Log::Log4perl::Appender - Log appender class


Log::Log4perl::Appender - Log appender class


  use Log::Log4perl;
      # Define a logger
  my $logger = Log::Log4perl->get_logger("abc.def.ghi");
      # Define a layout
  my $layout = Log::Log4perl::Layout::PatternLayout->new(
                   "%d (%F:%L)> %m");
      # Define an appender
  my $appender = Log::Log4perl::Appender->new(
                   name => 'dumpy');
      # Set the appender's layout


This class is a wrapper around the Log::Log4perl::Appender appender set.

It also supports the <Log::Dispatch::*> collections of appenders. The module hides the idiosyncrasies of Log::Dispatch (e.g. every dispatcher gotta have a name, but there's no accessor to retrieve it) from Log::Log4perl and yet re-uses the extremely useful variety of dispatchers already created and tested in Log::Dispatch.


Log::Log4perl::Appender->new($dispatcher_class_name, ...);

The constructor new() takes the name of the appender class to be created as a string (!) argument, optionally followed by a number of appender-specific parameters, for example:

      # Define an appender
  my $appender = Log::Log4perl::Appender->new(
      file => 'out.log');

In case of Log::Dispatch appenders, if no name parameter is specified, the appender object will create a unique one (format appNNN), which can be retrieved later via the name() method:

  print "The appender's name is ", $appender->name(), "\n";

Other parameters are specific to the appender class being used. In the case above, the file parameter specifies the name of the Log::Log4perl::Appender::File dispatcher used.

However, if, for instance, you're using a Log::Dispatch::Email dispatcher to send you email, you'll have to specify from and to email addresses. Every dispatcher is different. Please check the Log::Dispatch::* documentation for the appender used for details on specific requirements.

The new() method will just pass these parameters on to a newly created Log::Dispatch::* object of the specified type.

When it comes to logging, the Log::Log4perl::Appender will transparently relay all messages to the Log::Dispatch::* object it carries in its womb.


The layout() method sets the log layout used by the appender to the format specified by the Log::Log4perl::Layout::* object which is passed to it as a reference. Currently there's two layouts available:


Please check the the Log::Log4perl::Layout::SimpleLayout manpage and the Log::Log4perl::Layout::PatternLayout manpage manual pages for details.

Supported Appenders

Here's the list of appender modules currently available via Log::Dispatch, if not noted otherwise, written by Dave Rolsky:

       Log::Dispatch::DBI (by Tatsuhiko Miyagawa)
       Log::Dispatch::FileRotate (by Mark Pfeiffer)
       Log::Dispatch::Tk (by Dominique Dumont)

Log4perl doesn't care which ones you use, they're all handled in the same way via the Log::Log4perl::Appender interface. Please check the well-written manual pages of the Log::Dispatch hierarchy on how to use each one of them.

Parameters passed on to the appender's log() method

When calling the appender's log()-Funktion, Log::Log4perl will submit a list of key/value pairs. Entries to the following keys are guaranteed to be present:

Text of the rendered message

Name of the category of the logger that triggered the event.

Log::Log4perl level of the event


Since the Log::Dispatch::File appender truncates log files by default, and most of the time this is not what you want, we've instructed Log::Log4perl to change this behaviour by slipping it the mode => append parameter behind the scenes. So, effectively with Log::Log4perl 0.23, a configuration like

    log4perl.category = INFO, FileAppndr
    log4perl.appender.FileAppndr          = Log::Dispatch::File
    log4perl.appender.FileAppndr.filename = test.log
    log4perl.appender.FileAppndr.layout   = Log::Log4perl::Layout::SimpleLayout

will always append to an existing logfile test.log while if you specifically request clobbering like in

    log4perl.category = INFO, FileAppndr
    log4perl.appender.FileAppndr          = Log::Dispatch::File
    log4perl.appender.FileAppndr.filename = test.log
    log4perl.appender.FileAppndr.mode     = write
    log4perl.appender.FileAppndr.layout   = Log::Log4perl::Layout::SimpleLayout

it will overwrite an existing log file test.log and start from scratch.

Appenders Expecting Message Chunks

Instead of simple strings, certain appenders are expecting multiple fields as log messages. If a statement like

    $logger->debug($ip, $user, "signed in");

causes an off-the-shelf Log::Log4perl::Screen appender to fire, the appender will just concatenate the three message chunks passed to it in order to form a single string. The chunks will be separated by a string defined in $Log::Log4perl::JOIN_MSG_ARRAY_CHAR (defaults to the empty string ``'').

However, different appenders might choose to interpret the message above differently: An appender like Log::Log4perl::Appender::DBI might take the three arguments passed to the logger and put them in three separate rows into the DB.

The warp_message appender option is used to specify the desired behaviour. If no setting for the appender property

    # *** Not defined ***
    # log4perl.appender.SomeApp.warp_message

is defined in the Log4perl configuration file, the appender referenced by SomeApp will fall back to the standard behaviour and join all message chunks together, separating them by $Log::Log4perl::JOIN_MSG_ARRAY_CHAR.

If, on the other hand, it is set to a false value, like in

    log4perl.appender.SomeApp.warp_message = 0

then the message chunks are passed unmodified to the appender as an array reference. Please note that you need to set the appender's layout to Log::Log4perl::Layout::NoopLayout which just leaves the messages chunks alone instead of formatting them or replacing conversion specifiers.

Please note that the standard appenders in the Log::Dispatch hierarchy will choke on a bunch of messages passed to them as an array reference. You can't use warp_message = 0 (or the function name syntax defined below) on them. Only special appenders like Log::Log4perl::Appender::DBI can deal with this.

If (and now we're getting fancy) an appender expects message chunks, but we would like to pre-inspect and probably modify them before they're actually passed to the appender's log method, an inspection subroutine can be defined with the appender's warp_message property:

    log4perl.appender.SomeApp.warp_message = sub { \
                                           $#_ = 2 if @_ > 3; \
                                           return @_; }

The inspection subroutine defined by the warp_message property will receive the list of message chunks, like they were passed to the logger and is expected to return a corrected list. The example above simply limits the argument list to a maximum of three by cutting off excess elements and returning the shortened list.

Also, the warp function can be specified by name like in

    log4perl.appender.SomeApp.warp_message = main::filter_my_message

In this example, filter_my_message is a function in the main package, defined like this:

    my $COUNTER = 0;
    sub filter_my_message {
        my @chunks = @_;
        unshift @chunks, ++$COUNTER;
        return @chunks;

The subroutine above will add an ever increasing counter as an additional first field to every message passed to the SomeApp appender -- but not to any other appender in the system.




Mike Schilli, <log4perl@perlmeister.com>

 Log::Log4perl::Appender - Log appender class