Specifying the log file name at run time?

Hi,

I could use some help. I would like to set up log4cxx so that by default it 
logs to one file, but while running, I want to be able to start writing to 
another file, and the name of that file is not known up front, so I cannot set 
it up in the log4j.properties file.

(The second file name is based on configuration data that the user specifies in 
another config file, and it would not be acceptable to the users if they had to 
separately add an entry in the log4j.properties file.)

I've tried creating a separate instance of a RollingFileAppender, and adding 
that to a new logger instance that I created. The problem is that the output 
I'm writting to that new logger is also getting to the default log file.

Is there a way that I can create a new logger instance with a new log file, and 
have it only write to that, without writing to the default root log file?

Thanks
Bruce