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XML format

Log4j 2 config

How a Java application decides what to log and where, in a file whose element names are plugin names rather than a fixed vocabulary.

log4j2.xml configures appenders, layouts and loggers for one of the most widely deployed logging frameworks in Java. It has no schema in the ordinary sense: element names resolve to plugins by name, so <RollingFile> works because a plugin registered itself under that name, and a typo produces a component that silently does not exist rather than a validation error.

Logger configuration is hierarchical by dotted name, and additivity is the setting that catches people: a logger both handles an event and passes it to its parent unless told otherwise, which is how one message ends up in three files.

Constraints that bite

  • Element names are plugin names and are matched case-insensitively. A misspelling is not an error — the component is simply absent.
  • Set additivity="false" on a logger with its own appender, or events also go to the root logger's appenders.
  • status on the root element controls Log4j's own internal logging, and is the only way to see why a configuration did not load.
  • Property substitution uses ${...} with a prefix (${env:…}, ${sys:…}); an unresolved lookup is left as the literal text.
  • The file is watched only if monitorInterval is set, and the minimum effective interval is measured in seconds.
Root element
<Configuration>
Media type
application/xml
Extensions
.xml
Namespaces
None — unqualified elements
XMLDir label
—

Specification: Log4j 2 configuration

A minimal valid document

Every example on this site is checked against the same parser the workspace uses, so what you see below is known to be well-formed.

log4j2-config.xml
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN" monitorInterval="30">
  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{ISO8601} %-5level %logger{36} - %msg%n"/>
    </Console>
    <RollingFile name="File" fileName="logs/app.log"
                 filePattern="logs/app-%d{yyyy-MM-dd}.log.gz">
      <JsonTemplateLayout/>
      <TimeBasedTriggeringPolicy/>
    </RollingFile>
  </Appenders>
  <Loggers>
    <Logger name="com.example.audit" level="info" additivity="false">
      <AppenderRef ref="File"/>
    </Logger>
    <Root level="warn">
      <AppenderRef ref="Console"/>
    </Root>
  </Loggers>
</Configuration>
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