It would be really handy of Commons Configuration could support nested variable interpolation:
org.osgi.framework.system.packages=org.osgi.framework; version=1.4.0, \
org.osgi.service.packageadmin; version=1.2.0, \
org.osgi.service.startlevel; version=1.1.0, \
org.osgi.service.url; version=1.0.0, \
org.osgi.util.tracker; version=1.3.3 \
The process being to expand innermost variable references first and work outwards - this allows for very advance config, such as the above, which can dynamically detect the Java version and expand a JRE version specific property into the property being defined.
Care is needed to avoid recursion, which can been handled through use of a "Cycle Map" that detects of an inner variable has already been referenced.
An example of this can be found in the substVars method of the Apache Felix org.apache.felix.framework.util.Util class.