| Log::Log4perl - Log4j implementation for Perl |
Log::Log4perl - Log4j implementation for Perl
Log::Log4perl::init('/etc/log4perl.conf');
--or--
Log::Log4perl::init_and_watch('/etc/log4perl.conf',10);
--then--
$logger = Log::Log4perl->get_logger('house.bedrm.desk.topdrwr');
$logger->debug('this is a debug message');
$logger->info('this is an info message');
$logger->warn('etc');
$logger->error('..');
$logger->fatal('..');
#####/etc/log4perl.conf###################
log4perl.logger.house = WARN, FileAppndr1
log4perl.logger.house.bedroom.desk = DEBUG, FileAppndr1
log4perl.appender.FileAppndr1 = Log::Log4perl::Appender::File
log4perl.appender.FileAppndr1.filename = desk.log
log4perl.appender.FileAppndr1.layout = \
Log::Log4perl::Layout::SimpleLayout
###########################################
=head1 ABSTRACT
Log::Log4perl provides a powerful logging API to your application
Log::Log4perl lets you remote-control and fine-tune the logging behaviour of your system from the outside. It implements the widely popular (Java-based) Log4j logging package in pure Perl.
For a detailed tutorial on Log::Log4perl usage, please read [1].
A WORD OF CAUTION: Log::Log4perl IS STILL UNDER DEVELOPMENT. WE WILL ALWAYS MAKE SURE THE TEST SUITE (approx. 400 CASES) WILL PASS, BUT THERE MIGHT STILL BE BUGS. PLEASE CHECK http://log4perl.sourceforge.net REGULARILY FOR THE LATEST RELEASE. THE API HAS REACHED A MATURE STATE, WE WILL NOT CHANGE IT UNLESS FOR A GOOD REASON.
Logging beats a debugger if you want to know what's going on in your code during runtime. However, traditional logging packages are too static and generate a flood of log messages in your log files that won't help you.
Log::Log4perl is different. It allows you to control the amount of
logging messages generated at three different levels:
This is a very powerful and flexible mechanism. You can turn on and off your logs at any time, specify the level of detail and make that dependent on the subsystem that's currently executed.
Let me give you an example: You might
find out that your system has a problem in the
MySystem::Helpers::ScanDir
component. Turning on detailed debugging logs all over the system would
generate a flood of useless log messages and bog your system down beyond
recognition. With Log::Log4perl, however, you can tell the system:
``Continue to log only severe errors in the log file. Open a second
log file, turn on full debug logs in the MySystem::Helpers::ScanDir
component and dump all messages originating from there into the new
log file''. And all this is possible by just changing the parameters
in a configuration file, which your system can re-read even
while it's running!
The Log::Log4perl package can be initialized in two ways: Either
via Perl commands or via a log4j-style configuration file.
This is the easiest way to prepare your system for using
Log::Log4perl. Use a configuration file like this:
############################################################
# A simple root logger with a Log::Log4perl::Appender::File
# file appender in Perl.
# Mike Schilli 2002 m@perlmeister.com
############################################################
log4perl.rootLogger=ERROR, LOGFILE
log4perl.appender.LOGFILE=Log::Log4perl::Appender::File
log4perl.appender.LOGFILE.filename=/var/log/myerrs.log
log4perl.appender.LOGFILE.mode=append
log4perl.appender.LOGFILE.layout=PatternLayout
log4perl.appender.LOGFILE.layout.ConversionPattern=[%r] %F %L %c - %m%n
These lines define your standard logger that's appending severe
errors to /var/log/myerrs.log, using the format
[millisecs] source-filename line-number class - message newline
Check Configuration files for more details on how to control your loggers using a configuration file.
Assuming that this file is saved as log.conf, you need to
read it in in the startup section of your code, using the following
commands:
use Log::Log4perl;
Log::Log4perl->init("log.conf");
After that's done somewhere in the code, you can retrieve logger objects anywhere in the code. Note that there's no need to carry any logger references around with your functions and methods. You can get a logger anytime via a singleton mechanism:
package My::MegaPackage;
sub some_method {
my($param) = @_;
use Log::Log4perl;
my $log = Log::Log4perl->get_logger("My::MegaPackage");
$log->debug("Debug message");
$log->info("Info message");
$log->error("Error message");
...
}
With the configuration file above, Log::Log4perl will write
``Error message'' to the specified log file, but won't do anything for
the debug() and info() calls, because the log level has been set
to ERROR for all components in the first line of
configuration file shown above.
Why Log::Log4perl->get_logger and
not Log::Log4perl->new? We don't want to create a new
object every time. Usually in OO-Programming, you create an object
once and use the reference to it to call its methods. However,
this requires that you pass around the object to all functions
and the last thing we want is pollute each and every function/method
we're using with a handle to the Logger:
sub function { # Brrrr!!
my($logger, $some, $other, $parameters) = @_;
}
Instead, if a function/method wants a reference to the logger, it
just calls the Logger's static get_logger() method to obtain
a reference to the one and only possible logger object of
a certain category.
That's called a singleton if you're a Gamma fan.
How does the logger know
which messages it is supposed to log and which ones to suppress?
Log::Log4perl works with inheritence: The config file above didn't
specify anything about My::MegaPackage.
And yet, we've defined a logger of the category
My::MegaPackage.
In this case, Log::Log4perl will walk up the class hierarchy
(My and then the we're at the root) to figure out if a log level is
defined somewhere. In the case above, the log level at the root
(root always defines a log level, but not necessary an appender)
defines that
the log level is supposed to be ERROR -- meaning that debug
and info messages are suppressed.
Initializing the logger can certainly also be done from within Perl.
At last, this is what Log::Log4perl::Config does behind the scenes.
At the Perl level, we can specify exactly, which loggers work with
which appenders and which layouts.
