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

[Gunnar Morling]

2013/1/14 Emmanuel Bernard <emmanuel at hibernate.org>

> After letting this idea rest for a while, I still like it.
>
> In the issue, you also proposed to go a step further and get rid of
> @CrossParameterConstraint altogether. I sort of remember not being keen
> on the idea to protect future evolutions on the type-safe
> cross-parameter way. Have you thought about it more? What is your
> opinion?
>

Yes, I still like the idea of having only the @Constraint annotation for
defining constraints, but one problem I see is that one can not specify
different validators for return value arrays and method argument arrays in
cases like this:

    @MyCustomConstraint(validationAppliesTo = PARAMETERS)
    public void foo(int p1, int p2) { ... }

    @MyCustomConstraint(validationAppliesTo = ANNOTATED_ELEMENT)
    public Object[] bar() { ... }

With the different annotations @Constraint and @CrossParameterConstraint,
two different validators (both parameterized with <MyCustomConstraint,
Object[]>) could be specified here. Not sure, how relevant this is in
reality, though.

An alternative would be to extend @Constraint to accommodate the
specification of generic validator(s) and/or a cross-parameter validator:

    public @interface Constraint {
        Class<? extends ConstraintValidator<?, ?>>[] validatedBy() default
{};
        Class<? extends ConstraintValidator<?, ?>>
crossParameterValidatorType() default ConstraintValidator.class;
    }

The same rules would apply, i.e. the constraint must have a
"validationAppliesTo" attribute in case validators for both cases are
specified.

Thinking about Matt's IMPLICIT suggestion a bit more, I guess we could even
limit the cases where the constraint user must actually set the
"validationAppliesTo" attribute to those cases where it can't be inferred
automatically:

    //"validationAppliesTo" not required, since method is void
    @MyCustomConstraint
    public void foo(int p1, int p2) { ... }

    //not required, since method has no parameters
    @MyCustomConstraint
    public Object[] bar() { ... }

    //"validationAppliesTo" required if constraint has a (generic)
validator for Baz and cross-param validator for Object[]
    @MyCustomConstraint(validationAppliesTo = ANNOTATED_ELEMENT)
    public Baz foo(int p1, int p2) { ... }

    //"validationAppliesTo" required if constraint has a (generic)
validator for Object[] and cross-param validator for Object[]
    @MyCustomConstraint(validationAppliesTo = PARAMETERS)
    public Object[] foo(int p1, int p2) { ... }

The ValidationTarget enum would have the values IMPLICIT (default),
ANNOTATED_ELEMENT and PARAMETERS.

WDYT?

--Gunnar



>
> Emmanuel
>
> On Mon 2013-01-07 23:21, Gunnar Morling wrote:
> > 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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