The JavaDoc for Class PropertiesConfiguration lists a very handy feature of Configuration, which is variable substitution. The behavior can be seen in the example file on the JavaDoc (good).
But it's not in the preceding list of features. It would be nice to have a little writeup describing the intended behavior: What is supposed to happen if you have an undefined ref? What about nested refs? What about circular refs?
And it was interesting to find that the substitution occurred during get time rather than load (unexpected), and that if one calls the wrong get() method then the substitution will not happen. (Is that intended behavior?) This should be called out in the description as well as the individual method JavaDocs.