Save content of code bein executed by test into File

I want to write a program that counts the lines of code being executed when

a test, lets say TestA(), runs. More importantly, I need to store the

lines of code being executed

by TestA(), into a codeLines: String.

I find this concept really bizarre and so far, I did not have any luck with

online resources. Do you know of any way that such functionality could be achieved?