I don't think Appenders can be completely immutable. For one, you can invoke the close() method on an appender. You can also add remove filters or change the threshold of an active appender. The only appender which should be immutable is a closed appender.
It does not make much sense to me to harden appenders when one can easily yank out any appender from a logger and replace it with a different one...
At 07:03 PM 2/18/2005, Curt Arnold wrote:
I obviously haven't reviewed your last commit, but you did appear to tweak with activateOptions in a few classes and mention appender lifecycle which brought the topic to mind.
A very common pattern in log4j is to have a class that operates in a configuration mode early in its lifetime and then after a call to activate or activateOptions is ready for use. Unfortunately this pattern implies objects that are not inherently immutable. See "Effective Java", item 13 for the benefits of immutable classes. Since the objects using the activateOptions pattern are subject to change early in the object's lifetime, it is only developer etiquette and self-preservation that enforces the object does not change during use.
It would be better (in a perfect world) if distinct objects were used for the mutable configuration and immutable run-time behavior.
I don't have a path forward to move to greater use of immutable objects in log4j. However, I think it should be seriously considered on any substantial rework. When I finally get to my long standing date with LoggingEvent, it is likely to come out immutable.
When looking at appenders that change state over time, such as a database appender that can go into a failsafe mode if it can't connect, it would be preferable that these modes be represented by distinct immutable objects. So you would have an object that forever represented a working database and an object that forever represented the failover mode and the failover transition was accomplished by switching these objects out instead of changing an object that has potentially been shared.
One pattern that might allow a transition is to have activateOptions-like methods return an object pointer which is used from then on. Mutable objects could just return "this" since they don't change object identity when activating. However, if one wanted to have an immutable implementation, the initial instance could be of a "Configurator" or "Builder" variant and the value returned from activateOptions would be immutable. For example, an immutable FileAppender could be implemented with two classes: an immutable FileAppender and a FileAppenderBuilder, which allows setFile and the like, and would return a FileAppender on the call to activateOptions.
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