How to specify log file path using env variable

[Jitendra Kharche]

Hi All,
 
I am using log4j for looging my applocation log on windows. The log file
path I have specified in the log4j.xml file is currently a windows path
e.g. C:\myapp\logs\myapp.log.
I want to remove the hardcoded path (C:\myapp) and replace it with a
system environment variable MY_APP (=C:\myapp). How can I do that?
Furthermore, this will help me in migration my application from windows
to unix or other OSs.
 
Please help.
 
Regards,
Jitendra Kharche