NAME

Log::Log4perl - Log4j implementation for Perl

SYNOPSIS # Easy mode if you like it simple ...

use Log::Log4perl qw(:easy);
Log::Log4perl->easy_init($ERROR);

DEBUG "This doesn't go anywhere";
ERROR "This gets logged";

    # ... or standard mode for more features:

Log::Log4perl::init('/etc/log4perl.conf');

--or--

    # Check config every 10 secs
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
######################################################

ABSTRACT

Log::Log4perl provides a powerful logging API for your application

DESCRIPTION

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

http://www.perl.com/pub/a/2002/09/11/log4perl.html

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 number of logging messages generated at three different levels:

At a central location in your system (either in a configuration file or in the startup code) you specify which components (classes, functions) of your system should generate logs.
You specify how detailed the logging of these components should be by specifying logging levels.
You also specify which so-called appenders you want to feed your log messages to ("Print it to the screen and also append it to /tmp/my.log") and which format ("Write the date first, then the file name and line number, and then the log message") they should be in.

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 to 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!

How to use it

The Log::Log4perl package can be initialized in two ways: Either via Perl commands or via a log4j-style configuration file.

Initialize via a 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.
############################################################
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

Assuming that this configuration file is saved as log.conf, you need to read it 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;
use  Log::Log4perl;

sub some_method {
    my($param) = @_;

    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($category) 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 inheritance: 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 namespace hierarchy (My and then 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 necessarily an appender) defines that the log level is supposed to be ERROR -- meaning that DEBUG and INFO messages are suppressed. Note that this 'inheritance' is unrelated to Perl's class inheritance, it is merely related to the logger namespace. By the way, if you're ever in doubt about what a logger's category is, use $logger->category() to retrieve it.

Log Levels

There are six predefined log levels: FATAL, ERROR, WARN, INFO, DEBUG, and TRACE (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(), debug(), and trace() 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->trace("...");  # Log a trace message
$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;

$logger->log($TRACE, "...");
$logger->log($DEBUG, "...");
$logger->log($INFO, "...");
$logger->log($WARN, "...");
$logger->log($ERROR, "...");
$logger->log($FATAL, "...");

This form is rarely used, but it comes in handy if you want to log at different levels depending on an exit code of a function:

$logger->log( $exit_level{ $rc }, "...");

As for needing more logging levels than these predefined ones: It's usually best to steer your logging behaviour via the category mechanism instead.

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_trace()    # True if trace messages would go through
$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.

Also available are a series of more Java-esque functions which return the same values. These are of the format isLevelEnabled(), so $logger->isDebugEnabled() is synonymous to $logger->is_debug().

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.

Note that even if one of the methods above returns true, it doesn't necessarily mean that the message will actually get logged. What is_debug() checks is that the logger used is configured to let a message of the given priority (DEBUG) through. But after this check, Log4perl will eventually apply custom filters and forward the message to one or more appenders. None of this gets checked by is_xxx(), for the simple reason that it's impossible to know what a custom filter does with a message without having the actual message or what an appender does to a message without actually having it log it.

Log and die or warn

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:

$logger->logdie($err);

And if instead of using

warn($message);
$logger->warn($message);

to both issue a warning via Perl's warn() mechanism and make sure you have the same message in the log file as well, use:

$logger->logwarn($message);

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, in addition to logging, also pass the stringified message to their companions in the Carp package:

$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

Appenders

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::ScreenColoredLevels
Log::Log4perl::Appender::File
Log::Log4perl::Appender::Socket
Log::Log4perl::Appender::DBI
Log::Log4perl::Appender::Synchronized
Log::Log4perl::Appender::RRDs

to log to the screen, to files and to databases.

On CPAN, you can find additional appenders like

Log::Log4perl::Layout::XMLLayout

by Guido Carls <gcarls@cpan.org>. It allows for hooking up Log::Log4perl with the graphical Log Analyzer Chainsaw (see "Can I use Log::Log4perl with log4j's Chainsaw?" in Log::Log4perl::FAQ).

