Overwriting Super Variables Java

Question

I have a class Client, which has a variable privilege which I cannot edit. I can however edit a class Launcher which extends Client. A method in Client checks the privilege, and I would like to overwrite the privilege variable before that method.

public class Client { // I can't edit this class at all

    private int privilege = 0;

    public void chat(String msg) {
        if (privilege == 3) {
            // send chat packet with privileges to server
        }
    }
}

Then in a seperate file:

public class Launcher extends Client { // I can edit this class
    // This is what I tried... it didn't work
    @Override
    public void chat(String msg) {
        int privilege = 3;
        super.chat(msg);
    }
}

This is part of a game I've decompiled, and I'd like to overwrite the privilege variable (cheat), how can I accomplish this?

Once you learn what it means for a variable to be private, you will understand that what you are trying to accomplish is not possible — ControlAltDel, Jun 21 '16 at 18:40
If you're already decompiling the game, why *not* edit `Client`? — user2357112, Jun 21 '16 at 18:40
I'm voting to close this question as off-topic because there is no solution — ControlAltDel, Jun 21 '16 at 18:41
Yeah, I know what private variables are, I thought the community might have a workaround for this. — Nicholas Mattiacci, Jun 21 '16 at 18:41
@user2357112 The decompiler I used created an incorrect Java file, I can't recompile it so I can't use it. — Nicholas Mattiacci, Jun 21 '16 at 18:42
Yeah, I guess there isn't a way, I'll delete the question... nevermind I can't — Nicholas Mattiacci, Jun 21 '16 at 18:43
The point of the `private` keyword is that no one - including subclasses - besides the class itself can access those members. So no, there is no workaround. You'd have to modify the parent class or create and use a new one. — Chris Sprague, Jun 21 '16 at 18:43

score 1 · Accepted Answer · answered Jun 21 '16 at 18:51

1

You can use reflection to write private fields. Get the field, make it accessible, and assign the value you want. A similar approach can be used to invoke private methods.

@Override
public void chat(String msg)
{
  try {
    Field field = Client.class.getDeclaredField("privilege");
    field.setAccessible(true);
    field.setInt(this, 3);
  }
  catch (Exception ex) {
    throw new RuntimeException("Failed to modify field", ex);
  }
  super.chat(msg);
}

answered Jun 21 '16 at 18:51

erickson

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Worked great, thanks. – Nicholas Mattiacci Jun 21 '16 at 18:54
The reflection access modification seems incredibly unsafe (overall not in this specific example). Isn't the point of java's access modifiers to make variables safe in polymorphism? – Eli Sadoff Jun 21 '16 at 18:57
@EliSadoff Access modification is a privileged action, and the runtime can be configured to deny its use by untrusted code. – erickson Jun 21 '16 at 18:59
@erickson Ah, I didn't realize it was a privileged action; however, I guess I could have inferred that from the `RuntimeException` being thrown. – Eli Sadoff Jun 21 '16 at 19:01

Overwriting Super Variables Java

1 Answers1