The FieldValidator interface defines the methods to be implemented by FieldValidators,
which are used by the XWork validation framework to validate Action properties before
executing the Action.
This interceptor runs the action through the standard validation framework, which in turn checks the action against
any validation rules (found in files such as ActionClass-validation.xml) and adds field-level and action-level
error messages (provided that the action implements
com.opensymphony.xwork.ValidationAware ).