--- name: "Log::Log4perl::Layout::PatternLayout" section: "3pm" platform: "linux" package: "perl-log-log4perl" version: "1.58-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/Log::Log4perl::Layout::PatternLayout" --- # Log::Log4perl::Layout::PatternLayout(3pm) Pattern Layout ## NAME Log::Log4perl::Layout::PatternLayout - Pattern Layout ## SYNOPSIS ``` use Log::Log4perl::Layout::PatternLayout; my $layout = Log::Log4perl::Layout::PatternLayout->new( "%d (%F:%L)> %m"); ``` ## DESCRIPTION Creates a pattern layout according to http://jakarta.apache.org/log4j/docs/api/org/apache/log4j/PatternLayout.html and a couple of Log::Log4perl-specific extensions. The `new()` method creates a new PatternLayout, specifying its log format. 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 %d{...} Current date in customized format (see below) %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} Log message, stripped off a trailing newline %m{indent} Log message, multi-lines indented so they line up with first %m{indent=n} Log message, multi-lines indented by n spaces %M Method or function where the logging request was issued %n Newline (OS-independent) %p Priority/level of the logging event (%p{1} shows the first letter) %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)" in Log::Log4perl and "Mapped Diagnostic Context (MDC)" in Log::Log4perl. The granularity of time values is milliseconds if Time::HiRes is available. If not, only full seconds are used. Every once in a while, someone uses the "%m%n" pattern and additionally provides an extra newline in the log message (e.g. `"->log("message\n")"`. To avoid printing an extra newline in this case, the PatternLayout will chomp the message, printing only one newline. This option can be controlled by PatternLayout's `"message_chomp_before_newline"` option. See "Advanced options" for details. ### Quantify placeholders All placeholders can be extended with formatting instructions, just like in *printf*: ``` %20c Reserve 20 chars for the category, right-justify and fill with blanks if it is shorter %-20c Same as %20c, but left-justify and fill the right side with blanks %09r Zero-pad the number of milliseconds to 9 digits %.8c Specify the maximum field with and have the formatter cut off the rest of the value ``` ### Fine-tuning with curlies Some placeholders have special functions defined if you add curlies with content after them: ``` %c{1} Just show the right-most category compontent, useful in large class hierarchies (Foo::Baz::Bar -> Bar) %c{2} Just show the two right most category components (Foo::Baz::Bar -> Baz::Bar) %F Display source file including full path %F{1} Just display filename %F{2} Display filename and last path component (dir/test.log) %F{3} Display filename and last two path components (d1/d2/test.log) %M Display fully qualified method/function name %M{1} Just display method name (foo) %M{2} Display method name and last path component (main::foo) ``` In this way, you're able to shrink the displayed category or limit file/path components to save space in your logs. ### Fine-tune the date If you're not happy with the default `%d` format for the date which looks like ``` yyyy/MM/DD HH:mm:ss ``` (which is slightly different from Log4j which uses `"yyyy-MM-dd HH:mm:ss,SSS"`) you're free to fine-tune it in order to display only certain characteristics of a date, according to the SimpleDateFormat in the Java World (http://docs.oracle.com/javase/8/docs/api/java/text/SimpleDateFormat.html): ``` %d{HH:mm} "23:45" -- Just display hours and minutes %d{yy, EEEE} "02, Monday" -- Just display two-digit year and spelled-out weekday %d{e} "1473741760" -- Epoch seconds %d{h a} "12 PM" -- Hour and am/pm marker ... and many more ``` For an exhaustive list of all supported date features, look at Log::Log4perl::DateFormat. ### Custom cspecs First of all, "cspecs" is short for "conversion specifiers", which is the log4j and the [**printf**(3)](/printf/3.md) term for what Mike is calling "placeholders." I suggested "cspecs" for this part of the api before I saw that Mike was using "placeholders" consistently in the log4perl documentation. Ah, the joys of collaboration ;=) --kg If the existing corpus of placeholders/cspecs isn't good enough for you, you can easily roll your own: ``` #'U' a global user-defined cspec log4j.PatternLayout.cspec.U = sub { return "UID: $< "} #'K' cspec local to appndr1 (pid in hex) log4j.appender.appndr1.layout.cspec.K = sub { return sprintf "%1x", $$} #and now you can use them log4j.appender.appndr1.layout.ConversionPattern = %K %U %m%n ``` The benefit of this approach is that you can define and use the cspecs right next to each other in the config file. If you're an API kind of person, there's also this call: ``` Log::Log4perl::Layout::PatternLayout:: add_global_cspec('Z', sub {'zzzzzzzz'}); #snooze? ``` When the log message is being put together, your anonymous sub will be called with these arguments: ``` ($layout, $message, $category, $priority, $caller_level); layout: the PatternLayout object that called it message: the logging message (%m) category: e.g. groceries.beverages.adult.beer.schlitz priority: e.g. DEBUG|WARN|INFO|ERROR|FATAL caller_level: how many levels back up the call stack you have to go to find the caller ``` 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. *Also make sure* *these subroutines aren't using Log4perl, otherwise Log4perl will enter* *an infinite recursion.