How to make sure that the method body code is immutable

Let say there is a method V1Archiver.

Now we want to make sure that during development somebody should not modify this method at all., or if modified need to get it reviewed by 2 different departments.

Possible ways in my mind
[1] Now one way is to make sure before any release that there is no diff for this method or class, but this require manual check plus not salable.

[2] other way can be is to write Junit test case which somehow get the method body as a string and calc the hash and compare it with the predefined hash. But how to write such Junit test case ?

[3] Is there some maven plugin which help for the same?

Or is there a much better strategy to achieve my usecase ?