Here's the code for a root logger which sends error and
higher prioritized messages to the /tmp/my.log logfile:
# Initialize the logger
use Log::Log4perl qw(:levels); use Log::Log4perl::Appender::Screen; use Log::Log4perl::Appender;
my $app = Log::Log4perl::Appender->new("Log::Log4perl::Appender::Screen");
my $layout = Log::Log4perl::Layout::PatternLayout
->new("%d> %F %L %m %n");
$app->layout($layout);
my $logger = Log::Log4perl->get_logger("My.Component");
$logger->level($INFO);
$logger->add_appender($app);
And after this, we can, again, start logging anywhere in the system like this (remember, we don't want to pass around references, so we just get the logger via the singleton-mechanism):
# Use the logger
use Log::Log4perl;
my $log = Log::Log4perl->get_logger("My::Component");
$log->debug("Debug Message"); # Suppressed
$log->info("Info Message"); # Printed
$log->error("Error Message"); # Printed
There's five predefined log levels: FATAL, ERROR, WARN, INFO
and DEBUG (in descending priority). Your configured logging level
has to at least match the priority of the logging message.
If your configured logging level is WARN, then messages logged
with info() and debug() message will be suppressed.
fatal(), error() and warn() will make their way through,
because their priority is higher or equal than the configured setting.
Instead of calling the methods
$logger->debug("..."); # Log a debug message
$logger->info("..."); # Log a info message
$logger->warn("..."); # Log a warn message
$logger->error("..."); # Log a error message
$logger->fatal("..."); # Log a fatal message
you could also call the log() method with the appropriate level
using the constants defined in Log::Log4perl::Level:
use Log::Log4perl::Level;
$logger->log($DEBUG, "...");
$logger->log($INFO, "...");
$logger->log($WARN, "...");
$logger->log($ERROR, "...");
$logger->log($FATAL, "...");
But nobody does that, really. Neither does anyone need more logging levels than these predefined ones. If you think you do, I would suggest you look into steering your logging behaviour via the category mechanism.
If you need to find out if the currently configured logging
level would allow a logger's logging statement to go through, use the
logger's is_level() methods:
$logger->is_debug() # True if debug messages would go through
$logger->is_info() # True if info messages would go through
$logger->is_warn() # True if warn messages would go through
$logger->is_error() # True if error messages would go through
$logger->is_fatal() # True if fatal messages would go through
Example: $logger->is_warn() returns true if the logger's current
level, as derived from either the logger's category (or, in absence of
that, one of the logger's parent's level setting) is
$WARN, $ERROR or $FATAL.
These level checking functions will come in handy later, when we want to block unnecessary expensive parameter construction in case the logging level is too low to log the statement anyway, like in:
if($logger->is_error()) {
$logger->error("Erroneous array: @super_long_array");
}
If we had just written
$logger->error("Erroneous array: @super_long_array");
then Perl would have interpolated
@super_long_array into the string via an expensive operation
only to figure out shortly after that the string can be ignored
entirely because the configured logging level is lower than $ERROR.
The to-be-logged message passed to all of the functions described above can consist of an arbitrary number of arguments, which the logging functions just chain together to a single string. Therefore
$logger->debug("Hello ", "World", "!"); # and
$logger->debug("Hello World!");
are identical.
Often, when you croak / carp / warn / die, you want to log those messages. Rather than doing the following:
$logger->fatal($err) && die($err);
you can use the following:
$logger->logwarn();
$logger->logdie();
These print out log messages in the WARN and FATAL level, respectively,
and then call the built-in warn() and die() functions. Since there is
an ERROR level between WARN and FATAL, there are two additional helper
functions in case you'd like to use ERROR for either warn() or die():
$logger->error_warn();
$logger->error_die();
Finally, there's the Carp functions that do just what the Carp functions do, but with logging:
$logger->logcarp(); # warn w/ 1-level stack trace
$logger->logcluck(); # warn w/ full stack trace
$logger->logcroak(); # die w/ 1-level stack trace
$logger->logconfess(); # die w/ full stack trace
If you don't define any appenders, nothing will happen. Appenders will be triggered whenever the configured logging level requires a message to be logged and not suppressed.
Log::Log4perl doesn't define any appenders by default, not even the root
logger has one.
Log::Log4perl already comes with a standard set of appenders:
Log::Log4perl::Appender::Screen
Log::Log4perl::Appender::File
Log::Log4perl::Appender::DBI
to log to the screen, to files and to databases.
Log::Log4perl also supports Dave Rolskys excellent Log::Dispatch
framework which implements a wide variety of different appenders.
Here's the list of appender modules currently available via Log::Dispatch:
Log::Log4perl::Appender::DBI (by Kevin Goess)
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)
Please note that in order to use any of these additional appenders, you have to fetch Log::Dispatch from CPAN and install it. Also the particular appender you're using might require installing the particular module.
For additional information on appenders, please check the the Log::Log4perl::Appender manpage manual page.
Now let's assume that we want to log info() or
higher prioritized messages in the My::Category class
to both STDOUT and to a log file, say /tmp/my.log.
In the initialisation section of your system,
just define two appenders using the readily available
Log::Log4perl::Appender::File and Log::Log4perl::Appender::Screen
modules via the Log::Log4perl::Appender wrapper:
######################## # Initialisation section ######################## use Log::Log4perl; use Log::Log4perl::Layout; use Log::Log4perl::Level;
# Define a category logger
my $log = Log::Log4perl->get_logger("My::Category");
# Define a layout
my $layout = Log::Log4perl::Layout::PatternLayout->new("[%r] %F %L %m%n");
# Define a file appender
my $file_appender = Log::Log4perl::Appender->new(
"Log::Log4perl::Appender::File",
name => "filelog",
filename => "/tmp/my.log");
# Define a stdout appender
my $stdout_appender = Log::Log4perl::Appender->new(
"Log::Log4perl::Appender::Screen",
name => "screenlog",
stderr => 0);
# Have both appenders use the same layout (could be different)
$stdout_appender->layout($layout);
$file_appender->layout($layout);
$log->add_appender($stdout_appender); $log->add_appender($file_appender); $log->level($INFO);
Please note the class of the appender object is passed as a string to
Log::Log4perl::Appender in the first argument. Behind the scenes,
Log::Log4perl::Appender will create the necessary
Log::Log4perl::Appender::* (or Log::Dispatch::*) object and pass
along the name value pairs we provided to
Log::Log4perl::Appender->new() after the first argument.
