Layouts

Layouts are used by Appenders to format the LogEvent into a form that meets the needs of whoever will be consuming the log events. In Log4j 1.x and Logback Layouts were expected to transform an event into a String. In Log4j 2 Layouts return a byte array. This allows the result of the Layout to be useful in many more types of Appenders. However, this means most Layouts need to be configured with a Charset to insure the byte array contains correct values.

HTMLLayout

The HTMLLayout generates an HTML page and adds each LogEvent to a row in a table.

HTML Layout Parameters
Parameter Name Type Description
charset String The character set to use when converting the HTML String to a byte array. The value must be a valid Charset. If not specified, the default system Charset will be used.
contentType String The value to assign to the Content-Type header. The default is "text/html".
locationInfo boolean If true, the filename and line number will be included in the HTML output. The default value is false.
title String A String that will appear as the HTML title.

PatternLayout

A flexible layout configurable with pattern string. The goal of this class is to format a LogEvent and return the results. The format of the result depends on the conversion pattern.

The conversion pattern is closely related to the conversion pattern of the printf function in C. A conversion pattern is composed of literal text and format control expressions called conversion specifiers.

Note that any literal text may be included in the conversion pattern.

Each conversion specifier starts with a percent sign (%) and is followed by optional format modifiers and a conversion character. The conversion character specifies the type of data, e.g. category, priority, date, thread name. The format modifiers control such things as field width, padding, left and right justification. The following is a simple example.

Let the conversion pattern be "%-5p [%t]: %m%n" and assume that the log4j environment was set to use a PatternLayout. Then the statements 

 Logger logger = LogManager.getLogger("MyLogger");
 logger.debug("Message 1");
 logger.warn("Message 2");
would yield the output 
 DEBUG [main]: Message 1
 WARN  [main]: Message 2

Note that there is no explicit separator between text and conversion specifiers. The pattern parser knows when it has reached the end of a conversion specifier when it reads a conversion character. In the example above the conversion specifier %-5p means the priority of the logging event should be left justified to a width of five characters.

Pattern Layout Parameters
Parameter Name Type Description
charset String The character set to use when converting the syslog String to a byte array. The String must be a validCharset. If not specified, the default system Charset will be used.
pattern String
replace RegexReplacement Allows portions of the resulting String to be replaced. If configured, the replace element must specify the regular expression to match and the substitution. This performs a function similar to the RegexReplacement converter but applies to the whole message while the converter only applies to the String its pattern generates.

Patterns

The conversions that are provided with Log4j are:

Conversion Pattern Description
c{precision}
logger{precision}

Used to output the name of the logger that published the logging event. The logger conversion specifier can be optionally followed byprecision specifier, which consists of a decimal integer, or a pattern starting with a decimal integer.

If a precision specifier is given and it is an integer value, then only the corresponding number of right most components of the logger name will be printed. If the precision contains other non-integer characters then the name will be abbreviated based on the pattern. If the precision integer is less than one the right-most token will still be printed in full. By default the logger name is printed in full.

Conversion Pattern Logger Name Result
%c{1} org.apache.commons.Foo Foo
%c{2} org.apache.commons.Foo commons.Foo
%c{1.} org.apache.commons.Foo o.a.c.Foo
%c{1.1.~.~} org.apache.commons.test.Foo o.a.~.~.Foo
%c{.} org.apache.commons.test.Foo ....Foo
C{precision}
class{precision}

Used to output the fully qualified class name of the caller issuing the logging request. This conversion specifier can be optionally followed byprecision specifier, that follows the same rules as the logger name converter.

d{pattern}
date{pattern}

Used to output the date of the logging event. The date conversion specifier may be followed by a set of braces containing a date and time pattern strings SimpleDateFormat, ABSOLUTE, DATE or ISO8601 and a set of braces containing a time zone id per java.util.TimeZone.getTimeZone. For example,%d{HH:mm:ss,SSS},%d{dd MMM yyyy HH:mm:ss,SSS}, %d{DATE} or%d{HH:mm:ss}{GMT+0}. If no date format specifier is given then ISO8601 format is assumed.

ex{depth}
exception{depth}
throwable{depth}

Used to output the Throwable trace that has been bound to the LoggingEvent, by default this will output the full trace as one would normally find by a call to Throwable.printStackTrace(). The throwable conversion word can be followed by an option in the form %throwable{short} which will only output the first line of the Throwable or %throwable{n} where the first n lines of the stacktrace will be printed.

F
file 		Used to output the file name where the logging request was issued.
K{key}
map{key}
MAP{key}

Used to output the entries in a MapMessage, if one is present in the event. The K conversion character can be followed by the key for the map placed between braces, as in %K{clientNumber} where clientNumber is the key. The value in the Map corresponding to the key will be output. If no additional sub-option is specified, then the entire contents of the Map key value pair set is output using a format {{key1,val1},{key2,val2}}

l
location

Used to output location information of the caller which generated the logging event.

