R.java to xml file

Question

How does R.java work? In a compiled .apk file, does it contain all constants from the XML files prior to compilation? I've decompiled an app using several Java RE tools, and used apktool to extract the resources (xml files, images and others). However, when rebuilding the project in Eclipse, the R.java generated by Eclipse (in the gen folder) does not match the original R.java from the decompiled app. Is this a common occurrence when reversing android apps? In this case, the original R.java contains fields consistent with the rest of the code. However, this is not present in the generated R.java and hence this error.

The decompilers I used are

1) JEB (Commercial)

2) jadx (Open-source)

Answer 1

The R.java file created by jadx exists to be a reference between the resources id after compilation and the name of the resource mentioned throughout the code.

If you wish to recompile it, it is possible to recreate something that looks like the original xml files by creating a script that ignores a part of the prefix and searches for the resource's name. Once it is located inside the xml files, specify the correct one.

And I think that apktool provides a much better reference to the resources xml files.

But if what you wanna do is make just a small change, it will be faster to do it with smali and use apktool to put it all back together again.