The ability to override annotations without having to express the linkage to
the classes that use those annotations.
1. meta-data in xml
- overriding java classes
- hard to maintain the relationship between classes and config
- redundant config
2. annotation meta-data
- easy to maintain the relationship between classes and config
- overriding java classes
- redundant config in both annotation and xml
3. meta-annotations with annotation-level override
- easy to maintain the relationship between "config" and classes
- overriding annotations, not classes
- extremely brief config in both annotation and xml
