Question

Is it possible with JSR-303 bean validation to write a custom annotation that can talk to a back end service?

We accomplished this type of validation with the “old school” Spring validators. In that case, the validator was a Spring bean and could have other services injected into it. Then that validator is injected into the controller.

An example might be an annotation (perhaps @EmailExists) to verify if an email already exists. I can only do this with a SQL query using one of our services. I would prefer to “validate” this alongside the other annotations and check it as soon as possible and not have to explicity do it in a back end service.

NOTE: We are using iBatis/MyBatis so I can’t use any JPA/Hibernate tricks 🙂

thanks!

Answer 1

That’s definitely possible. Spring provides dependency injection support also within constraint validators. So you can simply inject any required services in your custom validators like this:

public class EmailExistsValidator implements ConstraintValidator<EmailExists, String> {

    @Inject
    private EmailValidationService service;

    @Override
    public void initialize(EmailExists constraintAnnotation) {}

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {

        if (value == null) {
            return true;
        }

        return service.exists(value);
    }
}

Depending on your concrete scenario it might be a good idea to first check “cheap” constraints such as @NotNull and only if these constraints are valid check more expensive constraints such as @EmailExists.

You can do this with help of group sequences and a redefined default group sequence for your type.