DO NOT REPLY [Bug 43403] - PatternLayout: new format modifer: prefix if non-empty

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------- Additional Comments From carnold@apache.org  2007-09-16 11:34 -------
I think the use case is reasonable, but I don't think the proposed solution is desirable.  First, the 
proposed solution isn't general enough. if there is a "prefix" why shouldn't there be a "suffix" too.  If it 
is desirable for NDC and MDC, why not for other fields as well. Second, implementing the solution 
would require hacking the pattern parser to support a new language feature.

ExtendedPatternLayout (the log4j 1.3 pattern layout backported for use with log4j 1.2 and provided in 
the extras companion) does support just %X which will print out all MDC values as key=value.

XSLTLayout (also in the extras) companion should also be capable of achieving your desired layout 
goals.  I haven't benchmarked XSLTLayout, but I would assume that it would have to be substantially 
slower than PatternLayout for equivalent tasks.  But it is hugely flexible and the performance different 
may not be an issue in your application.

If that isn't desirable, then possibly adding a new "replace" pattern to ExtendedPatternLayout in apache-
log4j-extras would be a reasonable approach that would not require hacking the pattern parser, just 
adding a new pattern specifier.  replace could look like:

%replace{layout pattern}{regex}{replacement}

So your NDC prefix would look like:

%c%replace{%x}{(.+)}{-$1}



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