Is there a way to use a java object as an argument for a function and return one of said objects’s values in GraalJS?

I want to use a Java object new Train() as an argument to pass into a JavaScript function, here is the Java code

public void execute() {

        File script = new File("/Test.js");

        ScriptEngine engine = new ScriptEngineManager().getEngineByName("graal.js");

        try {
            engine.eval(new FileReader(script));
            Invocable invocable = (Invocable)engine;
            
            Object result = invocable.invokeFunction("fun1", new Train());
            System.out.println(result);
        } catch(Exception e) {
            e.printStackTrace();
        }

}

public class Train {

     private double speed = 0.0D;
     
     public Train() {
          this.speed = 10.5D;
     }

     public double getSpeed() {
          return this.speed;
     }
}

JavaScript code

var fun1 = function test(train) {
    print(train.getSpeed());
    return train.getSpeed();
}

As of right now it puts this error in the console

[16:56:42] [INFO]: [STDERR]: javax.script.ScriptException: org.graalvm.polyglot.PolyglotException: TypeError: invokeMember (getSpeed) on ScriptExecutor$Train@429b2a7b failed due to: Unknown identifier: getSpeed [16:56:42] [INFO]: [STDERR]: at com.oracle.truffle.js.scriptengine.GraalJSScriptEngine.toScriptException(GraalJSScriptEngine.java:483) [16:56:42] [INFO]: [STDERR]: at com.oracle.truffle.js.scriptengine.GraalJSScriptEngine.invokeFunction(GraalJSScriptEngine.java:558)

Is there any way to make this work?

Answer

GraalJS (and GraalVM in general) has tight security/access controls by default. GraalJS is not exposing getSpeed() (or any other field or method) on the Train instance to JavaScript.

You can open up access to all host fields/methods with a configuration setting:

Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
bindings.put("polyglot.js.allowHostAccess", true);

or instead enable it on a more granular level by decorating getSpeed() in Train:

import org.graalvm.polyglot.HostAccess;
// ...
@HostAccess.Export
public double getSpeed() {
    return this.speed;
}