Log4j Format Specification

External

• http://logging.apache.org/log4j/2.x/manual/layouts.html

Internal

• log4j

Overview

log4j calls its output format layout. Most commonly used is the pattern layout.

Pattern Layout

http://logging.apache.org/log4j/2.x/manual/layouts.html#PatternLayout

The pattern layout is defined by a conversion pattern string.

Conversion Pattern

A conversion pattern string , where various elements of a log event can be referred and the way they are intended to be displayed in a log file specified. The conversion pattern is closely related to the conversion pattern of the printf function in C. The conversion pattern is composed of:

• literal text
• conversion specifiers

There is no explicit separator between literal text and conversion specifiers. The pattern parser knows when it has reached the end of a conversion specifier when it reads a conversion character.

Literal Text

Literal text may include special characters (\t, \n \r \f). '\\' can be used to insert a backslash in the output.

Conversion Specifier

Each conversion specifier starts with a percent sign ('%') and it is followed by an optional format modifier and a mandatory conversion character:

%-5c

Format Modifier

A format modifier modifies how the data generated by the conversion specifier is displayed: field with, padding and justification.

Conversion Character

The conversion character specifies the type of data:

• category
• priority
• date
• thread name

Throwable Handling

If the pattern string does not contain a specifier to handle a Throwable being logged, parsing of the pattern will act as if the "%xEx" specifier had been added to the end of the string. To suppress formatting of the Throwable completely, simply add "%ex{0}" as a specifier in the pattern string.