The name value is optional and if you don't provide one,
Log::Log4perl::Appender->new() will create a unique one for you.
The names and values of additional parameters are dependent on the requirements
of the particular appender class and can be looked up in their
manual pages.
A side note: In case you're wondering if
Log::Log4perl::Appender->new() will also take care of the
min_level argument to the Log::Dispatch::* constructors called
behind the scenes -- yes, it does. This is because we want the
Log::Dispatch objects to blindly log everything we send them
(debug is their lowest setting) because we in Log::Log4perl
want to call the shots and decide on when and what to log.
The call to the appender's layout() method specifies the format (as a
previously created Log::Log4perl::PatternLayout object) in which the
message is being logged in the specified appender. The format shown
above is logging not only the message but also the number of
milliseconds since the program has started (%r), the name of the file
the call to the logger has happened and the line number there (%F and
%L), the message itself (%m) and a OS-specific newline character (%n).
For more detailed info on layout formats, see Log Layouts. If you
don't specify a layout, the logger will fall back to
Log::Log4perl::SimpleLayout, which logs the debug level, a hyphen (-)
and the log message.
Once the initialisation shown above has happened once, typically in
the startup code of your system, just use this logger anywhere in
your system (or better yet, only in My::Category, since we
defined it this way) as often as you like:
##########################
# ... in some function ...
##########################
my $log = Log::Log4perl->get_logger("My::Category");
$log->info("This is an informational message");
Above, we chose to define a category logger (My::Category)
in a specific way. This will cause only messages originating from
this specific category logger to be logged in the defined format
and locations.
Instead, we could have configured the root logger with the appenders and layout shown above. Now
##########################
# ... in some function ...
##########################
my $log = Log::Log4perl->get_logger("My::Category");
$log->info("This is an informational message");
will trigger a logger with no layout or appenders or even a level
defined. This logger, however, will inherit the level from categories up
the hierarchy -- ultimately the root logger, since there's no My
logger. Once it detects that it needs to log a message, it will first
try to find its own appenders (which it doesn't have any of) and then
walk up the hierarchy (first My, then root) to call any appenders
defined there.
This will result in exactly the same behaviour as shown above -- with the exception that other category loggers could also use the root logger's appenders and layouts, but could certainly define their own categories and levels.
Log4perl doesn't only allow you to selectively switch on a category
of log messages, you can also use the mechanism to selectively disable
logging in certain components whereas logging is kept turned on in higher-level
categories. This mechanism comes in handy if you find that while bumping
up the logging level of a high-level (i. e. close to root) category,
that one component logs more than it should,
Here's how it works:
############################################################
# Turn off logging in a lower-level category while keeping
# it active in higher-level categories.
############################################################
log4perl.rootLogger=debug, LOGFILE
log4perl.logger.deep.down.the.hierarchy = error, LOGFILE
# ... Define appenders ...
This way, log messages issued from within
Deep::Down::The::Hierarchy and below will be
logged only if they're error or worse, while in all other system components
even debug messages will be logged.
As shown above, you can define Log::Log4perl loggers both from within
your Perl code or from configuration files. The latter have the unbeatable
advantage that you can modify your system's logging behaviour without
interfering with the code at all. So even if your code is being run by
somebody who's totally oblivious to Perl, they still can adapt the
module's logging behaviour to their needs.
Log::Log4perl has been designed to understand Log4j configuration
files -- as used by the original Java implementation. Instead of
reiterating the format description in [2], let me just list three
examples (also derived from [2]), which should also illustrate
how it works:
log4j.rootLogger=DEBUG, A1
log4j.appender.A1=org.apache.log4j.ConsoleAppender
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%-4r %-5p %c %x - %m%n
This enables messages of priority debug or higher in the root
hierarchy and has the system write them to the console.
ConsoleAppender is a Java appender, but Log::Log4perl jumps
through a significant number of hoops internally to map these to their
corresponding Perl classes, Log::Log4perl::Appender::Screen in this case.
Second example:
log4perl.rootLogger=DEBUG, A1
log4perl.appender.A1=Log::Log4perl::Appender::Screen
log4perl.appender.A1.layout=PatternLayout
log4perl.appender.A1.layout.ConversionPattern=%d %-5p %c - %m%n
log4perl.logger.com.foo=WARN
This defines two loggers: The root logger and the com.foo logger.
The root logger is easily triggered by debug-messages,
but the com.foo logger makes sure that messages issued within
the Com::Foo component and below are only forwarded to the appender
if they're of priority warning or higher.
Note that the com.foo logger doesn't define an appender. Therefore,
it will just propagate the message up the hierarchy until the root logger
picks it up and forwards it to the one and only appender of the root
category, using the format defined for it.
Third example:
log4j.rootLogger=debug, stdout, R
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p (%F:%L) - %m%n
log4j.appender.R=org.apache.log4j.RollingFileAppender
log4j.appender.R.File=example.log
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%p %c - %m%n
The root logger defines two appenders here: stdout, which uses
org.apache.log4j.ConsoleAppender (ultimately mapped by Log::Log4perl
to Log::Log4perl::Appender::Screen) to write to the screen. And
R, a org.apache.log4j.RollingFileAppender
(mapped by Log::Log4perl to
Log::Dispatch::FileRotate with the File attribute specifying the
log file.
See the Log::Log4perl::Config manpage for more examples and syntax explanations.
If the logging engine passes a message to an appender, because it thinks it should be logged, the appender doesn't just write it out haphazardly. There's ways to tell the appender how to format the message and add all sorts of interesting data to it: The date and time when the event happened, the file, the line number, the debug level of the logger and others.
There's currently two layouts defined in Log::Log4perl:
Log::Log4perl::Layout::SimpleLayout and
Log::Log4perl::Layout::Patternlayout:
Log::Log4perl::SimpleLayout"$level - $message, for example "FATAL - Can't open password file".
