Log4j2 one file per user session

In my vaadin application, I'd like to create one log file per user session. I started to create a customer file appender but then decided to investigate on ThreadContexts a bit more. I now store the user's uid inside the ThreadContext, which works fine within my PatternLayout (%X{uid}), but unfortunately not inside the FileAppender (maybe I was a bit naive).

Which way would you go for something like this? Simply put, I'd like to get this result for the time being:

User not logged in -> filename=debug.log
User logged in -> filename=[username]_debug.log

Any ideas or pointers highly appreciated!

Solution

Problem solved and lessons learned:

To write to different log files, I needed Routers, as described in the FAQ. No clue why I didn't come across this while googling.
There's no need for a custom appender to solve this problem.
Log4j2 is very, very picky about directories not being writable. If this occurs, scroll up in the stacktrace: ERROR Unable to invoke factory method in class org.apache.logging.log4j.core.appender.RollingFileAppender. You'll find the culprit, google won't.
As well, Log4j2 cares in which order the appenders appear. It's stated multiple times in the FAQ and in the appender docs, still I overread it multiple times.