man 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 CWLog::Log4perl::Appender appender set.

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


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

The constructor CWnew() 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 CWLog::Dispatch appenders, if no CWname parameter is specified, the appender object will create a unique one (format CWappNNN), which can be retrieved later via the CWname() 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 CWfile parameter specifies the name of the CWLog::Log4perl::Appender::File dispatcher used.

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

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

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


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


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

Supported Appenders

Here's the list of appender modules currently available via CWLog::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)

CWLog4perl doesn't care which ones you use, they're all handled in the same way via the CWLog::Log4perl::Appender interface. Please check the well-written manual pages of the CWLog::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 CWLog::Dispatch::File appender truncates log files by default, and most of the time this is not what you want, we've instructed CWLog::Log4perl to change this behaviour by slipping it the CWmode => append parameter behind the scenes. So, effectively with CWLog::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 CWtest.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 CWtest.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 CWLog::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 CW$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 CWLog::Log4perl::Appender::DBI might take the three arguments passed to the logger and put them in three separate rows into the DB.

The CWwarp_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 CWSomeApp will fall back to the standard behaviour and join all message chunks together, separating them by CW$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 CWLog::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 CBwarp_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 CWlog method, an inspection subroutine can be defined with the appender's CWwarp_message property:

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

The inspection subroutine defined by the CWwarp_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, CWfilter_my_message is a function in the CWmain 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 CWSomeApp appender but not to any other appender in the system.




Mike Schilli, <>