MyFaces Commons Validators contains some useful validators not provided by the spec, and a base class for create custom validators using myfaces-builder-plugin.
This base class ( org.apache.myfaces.commons.validator.ValidatorBase ) allow use ValueExpressions and in 2.0 version provides an implementation of StateHelper. This is useful in cases where parameters for the validator are bound to ValueExpression instance that are evaluated at runtime. By default, validator parameters are evaluated on build time (the first time the view is built), so they cannot change over the time (further web postback requests).
To understand this see the example on MyFaces Commons Converters section.
In summary, in this project you can found: