[hibernate-dev] [Validator] Applying constraints to property objects

[Emmanuel Bernard]

On Wed 2013-09-04  8:27, Gunnar Morling wrote:
> 2013/9/3 Emmanuel Bernard <emmanuel at hibernate.org>
> 
> > Something like c makes sense.
> >
> 
> Ok.
> 
> 
> > It similar to the notion of converter in JPA.
> >
> > But why not the following style of interfaces
> >
> >     interface Convert<From,To> {
> >         To convert(From);
> >     }
> >
> 
> Yes, thinking more about it, it probably makes sense to support multiple
> converters, e.g. in case someone works with UIInput and *Property in the
> same application. Then the "From" parameter makes sense to avoid casts
> within the converter implementation. Need to experiment with it a bit.

More than just the cast in the impl, its a way to statically know that
you have a converter that takes From and only From, so you can restrict
the list of converters to consider to 0 or 1 all the time (assuming we
don't support multiple converters of the same From.

> 
> Regarding the name, I find "Converter" a bit too generic, in particular
> since it needs not only to convert the actual property value but also the
> static type so you can reject this (because @Size can't be applied to
> Object):

Not sure I follow, before the core HV work, it converts a property or
class from From to To and from there you are good to go.

Are you saying that if From = UIInput<String> we would not be able to
separate it from UIInput<Object> statically? I *think* we can, just like
we do for collections in JPA.

> 
>     @Size(min=3, max=10)
>     UIInput<Object> name;
> 
> 
> On Tue 2013-09-03 15:58, Gunnar Morling wrote:
> > > Hi,
> > >
> > > Yesterday George Gastaldi from the Forge team approached me regarding the
> > > application of constraints to "wrapped" properties. Their situation is
> > the
> > > following:
> > >
> > >     ...
> > >     @Size(min=3, max=10)
> > >     UIInput<String> name;
> > >     ...
> > >
> > > Here, UInput is some kind of UI component, wrapping the actual property
> > > value. As is, validation of this property will fail since there is no
> > > validator for applying @Size to UIInput (only for String).
> > >
> > > A similar use case exists in JavaFX where one might want to apply
> > > constraints to the FX *Property types:
> > >
> > >     ...
> > >     @Min(5)
> > >     IntegerProperty count = new SimpleIntegerProperty(4);
> > >     ...
> > >
> > > Again, validation will fail out of the box, as no validator for applying
> > > @Min to IntegerProperty exists (but for int/Integer).
> > >
> > > How could this be solved? The following alternatives came to my mind:
> > >
> > > a) Create and register validators for these wrapper types, e.g.
> > > ConstraintValidator<Size, UIInput> etc.
> > >
> > > Pro: Works with the existing APIs without modification
> > > Con: Lots of code to write, either duplicating code or delegating to
> > > (internal) implementation types; doesn't automatically benefit from new
> > > built-in validators
> > >
> > > b) Apply constraints to getters instead of fields:
> > >
> > >     IntegerProperty count = new SimpleIntegerProperty(4);
> > >
> > >     @Min(5)
> > >     int getCount() {
> > >         return count.getValue();
> > >     }
> > >
> > > Pro: Works with the existing APIs without modification; benefits from any
> > > newly added built-in validators
> > > Con: There may be cases where there is no such getter, e.g. for parameter
> > > validation
> > >
> > > c) Provide an SPI which allows to plug in a custom "value processor"
> > > implementation, retrieving the wrapped object and its "declared" type:
> > >
> > >     public interface ValidationValueProcessor {
> > >         Object getValidatedValue(Object source);
> > >         Type getDeclaredType(Type sourceType);
> > >     }
> > >
> > > For the original example, the methods would return the name value and
> > > String.class, respectively.
> > >
> > > Note that validator resolution is done using the static type of a
> > property,
> > > so I think the original example above should be supported, but the
> > > following should not as no validator for @Size/Object exists:
> > >
> > >     @Size(min=3, max=10)
> > >     UIInput<Object> name;
> > >
> > > Pro: Benefits from any newly added built-in validators, allows directly
> > > annotating "wrapped" properties, requires no implementation by the user
> > > besides the ValidationValueProcessor
> > > Con: new HV-specific (at least for the time being) SPI
> > >
> > > I think a) creates prohibitively high efforts for the user/integrator, b)
> > > lacks support for method constraints, so I think c) should be
> > implemented,
> > > possibly making this a spec SPI later on.
> > >
> > > Does anyone have other preferences or alternatives? If you also think c)
> > > makes most sense, do you have a good/better idea for the interface and
> > > method names?
> > >
> > > Thanks,
> > >
> > > --Gunnar
> > > _______________________________________________
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> > > hibernate-dev at lists.jboss.org
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> >