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Log-Log4perl-0.24
River stage four • 376 direct dependents • 1144 total dependents
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/ Log::Log4perl

NAME

Log::Log4perl - Log4j implementation for Perl

SYNOPSIS

    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###################
    log4j.category.house              = WARN,  FileAppndr1
    log4j.category.house.bedroom.desk = DEBUG,  FileAppndr1
    
    log4j.appender.FileAppndr1          = Log::Dispatch::File
    log4j.appender.FileAppndr1.filename = desk.log 
    log4j.appender.FileAppndr1.layout   = \
                            Log::Log4perl::Layout::SimpleLayout
    ###########################################

ABSTRACT

    Log::Log4perl provides a powerful logging API to 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 [1].

A WORD OF CAUTION: Log::Log4perl IS STILL UNDER DEVELOPMENT. WE WILL ALWAYS MAKE SURE THE TEST SUITE (approx. 300 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 when 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:

  • 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 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!

How to use it

The Log::Log4perl package can be initialized in two ways: Either via Perl commands or via a lib4j-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::Dispatch file appender
    # in Perl.
    # Mike Schilli 2002 m@perlmeister.com
    ############################################################
    log4j.rootLogger=error, LOGFILE
    
    log4j.appender.LOGFILE=Log::Dispatch::File
    log4j.appender.LOGFILE.filename=/var/log/myerrs.log
    log4j.appender.LOGFILE.mode=append
    
    log4j.appender.LOGFILE.layout=org.apache.log4j.PatternLayout
    log4j.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.

Configuration within Perl

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;
  use Log::Dispatch::Screen;
  use Log::Log4perl::Appender;

  my $app = Log::Log4perl::Appender->new("Log::Dispatch::Screen");
  my $layout = Log::Log4perl::Layout::PatternLayout
                                        ->new("%d> %F %L %m %n");
  $app->layout($layout);

  my $logger = Log::Log4perl->get_logger("My.Component");
  $logger->add_appender($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");
  $log->info("Info Message");
  $log->error("Error Message");

Log Levels

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.

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->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

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 utilizes Dave Rolskys excellent Log::Dispatch module to implement a wide variety of different appenders. You can have your messages written to STDOUT, to a file or to a database -- or to all of them at once if you desire to do so.

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::Handle
       Log::Dispatch::Screen
       Log::Dispatch::Syslog
       Log::Dispatch::Tk (by Dominique Dumont)

For additional information on appenders, please check the Log::Log4perl::Appender manual page.

Now let's assume that we want to go overboard and 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::Dispatch::File and Log::Dispatch::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->new("[%r] %F %L %m%n");

     # Define a file appender
  my $file_appender = Log::Log4perl::Appender->new(
                          "Log::Dispatch::File",
                          name      => "filelog",
                          filename  => "/tmp/my.log");


     # Define a stdout appender
  my $stdout_appender =  Log::Log4perl::Appender->new(
                          "Log::Dispatch::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 Log::Dispatch 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::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 Log::Dispatch::* class and can be looked up in their manual pages.

On 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.

Turn off a component

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.
    ############################################################
    log4j.rootLogger=debug, LOGFILE
    log4j.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.

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 unbeatible 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=ConsoleAppender
    log4j.appender.A1.layout=org.apache.log4j.PatternLayout
    log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-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::Dispatch::Screen in this case.

Second example:

    log4j.rootLogger=DEBUG, A1
    log4j.appender.A1=Log::Dispatch::Screen
    log4j.appender.A1.layout=org.apache.log4j.PatternLayout
    log4j.appender.A1.layout.ConversionPattern=%d [%t] %-5p %c - %m%n
    log4j.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 [%t] (%F:%L) - %m%n
    log4j.appender.R=org.apache.log4j.FileAppender
    log4j.appender.R.File=example.log
    log4j.appender.R.layout=org.apache.log4j.PatternLayout
    log4j.appender.R.layout.ConversionPattern=%p %t %c - %m%n

The root logger defines two appenders here: stdout, which uses org.apache.log4j.ConsoleAppender (ultimately mapped by Log::Log4perl to Log::Dispatch::Screen) to write to the screen. And R, a org.apache.log4j.RollingFileAppender (ultimately mapped by Log::Log4perl to Log::Dispatch::File 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::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
    %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
    %r Number of milliseconds elapsed from program start to logging 
       event
    %% A literal percent (%) sign

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.

See Log::Log4perl::Layout::PatternLayout for details.

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::Dispatch::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

Penalties

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.

Categories

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.

Cool Tricks

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();

Alternative initialization

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 = (
        "log4j.rootLogger"       => "error, LOGFILE",
        "log4j.appender.LOGFILE" => "Log::Dispatch::File",
        ...
    );

    Log::Log4perl->init( \%key_value_pairs );

Incrementing and Decrementing the Log Levels

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

Custom Log Levels

First off, let me tell you that creating custom levels is heavily deprechiated 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.

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 :).

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 independantly, you'll love the features of Log::Log4perl, which is equally easy to use.

Using Log::Log4perl from wrapper classes

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.

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::Dispatch::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

Log::Log4perl needs Log::Dispatch (2.00 or better) from CPAN, which itself depends on Attribute-Handlers and Params-Validate. Everything's automatically fetched from CPAN if you're using the CPAN shell (CPAN.pm), because they're listed as dependencies. Also, Test::Simple, Test::Harness and File::Spec are needed, but they already come with fairly recent versions of perl.

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 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.

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://jakarta.apache.org/log4j/docs/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.sourceforge.net

SEE ALSO

Log::Log4perl::Config, Log::Log4perl::Appender, Log::Log4perl::Layout::PatternLayout, Log::Log4perl::Layout::SimpleLayout, Log::Log4perl::Level, Log::Log4perl::JavaMap

AUTHORS

Please send bug reports or requests for enhancements to the authors via our log4perl development mailing list:

    log4perl-devel@lists.sourceforge.net

    Log::Log4perl Authors: 
    Mike Schilli <m@perlmeister.com>
    Kevin Goess <cpan@goess.org>

    Log::Log4perl Contributors:
    Chris R. Donnelly <cdonnelly@digitalmotorworks.com>
    Erik Selberg <erik@selberg.com>

COPYRIGHT AND LICENSE

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.

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