Additional Appenders via Log::Dispatch

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::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 Log::Log4perl::Appender manual page.

Appender Example

Now let's assume that we want to log info() or higher prioritized messages in the Foo::Bar category to both STDOUT and to a log file, say test.log. In the initialization section of your system, just define two appenders using the readily available Log::Log4perl::Appender::File and Log::Log4perl::Appender::Screen modules:

use Log::Log4perl;

   # Configuration in a string ...
my $conf = q(
  log4perl.category.Foo.Bar          = INFO, Logfile, Screen

  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 = [%r] %F %L %m%n

  log4perl.appender.Screen         = Log::Log4perl::Appender::Screen
  log4perl.appender.Screen.stderr  = 0
  log4perl.appender.Screen.layout = Log::Log4perl::Layout::SimpleLayout
);

   # ... passed as a reference to init()
Log::Log4perl::init( \$conf );

Once the initialization shown above has happened once, typically in the startup code of your system, just use the defined logger anywhere in your system:

##########################
# ... in some function ...
##########################
my $log = Log::Log4perl::get_logger("Foo::Bar");

  # Logs both to STDOUT and to the file test.log
$log->info("Important Info!");

The layout settings specified in the configuration section define the format in which the message is going to be logged by the specified appender. The format shown for the file appender 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):

[187] ./myscript.pl 27 Important Info!

The screen appender above, on the other hand, uses a SimpleLayout, which logs the debug level, a hyphen (-) and the log message:

INFO - Important Info!

For more detailed info on layout formats, see "Log Layouts".

In the configuration sample above, we chose to define a category logger (Foo::Bar). This will cause only messages originating from this specific category logger to be logged in the defined format and locations.

Logging newlines

There's some controversy between different logging systems as to when and where newlines are supposed to be added to logged messages.

The Log4perl way is that a logging statement should not contain a newline:

$logger->info("Some message");
$logger->info("Another message");

If this is supposed to end up in a log file like

Some message
Another message

then an appropriate appender layout like "%m%n" will take care of adding a newline at the end of each message to make sure every message is printed on its own line.

Other logging systems, Log::Dispatch in particular, recommend adding the newline to the log statement. This doesn't work well, however, if you, say, replace your file appender by a database appender, and all of a sudden those newlines scattered around the code don't make sense anymore.

Assigning matching layouts to different appenders and leaving newlines out of the code solves this problem. If you inherited code that has logging statements with newlines and want to make it work with Log4perl, read the Log::Log4perl::Layout::PatternLayout documentation on how to accomplish that.

Configuration files

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 Log::Log4perl::Config for more examples and syntax explanations.

Log Layouts

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

formats a message in a simple way and just prepends it by the debug level and a hyphen: "$level - $message, for example "FATAL - Can't open password file".

Log::Log4perl::Layout::PatternLayout

on the other hand is very powerful and allows for a very flexible format in printf-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 (if Sys::Hostname is available)
%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{chomp} The message to be logged, stripped off a trailing newline
%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
%R Number of milliseconds elapsed from last logging event to
   current logging event 
%T A stack trace of functions called
%x The topmost NDC (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 Log::Log4perl::Layout::PatternLayout 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.

For more details on logging and how to use the flexible and the simple format, check out the original log4j website under

SimpleLayout and PatternLayout

Penalties

Logging comes with a price tag. Log::Log4perl has been 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 unnoticeable 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: $_");
}

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: $_");
    }
}

If you're afraid that generating the parameters to 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.

Cool Tricks

Here's a collection of useful tricks for the advanced Log::Log4perl user. For more, check the FAQ, either in the distribution (Log::Log4perl::FAQ) or on http://log4perl.sourceforge.net.

Shortcuts

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();

Please note this difference: To obtain the root logger, please use get_logger(""), call it without parameters (get_logger()), you'll get the logger of a category named after the current package. get_logger() is equivalent to get_logger(__PACKAGE__).

