The Apache Commons Logging Library.
Distributed on the Apache License, Version 2.0
When writing a library it is very useful to log information. However there are many logging implementations out there, and a library cannot impose the use of a particular one on the overall application that the library is a part of.
The Logging package is an ultra-thin bridge between different logging implementations. A library that uses the commons-logging API can be used with any logging implementation at runtime. Commons-logging comes with support for a number of popular logging implementations, and writing adapters for others is a reasonably simple task.
Applications (rather than libraries) may also choose to use commons-logging. While logging-implementation independence is not as important for applications as it is for libraries, using commons-logging does allow the application to change to a different logging implementation without recompiling code.
Note that commons-logging does not attempt to initialise or terminate the underlying logging implementation that is used at runtime; that is the responsibility of the application. However many popular logging implementations do automatically initialise themselves; in this case an application may be able to avoid containing any code that is specific to the logging implementation used.