I've been using butterknife for a few months and I've just noticed in its documentation that it says:
Now make sure you use R2 instead of R inside all Butter Knife annotations.
Why is that? I've been using R and everything works perfect.
Using R2 is only necessary for building Android Library projects.
https://github.com/JakeWharton/butterknife#library-projects
It has to do with fact that the values in the R.java generated class aren't declared as "final" when built as part of a library. The annotations used for @BindView() need these R. values to be final and not subject to change later on.
I'm sure someone could explain all of this better -- but the bottom line is - you're just fine using "R." values in @BindView in an Android application
来源：https://stackoverflow.com/questions/42362062/android-why-do-we-need-to-use-r2-instead-of-r-with-butterknife