For the underlying JSR303 Bean Validation specification, checkout the JCP
TBD...
The complete set of online JavaDocs for the provider and optional modules can be found online or downloaded from a Maven repository.
Online 0.3-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository - bval-jsr303-0.3-incubating-javadoc.jar bval-core-0.3-incubating-javadoc.jar
Online 0.2-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository - bval-jsr303-0.2-incubating-javadoc.jar bval-core-0.2-incubating-javadoc.jar
Online 0.1-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository - bval-jsr303-0.1-incubating-javadoc.jar bval-core-0.1-incubating-javadoc.jar
Since Apache BVal implements the JSR303 Bean Validation specification, most articles and HowTo guides for the RI (Hibernate Validator) can also be used as a reference: