Question

I’m calling Clojure from Java and calling eval on a string passed in. The Java code will hold the objects, and the client code can specify strings of Clojure code to run on an object. I know how to call the Clojure code from Java, but how do I pass a variable in?

Here’s what I have. First, a simple object to work on:

public class Helloer {
    public String getGreeting() { return "Hello"; }
}

Then some boilerplate code to call a Clojure method.

public static String call(Helloer helloer, String expression) throws Exception {
    RT.loadResourceScript("EvalObject.clj");
    final Var schrodEval = RT.var("eval-object", "eval-string");
    final String result = (String) schrodEval.invoke(expression, helloer);
    return result;
}

But then I get stuck on the Clojure code. The object is passed in fine, but how do I pass the value into the eval?

Here’s what I’ve tried:

(ns eval-object)

(defn eval-string [string this]
  (eval (read-string string)))

(defn eval-string2 [string value]
  (def this)
  (binding [this value]
    (eval (read-string string))))

(defn eval-string3 [string value]
  (def this)
  (eval (list 'binding (vector 'this 5) (read-string string))))

These give:

java.lang.Exception: Unable to resolve symbol: this in this context (NO_SOURCE_FILE:0)

So then I tried constructing a binding clause that defines this:

(defn eval-string4 [string value]
  (def this)
  (eval (list 'do (list 'defonce 'this nil)
        (list 'binding (vector 'this value) (read-string string)))))

But now I get:

java.lang.RuntimeException: Can't embed object in code, maybe print-dup not defined: Helloer@638bd7f1 (NO_SOURCE_FILE:0)

Am I missing something? Is it possible to pass objects from Java, to Clojure, and into an eval?

Answer 1

It turns out the error message was correct. I needed to define print-dup. I implemented a simple ID lookup for the Helloer class, and defined print-dup as:

(defmethod print-dup com.ziroby.Helloer [h stream]
  (.write stream "#=(com.ziroby.Helloer/getById ")
  (.write stream (str (.getId h)))
  (.write stream ")"))

Apparently, Clojure requires some way to print an object to be able to put it into an eval statement. This print-dup creates a Clojure statement to “create” the object by looking it up via Helloer.getById (which just looks up the object in a hashmap).

As @mikera says, you only need to jump thru these hoops if you’re accepting arbitrary code from your caller or dynamically generating code.