Nested lookup and substitute System properties in log4j2 properties file

Question

In log4j2.properties file we have following definition to accept dynamic file path via System property log.filePath

appender.file.fileName=${sys:log.filePath}

In JAVA layer before creation of Logger instance we are replacing log.filePath System property with actual value

System.setProperty("log.filePath" + logfilePath);

We would like to ensure if System property log.filePath is not available/replaced, then want to create the log file in java.io.tmpdir directory with name app.log.

As per Log4j2 System Properties Lookup doc https://logging.apache.org/log4j/2.x/manual/lookups.html following syntax exists fileName="${sys:logPath:-/var/logs}/app.log" to create default log in /var/logs/app.log

Considering both Unix and Windows environment we would like to use java.io.tmpdir System property instead of /var/logs directory

I have tried multiple combination of following syntaxes but not works

fileName="${sys:logPath:-sys:java.io.tmpdir}/app.log"

Can someone please suggest if using a System property for lookup default value is supported?

Answer 1

The fallback value after :- is treated as a literal, but accepts nested lookups. So you have two choices:

• use ${sys:logPath:-${sys:java.io.tmpdir}},
• or define a configuration property named logPath (the default fallback for ${sys:logPath} is ${logPath}):

  <Properties>
    <Property name="logPath" value="${sys:java.io.tmpdir}"/>
  </Properties>
  <Appenders>
    <File fileName="${sys:logPath}/app.log" .../>
  </Appenders>