How can I validate two or more fields in combination?

Question

I'm using JPA 2.0/Hibernate validation to validate my models. I now have a situation where the combination of two fields has to be validated:

public class MyModel {
    public Integer getValue1() {
        //...
    }
    public String getValue2() {
        //...
    }
}

The model is invalid if both getValue1() and getValue2() are null and valid otherwise.

How can I perform this kind of validation with JPA 2.0/Hibernate? With a simple @NotNull annotation both getters must be non-null to pass validation.

Answer 1

For multiple properties validation, you should use class-level constraints. From Bean Validation Sneak Peek part II: custom constraints:

Class-level constraints

Some of you have expressed concerns about the ability to apply a constraint spanning multiple properties, or to express constraint which depend on several properties. The classical example is address validation. Addresses have intricate rules:

• a street name is somewhat standard and must certainly have a length limit
• the zip code structure entirely depends on the country
• the city can often be correlated to a zipcode and some error checking can be done (provided that a validation service is accessible)
• because of these interdependencies a simple property level constraint does to fit the bill

The solution offered by the Bean Validation specification is two-fold:

• it offers the ability to force a set of constraints to be applied before an other set of constraints through the use of groups and group sequences. This subject will be covered in the next blog entry
• it allows to define class level constraints

Class level constraints are regular constraints (annotation / implementation duo) which apply on a class rather than a property. Said differently, class-level constraints receive the object instance (rather than the property value) in isValid.

@Address 
public class Address {
    @NotNull @Max(50) private String street1;
    @Max(50) private String street2;
    @Max(10) @NotNull private String zipCode;
    @Max(20) @NotNull String city;
    @NotNull private Country country;

    ...
}

@ConstraintValidator(MultiCountryAddressValidator.class)
@Target(TYPE)
@Retention(RUNTIME)
public @interface Address {
    String message() default "{error.address}";
    String[] groups() default {};
}

public class MultiCountryAddressValidator implements Constraint<Address> {
    public void initialize(Address constraintAnnotation) {
    // initialize the zipcode/city/country correlation service
    }

    /**
     * Validate zipcode and city depending on the country
     */
    public boolean isValid(Object object) {
        if (!(object instanceof Address)) {
            throw new IllegalArgumentException("@Address only applies to Address");
        }
        Address address = (Address) object;
        Country country = address.getCountry();
        if (country.getISO2() == "FR") {
            // check address.getZipCode() structure for France (5 numbers)
            // check zipcode and city correlation (calling an external service?)
            return isValid;
        } else if (country.getISO2() == "GR") {
            // check address.getZipCode() structure for Greece
            // no zipcode / city correlation available at the moment
            return isValid;
        }
        // ...
    }
}

The advanced address validation rules have been left out of the address object and implemented by MultiCountryAddressValidator. By accessing the object instance, class level constraints have a lot of flexibility and can validate multiple correlated properties. Note that ordering is left out of the equation here, we will come back to it in the next post.

The expert group has discussed various multiple properties support approaches: we think the class level constraint approach provides both enough simplicity and flexibility compared to other property level approaches involving dependencies. Your feedback is welcome.