Mix-ins do not work for `Enum`s

[kostousov-ds]

1. create java enum like this

public enum SomeEnum {
    none,
    tax10,
    tax20
}

2. create mixin for enum

public enum  SomeEnumMixin {
    @JsonProperty("zero")
    none,
    @JsonProperty("TypTyp")
    tax10,
    @JsonProperty("PytPyt")
    tax20
}

3. register mixin via .addMixIn(SomeEnum.class, SomeEnumMixin.class)

4. try to deserialize sometithing

ObjectMapper throws NullPointerException

java.lang.NullPointerException
	at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector._addFieldMixIns(AnnotatedFieldCollector.java:117)
	at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector._findFields(AnnotatedFieldCollector.java:94)
	at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector.collect(AnnotatedFieldCollector.java:48)
	at com.fasterxml.jackson.databind.introspect.AnnotatedFieldCollector.collectFields(AnnotatedFieldCollector.java:43)
	at com.fasterxml.jackson.databind.introspect.AnnotatedClass._fields(AnnotatedClass.java:366)
	at com.fasterxml.jackson.databind.introspect.AnnotatedClass.fields(AnnotatedClass.java:338)
	at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector._addFields(POJOPropertiesCollector.java:393)
	at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.collectAll(POJOPropertiesCollector.java:322)
	at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.getPropertyMap(POJOPropertiesCollector.java:287)
	at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.getProperties(POJOPropertiesCollector.java:186)
	at com.fasterxml.jackson.databind.introspect.BasicBeanDescription._properties(BasicBeanDescription.java:164)
	at com.fasterxml.jackson.databind.introspect.BasicBeanDescription.findProperties(BasicBeanDescription.java:239)
	at com.fasterxml.jackson.databind.deser.BasicDeserializerFactory._findCreatorsFromProperties(BasicDeserializerFactory.java:292)
	at com.fasterxml.jackson.databind.deser.BasicDeserializerFactory._constructDefaultValueInstantiator(BasicDeserializerFactory.java:276)
	at com.fasterxml.jackson.databind.deser.BasicDeserializerFactory.createEnumDeserializer(BasicDeserializerFactory.java:1472)
	at com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer2(DeserializerCache.java:371)
	at com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer(DeserializerCache.java:349)
	at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2(DeserializerCache.java:264)
	at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer(DeserializerCache.java:244)
	at com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer(DeserializerCache.java:142)
	at com.fasterxml.jackson.databind.DeserializationContext.findNonContextualValueDeserializer(DeserializationContext.java:481)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.resolve(BeanDeserializerBase.java:497)
	at com.fasterxml.jackson.databind.deser.std.DelegatingDeserializer.resolve(DelegatingDeserializer.java:58)
	at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2(DeserializerCache.java:293)
	at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer(DeserializerCache.java:244)
	at com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer(DeserializerCache.java:142)
	at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:491)
	at com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer(ObjectMapper.java:4669)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4478)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3434)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3402)

I've tested on 2.11.0 and 2.11.1

[cowtowncoder]

Ok I can reproduce this.

Also checked same failure occurs on 2.10 at least.

[cowtowncoder]

So, as per commit message, added failing test; and fixed NPE part.

However, functionality will not work as expected, most likely because of the way JDK implements Enums under the hood; entries may look like fields or perhaps instance methods but are neither if I remember correctly -- so mix-in handling functionality may not be able to attach annotations as expected to them yet (class annotations work fine and are tested).

I hope to resolve the second problem too, hence leaving this issue open.

And the problem itself is that when buffering content that can not yet be used (both for polymorphic subtype handling and for dealing with unwrapped content), it is not known that type will be needed as BigDecimal -- so it will be buffered as Double (with somewhat lower overhead).
While it would be possible to force storage of all floating-point values as BigDecimal, in theory, one nasty consequence but that binary formats with efficient storage format for 32- and 64-bit values would be heavily penalized by conversions between 2- and 10-based FP numbers. So it would be good to figure out something better; in case of textual format it might even make sense to defer number parsing.

But I do not know a good way yet; and changes likely need to go in a new minor version anyway (2.12.0 at earliest).

[cowtowncoder]

Ah. The problem is that unlike POJO properties that are discovered using AnnotatedField / -Method and so on -- on which mix-ins are applied -- enum names are detected directly from fields that Enum declares, by JacksonAnnotationIntrospector. This means mix-ins are not indeed applied.

This should be fixed but will be bit bigger undertaking as it requires changes to AnnotationIntrospector interface, for one.
That means it can not be fixed for versions earlier than 2.12.0.

[ianbrandt]

It looks like 2.15 will bring support for lowercasing of serialized enums per #3053, which is one use case I have for enum mix-ins. I'd rather configure the output case on a per-mix-in basis, so I'd still very much appreciate this feature.