Log4j is a logging package written in Java. Log4j allows you to log to a file, a java.io.Writer, a remote server, or a syslog daemon. The package is designed so that log statements can remain in shipped code without incurring a high performance cost. One distinctive feature of log4j is the notion of hierarchical loggers.

Logging system for the Java platform log4j from the Apache Software Foundation is the de facto standard application logging library for the Java platform. log4j is used by applications to log messages relating to internal events. The routing of the messages to relevant parties may be configured by the application administrator. more information

http://www.apache.org. Log4J is used for logging. See log4j logging for more details.