Using Lisp or Scheme for runtime configuration of Java programs

Question

I have now seen several projects ending at a point where the actual configuration depended on things only available at run-time.

The typical way to configure a Java prog

Answer 1

Good, much smaller solution is an embedded Scheme for Java. Out of many implementations, I found and tested JScheme. (API looks okay: http://jscheme.sourceforge.net/jscheme/doc/api/index.html)

A simple main program in Java:

import jscheme.JScheme;
import jscheme.SchemeException;
import java.io.*;
public class SchemeTest {
    public static void main(String[] args) {
        JScheme js = null;
        try {
            js = new JScheme();
            js.load(new FileReader("config.scm"));
        } catch (FileNotFoundException e) { return; }

        System.out.println("Message: '" + js.eval("msg") + "'");

        // Values
        int answer = (Integer) js.eval("answer");

        // Function calls
        System.out.println(js.call("countdown", 42));
    }
}

And an example config.scm:

(define (countdown x)
  (define (loop x acc)
    (if (= x 0)
      acc
      (loop (- x 1) (+ acc x))))
  (loop x 0))

;;; config variables
(define msg "Hello from JScheme!")
;; tail calls are optimized as required
(define answer (countdown 42))

The scheme is being interpreted at startup every time, so this is good choice for configurations. Advantages to JScheme include:

• good JVM integration
• apparently a complete Scheme implementation
• smallish size (572 kB)
• fast, somewhat easily called from Java

Benchmark update: this code runs in 0.13 seconds time. Compared to the Clojure version's time this is pretty fast.