[log4perl-devel] Log::Log4perl Recipe of the Week (#20)

[<log...@pe...>]

Welcome to the Log::Log4perl recipe of the week. Today:

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Log::Log4perl Recipe of the Week (#20):

I want to suppress certain messages based on their content!
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Let's assume you've plastered all your functions with
Log4perl statements like

    sub some_func {

        INFO("Begin of function");

        # ... Stuff happens here ...

        INFO("End of function");
    }

to issue two log messages, one at the beginning and one at
the end of each function. Now you want to suppress the
message at the beginning and only keep the one at the end,
what can you do? You can't use the category mechanism,
because both messages are issued from the same package.

Log::Log4perl's custom filters (0.30 or better) provide an
interface for the Log4perl user to step in right before a
message gets logged and decide if it should be written out
or suppressed, based on the message content or other
parameters:

    use Log::Log4perl qw(:easy);
    
    Log::Log4perl::init( \ <<'EOT' );
        log4perl.logger             = INFO, A1
        log4perl.appender.A1        = Log::Dispatch::Screen
        log4perl.appender.A1.layout = \
            Log::Log4perl::Layout::PatternLayout
        log4perl.appender.A1.layout.ConversionPattern = %m%n
    
        log4perl.filter.M1 = Log::Log4perl::Filter::StringMatch
        log4perl.filter.M1.StringToMatch = Begin
        log4perl.filter.M1.AcceptOnMatch = false
    
        log4perl.appender.A1.Filter = M1
EOT

The last four statements in the configuration above are
defining a custom filter "M1" of type
"Log::Log4perl::Filter::StringMatch", which comes with
Log4perl right out of the box and allows you to define a
text pattern to match (as a perl regular expression) and a
flag "AcceptOnMatch" indicating if a match is supposed to
suppress the message or let it pass through.

The last line then assigns this filter to the "A1" appender,
which will call it every time it receives a message to be
logged and throw all messages out *not* matching the regular
expression "Begin".

Instead of using the standard
"Log::Log4perl::Filter::StringMatch" filter, you can define
your own, simply using a perl subroutine:

    log4perl.filter.ExcludeBegin  = sub { !/Begin/ }
    log4perl.appender.A1.Filter   = ExcludeBegin

For details on custom filters, check Log::Log4perl::Filter.


Have fun! Until next week.

-- Mike

###################################
# Mike Schilli                    #
# log...@pe...        #
# http://perlmeister.com          #
# http://log4perl.sourceforge.net #
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