Add jackson-module-spi-subtypes module

[black-06]

Adds a module that allows subtype to be registered without annotating the parent class.

It is implemented on SPI.

See FasterXML/jackson-databind#2104

Its original version is https://github.com/black-06/jackson-modules-dynamic-subtype.

I removed the custom ServiceLoader and always use the standard library,
which means that pojo always needs a no-parameter constructor.

[JooHyukKim]

If the two lists are null-safe, no need for empty checking.

[cowtowncoder]

Quick note: due to timing, decided that I will not try to get this in Jackson 2.16 -- so will need to be re-based to 2.17.
This because there's a bit of work to be done here.

For example, I think that naming of @JsonSubType should change as the general rule is that only jackson-annotations has annotations with name starting with @Json (and for legacy reasons, some in jackson-databind).
Modules can provide annotations that start with @Jackson prefix (or anything else that makes sense).

But I need to read the PR in bit more thought to make sure my feedback is relevant.

[amricko0b]

Hello everyone!
Any updates on this one?

[cowtowncoder]

I totally forgot. If this was to be added, would need to re-base/re-create against 2.x branch.

[black-06]

I totally forgot. If this was to be added, would need to re-base/re-create against 2.x branch.

I've changed base to 2.x branch, and I checked the comments above, most of them have been modified, and the problems that haven't been solved include:

• findSubtypes: It's almost a copy of AnnotationIntrospectorPair.findSubtypes, so I tend not to modify it.
• annotation name JsonSubType: If the user understands JsonSubTypes, then he should be able to understand JsonSubType easily, but if there is a more appropriate name, I have no problem.

Because it's been a while. We can review the code again.

[cowtowncoder]

This seems to fail on Java 8: if we are targeting Jackson 2.x, module should be Java 8 compatible.

But looks like simple change to pom.xml solves that. I'll fix it.

EDIT: done

[cowtowncoder]

@black-06 Ok: I have some minor questions, but overall the only changes I do require are to naming:

1. Annotation name to avoid "@JSON" prefix: my proposal is "@JacksonSubType"
2. Module name: jackson-module-spi-subtypes (not just "jackson-module-subtype")
3. Module directory: spi-subtypes (not just "subtypes")

and matching changes to module-info.java and README (aside from obvious Java classes and pom.xml).
I am open to other names too, but want them to be slightly more specific.

Once naming is changed, I can get this merge in 2.x for inclusion in upcoming 2.21.0 release.

[cowtowncoder]

@black-06 One other (hopefully last) thing: if you haven't yet sent CLA (I don't think so but just in case), we would need it from:

https://github.com/FasterXML/jackson/blob/main/contributor-agreement.pdf

This only needs to be done once, before merging the first PR: one is valid for all future contributions.
The usual way is to print it, fill & sign, scan/photo, email to cla at fasterxml dot com.
Once we have it, merging can occur (assuming code review etc complete).

Looking forward to getting this PR merged; thank you again for providing it!

[black-06]

@black-06 Ok: I have some minor questions, but overall the only changes I do require are to naming:

1. Annotation name to avoid "@JSON" prefix: my proposal is "@JacksonSubType"
2. Module name: jackson-module-spi-subtypes (not just "jackson-module-subtype")
3. Module directory: spi-subtypes (not just "subtypes")

and matching changes to module-info.java and README (aside from obvious Java classes and pom.xml). I am open to other names too, but want them to be slightly more specific.

Once naming is changed, I can get this merge in 2.x for inclusion in upcoming 2.21.0 release.

The name has been changed. In addition,

1. rename package to com.fasterxml.jackson.module.spisubtypes.
2. rename SubtypeModule to SubtypesModule, SubtypeAnnotationIntrospector to SubtypesAnnotationIntrospector.

Thank you very much for your review, ps. CLA has been sent.

[cowtowncoder]

@black-06 Thank you for doing all of that. I am hoping to go over PR one more time, get it merged, and soon prepare for 2.21.0 release that includes the new module!