commons-logging in OSGI Environments
While working on a small toy project using Apache Felix and commons-httpclient, I kept running into the issue that there is no official OSGI bundle for Apache commons-logging out there. While most Apache commons projects either provide simple bundles or full blown OSGI implementations with Activators and Services, commons-logging is an interesting exception. If you scan the Commons OSGI status page, you will notice that there is no OSGI version for commons-logging available, and a separate section to explain why. I haven’t dug into why and how it conflicts with the OSGI classloading scheme, but I can see that being a problem. Be it as it may, there is no official OSGI bundle for commons logging.
The problem was that I needed commons-httpclient which requires commons-logging. The solution was rather simple, but took me way too long to figure out. As the Commons OSGI status page indicates, you should go ahead and use the Pax Logging bundle. It contains several logging APIs, including log4j, commons-logging, JDK logging, SLF4J, and a few others.
Once you drop the pax logging bundle in your application, you’ll notice that it exports a boatload of logging APIs, which will make using libraries like commons-httpclient easy:
Export-Package = org.apache.avalon.framework.logger;provider=paxlogging;uses:="org.apache.log";version="4.3",org.apache.commons.logging;provider=paxlogging;uses:="org.ops4j.pax.logging,org.osgi.framework";version="1.1.1", ...
Having the actual APIs exposed in the OSGI classspace is already a huge step forward. However, once you start using the httpclient, it will flood your console with log messages. That might be OK in a GUI app (
> /dev/null). I’ll examine logging configuration in my next post.