Apache log4j has three development branches: a stable branch, 1.2, a discontinued branch, 1.3 and an experimental branch 2.0.
Apache log4j 1.2 releases are widely deployed. Development on the 1.2 branch is generally limited to bug fixing and minor enhancements.
Apache log4j 1.3 alpha releases are in limited use. Apache log4j 1.3 added many interesting features, but was compatibility with log4j 1.2 was problematic. Many features original developed for log4j 1.3 have been back-ported as companions for log4j 1.2. No further development is anticipated.
Apache log4j 2.0 is an experimental development branch for logging services designed for Java 5 and later.