The location information depends on the JVM implementation but usually consists of the fully qualified name of the calling method followed by the callers source the file name and line number between parentheses.

L
line 		Used to output the line number from where the logging request was issued.
m
msg
message 		Used to output the application supplied message associated with the logging event.
M
method 		Used to output the method name where the logging request was issued.
marker The name of the marker, if one is present.
n

Outputs the platform dependent line separator character or characters.

This conversion character offers practically the same performance as using non-portable line separator strings such as "\n", or "\r\n". Thus, it is the preferred way of specifying a line separator.

p
level 		Used to output the level of the logging event.
r
relative 		Used to output the number of milliseconds elapsed since the JVM was started until the creation of the logging event.
replace{pattern}{regex}{substitution}

Replaces occurrences of 'regex', a regular expression, with its replacement 'substitution' in the string resulting from evaluation of the pattern. For example, "%replace(%msg}{\s}{}" will remove all spaces contained in the event message.

The pattern can be arbitrarily complex and in particular can contain multiple conversion keywords. For instance, "%replace{%logger %msg}{\.}{/}" will replace all dots in the logger or the message of the event with a forward slash.

rEx{depth}
rException{depth}
rThrowable{depth}

The same as the %throwable conversion word but the stack trace is printed starting with the first exception that was thrown followed by each subsequent wrapping exception.

sn
sequenceNumber 		Includes a sequence number that will be incremented in every event. The counter is a static variable so will only be unique within applications that share the same converter Class object.
t
thread 		Used to output the name of the thread that generated the logging event.
x
NDC 		Used to output the Thread Context Stack (also known as the Nested Diagnostic Context or NDC) associated with the thread that generated the logging event.
X{key}
mdc{key}
MDC{key}

Used to output the Thread Context Map (also known as the Mapped Diagnostic Context or MDC) associated with the thread that generated the logging event. The X conversion character can be followed by the key for the map placed between braces, as in %X{clientNumber} where clientNumber is the key. The value in the MDC corresponding to the key will be output. If no additional sub-option is specified, then the entire contents of the MDC key value pair set is output using a format {{key1,val1},{key2,val2}}

See the ThreadContext class for more details.

u{"RANDOM" | "TIME"}
uuid 		Includes either a random or a time-based UUID. The time-based UUID is a Type 1 UUID that can generate up to 10,000 unique ids per millisecond, will use the MAC address of each host, and to try to insure uniqueness across multiple JVMs and/or ClassLoaders on the same host a random number between 0 and 16,384 will be associated with each instance of the UUID generator Class and included in each time-based UUID generated. Because time-based UUIDs contain the MAC address and timestamp they should be used with care as they can cause a security vulnerability.
xEx{depth}
xException{depth}
xThrowable{depth}

The same as the %throwable conversion word but also includes class packaging information.

At the end of each stack element of the exception, a string containing the name of the jar file that contains the class or the directory the class is located in and the "Implementation-Version" as found in that jar's manifest will be added. If the information is uncertain, then the class packaging data will be preceded by a tilde, i.e. the '~' character.

% The sequence %% outputs a single percent sign.

By default the relevant information is output as is. However, with the aid of format modifiers it is possible to change the minimum field width, the maximum field width and justification.

The optional format modifier is placed between the percent sign and the conversion character.

The first optional format modifier is the left justification flag which is just the minus (-) character. Then comes the optional minimum field width modifier. This is a decimal constant that represents the minimum number of characters to output. If the data item requires fewer characters, it is padded on either the left or the right until the minimum width is reached. The default is to pad on the left (right justify) but you can specify right padding with the left justification flag. The padding character is space. If the data item is larger than the minimum field width, the field is expanded to accommodate the data. The value is never truncated.

This behavior can be changed using the maximum field width modifier which is designated by a period followed by a decimal constant. If the data item is longer than the maximum field, then the extra characters are removed from the beginning of the data item and not from the end. For example, it the maximum field width is eight and the data item is ten characters long, then the first two characters of the data item are dropped. This behavior deviates from the printf function in C where truncation is done from the end.

Below are various format modifier examples for the category conversion specifier.

Pattern Converters
Format modifier left justify minimum width maximum width comment
%20c false 20 none Left pad with spaces if the category name is less than 20 characters long.
%-20c true 20 none Right pad with spaces if the category name is less than 20 characters long.
%.30c NA none 30 Truncate from the beginning if the category name is longer than 30 characters.
%20.30c false 20 30 Left pad with spaces if the category name is shorter than 20 characters. However, if category name is longer than 30 characters, then truncate from the beginning.
%-20.30c true 20 30 Right pad with spaces if the category name is shorter than 20 characters. However, if category name is longer than 30 characters, then truncate from the beginning.

