On Feb 18, 2005, at 1:30 PM, Ceki G�lc� wrote:
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...
I was suggesting that if a substantial part of the framework is being refactored, trying to make the objects immutable should be considered as part of the refactoring. Maybe Appenders aren't a good fit with the pattern, maybe they are but it just isn't obvious. I wasn't suggesting that we try to refactor them now.
However, the state changes that you mentioned could be represented by a series of immutable objects. The advantage is that you would not need to synchronize the state changes only the swap of the new appender with the old appender. For example, if I'm removing one filter and adding another filter and I'm changing the object in place, the appender with just the first filter removed could be visible to other threads for a period of time. With immutable objects, I could do the state changes in isolation and then swap out the new appender for the old one as an atomic operation. It also can reduce the number of code points that you have to worry about synchronization. With immutable objects, you only have to synchronize the replacement of one object with another. With mutable objects, you have to synchronize every method that might be affected by the contents of a mutable field.
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