Context Map Lookup

The ContextMapLookup allows applications to store data in the Log4j ThreadContext Map and then retrieve the values in the Log4j configuration. In the example below, the application would store the current user's login id in the ThreadContext Map with the key "loginId". During initial configuration processing the first '$' will be removed. The PatternLayout supports interpolation with Lookups and will then resolve the variable for each event. Note that the pattern "%X{loginId}" would achieve the same result.

<File name="Application" fileName="application.log">
  <PatternLayout>
    <pattern>%d %p %c{1.} [%t] $${ctx:loginId} %m%n</pattern>
  </PatternLayout>
</File>

Date Lookup

The DateLookup is somewhat unusual from the other lookups as it doesn't use the key to locate an item. Instead, the key can be used to specify a date format string that is valid for SimpleDateFormat. The current date, or the date associated with the current log event will be formatted as specified.

<RollingFile name="Rolling-${map:type}" fileName="${filename}" filePattern="target/rolling1/test1-$${date:MM-dd-yyyy}.%i.log.gz">
  <PatternLayout>
    <pattern>%d %p %c{1.} [%t] %m%n</pattern>
  </PatternLayout>
  <SizeBasedTriggeringPolicy size="500" />
</RollingFile>

Docker Lookup

The DockerLookup can be used to lookup attributes from the Docker container the application is running in.

<JsonLayout properties="true" compact="true" eventEol="true">
  <KeyValuePair key="containerId" value="${docker:containerId}"/>
  <KeyValuePair key="containerName" value="${docker:containerName}"/>
  <KeyValuePair key="imageName" value="${docker:imageName}"/>
</JsonLayout>

This Lookup is subject to the requirements listed at Log4j Docker Support

Environment Lookup

The EnvironmentLookup allows systems to configure environment variables, either in global files such as /etc/profile or in the startup scripts for applications, and then retrieve those variables from within the logging configuration. The example below includes the name of the currently logged in user in the application log.

<File name="Application" fileName="application.log">
  <PatternLayout>
    <pattern>%d %p %c{1.} [%t] $${env:USER} %m%n</pattern>
  </PatternLayout>
</File>

This lookup also supports default value syntax. In the sample below, when the USER environment variable is undefined, the default value jdoe is used:

<File name="Application" fileName="application.log">
  <PatternLayout>
    <pattern>%d %p %c{1.} [%t] $${env:USER:-jdoe} %m%n</pattern>
  </PatternLayout>
</File>

Java Lookup

The JavaLookup allows Java environment information to be retrieved in convenient preformatted strings using the java: prefix.

For example:

<File name="Application" fileName="application.log">
  <PatternLayout header="${java:runtime} - ${java:vm} - ${java:os}">
    <Pattern>%d %m%n</Pattern>
  </PatternLayout>
</File>

Jndi Lookup

As of Log4j 2.13.3 JNDI operations require that log4j2.enableJndiLookup=true be set as a system property or the corresponding environment variable for this lookup to function. See the enableJndiLookup system property.

The JndiLookup allows variables to be retrieved via JNDI. By default the key will be prefixed with java:comp/env/, however if the key contains a ":" no prefix will be added.

<File name="Application" fileName="application.log">
  <PatternLayout>
    <pattern>%d %p %c{1.} [%t] $${jndi:logging/context-name} %m%n</pattern>
  </PatternLayout>
</File>

Java's JNDI module is not available on Android.

JVM Input Arguments Lookup (JMX)

Maps JVM input arguments -- but not main arguments -- using JMX to acquire the JVM arguments.

Use the prefix jvmrunargs to access JVM arguments.

See the Javadocs for java.lang.management.RuntimeMXBean.getInputArguments() .

Java's JMX module is not available on Android or on Google App Engine.

Log4j Configuration Location Lookup

Log4j configuration properties. The expressions ${log4j:configLocation} and ${log4j:configParentLocation} respectively provide the absolute path to the log4j configuration file and its parent folder.

The example below uses this lookup to place log files in a directory relative to the log4j configuration file.

<File name="Application" fileName="${log4j:configParentLocation}/logs/application.log">
  <PatternLayout>
    <pattern>%d %p %c{1.} [%t] %m%n</pattern>
  </PatternLayout>
</File>

Main Arguments Lookup (Application)

This lookup requires that you manually provide the main arguments of the application to Log4j:

import org.apache.logging.log4j.core.lookup.MainMapLookup;

public static void main(String args[]) {
  MainMapLookup.setMainArguments(args);
  ...
}

If the main arguments have been set, this lookup allows applications to retrieve these main argument values from within the logging configuration. The key that follows the main: prefix can either be a 0-based index into the argument list, or a string, where ${main:myString} is substituted with the value that follows myString in the main argument list.

For example, suppose the static void main String[] arguments are:

--file foo.txt --verbose -x bar

Then the following substitutions are possible:

Example usage:

<File name="Application" fileName="application.log">
  <PatternLayout header="File: ${main:--file}">
    <Pattern>%d %m%n</Pattern>
  </PatternLayout>
</File>

Map Lookup

The MapLookup serves several purposes.

