[bv-dev] Distinguishing cross-parameter and generic constraints

[Gunnar Morling]

Hi all,

As per the latest spec draft, a constraint must be either a cross-parameter
*or* a generic constraint, but not both at the same time, as otherwise it
would be ambiguous whether a constraint on a method refers to the method
parameters or return value.

Most of the time this does not really pose a limitation, but some
constraints actually might be both, cross-parameter *and* generic,
depending on the specific context. Examples are @ScriptAssert in Hibernate
Validator or generic constraints such as this:

@EqualPasswords
public void register(String userName, String password, String
confirmedPassword) {
}

@EqualPasswords
public class ResetPasswordRequest {
    String userName;
    String password;
    String confirmedPassword;
}

Based on a recent BVAL issue [1], I'm proposing to introduce a special
constraint annotation attribute, "validationAppliesTo", allowing to specify
the required behavior at the usage site:

@EqualPasswords(validationAppliesTo=PARAMETERS)
public void register(String username, String password, String
confirmPassword) {
}

@EqualPasswords(validationAppliesTo=ANNOTATED_ELEMENT)
public class ResetPasswordRequest {
    String password;
    String confirmedPassword;
}

The following rules would apply:

* If a constraint is annotated with @Constraint and
@CrossParameterConstraint, it must define a member "validationAppliesTo".
The default value should be ANNOTATED_ELEMENT.
* If a constraint is annotated with only one
of @Constraint/@CrossParameterConstraint, defining a "validationAppliesTo"
member doesn't have any special effect
* Specifying validationAppliesTo=PARAMETERS anywhere except a method causes
ConstraintDeclarationException

Any thoughts?

--Gunnar

[1] https://hibernate.onjira.com/browse/BVAL-340
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