Log::Log4perl::PatternLayoutprintf-style. The format
string can contain a number of placeholders which will be
replaced by the logging engine when it's time to log the message:
%c Category of the logging event.
%C Fully qualified package (or class) name of the caller
%d Current date in yyyy/MM/dd hh:mm:ss format
%F File where the logging event occurred
%H Hostname
%l Fully qualified name of the calling method followed by the
callers source the file name and line number between
parentheses.
%L Line number within the file where the log statement was issued
%m The message to be logged
%M Method or function where the logging request was issued
%n Newline (OS-independent)
%p Priority of the logging event
%P pid of the current process
%r Number of milliseconds elapsed from program start to logging
event
%x The elements of the NDC stack (see below)
%X{key} The entry 'key' of the MDC (see below)
%% A literal percent (%) sign
NDC and MDC are explained in Nested Diagnostic Context (NDC) and Mapped Diagnostic Context (MDC).
Also, %d can be fine-tuned to display only certain characteristics
of a date, according to the SimpleDateFormat in the Java World
(http://java.sun.com/j2se/1.3/docs/api/java/text/SimpleDateFormat.html)
In this way, %d{HH:mm} displays only hours and minutes of the current date,
while %d{yy, EEEE} displays a two-digit year, followed by a spelled-out
(like Wednesday).
Similar options are available for shrinking the displayed category or
limit file/path components, %F{1} only displays the source file name
without any path components while %F logs the full path. %c{2} only
logs the last two components of the current category, Foo::Bar::Baz
becomes Bar::Baz and saves space.
If those placeholders aren't enough, then you can define your own right in the config file like this:
log4perl.PatternLayout.cspec.U = sub { return "UID $<" }
See the Log::Log4perl::Layout::PatternLayout manpage for further details on customized specifiers.
Please note that the subroutines you're defining in this way are going
to be run in the main namespace, so be sure to fully qualify functions
and variables if they're located in different packages.
SECURITY NOTE: this feature means arbitrary perl code can be embedded in the config file. In the rare case where the people who have access to your config file are different from the people who write your code and shouldn't have execute rights, you might want to call
Log::Log4perl::Config->allow_code(0);
before you call init(). Alternatively you can supply a restricted set of Perl opcodes that can be embedded in the config file as described in Restricting what Opcodes can be in a Perl Hook.
All placeholders are quantifiable, just like in printf. Following this
tradition, %-20c will reserve 20 chars for the category and left-justify it.
Layouts are objects, here's how you create them:
# Create a simple layout
my $simple = Log::Log4perl::SimpleLayout();
# create a flexible layout:
# ("yyyy/MM/dd hh:mm:ss (file:lineno)> message\n")
my $pattern = Log::Log4perl::PatternLayout("%d (%F:%L)> %m%n");
Every appender has exactly one layout assigned to it. You assign
the layout to the appender using the appender's layout() object:
my $app = Log::Log4perl::Appender->new(
"Log::Log4perl::Appender::Screen",
name => "screenlog",
stderr => 0);
# Assign the previously defined flexible layout
$app->layout($pattern);
# Add the appender to a previously defined logger
$logger->add_appender($app);
# ... and you're good to go!
$logger->debug("Blah");
# => "2002/07/10 23:55:35 (test.pl:207)> Blah\n"
If you don't specify a layout for an appender, the logger will fall back
to SimpleLayout.
For more details on logging and how to use the flexible and the simple
format, check out the original log4j website under
http://jakarta.apache.org/log4j/docs/api/org/apache/log4j/SimpleLayout.html
http://jakarta.apache.org/log4j/docs/api/org/apache/log4j/PatternLayout.html
Logging comes with a price tag. Log::Log4perl is currently being optimized
to allow for maximum performance, both with logging enabled and disabled.
But you need to be aware that there's a small hit every time your code
encounters a log statement -- no matter if logging is enabled or not.
Log::Log4perl has been designed to keep this so low that it will
be unnoticable to most applications.
Here's a couple of tricks which help Log::Log4perl to avoid
unnecessary delays:
You can save serious time if you're logging something like
# Expensive in non-debug mode!
for (@super_long_array) {
$logger->debug("Element: $_\n");
}
and @super_long_array is fairly big, so looping through it is pretty
expensive. Only you, the programmer, knows that going through that for
loop can be skipped entirely if the current logging level for the
actual component is higher than debug.
In this case, use this instead:
# Cheap in non-debug mode!
if($logger->is_debug()) {
for (@super_long_array) {
$logger->debug("Element: $_\n");
}
}
If you're afraid that the way you're generating the parameters to the of the logging function is fairly expensive, use closures:
# Passed as subroutine ref
use Data::Dumper;
$logger->debug(sub { Dumper($data) } );
This won't unravel $data via Dumper() unless it's actually needed
because it's logged.
Also, Log::Log4perl lets you specify arguments to logger functions in message output filter syntax:
$logger->debug("Structure: ",
{ filter => \&Dumper,
value => $someref });
In this way, shortly before Log::Log4perl sending the message out to any appenders, it will be searching all arguments for hash references and treat them in a special way:
It will invoke the function given as a reference with the filter key
(Data::Dumper::Dumper()) and pass it the value that came with
the key named value as an argument.
The anonymous hash in the call above will be replaced by the return
value of the filter function.
Log::Log4perl uses categories to determine if a log statement in
a component should be executed or suppressed at the current logging level.
Most of the time, these categories are just the classes the log statements
are located in:
package Candy::Twix;
sub new {
my $logger = Log::Log4perl->new("Candy::Twix");
$logger->debug("Creating a new Twix bar");
bless {}, shift;
}
# ...
package Candy::Snickers;
sub new {
my $logger = Log::Log4perl->new("Candy.Snickers");
$logger->debug("Creating a new Snickers bar");
bless {}, shift;
}
# ...
package main;
Log::Log4perl->init("mylogdefs.conf") or
die "Whoa, cannot read mylogdefs.conf!";
# => "LOG> Creating a new Snickers bar"
my $first = Candy::Snickers->new();
# => "LOG> Creating a new Twix bar"
my $second = Candy::Twix->new();
Note that you can separate your category hierarchy levels using either dots like in Java (.) or double-colons (::) like in Perl. Both notations are equivalent and are handled the same way internally.