Alternative initialization

Instead of having init() read in a configuration file by specifying a file name or passing it a reference to an open filehandle (Log::Log4perl->init( \*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 initialize 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:

Using LWP to parse URLs

(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 LWP::UserAgent 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().

Automatic reloading of changed configuration files

Instead of just statically initializing Log::Log4perl via

Log::Log4perl->init($conf_file);

there's a way to have Log::Log4perl periodically check for changes in the configuration and reload it if necessary:

Log::Log4perl->init_and_watch($conf_file, $delay);

In this mode, Log::Log4perl will examine the configuration file $conf_file every $delay seconds for changes via the file's last modification timestamp. If the file has been updated, it will be reloaded and replace the current Log::Log4perl configuration.

The way this works is that with every logger function called (debug(), is_debug(), etc.), Log::Log4perl will check if the delay interval has expired. If so, it will run a -M file check on the configuration file. If its timestamp has been modified, the current configuration will be dumped and new content of the file will be loaded.

This convenience comes at a price, though: Calling time() with every logging function call, especially the ones that are "suppressed" (!), will slow down these Log4perl calls by about 40%.

To alleviate this performance hit a bit, init_and_watch() can be configured to listen for a Unix signal to reload the configuration instead:

Log::Log4perl->init_and_watch($conf_file, 'HUP');

This will set up a signal handler for SIGHUP and reload the configuration if the application receives this signal, e.g. via the kill command:

kill -HUP pid

where pid is the process ID of the application. This will bring you back to about 85% of Log::Log4perl's normal execution speed for suppressed statements. For details, check out "Performance". For more info on the signal handler, look for "SIGNAL MODE" in Log::Log4perl::Config::Watch.

If you have a somewhat long delay set between physical config file checks or don't want to use the signal associated with the config file watcher, you can trigger a configuration reload at the next possible time by calling Log::Log4perl::Config->watcher->force_next_check().

One thing to watch out for: If the configuration file contains a syntax or other fatal error, a running application will stop with die if this damaged configuration will be loaded during runtime, triggered either by a signal or if the delay period expired and the change is detected. This behaviour might change in the future.

To allow the application to intercept and control a configuration reload in init_and_watch mode, a callback can be specified:

Log::Log4perl->init_and_watch($conf_file, 10, { 
        preinit_callback => \&callback });

If Log4perl determines that the configuration needs to be reloaded, it will call the preinit_callback function without parameters. If the callback returns a true value, Log4perl will proceed and reload the configuration. If the callback returns a false value, Log4perl will keep the old configuration and skip reloading it until the next time around. Inside the callback, an application can run all kinds of checks, including accessing the configuration file, which is available via Log::Log4perl::Config->watcher()->file().

Variable Substitution

To avoid having to retype the same expressions over and over again, Log::Log4perl's configuration files support simple variable substitution. New variables are defined simply by adding

varname = value

lines to the configuration file before using

${varname}

afterwards to recall the assigned values. Here's an example:

layout_class   = Log::Log4perl::Layout::PatternLayout
layout_pattern = %d %F{1} %L> %m %n

log4perl.category.Bar.Twix = WARN, Logfile, Screen

log4perl.appender.Logfile  = Log::Log4perl::Appender::File
log4perl.appender.Logfile.filename = test.log
log4perl.appender.Logfile.layout = ${layout_class}
log4perl.appender.Logfile.layout.ConversionPattern = ${layout_pattern}

log4perl.appender.Screen  = Log::Log4perl::Appender::Screen
log4perl.appender.Screen.layout = ${layout_class}
log4perl.appender.Screen.layout.ConversionPattern = ${layout_pattern}

This is a convenient way to define two appenders with the same layout without having to retype the pattern definitions.

Variable substitution via ${varname} will first try to find an explicitly defined variable. If that fails, it will check your shell's environment for a variable of that name. If that also fails, the program will die().

