logging - How to use log levels in java

Question

I am developing an application where i need to use the logger functionality. I have read about different levels of logger which are:

• SEVERE (highest)
• WARNING
• INFO
• CONFIG
• FINE
• FINER
• FINEST

I am not able to understand the usage of each logging level.

Can someone give me a good example showing all the logging levels and their usage?

Answer 1

Generally, you don't need all those levels, SEVERE, WARNING, INFO, FINE might be enough. We're using Log4J (not java.util.logging directly) and the following levels (which might differ in name from other logging frameworks):

• ERROR: Any error/exception that is or might be critical. Our Logger automatically sends an email for each such message on our servers (usage: logger.error("message"); )

• WARN: Any message that might warn us of potential problems, e.g. when a user tried to log in with wrong credentials - which might indicate an attack if that happens often or in short periods of time (usage: logger.warn("message"); )

• INFO: Anything that we want to know when looking at the log files, e.g. when a scheduled job started/ended (usage: logger.info("message"); )

• DEBUG: As the name says, debug messages that we only rarely turn on. (usage: logger.debug("message"); )

The beauty of this is that if you set the log level to WARN, info and debug messages have next to no performance impact. If you need to get additional information from a production system you just can lower the level to INFO or DEBUG for a short period of time (since you'd get much more log entries which make your log files bigger and harder to read). Adjusting log levels etc. can normally be done at runtime (our JBoss instance checks for changes in that config every minute or so).