An open source logging utility for the Java language. Log4J was created by IBM and released originally under their proprietary license, and was recently released to the Apache Group.
The Log4J system uses a fairly simple API to expose logging functionality to any Java application that needs to write log messages to standard output, a remote network socket, a file output stream, and many other targets. The user also has the ability to subclass the prebuilt logging targets and design a completely new one.