Perl Hooks in the Configuration File

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 Log::Log4perl::Layout::PatternLayout: 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.

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".

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->allow_code() is called with a value which is a key of the map previously defined with Log::Log4perl::Config->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(): Returns the entire convenience name map as a hash reference in scalar context or a hash in list context.
allowed_code_ops_convenience_map( \%map ): Replaces the entire convenience name map with the supplied hash reference.
allowed_code_ops_convenience_map( $name ): Returns the opcode mask for the given convenience name, or undef if no such name is defined in the map.
allowed_code_ops_convenience_map( $name, \@mask ): Adds the given name/mask pair to the convenience name map. If the name already exists in the map, it's value is replaced with the new mask.

as can vars_shared_with_safe_compartment():

vars_shared_with_safe_compartment(): Return the entire map of packages to variables as a hash reference in scalar context or a hash in list context.
vars_shared_with_safe_compartment( \%packages ): Replaces the entire map of packages to variables with the supplied hash reference.
vars_shared_with_safe_compartment( $package ): Returns the arrayref of variables to be shared for a specific package.
vars_shared_with_safe_compartment( $package, \@vars ): Adds the given package / varlist pair to the map. If the package already exists in the map, it's value is replaced with the new arrayref of variable names.

For more information on opcodes and Safe Compartments, see Opcode and Safe.

Changing the Log Level on a Logger

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).

You get the log level from a logger object with:

$current_level = $logger->level();

and you may set it with the same method, provided you first imported the log level constants, with:

use Log::Log4perl::Level;

Then you can set the level on a logger to one of the constants,

$logger->level($ERROR); # one of DEBUG, INFO, WARN, ERROR, FATAL

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, ...);

if( $v ) {
  $logger->more_logging($v); # inc logging level once for each -v in ARGV
}

Custom Log Levels

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 L4p::Level::to_level()
$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.

Since Log4perl translates log levels to syslog and Log::Dispatch if their appenders are used, you may add mappings for custom levels as well:

Log::Log4perl::Level::add_priority("NOTIFY", "WARN",
                                   $syslog_equiv, $log_dispatch_level);

For example, if your new custom "NOTIFY" level is supposed to map to syslog level 2 ("LOG_NOTICE") and Log::Dispatch level 2 ("notice"), use:

Log::Log4perl::Logger::create_custom_level("NOTIFY", "WARN", 2, 2);

System-wide log levels

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.

Easy Mode

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 :).

Stealth loggers

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 and introduce the convenience functions TRACE, DEBUG(), INFO(), WARN(), ERROR(), FATAL(), and ALWAYS into the package namespace. These functions simply take messages as arguments and forward them to the stealth loggers methods (debug(), info(), and so on).

If a message should never be blocked, regardless of the log level, use the ALWAYS function which corresponds to a log level of OFF:

ALWAYS "This will be printed regardless of the log level";

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",
                            utf8     => 1,
                            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 utf8 flag, if set to a true value, runs a binmode command on the file handle to establish a utf8 line discipline on the file, otherwise you'll get a 'wide character in print' warning message and probably not what you'd expect as output.

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(), LOGDIE(), and LOGEXIT(), combining a logging request with a subsequent Perl warn() or die() or exit() 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.

Similarily, LOGCARP(), LOGCLUCK(), LOGCROAK(), and LOGCONFESS() are provided in :easy mode, facilitating the use of logcarp(), logcluck(), logcroak(), and logconfess() with 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.

Nested Diagnostic Context (NDC)

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 Log::Log4perl::NDC.

Mapped Diagnostic Context (MDC)

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 undef.

Typically, MDC values are retrieved later on via the "%X{...}" placeholder in Log::Log4perl::Layout::PatternLayout. If the get() method returns undef, the placeholder will expand to the string [undef].

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 Log::Log4perl::MDC.

