http://mail.openjdk.java.net/pipermail/nashorn-dev/2014-July/003204.html

Inconsistency about conversion to ScriptObjectMirror with getInterface and invokeFunction makes it difficult know which kind of object is being worked with in Nashorn. Example;

package mirrors;

import java.util.*;
import javax.script.*;

public class Mirrors {
    public interface Example {
        Object test1(Object arg);
        Object test2(Object arg);
    }
    
    public static void main(String[] args) throws Exception {
        ScriptEngineManager engineManager = new ScriptEngineManager();
        ScriptEngine engine = engineManager.getEngineByName("nashorn");
        Invocable invocable = (Invocable)engine;

        engine.eval("function test1(arg) { print(arg.class); return { arg: arg }; }");
        engine.eval("function test2(arg) { print(arg.class); return arg; }");

        Map<String, Object> map = new HashMap<>();
        map.put("option", true);

        Example example = invocable.getInterface(Example.class);
        
        Object value1 = invocable.invokeFunction("test1", map);
        Object value2 = example.test1(map);
        Object value3 = invocable.invokeFunction("test2", value2);
        Object value4 = example.test2(value2);

        System.out.println(value1.getClass());
        System.out.println(value2.getClass());
        System.out.println(value3.getClass());
        System.out.println(value4.getClass());
    }
}


Output:

class java.util.HashMap
class java.util.HashMap
undefined
undefined
class jdk.nashorn.api.scripting.ScriptObjectMirror
class jdk.nashorn.internal.scripts.JO4
class jdk.nashorn.api.scripting.ScriptObjectMirror
class jdk.nashorn.internal.scripts.JO4