Java - Validating more than one field with only one annotation

Question

I found a similar question and answer -> How can I validate two or more fields in combination?

But I want to make it reusable way without using inheritance.

For example, I want to write an annotation that checks that end date must be greater than start date.

class A {
    Date startDate;
    Date endDate;
}

class B {
    Date startDate;
    Date endDate;
}

According to the answer I linked, I need to write 2 separate annotations. How can I do it with single annotation?

Answer 1

You can use reflection to get fields value.

1. In example you linked you need to add 2 fields to @AddressAnnotation:

• startDateFieldName
• endDateFieldName

2. Store them in MultiCountryAddressValidator in initialize method.
3. Change implements ConstraintValidator<AddressAnnotation, Address> to implements ConstraintValidator<AddressAnnotation, Object>
4. In isValid method use reflection to get actual values of start and end date.