Apache Karaf provides a powerful logging system based in OPS4j Pax Logging. Besides implementing a standard OSGi Log Service, it also supports all major logging APIs (Apache Commons Logging, SLF4J, Apache Log4j, Java Util Logging). So no matter what API your libraries are using, Karaf can log messages from them.
For new bundles I suppose that a good choice is SLF4J. This is a modern API that offers parameterized log messages. In Java code one usually creates a static logger:
Stay informed on our latest news!
Only for users who has an user and a password sent by the administrator.