However, categories are just there to make use of inheritance: if you invoke a logger in a sub-category, it will bubble up the hierarchy and call the appropriate appenders. Internally, categories are not related to the class hierarchy of the program at all -- they're purely virtual. You can use arbitrary categories -- for example in the following program, which isn't oo-style, but procedural:
sub print_portfolio {
my $log = Log::Log4perl->new("user.portfolio");
$log->debug("Quotes requested: @_");
for(@_) {
print "$_: ", get_quote($_), "\n";
}
}
sub get_quote {
my $log = Log::Log4perl->new("internet.quotesystem");
$log->debug("Fetching quote: $_[0]");
return yahoo_quote($_[0]);
}
The logger in first function, print_portfolio, is assigned the
(virtual) user.portfolio category. Depending on the Log4perl
configuration, this will either call a user.portfolio appender,
a user appender, or an appender assigned to root -- without
user.portfolio having any relevance to the class system used in
the program.
The logger in the second function adheres to the
internet.quotesystem category -- again, maybe because it's bundled
with other Internet functions, but not because there would be
a class of this name somewhere.
However, be careful, don't go overboard: if you're developing a system in object-oriented style, using the class hierarchy is usually your best choice. Think about the people taking over your code one day: The class hierarchy is probably what they know right up front, so it's easy for them to tune the logging to their needs.
All logging methods return values indicating if their message
actually reached one or more appenders. If the message has been
suppressed because of level constraints, undef is returned.
For example,
my $ret = $logger->info("Message");
will return undef if the system debug level for the current category
is not INFO or more permissive.
If Log::Log4perl
forwarded the message to one or more appenders, the number of appenders
is returned.
If appenders decide to veto on the message with an appender threshold,
the log method's return value will have them excluded. This means that if
you've got one appender holding an appender threshold and you're
logging a message
which passes the system's log level hurdle but not the appender threshold,
0 will be returned by the log function.
The bottom line is: Logging functions will return a true value if the message made it through to one or more appenders and a false value if it didn't. This allows for constructs like
$logger->fatal("@_") or print STDERR "@_\n";
which will ensure that the fatal message isn't lost if the current level is lower than FATAL or printed twice if the level is acceptable but an appender already points to STDERR.
Be careful with just blindly reusing the system's packages as categories. If you do, you'll get into trouble with inherited methods. Imagine the following class setup:
use Log::Log4perl;
###########################################
package Bar;
###########################################
sub new {
my($class) = @_;
my $logger = Log::Log4perl::get_logger(__PACKAGE__);
$logger->debug("Creating instance");
bless {}, $class;
}
###########################################
package Bar::Twix;
###########################################
our @ISA = qw(Bar);
###########################################
package main;
###########################################
Log::Log4perl->init(\ qq{
log4perl.category.Bar.Twix = DEBUG, Screen
log4perl.appender.Screen = Log::Log4perl::Appender::Screen
log4perl.appender.Screen.layout = SimpleLayout
});
my $bar = Bar::Twix->new();
Bar::Twix just inherits everything from Bar, including the constructor
new().
Contrary to what you might be thinking at first, this won't log anything.
Reason for this is the get_logger() call in package Bar, which
will always get a logger of the Bar category, even if we call new() via
the Bar::Twix package, which will make perl go up the inheritance
tree to actually execute Bar::new(). Since we've only defined logging
behaviour for Bar::Twix in the configuration file, nothing will happen.
This can be fixed by changing the get_logger() method in Bar::new()
to obtain a logger of the category matching the
actual class of the object, like in
# ... in Bar::new() ...
my $logger = Log::Log4perl::get_logger($class);
This way, you'll make sure the logger logs appropriately,
no matter if the method is inherited or called directly.
new() always gets the
real class name as an argument and all other methods can determine it
via ref($self)), so it shouldn't be a problem to get the right class
every time.
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.
Check out the Log::Log4perl::Filter manpage for detailed instructions on how to use them.
When getting an instance of a logger, instead of saying
use Log::Log4perl;
my $logger = Log::Log4perl->get_logger();
it's often more convenient to import the get_logger method from
Log::Log4perl into the current namespace:
use Log::Log4perl qw(get_logger);
my $logger = get_logger();
Instead of having init() read in a configuration file, you can
also pass in a reference to a string, containing the content of
the file:
Log::Log4perl->init( \$config_text );
Also, if you've got the name=value pairs of the configuration in
a hash, you can just as well initialized Log::Log4perl with
a reference to it:
my %key_value_pairs = (
"log4perl.rootLogger" => "error, LOGFILE",
"log4perl.appender.LOGFILE" => "Log::Log4perl::Appender::File",
...
);
Log::Log4perl->init( \%key_value_pairs );
Or also you can use a URL, see below:
(This section borrowed from XML::DOM::Parser by T.J. Mather).
The init() function now also supports URLs, e.g. http://www.erols.com/enno/xsa.xml.
It uses LWP to download the file and then calls parse() on the resulting string.
By default it will use a the LWP::UserAgent manpage that is created as follows:
use LWP::UserAgent; $LWP_USER_AGENT = LWP::UserAgent->new; $LWP_USER_AGENT->env_proxy;
Note that env_proxy reads proxy settings from environment variables, which is what I need to do to get thru our firewall. If you want to use a different LWP::UserAgent, you can set it with
Log::Log4perl::Config::set_LWP_UserAgent($my_agent);
Currently, LWP is used when the filename (passed to parsefile) starts with one of the following URL schemes: http, https, ftp, wais, gopher, or file (followed by a colon.)
Don't use this feature with init_and_watch().
