code: @Override
      public boolean isValid(String value, ConstraintValidatorContext context) {
          try {
              this.duracloudUserService.checkUsername(value);
              return false;
          } catch (InvalidUsernameException ex) {
              return false;
          } catch (UserAlreadyExistsException ex) {
              return true;
          }
      }
docs: /* (non-Javadoc)
  @see javax.validation.ConstraintValidator#isValid(java.lang.Object, javax.validation.ConstraintValidatorContext)