Here are a couple links to start out with:
- http://svn.geotools.org/geotools/trunk/gt/ext/validation/src/org/geotools/validation/relate/
- http://svn.geotools.org/geotools/trunk/gt/ext/validation/src/org/geotools/validation/spatial/
Each validation check operates similar to a "function" with the following additions:
- packaged as a bean, with a locale aware bean info for ui generation in geoserver
- user defined tests are grouped into xml files in a manner similar to junit and its test suites
- there is a disjunct between a "test" in the xml file and the implementation, so code can be modified without breaking end users tests suites
- both per feature (FeatureValidation) and inter featureType (IntegrityValidation) api is available
There is way more then this going on of course, including an implementation of join (as nested for loops), the ability to escape into SQL for JDBC based datastores, enough metadata about the validation tests that the engine can figure out what tests to run in the face of a modification, and finally the ability to hook into a full network builder (ported from JUMP) for topology based tests.
Jody