RFC5424Layout

As the name implies, the RFC5424Layout formats LogEvents in accordance with RFC 5424, the enhanced Syslog specification. Although the specification is primarily directed at sending messages via Syslog, this format is quite useful for other purposes since items are passed in the message as self-describing key/value pairs.

RFC5424Layout Parameters
Parameter Name Type Description
appName String The value to use as the APP-NAME in the RFC 5424 syslog record.
charset String The character set to use when converting the syslog String to a byte array. The String must be a valid Charset. If not specified, the default system Charset will be used.
enterpriseNumber integer The IANA enterprise number as described in RFC 5424
facility String The facility is used to try to classify the message. The facility option must be set to one of "KERN", "USER", "MAIL", "DAEMON", "AUTH", "SYSLOG", "LPR", "NEWS", "UUCP", "CRON", "AUTHPRIV", "FTP", "NTP", "AUDIT", "ALERT", "CLOCK", "LOCAL0", "LOCAL1", "LOCAL2", "LOCAL3", "LOCAL4", "LOCAL5", "LOCAL6", or "LOCAL7". These values may be specified as upper or lower case characters.
format String If set to "RFC5424" the data will be formatted in accordance with RFC 5424. Otherwise, it will be formatted as a BSD Syslog record. Note that although BSD Syslog records are required to be 1024 bytes or shorter the SyslogLayout does not truncate them. The RFC5424Layout also does not truncate records since the receiver must accept records of up to 2048 bytes and may accept records that are longer.
id String The default structured data id to use when formatting according to RFC 5424. If the LogEvent contains a StructuredDataMessage the id from the Message will be used instead of this value.
immediateFlush boolean When set to true, each write will be followed by a flush. This will guarantee the data is written to disk but could impact performance.
includeMDC boolean Indicates whether data from the ThreadContextMap will be included in the RFC 5424 Syslog record. Defaults to true.
mdcExcludes String A comma separated list of mdc keys that should be excluded from the LogEvent. This is mutually exclusive with the mdcIncludes attribute. This attribute only applies to RFC 5424 syslog records.
mdcIncludes String A comma separated list of mdc keys that should be included in the FlumeEvent. Any keys in the MDC not found in the list will be excluded. This option is mutually exclusive with the mdcExcludes attribute. This attribute only applies to RFC 5424 syslog records.
mdcRequired String A comma separated list of mdc keys that must be present in the MDC. If a key is not present a LoggingException will be thrown. This attribute only applies to RFC 5424 syslog records.
mdcPrefix String A string that should be prepended to each MDC key in order to distinguish it from event attributes. The default string is "mdc:". This attribute only applies to RFC 5424 syslog records.
messageId String The default value to be used in the MSGID field of RFC 5424 syslog records.
newLine boolean If true, a newline will be appended to the end of the syslog record. The default is false.

SerializedLayout

The SerializedLayout simply serializes the LogEvent into a byte array. This is useful when sending messages via JMS or via a Socket connection. The SerializedLayout accepts no parameters.

SyslogLayout

The SyslogLayout formats the LogEvent as BSD Syslog records matching the same format used by Log4j 1.2.

SyslogLayout Parameters
Parameter Name Type Description
charset String The character set to use when converting the syslog String to a byte array. The String must be a valid Charset. If not specified, the default system Charset will be used.
facility String The facility is used to try to classify the message. The facility option must be set to one of "KERN", "USER", "MAIL", "DAEMON", "AUTH", "SYSLOG", "LPR", "NEWS", "UUCP", "CRON", "AUTHPRIV", "FTP", "NTP", "AUDIT", "ALERT", "CLOCK", "LOCAL0", "LOCAL1", "LOCAL2", "LOCAL3", "LOCAL4", "LOCAL5", "LOCAL6", or "LOCAL7". These values may be specified as upper or lower case characters.
newLine boolean If true, a newline will be appended to the end of the syslog record. The default is false.

XMLLayout

The output of the XMLLayout consists of a series of log4j:event elements as defined in thelog4j.dtd. If configured to do so it will output a complete well-formed XML file. The output is designed to be included as an external entity in a separate file to form a correct XML file.

For example, if abc is the name of the file where the XMLLayout ouput goes, then a well-formed XML file would be: 

            <?xml version="1.0" ?>
            *
            <!DOCTYPE log4j:eventSet SYSTEM "log4j.dtd" [<!ENTITY data SYSTEM "abc">]>
            *
            <log4j:eventSet version="2.0" xmlns:log4j="http://logging.apache.org/log4j/">
              
            </log4j:eventSet>

This approach enforces the independence of the XMLLayout and the appender where it is embedded.

The version attribute helps components to correctly intrepret output generated by XMLLayout. The value of this attribute should be "2.0".

Appenders using this layout should have their encoding set to UTF-8 or UTF-16, otherwise events containing non ASCII characters could result in corrupted log files.