Allow failing on null values for creator (DeserializationFeature.FAIL_ON_NULL_CREATOR_PROPERTIES)

[NTCoding]

In relation to this discussion: #988 and this issue in Scala: FasterXML/jackson-module-scala#203 I would like to provide a way for deserialization to fail when a value is missing for a property except when the property is a type of optional.

In Scala the default value should be a None, but that is handled by the custom deserializers in the Jackson Scala Module and isn't a concern here. What I've done here is added a hook that will fail on null default values - ensuring that no values can be null. That's what we are really trying to achieve with optionals - to enforce there are no nulls in the codebase.

I belive this pull request is useful to anyone - using Java or Scala - that wants to avoid nulls in their codebase. It's still possible to return a different value from getNullValue so it won't just work for optionals - any type who's getNullValue doesn't return a null.

This is just an example pull request. I'm happy to put more effort into it if you are happy to go ahead with the idea.

[cowtowncoder]

Ok, I have think of this a bit more, but it seems reasonable enough on surface.

I am not big believer of "no nulls" approach myself, but since many users like such approaches (and inevitable overuse of Optionals in all kinds of wrong places :) ) it seems reasonable to try to support as conveniently as possible.

[NTCoding]

Great :) I'm happy to work with you and find the best solution if I can. I'm more than happy to do the work if you point me in the right direction.

[potmo]

👍
I have been looking for this for quite some time. Right now I have to do validation manually after deserialising and that is a pain. I think that this is a really great PR.

[sommestad]

👍

[petrenkotino]

👍

[kotlinski]

👍

[petruswrango]

👍

[cowtowncoder]

@NTCoding One wish: it would be great to get a unit test for this. Partly this because I think patch does not actually cover the case of explicit null, which would have to be done in method "assignParameter(...)" I think.
If this could be added, I would be able to get patch checked in for inclusion in 2.7.0.

[NTCoding]

@cowtowncoder Hi, I'll be more than happy to add some tests for this. The reason that I did not initially was because this is just an example of what I was trying to achieve.

If you're happy for me to proceed then I'll definitely spend some time and make this a robust implementation.

Could you tell me when you need it by please?

[cowtowncoder]

@NTCoding a simple unit tests that cover both missing creator property and explicit null would be good. I think I can handle the missing case (if it's missing like I think). But tests are also good to verify that I didn't misunderstand expected functionality (even if it is quite straightforward here).

[cowtowncoder]

Implemented, will be in 2.8.0.