If some of the values used in the Log4perl configuration file need to be dynamically modified by the program, use Perl hooks:
log4perl.appender.File.filename = \
sub { return getLogfileName(); }
Each value starting with the string sub {... is interpreted as Perl code to
be executed at the time the application parses the configuration
via Log::Log4perl::init(). The return value of the subroutine
is used by Log::Log4perl as the configuration value.
The Perl code is executed in the main package, functions in
other packages have to be called in fully-qualified notation.
Here's another example, utilizing an environment variable as a username for a DBI appender:
log4perl.appender.DB.username = \
sub { $ENV{DB_USER_NAME } }
However, please note the difference between these code snippets and those
used for user-defined conversion specifiers as discussed in
the Log::Log4perl::PatternLayout manpage: While the snippets above are run once
when Log::Log4perl::init() is called, the conversion specifier
snippets are executed each time a message is rendered according to
the PatternLayout.
SECURITY NOTE: this feature means arbitrary perl code can be embedded in the config file. In the rare case where the people who have access to your config file are different from the people who write your code and shouldn't have execute rights, you might want to set
Log::Log4perl::Config->allow_code(0);
before you call init(). Alternatively you can supply a restricted set of Perl opcodes that can be embedded in the config file as described in Restricting what Opcodes can be in a Perl Hook.
The value you pass to Log::Log4perl::Config->allow_code() determines whether
the code that is embedded in the config file is eval'd unrestricted, or
eval'd in a Safe compartment. By default, a value of '1' is assumed,
which does a normal 'eval' without any restrictions. A value of '0'
however prevents any embedded code from being evaluated.
If you would like fine-grained control over what can and cannot be included in embedded code, then please utilize the following methods:
Log::Log4perl::Config->allow_code( $allow ); Log::Log4perl::Config->allowed_code_ops($op1, $op2, ... ); Log::Log4perl::Config->vars_shared_with_safe_compartment( [ \%vars | $package, \@vars ] ); Log::Log4perl::Config->allowed_code_ops_convenience_map( [ \%map | $name, \@mask ] );
Log::Log4perl::Config->allowed_code_ops() takes a list of opcode masks
that are allowed to run in the compartment. The opcode masks must be
specified as described in Opcode:
Log::Log4perl::Config->allowed_code_ops(':subprocess');
This example would allow Perl operations like backticks, system, fork, and
waitpid to be executed in the compartment. Of course, you probably don't
want to use this mask -- it would allow exactly what the Safe compartment is
designed to prevent.
Log::Log4perl::Config->vars_shared_with_safe_compartment()
takes the symbols which
should be exported into the Safe compartment before the code is evaluated.
The keys of this hash are the package names that the symbols are in, and the
values are array references to the literal symbol names. For convenience,
the default settings export the '%ENV' hash from the 'main' package into the
compartment:
Log::Log4perl::Config->vars_shared_with_safe_compartment( main => [ '%ENV' ], );
Log::Log4perl::Config->allowed_code_ops_convenience_map() is an accessor
method to a map of convenience names to opcode masks. At present, the
following convenience names are defined:
safe = [ ':browse' ] restrictive = [ ':default' ] For convenience, if Log::Log4perl::Config-E<gt>allow_code() is called with a value which is a key of the map previously defined with Log::Log4perl::Config-E<gt>allowed_code_ops_convenience_map(), then the allowed opcodes are set according to the value defined in the map. If this is confusing, consider the following:
use Log::Log4perl;
my $config = <<'END';
log4perl.logger = INFO, Main
log4perl.appender.Main = Log::Log4perl::Appender::File
log4perl.appender.Main.filename = \
sub { "example" . getpwuid($<) . ".log" }
log4perl.appender.Main.layout = Log::Log4perl::Layout::SimpleLayout
END
$Log::Log4perl::Config->allow_code('restrictive');
Log::Log4perl->init( \$config ); # will fail
$Log::Log4perl::Config->allow_code('safe');
Log::Log4perl->init( \$config ); # will succeed
The reason that the first call to ->init() fails is because the
'restrictive' name maps to an opcode mask of ':default'. getpwuid() is not
part of ':default', so ->init() fails. The 'safe' name maps to an opcode
mask of ':browse', which allows getpwuid() to run, so ->init() succeeds.
allowed_code_ops_convenience_map() can be invoked in several ways:
allowed_code_ops_convenience_map()as can vars_shared_with_safe_compartment():
vars_shared_with_safe_compartment()For more information on opcodes and Safe Compartments, see Opcode and Safe.
Log4perl provides some internal functions for quickly adjusting the log level from within a running Perl program.
Now, some people might argue that you should adjust your levels from within an external Log4perl configuration file, but Log4perl is everybody's darling.
Typically run-time adjusting of levels is done at the beginning, or in response to some external input (like a ``more logging'' runtime command for diagnostics).
To increase the level of logging currently being done, use:
$logger->more_logging($delta);
and to decrease it, use:
$logger->less_logging($delta);
$delta must be a positive integer (for now, we may fix this later ;).
There are also two equivalent functions:
$logger->inc_level($delta);
$logger->dec_level($delta);
They're included to allow you a choice in readability. Some folks
will prefer more/less_logging, as they're fairly clear in what they
do, and allow the programmer not to worry too much about what a Level
is and whether a higher Level means more or less logging. However,
other folks who do understand and have lots of code that deals with
levels will probably prefer the inc_level() and dec_level() methods as
they want to work with Levels and not worry about whether that means
more or less logging. :)
That diatribe aside, typically you'll use more_logging() or inc_level()
as such:
my $v = 0; # default level of verbosity.
GetOptions("v+" => \$v, ...);
$logger->more_logging($v); # inc logging level once for each -v in ARGV
First off, let me tell you that creating custom levels is heavily deprecated by the log4j folks. Indeed, instead of creating additional levels on top of the predefined DEBUG, INFO, WARN, ERROR and FATAL, you should use categories to control the amount of logging smartly, based on the location of the log-active code in the system.
Nevertheless,
Log4perl provides a nice way to create custom levels via the
create_custom_level() routine function. However, this must be done
before the first call to init() or get_logger(). Say you want to create
a NOTIFY logging level that comes after WARN (and thus before INFO).
