NAME¶
Log::Log4perl::Appender - Log appender class
SYNOPSIS¶
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(
"Log::Log4perl::Appender::Screen",
name => 'dumpy');
# Set the appender's layout
$appender->layout($layout);
$logger->add_appender($appender);
DESCRIPTION¶
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".
FUNCTIONS¶
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(
"Log::Log4perl::Appender::File"
filename => '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 "filename" 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.
$appender->layout($layout);¶
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:
Log::Log4perl::Layout::SimpleLayout
Log::Log4perl::Layout::PatternLayout
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
"Log::Dispatch", if not noted otherwise, written by Dave Rolsky:
Log::Dispatch::ApacheLog
Log::Dispatch::DBI (by Tatsuhiko Miyagawa)
Log::Dispatch::Email,
Log::Dispatch::Email::MailSend,
Log::Dispatch::Email::MailSendmail,
Log::Dispatch::Email::MIMELite
Log::Dispatch::File
Log::Dispatch::FileRotate (by Mark Pfeiffer)
Log::Dispatch::Handle
Log::Dispatch::Screen
Log::Dispatch::Syslog
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:
- message
- Text of the rendered message
- log4p_category
- Name of the category of the logger that triggered the event.
- log4p_level
- Log::Log4perl level of the event
Pitfalls¶
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 behavior 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::Appender::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
behavior. 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 behavior 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.layout=NoopLayout
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.layout=NoopLayout
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.layout=NoopLayout
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.
Composite Appenders¶
Composite appenders relay their messages to sub-appenders after providing some
filtering or synchronizing functionality on incoming messages. Examples are
Log::Log4perl::Appender::Synchronized, Log::Log4perl::Appender::Limit, and
Log::Log4perl::Appender::Buffer. Check their manual pages for details.
Composite appender objects are regular Log::Log4perl::Appender objects, but they
have the composite flag set:
$app->composite(1);
and they define a
post_init() method, which sets the appender it relays
its messages to:
###########################################
sub post_init {
############################################
my($self) = @_;
if(! exists $self->{appender}) {
die "No appender defined for " . __PACKAGE__;
}
my $appenders = Log::Log4perl->appenders();
my $appender = Log::Log4perl->appenders()->{$self->{appender}};
if(! defined $appender) {
die "Appender $self->{appender} not defined (yet) when " .
__PACKAGE__ . " needed it";
}
$self->{app} = $appender;
}
The reason for this post-processing step is that the relay appender might not be
defined yet when the composite appender gets defined. This can happen if
Log4perl is initialized with a configuration file (which is the most common
way to initialize Log4perl), because appenders spring into existence in
unpredictable order.
For example, if you define a Synchronized appender like
log4perl.appender.Syncer = Log::Log4perl::Appender::Synchronized
log4perl.appender.Syncer.appender = Logfile
then Log4perl will set the appender's "appender" attribute to the
name of the appender to finally relay messages to. After the Log4perl
configuration file has been processed, Log4perl will remember to call the
composite appender's
post_init() method, which will grab the relay
appender instance referred to by the name (Logfile) and set it in its
"app" attribute. This is exactly what the code snippet above does.
But if you initialize Log4perl by its API, you need to remember to perform these
steps. Here's the lineup:
use Log::Log4perl qw(get_logger :levels);
my $fileApp = Log::Log4perl::Appender->new(
'Log::Log4perl::Appender::File',
name => 'MyFileApp',
filename => 'mylog',
mode => 'append',
);
$fileApp->layout(
Log::Log4perl::Layout::PatternLayout::Multiline->new(
'%d{yyyy-MM-dd HH:mm:ss} %p [%c] #%P> %m%n')
);
# Make the appender known to the system (without assigning it to
# any logger
Log::Log4perl->add_appender( $fileApp );
my $syncApp = Log::Log4perl::Appender->new(
'Log::Log4perl::Appender::Synchronized',
name => 'MySyncApp',
appender => 'MyFileApp',
key => 'nem',
);
$syncApp->post_init();
$syncApp->composite(1);
# The Synchronized appender is now ready, assign it to a logger
# and start logging.
get_logger("")->add_appender($syncApp);
get_logger("")->level($DEBUG);
get_logger("wonk")->debug("waah!");
The composite appender's
log() function will typically cache incoming
messages until a certain trigger condition is met and then forward a bulk of
messages to the relay appender.
Caching messages is surprisingly tricky, because you want them to look like they
came from the code location they were originally issued from and not from the
location that triggers the flush. Luckily, Log4perl offers a cache mechanism
for messages, all you need to do is call the base class'
log() function
with an additional reference to a scalar, and then save its content to your
composite appender's message buffer afterwards:
###########################################
sub log {
###########################################
my($self, %params) = @_;
# ... some logic to decide whether to cache or flush
# Adjust the caller stack
local $Log::Log4perl::caller_depth =
$Log::Log4perl::caller_depth + 2;
# We need to cache.
# Ask the appender to save a cached message in $cache
$self->{relay_app}->SUPER::log(\%params,
$params{log4p_category},
$params{log4p_level}, \my $cache);
# Save it in the appender's message buffer
push @{ $self->{buffer} }, $cache;
}
Note that before calling the
log() method of the relay appender's base
class (and thus introducing two additional levels on the call stack), we need
to adjust the call stack to allow Log4perl to render cspecs like the %M or %L
correctly. The cache will then contain a correctly rendered message, according
to the layout of the target appender.
Later, when the time comes to flush the cached messages, a call to the relay
appender's base class'
log_cached() method with the cached message as
an argument will forward the correctly rendered message:
###########################################
sub log {
###########################################
my($self, %params) = @_;
# ... some logic to decide whether to cache or flush
# Flush pending messages if we have any
for my $cache (@{$self->{buffer}}) {
$self->{relay_app}->SUPER::log_cached($cache);
}
}
SEE ALSO¶
Log::Dispatch
LICENSE¶
Copyright 2002-2013 by Mike Schilli <m@perlmeister.com> and Kevin Goess
<cpan@goess.org>.
This library is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.
AUTHOR¶
Please contribute patches to the project on Github:
http://github.com/mschilli/log4perl
Send bug reports or requests for enhancements to the authors via our
MAILING LIST (questions, bug reports, suggestions/patches):
log4perl-devel@lists.sourceforge.net
Authors (please contact them via the list above, not directly): Mike Schilli
<m@perlmeister.com>, Kevin Goess <cpan@goess.org>
Contributors (in alphabetical order): Ateeq Altaf, Cory Bennett, Jens Berthold,
Jeremy Bopp, Hutton Davidson, Chris R. Donnelly, Matisse Enzer, Hugh Esco,
Anthony Foiani, James FitzGibbon, Carl Franks, Dennis Gregorovic, Andy
Grundman, Paul Harrington, Alexander Hartmaier David Hull, Robert Jacobson,
Jason Kohles, Jeff Macdonald, Markus Peter, Brett Rann, Peter Rabbitson, Erik
Selberg, Aaron Straup Cope, Lars Thegler, David Viner, Mac Yang.