Log4J, a fast and flexible logging library for Java.

Log4J is an open source library based on the work of many authors.
It allows the developer to control which log statements are output
with arbitrary granularity. It is fully configurable at runtime
using external configuration files. Best of all, Log4J has a gentle
learning curve. Beware: judging from user feedback, it is also quite
addictive.

WWW: http://logging.apache.org/log4j/