Create custom hibernate Validator based on existing Hibernate Validator

Question

I would like to create a custom constraint @LengthIfNotBlank(min=,max=) based on Hibernates @Length constraint:

@Target( { METHOD, FIELD, ANNOTATION_TYPE })
@Retention(RUNTIME)
@Constraint(validatedBy = LengthIfNotBlankValidator.class)
@Documented
public @interface LengthIfNotBlank {
    String message() default "{com.mycompany.LengthIfNotBlank}";
    Class<? extends Payload>[] payload() default {};
    int max() default Integer.MAX_VALUE;
    Class<?>[] groups() default {};
    int min() default 0;
}

public class LengthIfNotBlankValidator extends LengthValidator {
    @Override
    public boolean isValid(String value, ConstraintValidatorContext constraintValidatorContext) {
        return StringUtils.isBlank(value) || super.isValid(value, constraintValidatorContext);
    }
}

Exception:

[java] javax.validation.ValidationException: Unable to initialize com.example.constraints.LengthIfNotBlankValidator
[java] Caused by: java.lang.ClassCastException: $Proxy32 cannot be cast to org.hibernate.validator.constraints.Length

UPDATE

Still getting this exception :-(

public class LengthIfNotBlankValidator extends LengthValidator {
    public void initialize(LengthIfNotBlank parameters) {
        Map<String,Object> elements = new HashMap<String,Object>();
        elements.put("message", parameters.message());
        elements.put("payload", parameters.payload());
        elements.put("groups", parameters.groups());
        elements.put("min", parameters.min());
        elements.put("max", parameters.max());

        AnnotationDescriptor<Length> descriptor = AnnotationDescriptor.getInstance(Length.class, elements);
        Length length = AnnotationFactory.create(descriptor);
        super.initialize(length);
    }

    @Override
    public boolean isValid(String value, ConstraintValidatorContext constraintValidatorContext) {
        return StringUtils.isBlank(value) || super.isValid(value, constraintValidatorContext);
    }
}

Answer 1

Since annotation types cannot be inherited, you should override initialize() and pass an actual instance of type Length to super.initialize().

Note that you cannot create an instance of annotation type directly, therefore you have to use something like AnnotationFactory.

Also I'm not sure that compiler would allow you to do it due to type safety constraints. If it doesn't, use composition instead of inheritance (i.e. create a field of type LengthValidator).