1. Provide the base for Properties declared in the configuration file.
2. Retrieve values from MapMessages in LogEvents.
3. Retrieve values set with MapLookup.setMainArguments(String[])

The first item simply means that the MapLookup is used to substitute properties that are defined in the configuration file. These variables are specified without a prefix - e.g. ${name}. The second usage allows a value from the current MapMessage, if one is part of the current log event, to be substituted. In the example below the RoutingAppender will use a different RollingFileAppender for each unique value of the key named "type" in the MapMessage. Note that when used this way a value for "type" should be declared in the properties declaration to provide a default value in case the message is not a MapMessage or the MapMessage does not contain the key. See the Property Substitution section of the Configuration page for information on how to set the default values.

<Routing name="Routing">
  <Routes pattern="$${map:type}">
    <Route>
      <RollingFile name="Rolling-${map:type}" fileName="${filename}"
                   filePattern="target/rolling1/test1-${map:type}.%i.log.gz">
        <PatternLayout>
          <pattern>%d %p %c{1.} [%t] %m%n</pattern>
        </PatternLayout>
        <SizeBasedTriggeringPolicy size="500" />
      </RollingFile>
    </Route>
  </Routes>
</Routing>

Marker Lookup

The marker lookup allows you to use markers in interesting configurations like a routing appender. Consider the following YAML configuration and code that logs to different files based on markers:

Configuration:
  status: debug

  Appenders:
    Console:
    RandomAccessFile:
      - name: SQL_APPENDER
        fileName: logs/sql.log
        PatternLayout:
          Pattern: "%d{ISO8601_BASIC} %-5level %logger{1} %X %msg%n"
      - name: PAYLOAD_APPENDER
        fileName: logs/payload.log
        PatternLayout:
          Pattern: "%d{ISO8601_BASIC} %-5level %logger{1} %X %msg%n"
      - name: PERFORMANCE_APPENDER
        fileName: logs/performance.log
        PatternLayout:
          Pattern: "%d{ISO8601_BASIC} %-5level %logger{1} %X %msg%n"

    Routing:
      name: ROUTING_APPENDER
      Routes:
        pattern: "$${marker:}"
        Route:
        - key: PERFORMANCE
          ref: PERFORMANCE_APPENDER
        - key: PAYLOAD
          ref: PAYLOAD_APPENDER
        - key: SQL
          ref: SQL_APPENDER

  Loggers:
    Root:
      level: trace
      AppenderRef:
        - ref: ROUTING_APPENDER

public static final Marker SQL = MarkerFactory.getMarker("SQL");
public static final Marker PAYLOAD = MarkerFactory.getMarker("PAYLOAD");
public static final Marker PERFORMANCE = MarkerFactory.getMarker("PERFORMANCE");

final Logger logger = LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);

logger.info(SQL, "Message in Sql.log");
logger.info(PAYLOAD, "Message in Payload.log");
logger.info(PERFORMANCE, "Message in Performance.log");

Note the key part of the configuration is pattern: "$${marker:}". This will produce three log files, each with a log event for a specific marker. Log4j will route the log event with the SQL marker to sql.log, the log event with the PAYLOAD marker to payload.log, and so on.

You can use the notation "${marker:name}" and "$${marker:name}" to check for the existence of a marker where name is the marker name. If the marker exists, the expression returns the name, otherwise null.

Structured Data Lookup

The StructuredDataLookup is very similar to the MapLookup in that it will retrieve values from StructuredDataMessages. In addition to the Map values it will also return the name portion of the id (not including the enterprise number) and the type field. The main difference between the example below and the example for MapMessage is that the "type" is an attribute of the StructuredDataMessage while "type" would have to be an item in the Map in a MapMessage.

<Routing name="Routing">
  <Routes pattern="$${sd:type}">
    <Route>
      <RollingFile name="Rolling-${sd:type}" fileName="${filename}"
                   filePattern="target/rolling1/test1-${sd:type}.%i.log.gz">
        <PatternLayout>
          <pattern>%d %p %c{1.} [%t] %m%n</pattern>
        </PatternLayout>
        <SizeBasedTriggeringPolicy size="500" />
      </RollingFile>
    </Route>
  </Routes>
</Routing>

System Properties Lookup

As it is quite common to define values inside and outside the application by using System Properties, it is only natural that they should be accessible via a Lookup. As system properties are often defined outside the application it would be quite common to see something like:

<Appenders>
  <File name="ApplicationLog" fileName="${sys:logPath}/app.log"/>
</Appenders>

This lookup also supports default value syntax. In the sample below, when the logPath system property is undefined, the default value /var/logs is used:

<Appenders>
  <File name="ApplicationLog" fileName="${sys:logPath:-/var/logs}/app.log"/>
</Appenders>

Web Lookup

The WebLookup allows applications to retrieve variables that are associated with the ServletContext. In addition to being able to retrieve various fields in the ServletContext, WebLookup supports looking up values stored as attributes or configured as initialization parameters. The following table lists various keys that can be retrieved:

Any other key names specified will first be checked to see if a ServletContext attribute exists with that name and then will be checked to see if an initialization parameter of that name exists. If the key is located then the corresponding value will be returned.

<Appenders>
  <File name="ApplicationLog" fileName="${web:rootDir}/app.log"/>
</Appenders>