The Simple Logging Facade for Java or (SLF4J) serves as a simple facade or abstraction for various logging frameworks, e.g. java.util.logging, log4j and logback, allowing the end-user to plug in the desired logging framework at deployment time. 

You need the slf4j-api + one of the other packages in the classpath to make it work. For example use slf4j-log4j12 to make slf4j log through log4j.