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.
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.