: https://github.com/mikera/clojure-utils

– Compiled version on Clojars: https://clojars.org/net.mikera/clojure-utils