Apache Log4j 2
Apache Log4j 2 is an upgrade to Log4j that provides significant improvements over its predecessor, Log4j 1.x, and provides many of the improvements available in Logback while fixing some inherent problems in Logback's architecture.
Documentation
The Log4j 2 User's Guide is available here or as a downloadable PDF.
Requirements
Log4j 2.4 and greater requires Java 7, versions 2.0-alpha1 to 2.3 required Java 6. Some features require optional dependencies; the documentation for these features specifies the dependencies.
License
Apache Log4j 2 is distributed under the Apache License, version 2.0.
Download
How to download Log4j, and how to use it from Maven, Ivy and Gradle.