SLF4J is somewhat of an interface. It delegates the actual logging to another framework such as log4j or logback. When you get that message you didn't provided an implementation and SLF4J doesn't know where to log to. So it defaults to not logging. You'll need to provide an implementation on the classpath. I'd recommend logback (successor of log4j by the maker of log4j).
"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." --- Martin Fowler
Please correct my English.