PatternFormatter offers complete control over the appearance of Log4r log events without having to write custom Formatter classes. In order to take advantage of PatternFormatter, some familarity with Kernel#sprintf or the C printf function is recommended. For time formatting, please look at Time.strftime.

PatternFormatter accepts three hash arguments:

pattern:Log event format string.
date_pattern:Date format string.
date_method:Time method to call (instead of using date_pattern).

The pattern format string is something like “%l [%d] %80M”, which resembles a pattern one would normally pass to Kernel#sprintf. However, the directives are specific to Log4r. Before we go on, let’s cover some terminology.


The directive identifier. Everything after this up to and including one of the directive letters defines a directive.
directive letter
Letters in the set [cCdtmMl%]. These identify what kind of data we’re interested in. They are detailed below.
format directive
The numbers and assorted symbols that appears between % and a directive letter is a format directive. It is comprised of an integer specifying the field width followed optionally by a period and an integer specifying the precision. The field width is the minimum number of characters to copy from the data string while the precision is the maximum number to copy. If the field width is preceded by a - sign, the data will be left-justified. Otherwise, it is right-justified.
A statement that says, “I want this data to appear with this (optional) particular format.” A directive starts with a % and is followed by a format directive and terminates in a directive letter.

What the Directive Letters mean

Produces a logger’s name. Fast.
Produces a logger’s full name. Fast.
Produces the time in a format specified by date_pattern or by date_method. If neither is specified, the default will be used (ISO8601). Slow.
Produces the file and line number of the log event. The appearance varies by Ruby version, but it is the same output returned by Kernel#caller[0]. Slow.
The non-inspected log message. That is, to_s called on the object passed into a log method. Fast.
The message formatted by the format_object method in BasicFormatter. It will pretty-print Exceptions, print Strings and inspect everything else. Slow.
The name of the level. That’s l as in Lambda. Fast.
%% just prints a %. Any formatting is probably ignored. Fast.

Examples of directives:

Prints out the date according to our date_pattern or date_method. By default, it looks like this: 2001-01-12 13:15:50
Prints out at most 120 characters of the log message.
Prints the execution trace and pads it on the left with enough whitespace to make the whole thing 15 chars.

Pattern String

A pattern string is simply a bunch of directives combined with the desired format. For instance, to show the level in brackets followed by the date and then the log message trimmed to 15 characters, we use the following pattern:

  "[%l] %d :: %.15m"     #=>     [DEBUG] 2001-01-12 13:15:50 :: This is a messa

To create a PatternFormatter with this format:

  p = => "[%l] %d :: %.15m")

Formatting time

To format time, do one of the following:

If neither date_pattern nor date_method is specified, the default date format will be used. Currently, that would be ISO8601,

The date_pattern is exactly what one would pass to Time.strftime. To specify a date_pattern, pass :date_pattern=>"pattern" to

Alternatively, date_method can be specified to produce the output of a specific Time method, such as usec or to_s or any other zero argument Time method that produces a time. More precisely, the method to call will be invoked on To specify a date_method, pass :date_method=>'methodname' (or a Symbol equivalent) to

XML Configuration

As explained in log4r/configurator.rb, the hash arguments to PatternFormatter are XML parameters. Here’s an example:

  <formatter type="PatternFormatter" pattern="[%l] %d :: %.15m">

Performace considerations

The performance impact of using a particular directive letter is noted in the What the Directives Letters mean section.

The performance impact of time formatting merits special attention. If you aren’t aware yet, the Time class is kind of a kludge. happens to be faster than If you’re concerned about performance, please profile the various time methods and patterns.


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