Resurrecting hidden Log4perl Statements

Sometimes scripts need to be deployed in environments without having Log::Log4perl installed yet. On the other hand, you don't want to live without your Log4perl statements -- they're gonna come in handy later.

So, just deploy your script with Log4perl statements commented out with the pattern ###l4p, like in

###l4p DEBUG "It works!";
# ...
###l4p INFO "Really!";

If Log::Log4perl is available, use the :resurrect tag to have Log4perl resurrect those buried statements before the script starts running:

use Log::Log4perl qw(:resurrect :easy);

###l4p Log::Log4perl->easy_init($DEBUG);
###l4p DEBUG "It works!";
# ...
###l4p INFO "Really!";

This will have a source filter kick in and indeed print

2004/11/18 22:08:46 It works!
2004/11/18 22:08:46 Really!

In environments lacking Log::Log4perl, just comment out the first line and the script will run nevertheless (but of course without logging):

# use Log::Log4perl qw(:resurrect :easy);

###l4p Log::Log4perl->easy_init($DEBUG);
###l4p DEBUG "It works!";
# ...
###l4p INFO "Really!";

because everything's a regular comment now. Alternatively, put the magic Log::Log4perl comment resurrection line into your shell's PERL5OPT environment variable, e.g. for bash:

set PERL5OPT=-MLog::Log4perl=:resurrect,:easy
export PERL5OPT

This will awaken the giant within an otherwise silent script like the following:

#!/usr/bin/perl

###l4p Log::Log4perl->easy_init($DEBUG);
###l4p DEBUG "It works!";

As of Log::Log4perl 1.12, you can even force all modules loaded by a script to have their hidden Log4perl statements resurrected. For this to happen, load Log::Log4perl::Resurrector before loading any modules:

use Log::Log4perl qw(:easy);
use Log::Log4perl::Resurrector;

use Foobar; # All hidden Log4perl statements in here will
            # be uncommented before Foobar gets loaded.

Log::Log4perl->easy_init($DEBUG);
...

Check the Log::Log4perl::Resurrector manpage for more details.

Access defined appenders

All appenders defined in the configuration file or via Perl code can be retrieved by the appender_by_name() class method. This comes in handy if you want to manipulate or query appender properties after the Log4perl configuration has been loaded via init().

Note that internally, Log::Log4perl uses the Log::Log4perl::Appender wrapper class to control the real appenders (like Log::Log4perl::Appender::File or Log::Dispatch::FileRotate). The Log::Log4perl::Appender class has an appender attribute, pointing to the real appender.

The reason for this is that external appenders like Log::Dispatch::FileRotate don't support all of Log::Log4perl's appender control mechanisms (like appender thresholds).

The previously mentioned method appender_by_name() returns a reference to the real appender object. If you want access to the wrapper class (e.g. if you want to modify the appender's threshold), use the hash $Log::Log4perl::Logger::APPENDER_BY_NAME{...} instead, which holds references to all appender wrapper objects.

Modify appender thresholds

To set an appender's threshold, use its threshold() method:

$app->threshold( $FATAL );

To conveniently adjust all appender thresholds (e.g. because a script uses more_logging()), use

   # decrease thresholds of all appenders
Log::Log4perl->appender_thresholds_adjust(-1);

This will decrease the thresholds of all appenders in the system by one level, i.e. WARN becomes INFO, INFO becomes DEBUG, etc. To only modify selected ones, use

   # decrease thresholds of all appenders
Log::Log4perl->appender_thresholds_adjust(-1, ['AppName1', ...]);

and pass the names of affected appenders in a ref to an array.

Advanced configuration within Perl

Initializing Log::Log4perl can certainly also be done from within Perl. At last, this is what Log::Log4perl::Config does behind the scenes. Log::Log4perl's configuration file parsers are using a publically available API to set up Log::Log4perl's categories, appenders and layouts.

