Lookups

Lookups provide a way to add values to the Log4j configuration at arbitrary places. They are a particular type of Plugin that implements the StrLookup interface. Information on how to use Lookups in configuration files can be found in the Property Substitution section of the Configuration page.

ContextMapLookup

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>

DateLookup

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>

EnvironmentLookup

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>

MapLookup

The MapLookup serves two purposes.

  1. Provide the base for Properties declared in the configuration file.
  2. Retrieve values from MapMessages in LogEvents.
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>

StructuredDataLookup

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>

SystemPropertiesLookup

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>