Logging? What’s there to talk about logging? It’s straightforward. Use the one of the log levels and log it.
log4j‘s website says …
ERROR level designates error events that might still allow the application to continue running
FATAL level designates very severe error events that will presumably lead the application to abort
INFO level designates informational messages that highlight the progress of the application at coarse-grained level
WARN level designates potentially harmful situations
That is the theory of it. Let me tell you my opinion on a more practical matter.