Here's an example on how to configure two appenders with the same layout in Perl, without using a configuration file at all:

########################
# Initialization section
########################
use Log::Log4perl;
use Log::Log4perl::Layout;
use Log::Log4perl::Level;

   # Define a category logger
my $log = Log::Log4perl->get_logger("Foo::Bar");

   # 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::Layout::PatternLayout object) in which the message is being logged in the specified appender. 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.

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::Layout::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"

It's also possible to remove appenders from a logger:

$logger->remove_appender($appender_name);

will remove an appender, specified by name, from a given logger. Please note that this does not remove an appender from the system.

To eradicate an appender from the system, you need to call Log::Log4perl->eradicate_appender($appender_name) which will first remove the appender from every logger in the system and then will delete all references Log4perl holds to it.

To remove a logger from the system, use Log::Log4perl->remove_logger($logger). After the remaining reference $logger goes away, the logger will self-destruct. If the logger in question is a stealth logger, all of its convenience shortcuts (DEBUG, INFO, etc) will turn into no-ops.

How about Log::Dispatch::Config?

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:

You can't assign categories to loggers. For small systems that's fine, but if you can't turn off and on detailed logging in only a tiny subsystem of your environment, you're missing out on a majorly useful log4j feature.
Defining appender thresholds. Important if you want to solve problems like "log all messages of level FATAL to STDERR, plus log all DEBUG messages in 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 independently, you'll love the features of Log::Log4perl, which is equally easy to use.

Using Log::Log4perl with wrapper functions and classes

If you don't use Log::Log4perl as described above, but from a wrapper function, 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 functions, Log::Log4perl will indicate where your logger function called the loggers, not where your application called your wrapper:

use Log::Log4perl qw(:easy);
Log::Log4perl->easy_init({ level => $DEBUG, 
                           layout => "%M %m%n" });

sub mylog {
    my($message) = @_;

    DEBUG $message;
}

sub func {
    mylog "Hello";
}

func();

prints

main::mylog Hello

but that's probably not what your application expects. Rather, you'd want

main::func Hello

because the func function called your logging function.

But don't despair, there's a solution: Just register your wrapper package with Log4perl beforehand. If Log4perl then finds that it's being called from a registered wrapper, it will automatically step up to the next call frame.

Log::Log4perl->wrapper_register(__PACKAGE__);

sub mylog {
    my($message) = @_;

    DEBUG $message;
}

Alternatively, you can increase the value of the global variable $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:

sub mylog {
    my($message) = @_;

    local $Log::Log4perl::caller_depth =
          $Log::Log4perl::caller_depth + 1;
    DEBUG $message;
}

Also, note that if you're writing a subclass of Log4perl, like

package MyL4pWrapper;
use Log::Log4perl;
our @ISA = qw(Log::Log4perl);

and you want to call get_logger() in your code, like

use MyL4pWrapper;

sub get_logger {
    my $logger = Log::Log4perl->get_logger();
}

then the get_logger() call will get a logger for the MyL4pWrapper category, not for the package calling the wrapper class as in

package UserPackage;
my $logger = MyL4pWrapper->get_logger();

To have the above call to get_logger return a logger for the "UserPackage" category, you need to tell Log4perl that "MyL4pWrapper" is a Log4perl wrapper class:

use MyL4pWrapper;
Log::Log4perl->wrapper_register(__PACKAGE__);

sub get_logger {
      # Now gets a logger for the category of the calling package
    my $logger = Log::Log4perl->get_logger();
}

This feature works both for Log4perl-relaying classes like the wrapper described above, and for wrappers that inherit from Log4perl use Log4perl's get_logger function via inheritance, alike.

Access to Internals

The following methods are only of use if you want to peek/poke in the internals of Log::Log4perl. Be careful not to disrupt its inner workings.

Log::Log4perl->appenders(): To find out which appenders are currently defined (not only for a particular logger, but overall), a appenders() method is available to return a reference to a hash mapping appender names to their Log::Log4perl::Appender object references.

