CODEWITHSUNDEEP
javaclojure
Marcus Junius Brutus
Marcus Junius Brutus

Reputation: 27276

Java -> Clojure -> Java

I am trying to use Clojure as a scripting language from a host Java program. The idea being that the end user will be able to write Clojure scripting code that will call a domain-specific Java API. At runtime, the host Java program will evaluate the end-user's Clojure script (which will in turn call the domain APIs). So I started with a dead-simple prototype to explore the terrain.

domain

package a.problem.domain;

public class Domain {

    public Domain() { }

    public String defaultMsg() {
        return "default";
    }

    public String passBackMsg(String s) {
        return s;
    }
}

Host Java program

(end user's Clojure script hard-coded for simplicity)

String script = "(do                                    "+
                "  (import '(a.problem.domain Domain))  "+
                "  (.defaultMsg (Domain.))              "+
                ")                                      ";
System.out.println(RT.var("clojure.core", "eval").invoke(RT.var("clojure.core","read-string").invoke(script)));

(code snippet taken from here)

So far so good.

However I couldn't find a way to invoke the second method (the one that requires an argument). Instead I resorted to dynamically generating the Clojure script at runtime and replacing a placeholder with a literal that represents the result of calling the domain method passBackMsg. Obviously, this is unsatisfactory and doesn't go very far (what if I want to pass a java.sql.Connection to my Clojure script ?).

So, how do I invoke the passBackMsg method from the host Java program?

When I try the following:

String script = "(ns foo)                                   "+ 
                "(import '(a.problem.domain Domain))        "+
                "(defn numberToString [s]  (                "+
                "  (.passBackMsg (Domain.) s)               "+
                "))                                         ";
RT.var("clojure.core", "eval").invoke(RT.var("clojure.core","read-string").invoke(script)); // line-A
System.out.println(RT.var("foo", "numberToString").invoke(33)); // line-B

I get:

java.lang.IllegalStateException: Can't change/establish root binding of: *ns* with set

... on line-A. When I try without the ns and with:

RT.var("user", "numberToString").invoke(33)

("user" being a wild guess as I don't see a var method without a namespace argument)

I get an:

java.lang.IllegalStateException: Attempting to call unbound fn: #'user/numberToString"

... on line-B.

Upvotes: 7

Views: 1120

Answers (1)

Ankur
Ankur

Reputation: 33637

Try this:

String script = "(do                                    "+
                "  (import '(a.problem.domain Domain))  "+
                "  (fn [s]                " +
                "   (.passBackMsg (Domain.) s)               "+
                "))                                         ";

IFn fn = (IFn)RT.var("clojure.core", "eval").invoke(RT.var("clojure.core","read-string").invoke(script));

fn.invoke("hello");

UPDATED: Below sample code works fine:

package hello_clj;

import clojure.lang.RT;
import clojure.lang.IFn;

public class Main {

    public String passBackMsg(String s) {
        return s;
    }

    public static void main(String[] args) {
        String script = "(do (import 'hello_clj.Main) (fn [s] " + 
                        "(.passBackMsg (Main.) s) ))";

        IFn fn = (IFn)RT.var("clojure.core", "eval").invoke(RT.var("clojure.core","read-string").invoke(script));
        System.out.print(fn.invoke("Hello"));
    }

}

Upvotes: 4

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