Java -> Clojure -> Java

Question

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.

• Marko Topolnik: Couldn't you just defn a function through a script string and then invoke it just like you invoked read-string, by derefing its var?
• Marcus Junius Brutus: I tried that but I couldn't get it to work. First it would accept defn and had to use (let (fn )) instead. Not a blocking problem but weird. Then I ran into some other problem. I didn't want to copy the failing code so as not to make the post too long, but after your message I might add it.
• Marko Topolnik: I have this working in a project of mine: clojure.lang.RT.load(filename); clojure.lang.RT.var(mainNamespace, "main").invoke( stringArg ); Loading from a file shouldn't make a fundamental difference.
• Marcus Junius Brutus: update on defn: it turns out that using a defn was not a problem after all and have updated the script shown, accordingly, the "Can't change/establish root binding of: ns with set" still persists however.
• Marko Topolnik: You definitely need a do around everything because read-string only reads one form. Also, read-string doesn't evaluate it, it just returns what was read in. This is in contrast to my technique with load.

Answer 1

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"));
 }
}

• Marko Topolnik: It would be more reliable to do it without the import as it may not take effect within the same form as it is being evaluated. Did you try it?
• Marcus Junius Brutus: java.lang.ClassCastException: java.lang.String cannot be cast to clojure.lang.IFn (thrown on line-A)
• Ankur: Updated the answer with a running sample code. The older code has extra params around fn and that was the issue.