Log::Log4perl::Filter - Log4perl Custom Filter Base Class

  use Log::Log4perl;

  Log::Log4perl->init(\ <<'EOT');
    log4perl.logger = INFO, Screen
    log4perl.filter.MyFilter        = sub { /let this through/ }
    log4perl.appender.Screen        = Log::Log4perl::Appender::Screen
    log4perl.appender.Screen.Filter = MyFilter
    log4perl.appender.Screen.layout = Log::Log4perl::Layout::SimpleLayout
  EOT

      # Define a logger
  my $logger = Log::Log4perl->get_logger("Some");

      # Let this through
  $logger->info("Here's the info, let this through!");

      # Suppress this
  $logger->info("Here's the info, suppress this!");

  #################################################################
  # StringMatch Filter:
  #################################################################
  log4perl.filter.M1               = Log::Log4perl::Filter::StringMatch
  log4perl.filter.M1.StringToMatch = let this through
  log4perl.filter.M1.AcceptOnMatch = true

  #################################################################
  # LevelMatch Filter:
  #################################################################
  log4perl.filter.M1               = Log::Log4perl::Filter::LevelMatch
  log4perl.filter.M1.LevelToMatch  = INFO
  log4perl.filter.M1.AcceptOnMatch = true

Log4perl allows the use of customized filters in its appenders to control the output of messages. These filters might grep for certain text chunks in a message, verify that its priority matches or exceeds a certain level or that this is the 10th time the same message has been submitted and come to a log/no log decision based upon these circumstantial facts.

Filters have names and can be specified in two different ways in the Log4perl configuration file: As subroutines or as filter classes. Here's a simple filter named CWMyFilter which just verifies that the oncoming message matches the regular expression CW/let this through/i:

    log4perl.filter.MyFilter        = sub { /let this through/i }

It exploits the fact that when CWok() is called on a message, Perl's special CW$_ variable will be set to the message text (prerendered, i.e. concatenated but not layouted) to be logged. The CWok() subroutine is expected to return a true value if it wants the message to be logged or a false value if doesn't.

Also, Log::Log4perl will pass a hash to the CWok() method, containing all key/value pairs that it would pass to the corresponding appender, as specified in Log::Log4perl::Appender. Here's an example of a filter checking the priority of the oncoming message:

  log4perl.filter.MyFilter        = sub {    \
       my %p = @_;                           \
       $p{log4p_level} eq "WARN" or          \
       $p{log4p_level} eq "INFO"             \
                                          }

If the message priority equals CWWARN or CWINFO, it returns a true value, causing the message to be logged.

For common tasks like verifying that the message priority matches a certain priority, there's already a set of predefined filters available. To perform an exact level match, it's much cleaner to use Log4perl's CWLevelMatch filter instead:

  log4perl.filter.M1               = Log::Log4perl::Filter::LevelMatch
  log4perl.filter.M1.LevelToMatch  = INFO
  log4perl.filter.M1.AcceptOnMatch = true

This will let the message through if its priority is INFO and suppress it otherwise. The statement can be negated by saying

  log4perl.filter.M1.AcceptOnMatch = false

instead. This way, the message will be logged if its priority is anything but INFO.

On a similar note, Log4perl's CWStringMatch filter will check the oncoming message for strings or regular expressions:

  log4perl.filter.M1               = Log::Log4perl::Filter::StringMatch
  log4perl.filter.M1.StringToMatch = bl.. bl..
  log4perl.filter.M1.AcceptOnMatch = true

This will open the gate for messages like CWblah blah because the regular expression in the CWStringToMatch matches them. Again, the setting of CWAcceptOnMatch determines if the filter is defined in a positive or negative way.

All class filter entries in the configuration file have to adhere to the following rule: Only after a filter has been defined by name and class/subroutine, its attribute values can be assigned, just like the CWtrue value above gets assigned to the CWAcceptOnMatch attribute after the filter CWM1 has been defined.

Attaching a filter to an appender is as easy as assigning its name to the appender's CWFilter attribute:

    log4perl.appender.MyAppender.Filter = MyFilter

This will cause CWLog::Log4perl to call the filter subroutine/method every time a message is supposed to be passed to the appender. Depending on the filter's return value, CWLog::Log4perl will either continue as planned or withdraw immediately.

Sometimes, it's useful to combine the output of various filters to arrive at a log/no log decision. While Log4j, Log4perl's mother ship, has chosen to implement this feature as a filter chain, similar to Linux' IP chains, Log4perl tries a different approach.

Typically, filter results will not need to be bumped along chains but combined in a programmatic manner using boolean logic. Log if this filter says 'yes' and that filter says 'no' is a fairly common requirement, but hard to implement as a chain.

CWLog::Log4perl::Filter::Boolean is a specially predefined custom filter for Log4perl. It combines the results of other custom filters in arbitrary ways, using boolean expressions:

    log4perl.logger = WARN, AppWarn, AppError

    log4perl.filter.Match1       = sub { /let this through/ }
    log4perl.filter.Match2       = sub { /and that, too/ }
    log4perl.filter.MyBoolean       = Log::Log4perl::Filter::Boolean
    log4perl.filter.MyBoolean.logic = Match1 || Match2

    log4perl.appender.Screen        = Log::Log4perl::Appender::Screen
    log4perl.appender.Screen.Filter = MyBoolean
    log4perl.appender.Screen.layout = Log::Log4perl::Layout::SimpleLayout

CWLog::Log4perl::Filter::Boolean's boolean expressions allow for combining different appenders by name using AND (&& or &), OR (|| or |) and NOT (!) as logical expressions. Also, parentheses can be used for defining precedences. Operator precedence follows standard Perl conventions. Here's a bunch of examples:

    Match1 && !Match2            # Match1 and not Match2
    !(Match1 || Match2)          # Neither Match1 nor Match2
    (Match1 && Match2) || Match3 # Both Match1 and Match2 or Match3

If none of Log::Log4perl's predefined filter classes fits your needs, you can easily roll your own: Just define a new class, derive it from the baseclass CWLog::Log4perl::Filter, and define its CWnew and CWok methods like this:

    package Log::Log4perl::Filter::MyFilter;

    use base Log::Log4perl::Filter;

    sub new {
        my ($class, %options) = @_;

        my $self = { %options,
                   };

        bless $self, $class;

        return $self;
    }

    sub ok {
         my ($self, %p) = @_;

         # ... decide and return 1 or 0
    }

    1;

Values you've defined for its attributes in Log4perl's configuration file, will be received through its CWnew method:

    log4perl.filter.MyFilter       = Log::Log4perl::Filter::MyFilter
    log4perl.filter.MyFilter.color = red

will cause CWLog::Log4perl::Filter::MyFilter's constructor to be called like this:

    Log::Log4perl::Filter::MyFilter->new( name  => "MyFilter",
                                          color => "red" );

The custom filter class should use this to set the object's attributes, to have them available later to base log/nolog decisions on it.

CWok() is the filter's method to tell if it agrees or disagrees with logging the message. It will be called by Log::Log4perl whenever it needs the filter to decide. A false value returned by CWok() will block messages, a true value will let them through.

See Log::Log4perl::FAQ for this.

Log::Log4perl::Filter::LevelMatch, Log::Log4perl::Filter::LevelRange, Log::Log4perl::Filter::StringRange, Log::Log4perl::Filter::Boolean

Mike Schilli, <log4perl@perlmeister.com>, 2003