Same Appender log into 2 different files with Log4J2 - configuration

Question

I would like to define 1 single Appender in my log4j2.xml configuration file, and using the magic of the Properties Substitution of Log4J2, be able to somehow log into 2 different files.
I imagine the Appender would look something like:
<RollingFile name="Rolling-${filename}" fileName="${filename}" filePattern="${filename}.%i.log.gz">
<PatternLayout>
<pattern>%d %p %c{1.} [%t] %m%n</pattern>
</PatternLayout>
<SizeBasedTriggeringPolicy size="500" />
</RollingFile>
Is there a way for a Logger to use this appender and to pass the filename property?
Or is there a way to pass it when we fetch the Logger with LogManager.getLogger?
Note that those logger may or may not be in the same Thread, it has to support both cases, so I don't think it's possible to use ThreadContext nor System properties.

Answer 1

The closest thing I can think of is RoutingAppender. RoutingAppender allows the log file to be dynamically selected based on values in some lookup. A popular built-in lookup is the ThreadContext map (see the example on the FAQ page), but you can create a custom lookup. Example code:
ThreadContext.put("ROUTINGKEY", "foo");
logger.debug("This message gets sent to route foo");
// Do some work, including logging by various loggers.
// All logging done in this thread is sent to foo.
// Other threads can also log to foo at the same time by setting ROUTINGKEY=foo.
logger.debug("... and we are done");
ThreadContext.remove("ROUTINGKEY"); // this thread no longer logs to foo
Example config that creates log files on the fly:
<Routing name="Routing">
<Routes pattern="$${ctx:ROUTINGKEY}">
<!-- This route is chosen if ThreadContext has a value for ROUTINGKEY.
The value dynamically determines the name of the log file. -->
<Route>
<RollingFile name="Rolling-${ctx:ROUTINGKEY}" fileName="logs/other-${ctx:ROUTINGKEY}.log"
filePattern="./logs/${date:yyyy-MM}/${ctx:ROUTINGKEY}-other-%d{yyyy-MM-dd}-%i.log.gz">
<PatternLayout>
<pattern>%d{ISO8601} [%t] %p %c{3} - %m%n</pattern>
</PatternLayout>
<Policies>
<TimeBasedTriggeringPolicy interval="6" modulate="true" />
<SizeBasedTriggeringPolicy size="10 MB" />
</Policies>
</RollingFile>
</Route>
</Routes>
<!-- This route is chosen if ThreadContext has no value for key ROUTINGKEY. -->
<Route key="$${ctx:ROUTINGKEY}">
<RollingFile name="Rolling-default" fileName="logs/default.log"
filePattern="./logs/${date:yyyy-MM}/default-%d{yyyy-MM-dd}-%i.log.gz">
<PatternLayout>
<pattern>%d{ISO8601} [%t] %p %c{3} - %m%n</pattern>
</PatternLayout>
<Policies>
<TimeBasedTriggeringPolicy interval="6" modulate="true" />
<SizeBasedTriggeringPolicy size="10 MB" />
</Policies>
</RollingFile>
</Route>
</Routing>
An alternative is to configure multiple loggers, each pointing to a separate appender (with additivity="false"). This allows your application to control the destination file by obtaining a logger by its name. However, in that case you would need to configure separate appenders so this does not fulfill your requirement, I mention it for completeness.

Answer 2

I am using the logger name to pass arguments to the appender.
It's hacky but it works:
LogManager.getLogger("com.company.test.Test.logto.xyz.log")
A custom StrLookup is necessary to extract the filename from the logger name.