* With Log4perl 1.20 and better, cspecs can be written with parameters in curly braces. Writing something like ``` log4perl.appender.Screen.layout.ConversionPattern = %U{user} %U{id} %m%n ``` will cause the cspec function defined for `%U` to be called twice, once with the parameter 'user' and then again with the parameter 'id', and the placeholders in the cspec string will be replaced with the respective return values. The parameter value is available in the 'curlies' entry of the first parameter passed to the subroutine (the layout object reference). So, if you wanted to map `%U`{xxx} to entries in the POE session hash, you'd write something like: ``` log4perl.PatternLayout.cspec.U = sub { \ POE::Kernel->get_active_session->get_heap()->{ $_[0]->{curlies} } } ``` **SECURITY NOTE** This feature means arbitrary perl code can be embedded in the config file. In the rare case where the people who have access to your config file are different from the people who write your code and shouldn't have execute rights, you might want to set ``` $Log::Log4perl::Config->allow_code(0); ``` before you call **init()**. Alternatively you can supply a restricted set of Perl opcodes that can be embedded in the config file as described in "Restricting what Opcodes can be in a Perl Hook" in Log::Log4perl. ### Advanced Options The constructor of the `"Log::Log4perl::Layout::PatternLayout"` class takes an optional hash reference as a first argument to specify additional options in order to (ab)use it in creative ways: ``` my $layout = Log::Log4perl::Layout::PatternLayout->new( { time_function => \&my_time_func, }, "%d (%F:%L)> %m"); ``` Here's a list of parameters: - time\_function Takes a reference to a function returning the time for the time/date fields, either in seconds since the epoch or as an array, carrying seconds and microseconds, just like `"Time::HiRes::gettimeofday"` does. - message\_chomp\_before\_newline If a layout contains the pattern "%m%n" and the message ends with a newline, PatternLayout will chomp the message, to prevent printing two newlines. If this is not desired, and you want two newlines in this case, the feature can be turned off by setting the `"message_chomp_before_newline"` option to a false value: ``` my $layout = Log::Log4perl::Layout::PatternLayout->new( { message_chomp_before_newline => 0 }, "%d (%F:%L)> %m%n"); ``` In a Log4perl configuration file, the feature can be turned off like this: ``` log4perl.appender.App.layout = PatternLayout log4perl.appender.App.layout.ConversionPattern = %d %m%n # Yes, I want two newlines log4perl.appender.App.layout.message_chomp_before_newline = 0 ``` ### Getting rid of newlines If your code contains logging statements like ``` # WRONG, don't do that! $logger->debug("Some message\n"); ``` then it's usually best to strip the newlines from these calls. As explained in "Logging newlines" in Log::Log4perl, logging statements should never contain newlines, but rely on appender layouts to add necessary newlines instead. If changing the code is not an option, use the special PatternLayout placeholder `%m`{chomp} to refer to the message excluding a trailing newline: ``` log4perl.appender.App.layout.ConversionPattern = %d %m{chomp}%n ``` This will add a single newline to every message, regardless if it complies with the Log4perl newline guidelines or not (thanks to Tim Bunce for this idea). ### Multi Lines If a log message consists of several lines, like ``` $logger->debug("line1\nline2\nline3"); ``` then by default, they get logged like this (assuming the the layout is set to "%d\>%m%n"): ``` # layout %d>%m%n 2014/07/27 12:46:16>line1 line2 line3 ``` If you'd rather have the messages aligned like ``` # layout %d>%m{indent}%n 2014/07/27 12:46:16>line1 line2 line3 ``` then use the `%m{indent}` option for the `%m` specifier. This option can also take a fixed value, as in `%m{indent=2}`, which indents subsequent lines by two spaces: ``` # layout %d>%m{indent=2}%n 2014/07/27 12:46:16>line1 line2 line3 ``` Note that you can still add the `"chomp"` option for the `%m` specifier in this case (see above what it does), simply add it after a separating comma, like in `%m{indent=2,chomp}`. ## LICENSE Copyright 2002-2026 by Mike Schilli \ and Kevin Goess \. This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.