JsonEnumDefaultValue support

[tfactor2]

We check the backward compatibility of payloads based on schemas. There is a case that we want to mark as backward compatible, particularly when the enum set is reduced but there was a default enum value set. This will allow developers to fallback to it even if some enum is deleted. Here is the PR to support it in the Jackson module.

     enum TestEnumWithJsonEnumDefaultAnnotations {
         A, @JsonEnumDefaultValue B, C
     }

->

      "enumValueWithJsonEnumDefaultValueAnnotation": {
            "type": "string",
            "enum": ["A", "B", "C"],
            "default": "B"
        },

PS: Not sure about using DefaultResolver in the module.

[CarstenWickner]

Hi @tfactor2,

First of all: Thank you for your contribution!
Generally, I don't mind adding support for this Jackson annotation.

A challenge for such additions, like your proposed JsonEnumDefaultValueResolver, is that it is probably working fine in your particular use-case but doesn't cover several other scenarios (namely: the variety of representations for a given enum value – for which there have been surprisingly many questions raised in the context of this project in the past).
Now I'm curious how widely used it is to declare a default enum value on the enum itself and not in the context of a particular field/method where it is used.
I'm wondering because I see two options here:

1. We add some complexity to the JsonEnumDefaultValueResolver, accepting the fact that this would never cover all scenarios, but would at least cover those already supported by the schema generator today.
2. Or you leave this piece of straightforward configuration in your own code base, where you can make simplifying assumptions such as: "enum values in JSON match the Java/Kotlin enum values".

Is it worth tackling all that complexity or do you rather keep this on your side and we only add this as another example somewhere in the documentation, where others will still be able to benefit from your contribution?
What do you reckon?

[CarstenWickner]

suggestion: Since MethodScope.getType() can return null for void methods and you're only applying this to fields, I'd suggest making the declared parameter type FieldScope instead.
Also getErasedType() may return null, I believe, if the given type is merely a generic type placeholder.

Suggested change

	public static String apply(TypeScope typeScope) {
	if (typeScope.getType().getErasedType().isEnum()) {
	return Arrays.stream(typeScope.getType().getErasedType().getDeclaredFields())
	public static String apply(FieldScope scope) {
	Class<?> erasedType = scope.getType().getErasedType();
	if (erasedType != null && erasedType.isEnum()) {
	return Arrays.stream(erasedType.getDeclaredFields())

[CarstenWickner]

As per below suggestion, declaring the more specific FieldScope instead of TypeScope as method parameter type.

Suggested change

	import com.github.victools.jsonschema.generator.TypeScope;
	import com.github.victools.jsonschema.generator.FieldScope;

[CarstenWickner]

suggestion (blocking): This is too simplistic.
There are several ways to represent an enum value in the JSON (Schema). The declared name in the code may therefore not be the appropriate one to declare as default here.
We should at least attempt to cover the handful of standard options already supported by the main schema generator and this JacksonModule (e.g., considering @JsonValue annotations).

[CarstenWickner]

nitpick: The alignment seems a bit off. 😉