Log4j version 1.4 or 2.0 is likely to introduce the notion of multiple domains for categorizing logging statements. When that happens, the notion of logging levels will be looked at very differently.
Commons-logging promises to abstract different logging APIs such as log4j, Avalon logkit and java.util.logging API. However, such a task is near impossible to fulfill, while Avalon Logkit is nowhere to be seen. In the history of software, no one has ever managed to abstract competing and divergent APIs without their active cooperation. Chances are it won't happen this time around either.
User who currently use commons-logging are likely to go through a lengthy and painful conversion process when they realize that log4j offers must-have features.
I think this added functionality that is coming in Log4J demonstrates another reason to leave Log as an interface rather than converting it to an abstract class. Assuming we make LocalizedLog an interface that extends Log, when Log4J introduces support for their new "domain" logging (if you will), JCL can just introduce a DomainLog interface that extends Log and has nothing to do with LocalizedLog. A logging implementation may or may not support internationalization, and may or may not support this new "domain" concept. In this way, we can have Log implementations describe which features they support by implementing certain interfaces and not others.
If Log becomes an abstract class, then instead of just introducing a LocalizedLog interface and a DomainLog interface, we would would have a factorial explosion of abstract Log subclasses:
DomainLog extends Log LocalizedLog extends Log DomainLocalizedLog extends Log
If we need to introduce FeatureNumberThree we will have all the above abstract classes plus:
FeatureNumberThreeLog extends Log DomainFeatureNumberThreeLog extends Log LocalizedFeatureNumberThreeLog extends Log LocalizedDomainFeatureNumberThreeLog extends Log
etc.
Conclusion: Switching to an abstract class doesn't just hamper the flexibility of users of JCL; it also decreases JCL's ability to expose more logging features by simply introducing new interfaces.
Matt
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