How to obtain a caller object reference from within a called method?

[mayngontay]

I have to execute a given String as JavaScript code, e.g. eval('Foo.setMessage("Hello!")'), from within a class called Engine. Here, setMessage() is a public static method of the Foo class. Because I want to access some properties of the Engine object from within the setMessage() method, how can I obtain a reference to the MyApp object?

I do know how to get the caller class name using Reflection or StackTrace or Throwable, but these do not return a caller object reference.

Please help, thanks,
John

[DarrylBurke]

You can't obtain a reference unless you pass a reference. So change setMessage() to setMessage(Engine object)

db

[mayngontay]

You can't obtain a reference unless you pass a reference. So change setMessage() to setMessage(Engine object)

db

Ok, assuming that I change setMessage(String msg) to setMessage(Engine object, String msg). How can I change the call to this method. The original call is eval('Foo.setMessage("Hello!")'). Note that, the call is made via the evaluation of a JavaScript string, it is not a direct call. Do you think it should be eval('Foo.setMessage(this, "Hello!")')? This will not work. Please carefully read my question. Thanks.

[KevinWorkman]

Please carefully read my question. Thanks.

Please ask your question more clearly. Thanks.

Saying "this will not work" is as useful to us as us saying "then fix it" is useful to you.

[mayngontay]

Ok, thanks. Here is the code to make my question clearer.

import javax.script.*;

public class Engine {
    public static ScriptEngine scriptEngine = new ScriptEngineManager().getEngineByName("ECMAScript");
    public String engineName;
    public String message;

    public Engine(String name) {
        this.engineName = name;
    }

    public void eval(String script) {
        try {
            Engine.scriptEngine.eval(script);
            //to do something more
        } catch (ScriptException e) {
            System.out.println(e.toString());
        }
    }

    public static void main(String[] args) {
        Engine firstEngine = new Engine("First engine");
        firstEngine.eval("Packages.Foo.setMessage('Hello!');");

        Engine secondEngine = new Engine("Second engine");
        secondEngine.eval("Packages.Foo.setMessage('Hello!');");
    }
}

public class Foo {
    public static void setMessage(String msg){
        //here: how to get which engine is calling this method: the first engine or the second engine?
        //callerEngine.message = msg;
        //System.out.println(callerEngine.engineName);
    }
}

[DarrylBurke]

In the main method, you already have variable references for firstEngine and secondEngine. What prevents you from changing the signature of Foo.setMessage (or creating an overload of the method) and passing those references as parameters?

db

[toadaly]

There is no 'give me a reference to the object that called me' mechanism in Java. Change your design, or use a different language.

[mayngontay]

There is no 'give me a reference to the object that called me' mechanism in Java.

If the above is true, then I would like to say thank you very much for poiting this out! I will finish looking for such the mechanism in Java.

[mayngontay]

In the main method, you already have variable references for firstEngine and secondEngine. What prevents you from changing the signature of Foo.setMessage (or creating an overload of the method) and passing those references as parameters?
db

You are right that we can change the signature of

Foo.setMessage()

to

setMessage(Engine callerEngine, String msg)

and call it by

firstEngine.eval("Packages.Foo.setMessage(firstEngine, 'Hello!');");

. However, the script text

"Packages.Foo.setMessage(firstEngine, 'Hello!');"

is actually from a file. The one who created this file may not know the engine object reference which will execute this script (e.g., firstEngine or myEngine or whatever).

Anyway, I have changed my application design. Thank you very much.