Dirty Tricks

infiltrate_lwp()

The famous LWP::UserAgent module isn't Log::Log4perl-enabled. Often, though, especially when tracing Web-related problems, it would be helpful to get some insight on what's happening inside LWP::UserAgent. Ideally, LWP::UserAgent would even play along in the Log::Log4perl framework.

A call to Log::Log4perl->infiltrate_lwp() does exactly this. In a very rude way, it pulls the rug from under LWP::UserAgent and transforms its debug/conn messages into debug() calls of loggers of the category "LWP::UserAgent". Similarily, LWP::UserAgent's trace messages are turned into Log::Log4perl's info() method calls. Note that this only works for LWP::UserAgent versions < 5.822, because this (and probably later) versions miss debugging functions entirely.

Suppressing 'duplicate' LOGDIE messages

If a script with a simple Log4perl configuration uses logdie() to catch errors and stop processing, as in

use Log::Log4perl qw(:easy) ;
Log::Log4perl->easy_init($DEBUG);

shaky_function() or LOGDIE "It failed!";

there's a cosmetic problem: The message gets printed twice:

2005/07/10 18:37:14 It failed!
It failed! at ./t line 12

The obvious solution is to use LOGEXIT() instead of LOGDIE(), but there's also a special tag for Log4perl that suppresses the second message:

use Log::Log4perl qw(:no_extra_logdie_message);

This causes logdie() and logcroak() to call exit() instead of die(). To modify the script exit code in these occasions, set the variable $Log::Log4perl::LOGEXIT_CODE to the desired value, the default is 1.

Redefine values without causing errors

Log4perl's configuration file parser has a few basic safety mechanisms to make sure configurations are more or less sane.

One of these safety measures is catching redefined values. For example, if you first write

log4perl.category = WARN, Logfile

and then a couple of lines later

log4perl.category = TRACE, Logfile

then you might have unintentionally overwritten the first value and Log4perl will die on this with an error (suspicious configurations always throw an error). Now, there's a chance that this is intentional, for example when you're lumping together several configuration files and actually want the first value to overwrite the second. In this case use

use Log::Log4perl qw(:nostrict);

to put Log4perl in a more permissive mode.

Prevent croak/confess from stringifying

The logcroak/logconfess functions stringify their arguments before they pass them to Carp's croak/confess functions. This can get in the way if you want to throw an object or a hashref as an exception, in this case use:

$Log::Log4perl::STRINGIFY_DIE_MESSAGE = 0;

eval {
      # throws { foo => "bar" }
      # without stringification
    $logger->logcroak( { foo => "bar" } );
};

EXAMPLE

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.

INSTALLATION

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 Log::Dispatch installation process.

Log::Log4perl needs Test::More, 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

DEVELOPMENT

Log::Log4perl is still being actively developed. We will always make sure the test suite (approx. 500 cases) will pass, but there might still be bugs. please check http://github.com/mschilli/log4perl for the latest release. The api has reached a mature state, we will not change it unless for a good reason.

Bug reports and feedback are always welcome, just email them to our mailing list shown in the AUTHORS section. We're usually addressing them immediately.

REFERENCES

[1]: Michael Schilli, "Retire your debugger, log smartly with Log::Log4perl!", Tutorial on perl.com, 09/2002, http://www.perl.com/pub/a/2002/09/11/log4perl.html
[2]: Ceki Gülcü, "Short introduction to log4j", http://logging.apache.org/log4j/1.2/manual.html
[3]: Vipan Singla, "Don't Use System.out.println! Use Log4j.", http://www.vipan.com/htdocs/log4jhelp.html
[4]: The Log::Log4perl project home page: http://log4perl.com

AUTHORS

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.

LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

To install Log::Log4perl, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Log::Log4perl

CPAN shell

perl -MCPAN -e shell
install Log::Log4perl

For more information on module installation, please visit the detailed CPAN module installation guide.

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