You'd do such as follows:
use Log::Log4perl;
use Log::Log4perl::Level;
Log::Log4perl::Logger::create_custom_level("NOTIFY", "WARN");
And that's it! create_custom_level() creates the following functions /
variables for level FOO:
$FOO_INT # integer to use in toLevel()
$logger->foo() # log function to log if level = FOO
$logger->is_foo() # true if current level is >= FOO
These levels can also be used in your config file, but note that your config file probably won't be portable to another log4perl or log4j environment unless you've made the appropriate mods there too.
As a fairly drastic measure to decrease (or increase) the logging level
all over the system with one single configuration option, use the threshold
keyword in the Log4perl configuration file:
log4perl.threshold = ERROR
sets the system-wide (or hierarchy-wide according to the log4j documentation) to ERROR and therefore deprives every logger in the system of the right to log lower-prio messages.
For teaching purposes (especially for [1]), I've put :easy mode into
Log::Log4perl, which just initializes a single root logger with a
defined priority and a screen appender including some nice standard layout:
### Initialization Section
use Log::Log4perl qw(:easy);
Log::Log4perl->easy_init($ERROR); # Set priority of root logger to ERROR
### Application Section
my $logger = get_logger();
$logger->fatal("This will get logged.");
$logger->debug("This won't.");
This will dump something like
2002/08/04 11:43:09 ERROR> script.pl:16 main::function - This will get logged.
to the screen. While this has been proven to work well familiarizing people
with Log::Logperl slowly, effectively avoiding to clobber them over the
head with a
plethora of different knobs to fiddle with (categories, appenders, levels,
layout), the overall mission of Log::Log4perl is to let people use
categories right from the start to get used to the concept. So, let's keep
this one fairly hidden in the man page (congrats on reading this far :).
Sometimes, people are lazy. If you're whipping up a 50-line script and want the comfort of Log::Log4perl without having the burden of carrying a separate log4perl.conf file or a 5-liner defining that you want to append your log statements to a file, you can use the following features:
use Log::Log4perl qw(:easy);
Log::Log4perl->easy_init( { level => $DEBUG,
file => ">>test.log" } );
# Logs to test.log via stealth logger
DEBUG("Debug this!");
INFO("Info this!");
WARN("Warn this!");
ERROR("Error this!");
some_function();
sub some_function {
# Same here
FATAL("Fatal this!");
}
In :easy mode, Log::Log4perl will instantiate a stealth logger
named $_default_logger and import it into the current package. Also,
it will introduce the
convenience functions DEBUG(), INFO(), WARN(),
ERROR() and FATAL() into the package namespace,
which take arguments and forward them to _default_logger->debug(),
_default_logger->info() and so on.
The easy_init method can be called with a single level value to
create a STDERR appender and a root logger as in
Log::Log4perl->easy_init($DEBUG);
or, as shown below (and in the example above)
with a reference to a hash, specifying values
for level (the logger's priority), file (the appender's data sink),
category (the logger's category> and layout for the appender's
pattern layout specification.
All key-value pairs are optional, they
default to $DEBUG for level, STDERR for file,
"" (root category) for category and
%d %m%n for layout:
Log::Log4perl->easy_init( { level => $DEBUG,
file => ">test.log",
category => "Bar::Twix",
layout => '%F{1}-%L-%M: %m%n' } );
The file parameter takes file names preceded by ">"
(overwrite) and ">>" (append) as arguments. This will
cause Log::Log4perl::Appender::File appenders to be created behind
the scenes. Also the keywords STDOUT and STDERR (no > or
>>) are recognized, which will utilize and configure
Log::Log4perl::Appender::Screen appropriately.
The stealth loggers can be used in different packages, you just need to make
sure you're calling the ``use'' function in every package you're using
Log::Log4perl's easy services:
package Bar::Twix;
use Log::Log4perl qw(:easy);
sub eat { DEBUG("Twix mjam"); }
package Bar::Mars;
use Log::Log4perl qw(:easy);
sub eat { INFO("Mars mjam"); }
package main;
use Log::Log4perl qw(:easy);
Log::Log4perl->easy_init( { level => $DEBUG,
file => ">>test.log",
category => "Bar::Twix",
layout => '%F{1}-%L-%M: %m%n' },
{ level => $DEBUG,
file => "STDOUT",
category => "Bar::Mars",
layout => '%m%n' },
);
Bar::Twix::eat();
Bar::Mars::eat();
As shown above, easy_init() will take any number of different logger
definitions as hash references.
Also, stealth loggers feature the functions LOGWARN() and LOGDIE(),
combining a logging request with a subsequent Perl warn() or die()
statement. So, for example
if($all_is_lost) {
LOGDIE("Terrible Problem");
}
will log the message if the package's logger is at least FATAL but
die() (including the traditional output to STDERR) in any case afterwards.
See Log and die or warn for the similar logdie() and logwarn()
functions of regular (i.e non-stealth) loggers.
When using Log::Log4perl in easy mode, please make sure you understand the implications of Pitfalls with Categories.
By the way, these convenience functions perform exactly as fast as the standard Log::Log4perl logger methods, there's no performance penalty whatsoever.
If you find that your application could use a global (thread-specific) data stack which your loggers throughout the system have easy access to, use Nested Diagnostic Contexts (NDCs). Also check out Mapped Diagnostic Context (MDC), this might turn out to be even more useful.
For example, when handling a request of a web client, it's probably useful to have the user's IP address available in all log statements within code dealing with this particular request. Instead of passing this piece of data around between your application functions, you can just use the global (but thread-specific) NDC mechanism. It allows you to push data pieces (scalars usually) onto its stack via
Log::Log4perl::NDC->push("San");
Log::Log4perl::NDC->push("Francisco");
and have your loggers retrieve them again via the ``%x'' placeholder in the PatternLayout. With the stack values above and a PatternLayout format like ``%x %m%n'', the call
$logger->debug("rocks");
will end up as
San Francisco rocks
in the log appender.
