Each Appender must have a layout which is an instance of the class EsPatternLayout or one of its sub classes. Each layout object has a pattern which is a string that describes what parts of the EsLoggingEvent object should be logged as well as how they should be formatted into a string before being output by the Appender.

The pattern string consists of conversion specifiers and literals in any order. For example, '%m' logs only the EsLoggingEvent message.

Log4s does not support all the conversion specifiers that Log4j does; for instance, there is no line number specifier because Smalltalk does not have line numbers. Conversion specifiers are mapped to EsPatternConverter objects which do the actual formatting work. The mapping table is in EsPatternParser class>> initializePatternLayoutRules.

A conversion specifier that is new in log4s is the '%o' or '%object' specifier, which maps to the EsObjectPatternConverter class. This specifer attempts to mimic the object rendering in Log4j, where you can specify an object that will render another object in a custom format.

The EsObjectPatternConverter asks the object held in the EsLoggingEvent instance variable object if it responds to the message printLog4s; if so, it sends the object instance variable the printLog4s message; if not the EsObjectPatternConverter sends the object instance variable the printString message. For example, if the object instance variable is an instance of class Foo, creating a #printLog4s method in class Foo will customize its output when using this conversion specifier.

Format modifiers can optionally be added to the pattern string to change the minimum and maximum field width and to control justification. The format modifier character is placed between the percent sign and the conversion specifier.

The left justification flag is the minus (-) character, e.g. '%-m'.

The minimum field width modifier is a decimal constant that specifies the minimum number of characters to output, e.g., '%15m'. If the data is smaller than the minimum field width, then it will be padded on the left with spaces, unless the left justification flag is specified. The data will not be truncated if it is larger than the minimum field width.

The maximum field width modifier is a period followed by a decimal constant that specifies the maximum number of characters to output, e.g. '%.20m'. If the data is longer than the maximum field width, then the extra characters are removed from the beginning of the data, not the end.

The date and time formatting in log4s is similar to that in log4j.

The pattern '%d' will print the date of the EsLoggingEvent in the format dd MMM yyyy HH:mm:ss,SSS, e.g., 28 Aug 1989 08:15:07,000.

Date and time formatting is done by the EsDatePatternConverter class. Formatting options can be passed to the class by enclosing them in curly braces {} after the %d pattern specifier, e.g., '%d{dd MMM yyyy HH:mm:ss,SSS}'.

In addition to using a custom formatting option, you can also use one of these predefined aliases:

'%d{DATE}' : dd MMM yyyy HH:mm:ss,SSS, e.g., 28 Aug 1989 08:15:07,000

'%d{ABSOLUTE}' : HH:mm:ss,SSS, e.g., 08:15:07,000

'%d{ISO8601}' : yyyy-MM-dd HH:mm:ss,SSS, e.g., 1989-08-28 08:15:07,000

The following table describes the date formatting options available in log4s.

The following table describes the time formatting options available in log4s.