How to access Java object functions from nashorn

Question

I would like to make an interface that will allow users to supply arbitrary filter functions to process records in a Java application. I decided to use Java Scripting utilities for this, specifically nashorn and JavaScript.

My problem is that when I pass in objects to evaluate I get TypeError: org.XXX.XXX.MyClass has no such function xxx ...

public class FilterClass{
    ScriptEngine engine;
    Invocable inv;
    public FilterClass(File file){
        ScriptEngineManager manager = new ScriptEngineManager();
        engine = manager.getEngineByName("JavaScript");
        engine.eval(new FileReader(file));
        inv = (Invocable) engine;
    }
    public boolean passes(MyObject object){
        Object result = inv.invokeFunction("passes", object);
        return (Boolean) result;
    }
}

object factory definition to make MyObject

public class MyObjectFactory{

    private class MyObject{
        private final int myint;
        public MyObject(int i){
            myint = i;
        }
        public int getValue(){
            return myint;
        }
    }

    public MyObject makeObject(int i){
        return new MyObject(i);
    }
}

javascript file

function passes(o){
    if(o.getValue() > 10){
        return true;
    } else {
        return false;
    }
}

score 3 · Answer 1 · answered Oct 08 '16 at 12:52

It turns out that nashorn is unable to access functions defined in private classes like the MyObject class in the example.

Didn't want to expose my nested class so I ended up writing a wrapper class around that to expose the MyObject fields:

public class MyObjectWrapper{
    private final MyObject myobject;
    public MyObjectWrapper(MyObject o){
        myobject = o;
    }
    public int getValue(){
        return myobject.getValue();
    }
}

Then I could access the functions by wrapping:

    public boolean passes(MyObject object){
        Object result = inv.invokeFunction("passes", new MyObjectWrapper(object));
        return (Boolean) result;
    }

How to access Java object functions from nashorn

1 Answers1