The stack mechanism allows for nested structures. Just make sure that at the end of the request, you either decrease the stack one by one by calling
Log::Log4perl::NDC->pop();
Log::Log4perl::NDC->pop();
or clear out the entire NDC stack by calling
Log::Log4perl::NDC->remove();
Even if you should forget to do that, Log::Log4perl won't grow the stack
indefinitely, but limit it to a maximum, defined in Log::Log4perl::NDC
(currently 5). A call to push() on a full stack will just replace
the topmost element by the new value.
Again, the stack is always available via the ``%x'' placeholder in the Log::Log4perl::Layout::PatternLayout class whenever a logger fires. It will replace ``%x'' by the blank-separated list of the values on the stack. It does that by just calling
Log::Log4perl::NDC->get();
internally. See details on how this standard log4j feature is implemented in the Log::Log4perl::NDC manpage.
Just like the previously discussed NDC stores thread-specific information in a stack structure, the MDC implements a hash table to store key/value pairs in.
The static method
Log::Log4perl::MDC->put($key, $value);
stores $value under a key $key, with which it can be retrieved later
(possibly in a totally different part of the system) by calling
the get method:
my $value = Log::Log4perl::MDC->get($key);
If no value has been stored previously under $key, the get method
will return the string "[undef]" to allow for easy string interpolation
later on.
Typically, MDC values are retrieved later on via the "%X{...}" placeholder
in Log::Log4perl::Layout::PatternLayout.
For example, an application taking a web request might store the remote host
like
Log::Log4perl::MDC->put("remote_host", $r->headers("HOST"));
at its beginning and if the appender's layout looks something like
log4perl.appender.Logfile.layout.ConversionPattern = %X{remote_host}: %m%n
then a log statement like
DEBUG("Content delivered");
will log something like
adsl-63.dsl.snf.pacbell.net: Content delivered
later on in the program.
For details, please check the Log::Log4perl::MDC manpage.
Tatsuhiko Miyagawa's Log::Dispatch::Config is a very clever
simplified logger implementation, covering some of the log4j
functionality. Among the things that
Log::Log4perl can but Log::Dispatch::Config can't are:
Foo::Bar to a log file''. If you don't have appenders
thresholds, there's no way to prevent cluttering STDERR with DEBUG messages.
PatternLayout specifications in accordance with the standard
(e.g. ``%d{HH:mm}'').
Bottom line: Log::Dispatch::Config is fine for small systems with
simple logging requirements. However, if you're
designing a system with lots of subsystems which you need to control
independantly, you'll love the features of Log::Log4perl,
which is equally easy to use.
If you don't use Log::Log4perl as described above,
but from a wrapper class (like your own Logging class which in turn uses
Log::Log4perl),
the pattern layout will generate wrong data for %F, %C, %L and the like.
Reason for this is that Log::Log4perl's loggers assume a static
caller depth to the application that's using them. If you're using
one (or more) wrapper classes, Log::Log4perl will indicate where
your logger classes called the loggers, not where your application
called your wrapper, which is probably what you want in this case.
But don't dispair, there's a solution: Just increase the value
of $Log::Log4perl::caller_depth (defaults to 0) by one for every
wrapper that's in between your application and Log::Log4perl,
then Log::Log4perl will compensate for the difference.
A simple example to cut-and-paste and get started:
use Log::Log4perl qw(get_logger);
my $conf = q(
log4perl.category.Bar.Twix = WARN, Logfile
log4perl.appender.Logfile = Log::Log4perl::Appender::File
log4perl.appender.Logfile.filename = test.log
log4perl.appender.Logfile.layout = \
Log::Log4perl::Layout::PatternLayout
log4perl.appender.Logfile.layout.ConversionPattern = %d %F{1} %L> %m %n
);
Log::Log4perl::init(\$conf);
my $logger = get_logger("Bar::Twix");
$logger->error("Blah");
This will log something like
2002/09/19 23:48:15 t1 25> Blah
to the log file test.log, which Log4perl will append to or
create it if it doesn't exist already.
If you want to use external appenders provided with Log::Dispatch,
you need to install Log::Dispatch (2.00 or better) from CPAN,
which itself depends on Attribute-Handlers and
Params-Validate. And a lot of other modules, that's the reason
why we're now shipping Log::Log4perl with its own standard appenders
and only if you wish to use additional ones, you'll have to go through
the grueling Log::Dispatch installation process.
Log::Log4perl needs Test::Simple, Test::Harness and File::Spec,
but they already come with fairly recent versions of perl.
If not, everything's automatically fetched from CPAN if you're using the CPAN
shell (CPAN.pm), because they're listed as dependencies.
Time::HiRes (1.20 or better) is required only if you need the
fine-grained time stamps of the %r parameter in
Log::Log4perl::Layout::PatternLayout.
Manual installation works as usual with
perl Makefile.PL
make
make test
make install
If you're running Windows (98, 2000, NT, XP etc.), and you're too lazy to rummage through all of Log-Log4perl's dependencies, don't despair: We're providing a PPM package which installs easily with your Activestate Perl. Check how_can_i_install_log__log4perl_on_microsoft_windows in the Log::Log4perl::FAQ manpage for details.
Log::Log4perl is under heavy development. The latest CVS tarball
can be obtained from SourceForge, check http://log4perl.sourceforge.net
for details. Bug reports and feedback are always welcome, just email
to our mailing list shown in the AUTHORS section.
Log::Log4perl::Config, Log::Log4perl::Appender, Log::Log4perl::Layout::PatternLayout, Log::Log4perl::Layout::SimpleLayout, Log::Log4perl::Level, Log::Log4perl::JavaMap Log::Log4perl::NDC,
Please 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:
Chris R. Donnelly <cdonnelly@digitalmotorworks.com>
James FitzGibbon <james.fitzgibbon@target.com>
Paul Harrington <Paul-Harrington@deshaw.com>
Erik Selberg <erik@selberg.com>
Aaron Straup Cope <asc@vineyard.net>
Copyright 2002 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.
| Log::Log4perl - Log4j implementation for Perl |