How to extend a final class?(Reflection, Javassist)

Question

I have a .JAR file and it has tons of classes. One, that I need is set as final, so I cannot extend it. There is one method, that I basically have to extend and fix, otherwise everything breaks. How can I do that? I know Reflection and Javassist can be used for this, but I have no idea how. Any other tool is also acceptable, as long as it works.

Have a look over http://stackoverflow.com/questions/13294629/how-to-extend-a-final-class-in-java — Ankit Tripathi, Sep 19 '16 at 17:53
Seen it, there is no example. I know it's not possible using Java normally, but I still need to get it done — Gintas_, Sep 19 '16 at 17:54

score 4 · Answer 1 · answered Sep 19 '16 at 18:01

You can use a wrapper around the Final class for example and extend the functionality,for example :

public class YourClass {
    private FinalClass finalClass;

    public YourClass {
        finalClass = new FinalClass():
    }

    public void yourMethod() {
        finalClass.methodInFinalClass();
    }
}

GhostCat · Accepted Answer · 2016-09-19T17:51:57.280

3

You can't use reflection to modify existing class definitions.

If the licence of that JAR allows you to, I would suggest a different solution: decompile that one class; drop the final keyword; and rebuild class and JAR file.

That is the only robust way to solve your problem.

edited Sep 19 '16 at 17:51

answered Sep 19 '16 at 17:50

GhostCat

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Which one did you try? And just for the record - we are not talking open source, where you could simply download the source? – GhostCat Sep 19 '16 at 17:52
Using JD-GUI, Luyten. Actually, it's a little bit of white-hat hacking :) – Gintas_ Sep 19 '16 at 17:53
One other thought: do you know about the **implementation** of that class? What I mean is: would there be a chance to completely replace it with your own code? Or are the "real" functions in that class important to you? – GhostCat Sep 19 '16 at 17:56
Thought about it. But there is a signature algorithm in that class, I thought about copy pasting it in my code, but decompiler messes it up real good. Now that I looked closer at it, I think my problem is quite different - I need to actually remove a method from that class and it should work fine. Because now I'm getting NoClassDefFoundError for some random class. I think I will create a new StackOverflow post, it will get too off-topic – Gintas_ Sep 19 '16 at 18:00
@Gintas_ Just use the Krakatau disassembler. That can handle anything, even classes which are impossible to decompile. If you're feeling brave, you can also use a hex editor. Removing the final flag is a single bit flip at a fixed offset, so it's easy to do in a hex editor. – Antimony Sep 20 '16 at 02:07

score 0 · Answer 3 · answered Sep 19 '16 at 17:55

0

Final classes are marked final for a reason, so ideally you should not try to modify the class. You should consider moving to other APIs or write your own solution class for your case.

answered Sep 19 '16 at 17:55

kazzaki

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How to extend a final class?(Reflection, Javassist)

3 Answers3

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