Java更改方法返回而不访问类Class

Question

I'm modifying someone else's code to implement a new functionality and I can't do it without changing the return of one of the functions. As I've said, this is not my code, so I can't change a single line of code.

The function itself is the following:

package me.Mohamad82.MineableGems.Core;

public class DropReader {
  ...
  public DropReader() {}
  public CustomDrop readCustomDrop(ConfigurationSection section, String mined, @Nullable String sectionNumber) {
    ...
  }
}

And I'm trying to do something like this:

package com.rogermiranda1000.mineit;

public class DropReader extends me.Mohamad82.MineableGems.Core.DropReader {
    public DropReader() {
        super();
    }

    @Override
    public CustomDrop readCustomDrop(ConfigurationSection section, String mined, @Nullable String sectionNumber) {
        CustomDrop drop = super.readCustomDrop(section, mined, sectionNumber);
        if (drop != null) {...}
        return drop;
    }
}

The problem is that I have no idea where to start. I can't change their code to call the other object, and I can't change the function call either.

The object is created every time (inside the functions that uses DropReader) so Reflection won't work, and searching I found something named Javassist but it won't work if the class is already loaded. I should be able to load by code before, but even with that I don't know if I'm approaching to the problem correctly, what do you think?

Edit: I'll try to explain better the situation. There's a class named Commands that runs the command new DropReader().readCustomDrop(section, mined, sectionNumber) . The problem is that if section.getString("mine") != null I need to change the readCustomDrop return (I need to add an aditional property).

I can't change Commands's code, nor DropReader. Commands MUST get the modified object.

Edit2: It should work on both Java 8 and 17.

Answer 1

Probably the best way to do this is Java Instrumentation, but it was too complex for me so I did the following (assuming you have the .jar, and the .jar it's not already loaded):

1. Open the .jar as a Zip using ZipFile class
2. Send the .java to ClassFileToJavaSourceDecompiler (from JD-Core ) and decompile it
3. Change the .java code
4. Compile the new code running javac with Runtime.getRuntime().exec (note: you'll probably need to add the dependencies using -classpath )
5. Add the compiled .java to the .jar