Log4j DispatcherAppender

A Log4j Appender that dispatches to a different Appender instance depending on the value of the Nested Diagnostic Context. The delegate appender is copied and a single property is overriden (e.g. a file location) based on a layout pattern. A typical use case would be directing logs to a different file based on the business data that are being processed.

Note that adding an appender to an existing one cannot be done using the Log4j PropertiesConfigurator so you have to use XML or Java to configure a DispatcherAppender.

License: Apache 2.0

Usage

Sample log4j.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">

<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/" debug="false">

  <appender name="LOGGER" class="org.springframework.util.log4j.DispatcherAppender">
    <param name="propertyName" value="file" />
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern" value="target/logs/%x.log" />
    </layout>
    <appender-ref ref="FILE" />
  </appender>

  <appender name="FILE" class="org.apache.log4j.FileAppender">
    <param name="file" value="target/logs/default.log" />
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern" value="%5p: %m%n" />
    </layout>
  </appender>

  <root>
    <priority value="info" />
    <appender-ref ref="LOGGER" />
  </root>

</log4j:configuration>

Application code:

logger.info("foo");
NDC.push("alt");
try {
  logger.info("foo");
} finally {
  NDC.clear();
}

Result:

$ ls target/logs
default.log    alt.log