Apache log4j is a Java-based logging utility. It was originally written by Ceki Gülcü and is now a project of the Apache Software Foundation. log4j is one of several Java logging frameworks.Gülcü has since started the SLF4J and Logback projects, with the intention of offering a successor to log4j.The log4j team has created a successor to log4j with version number 2.0.