Currently the SubstituteLoggerFactory returns a NOPLogger while the
LoggerFactory is getting initialzed. Due to this any Logger created during
initialization period are not useful for the lifetime of the system. This is
also explained here [1]. We are facing this issue while using Slf4j with
logback in Sling which is an OSGi applicatopn [2]

The fix suggested involves changes in the application using Slf4j. It would be
simpler if the SubstituteLoggerFactory returns some forms of
SubstitutableLogger which can delegate to a different logger impl. By default
it uses NOPlogger. But later the delegate can be set to actual logger.

[1] http://www.slf4j.org/codes.html#substituteLogger
[2] https://issues.apache.org/jira/browse/SLING-3340