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https://issues.apache.org/jira/browse/LOG4J2-555?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13925443#comment-13925443
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Remko Popma commented on LOG4J2-555:
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The enums (like MsgBuilder) use the double-dispatch pattern as a clean way to
avoid introducing a conditional. Avoiding a branch means the CPU doesn't need
to do branch prediction (potential cache miss/pipeline stall).
The purpose of the refactoring in the patch was to
# reduce semi-duplicate code in many AbstractLogger methods
# extract elements of the {{if(isEnabled(...){log(...);\}_}} pattern so these
elements can be re-used in extended/custom loggers
# these reusable elements in turn make managing the FQCN less onerous for
custom/extended loggers: these loggers don't need to repeat the
{{if(isEnabled(...){log(...);\}_}} pattern but instead can have a one-line
implementation that calls the {{doLog(...)}} method.
So to me, this patch solves the FQCN problem this ticket was created for. If we
decide to reject this patch I don't mind not having a solution but just keeping
things the way they are. I don't think the FQCN is a huge problem and I would
not be in favor of much added complexity just to make FQCN easier to manage for
the rare use case of custom/extended loggers. Any extra classes/interfaces
should carry their own weight so to speak and have benefits other than FQCN,
IMHO.
Would the stream/writer loggers benefit from the patch attached here? (I
haven't checked...)
> Location-based functionality broken in AbstractLoggerWrapper subclasses
> -----------------------------------------------------------------------
>
> Key: LOG4J2-555
> URL: https://issues.apache.org/jira/browse/LOG4J2-555
> Project: Log4j 2
> Issue Type: Bug
> Components: API, Core
> Affects Versions: 2.0-rc1
> Reporter: Remko Popma
> Assignee: Remko Popma
> Fix For: 2.0-rc2
>
> Attachments: LOG4J2-555-delegate.patch
>
>
> *How to reproduce*
> * Create a custom logger that extends {{AbstractLoggerWrapper}} (or generate
> one with the tool attached to LOG4J2-519)
> * In the custom logger provide a public method that invokes the {{log(Level,
> String)}} method
> * Configure a pattern layout that uses location, like %C for the logger FQCN
> * From a sample app, call the public method on your custom logger.
> * The output will show the class name of the custom logger instead of the
> class name of the calling class in the sample application.
> *Cause*
> {{AbstractLogger}}'s FQCN field is {{static final}} and initialized to
> {{AbstractLogger.class.getName()}}. Then, in
> {{Log4jLogEvent#calcLocation()}}, when walking over the stack trace elements,
> the element _following_ the FQCN is returned. So only loggers that directly
> subclass from {{AbstractLogger}} will work correctly. Loggers that inherit
> from {{AbstractLoggerWrapper}} are two levels removed from {{AbstractLogger}}
> and the {{calcLocation()}} method will not work correctly.
> *Solution*
> I think {{AbstractLogger}}'s FQCN field should be made non-static, and
> initialized to {{getClass().getName()}} in the constructor of
> {{AbstractLogger}}. {{Log4jLogEvent#calcLocation()}} can then be modified to
> return the {{StackElement}} whose class name matches the FQCN, instead of the
> next element. Location-based functionality should then work for arbitrarily
> deep subclass hierarchies of AbstractLogger.
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