Annotations in mix-ins added to ObjectMapper are not considered by the generator

[bmaizel]

I expected, that if I add a mix-in (with ObjectMapper.addMixIn() or ObjectMapper.setMixInResolver()) to the ObjectMapper instance and supply it to the SchemaGeneratorConfigBuilder constructor, it will be considered by the generator. (Yes, I know that this expectation is not supported by the documentation :)

It partially works. Mix-ins are considered during introspection, so, per example, adding @JsonProperty annotation via mix-in can make the annotated field visible to the generator. But the annotation access methods in the FieldScope and MethodScope do not consider mix-ins. So adding @JsonProperty("changedName") to the mix-in will not affect the property name in the generated schema.

I believe, it would be good to support mix-ins in the FieldScope, MethodScope and TypeScope.

For a while I can live with a workaround, which is hacky and shaky, and I would like to remove it:

SchemaGeneratorConfigBuilder configBuilder = new SchemaGeneratorConfigBuilder(
        objectMapper,
        schemaVersion,
        OptionPreset.PLAIN_JSON)
        .with(builder -> {
            builder.forFields().withPropertyNameOverrideResolver(fieldScope -> mixIn(fieldScope));
        })
        .with(jacksonModule)
        .with(javaxValidationModule)
        .with(jakartaValidationModule);

private String mixIn(FieldScope fieldScope) {
    Class<?> mixIn = objectMapper.findMixInClassFor(fieldScope.getDeclaringType().getErasedType());
    if(mixIn != null) {
        try {
            Field declaredField = mixIn.getDeclaredField(fieldScope.getDeclaredName());
            Annotation[] declaredAnnotations = declaredField.getDeclaredAnnotations();
            for(Annotation a : declaredAnnotations) {
                fieldScope.getMember().getAnnotations().add(a);
            }
        } catch (NoSuchFieldException e) {
            // NOOP
        }
    }
    return null;
}

[CarstenWickner]

Hi @bmaizel,

Three thoughts on this:

1. Supporting MixIns would be nice, as they are a standard Jackson feature.
2. I'm glad you found a workaround for now. That looks awfully Jackson-specific though and would therefore need to be considered carefully (and allowed to be toggled on/off). Doing this in the schema generator is not the ideal place to start.
3. Since the ResolvedField you're adding the annotation one yourself is returned by the classmate library, which happens to be maintained by the same person as the jackson libraries, it'd be a good idea to approach them for suggestions how to consider things like MixIns in classmate. If that requires changes inside the schema generator, I'm open to that.

Will you approach Tatu through https://github.com/FasterXML/java-classmate?

[bmaizel]

Hi @CarstenWickner,

I could do it, but you can do it better (please!). You definitely have much deeper understanding of the problem, and your code is going to use the (new) functionality, so better you start the discussion. Of course I will follow, and I will try to help if I see how.

[CarstenWickner]

Hi @bmaizel,

I had a brief look into the classmate code and figured that this is the place, where one would have to make a change to get mix-ins to work:

jsonschema-generator/jsonschema-generator/src/main/java/com/github/victools/jsonschema/generator/TypeContext.java

Line 146 in ea90e6c

return this.memberResolver.resolve(resolvedType, this.annotationConfig, null);

The third parameter on the classmate MemberResolver.resolve() method is of type AnnotationOverrides.
Currently, I'm always sending null. But providing an AnnotationOverrides instance would allow mix-ins to be respected.

Perhaps creating a subtype of the AnnotationOverrides, which delegates to the ObjectMapper's findMixInClassFor() method, would do the trick.
I've never used mix-ins myself, so I feel a little out of my depth here. Feel free to give it a shot and create a PR to move this along.

[CarstenWickner]

I've raised the question now: FasterXML/java-classmate#81
Let's see, what comes out of it.

[bmaizel]

Subscribed.
BTW, don't forget, that mix-ins can extend not only annotations on the class members, but also on the class itself.

[CarstenWickner]

One challenge at a time. 😆
Feel free to provide PRs to extend the mix-in support going forward. 😉

[bmaizel]

I have a workaround 😆
It seams to be more complicated then I thought. I will try to find time for it next week.