Custom Layout Configuration Help using Log4j2-beta7

[Andrew Berman]

Hello,

I've written a custom layout and a helper class.  It all works great,
however, my question has to do with how to interpolate the variables inside
of an attribute in the configuration.  For example:

<Socket name="socket" host="foo.com" port="5544" protocol="TCP">
       <MyLayout>
  <additionalField name="program" value="${sys:mySysProperty}"/>
 <additionalField name="environment" value="qa"/>
</MyLayout>
     </Socket>

What I'm trying to figure out is how I can get the $[sys:mySysProperty} to
resolve to the proper value.  I've looked through the code at
DailyRollingFileAppender to see how it does it, but I can't seem to find
anything actually performing the resolution.  Can someone point me in the
right direction?

Thanks,

Andrew