log4j

Log4j by default can log messages with five priority levels.

1. Use debug to write debugging messages which should not be printed when the application is in production.
2. Use info for messages similar to the "verbose" mode of many applications.
3. Use warn for warning messages which are logged to some log but the application is able to carry on without a problem.
4. Use error for application error messages which are also logged to some log but, still, the application can hobble along. Such as when some administrator-supplied configuration parameter is incorrect and you fall back to using some hard-coded default value.
5. Use fatal for critical messages, after logging of which the application quits abnormally.