I generally like this; I think the proposal is in quite good shape. My reactions to the open questions:

1. Should nested containers be supported?
  Given the Optional<List<@Email String>> example, it's probably a good idea (certainly it adds some complexity, but anything worth doing is worth doing right, as the saying goes).

2. Should @ConstraintsApplyTo be usable per-constraint?
  Doing such seems like it could be a bit clumsy, but it might be okay if @ConstraintsApplyTo were repeatable and included Class<? extends Annotation>[] constraintTypes() default {} element which, if non-empty, could differentiate which constraints applied to the wrapper vs. the extracted value.

3. Should @ConstraintsApplyTo also be used for tagging extractors triggering "auto-extraction"?
  I'm not sure I understand this question. Is this referring to classpath scanning, or "inline" specification of extractors? In either case a separate annotation might be warranted, particularly if the annotation is expanded for other reasons (see #2).

4. Should a path node be added for type argument constraints of Optional and similar types?
  I don't personally feel this is necessary, but I wouldn't oppose it if others strongly desired it.

5. Should value extractors be discoverable via ServiceLoader?
  I would personally really like to see this be possible. WRT being able to override these, it seems simple enough that value extractors registered programmatically or in validation.xml could preempt any discovered via ServiceLoader.

6. What to return from PropertyDescriptor#getElementClass() if field type does not match method RT?
  The Javadoc of PropertyDescriptor references "Java Beans." Java Bean properties are defined by method signatures rather than fields, so it seems an easy decision to choose the method RT over the field signature.

7. Should the presence of type argument constraints alone trigger nested validation?
  I can appreciate the sense of the consistency argument to requiring @Valid, but in practice it seems reasonable to me that @Valid be implied. It probably would be much simpler to require @Valid from an implementation perspective, however.

8. The core question seems to me to be whether there should be multiple valid mechanisms for specifying constraints on wrapped elements.
  Since it's necessary to support the inverse for wrapped elements of non-parameterized types, the effort required to support this is probably similar to that required to disallow it.

9. Should we allow extractors to be defined for specific parameterized types?
  It's tempting to support this, extends bounds and all, just to feel that the feature is as complete as possible. At the same time I can't make a compelling case why such a feature would really be *necessary*. This suggests support for things like: class FooToBarMapValueExtractor implements ValueExtractor<Map<? extends Foo, @ExtractedValue ? extends Bar>> . In this case it might even be reasonable to equivalently support: class FToBMapValueExtractor<F extends Foo, B extends Bar> implements ValueExtractor<Map<F, @ExtractedValue B>> (i.e. BV would not care whether the bounds are specified on the type parameter of the extractor class or on the argument to the wrapper type).  Not to say that these might not be valid use cases, but we should enter any such complexity with our eyes open, to be sure.

10. I have no suggestions for alternatives to "type argument constraints." "Type constraints" suggests the argument to a type parameter itself. "Type use constraints" is fine with me, though. If people don't know, they can learn. ;)

11. No input on this from me (for now, anyway).

12. I don't see why it would be necessary for an extractor to refer to a parameter from a super type. The super type parameter can(/should) be mapped to a type parameter from the extractor type. Or am I missing something?

13/14. I am in favor of the variant proposals that would permit the type parameter to be included as part of a Path node.

Additional notes:

* I think we ought to consider whether to provide ValueReceiver methods that omit the nodeName parameter rather than having ValueExtractor implementations pass null. We could potentially then require non-blank values when the nodeName-bearing method variants are called. This would make the API more explicit and we're not talking about an interface that will conceivably have a lot of implementations.

* Some of the features that might come under debate, e.g. ServiceLoader-discovered ValueExtractors, could be enabled via implementation-specific configuration parameters or e.g. CDI extensions, etc., if the community can't reach a consensus or decides to omit them.


On Fri, Jan 6, 2017 at 9:43 AM, Gunnar Morling <gunnar@hibernate.org> wrote:
Hi all,

I've pushed the consolidated proposal on the validation of container elements as an appendix to the spec draft now:

Can you please give it a read and raise your opinion on it. I know that Hendrik and Christian had some remarks/concerns originally, I'd hope the new one can address them appropriately. Apart from the more formal specification I recommend you take a look at the examples [1] to get a feeling for the indented semantics.

Towards the end of the appendix there's a list of open questions [2]. Any input on those is highly welcome. E.g. 1, 2 and 5 would deserve some more opinions. Also what's your take on the ValueExtractor API in general? I'll wait a few more days to let you form some thoughts and then start a specific thread per open question.

In the meantime we'll continue with implementing this proposal in the RI as far as it's defined.



2016-12-22 17:15 GMT+01:00 Gunnar Morling <gunnar@hibernate.org>:

It has been silent on the surface, but that doesn't mean we haven't been busy :)

I've spent some time working on a proof of concept implementation of
the value extraction proposal (BVAL-508) which has been added to the
RI earlier this week [1]. This effort brought a fair bit of insights
and clarity which is reflected in an updated proposal that you can
find at [2].

So if you can spend some time and review it, that'd be much
appreciated. It's a bit more formalized than the original proposal and
also takes an opinion on some of the original questions. I tried to
cut down a bit on the many loose ends to advance the matter a bit, but
if you are doubtful about any of the directions taken, please speak

Also the ValueExtractor contract has evolved a bit. It now allows for
a nicely uniform, yet efficient implementation of the single value and
multi value case. Refer to the pull request for the details. There are
some open questions at the end of the document. Feedback on these (and
of course any other parts of the document) are highly welcome. I've
checked on the previous threads and discussions of the matter (e.g.
Hendrik's extensive feedback) and hope the proposal covers all the
essential items. Please let me know if anything is missing.

The idea is that we add this version of the proposal as an appendix to
the spec, adjust the RI to conform with it (it already does more or
less) so we can aim for a first alpha release of spec and RI very
early next year. That will allow people to get their hands on this
feature and play around with it a bit, hopefully resulting in some
more feedback from the outside, too.

My main remaining questions are:

* Is allowing to put constraints to an element but automatically
applying them to the wrapped value the right thing to do? I can see
the concerns about lacking comprehensibility (it's working based on
the presence of an automatically unwrapping value extractor), but then
it's needed to support "@Email StringProperty email"
* Should we support nested collections (e.g. List<Map<String, @NotNull
String>> addressesByType). It hasn't been supported before, but I can
see how it's more appealing with type parameter constraints. But it
adds complexity, too.

Looking forward to your feedback,


[1] https://github.com/hibernate/hibernate-validator/pull/592
[2] https://github.com/beanvalidation/beanvalidation-spec/pull/116

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