Log4j config: Use Tomcat context params

I wanted to use parameters defined in context.xml on Tomcat 8 in configuration of Log4j2. My context was defined like:

<?xml version="1.0" encoding="UTF-8"?> 
    <Parameter name="instanceName" value="testInstance" override="false"/> 
    <Resource .../> 

Now I would like to use parameter instanceName in log filename. I spent many hours trying various approaches and googling. I wonder why is this not better documented - Using Log4j 2 in Web Applications does not really help. In the end it is easy - just use web: prefix like this:

<?xml version="1.0" encoding="UTF-8"?> 
<Configuration status="debug"> 
        <Property name="log-path">${sys:catalina.base}/logs/${web:instanceName}</Property> 
        <RollingFile name="file" fileName="${log-path}/test.log" filePattern="${log-path}/test-%d{yyyy-MM-dd}.log"> ... ... 

That’s it folks!

Tags:  Log4j  Tomcat  Java