Log4J Powers Unveiled-IV - Direct log output to different appenders by level.

Yes it is. Setting the Threshold option of any appender extending AppenderSkeleton, (most log4j appenders extend AppenderSkeleton) to filter out all log events with lower level than the value of the threshold option.

For example, setting the threshold of an appender to DEBUG also allow INFO, WARN, ERROR and FATAL messages to log along with DEBUG messages. This is usually acceptable as there is little use for DEBUG messages without the surrounding INFO, WARN, ERROR and FATAL messages. Similarly, setting the threshold of an appender to ERROR will filter out DEBUG, INFO and WARN messages but not ERROR or FATAL messages.

This policy usually best encapsulates what the user actually wants to do, as opposed to her mind-projected solution.

See examples/sort4.lcf for an example threshold configuration.

If you must filter events by exact level match, then you can attach a LevelMatchFilter to any appender to filter out logging events by exact level match.