code: protected void initRules() {
          this.validationRules = new Rules( getClass() ) {
              protected void initRules() {
                  add( PROPERTY_USERNAME, all( new Constraint[] { required(), minLength( getUsernameMinLength() ) } ) );
                  add( PROPERTY_PASSWORD, all( new Constraint[] { required(), minLength( getPasswordMinLength() ) } ) );
              }
  
              protected int getUsernameMinLength() {
                  return 2;
              }
  
              protected int getPasswordMinLength() {
                  return 2;
              }
  
          };
      }
docs: Initialize the field constraints for our properties. Minimal constraints are
  enforced here. If you need more control, you should override this in a subtype.