Creator properties are ignored on abstract types when collecting bean properties, breaking AsExternalTypeDeserializer

[zhenlin-pay2]

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Describe the bug

When using Jackson with frameworks such as Immutables where users are expected to write bean definitions as abstract types and let the framework generate concrete implementations, Jackson annotations on the abstract type sometimes have no effect, or worse, trigger bugs. This appears to be due to the assumption in BeanDeserializerFactory.addBeanProps that creator properties do not need to be considered for abstract types. In particular, deserialisation fails with a bizarre exception in in the following situation:

• The type to be deserialised is abstract but with a defined ValueInstantiator (e.g. from a@JsonCreator annotated factory method).
• The property to be deserialised has a getter but no setter.
• The type of the property to be deserialised is also abstract.
• The getter has a @JsonTypeInfo annotation with include = JsonTypeInfo.As.EXTERNAL_PROPERTY.

Version Information

2.17.1

Reproduction

See attached: example.zip

Expected behavior

No response

Additional context

Deserialisation fails with the following exception:

Unexpected token (VALUE_STRING), expected START_ARRAY: need Array value to contain `As.WRAPPER_ARRAY` type information for class example.TypedData$Value
 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 2, column: 20] (through reference chain: example.TypedData["value"])
com.fasterxml.jackson.databind.exc.MismatchedInputException: Unexpected token (VALUE_STRING), expected START_ARRAY: need Array value to contain `As.WRAPPER_ARRAY` type information for class example.TypedData$Value
 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 2, column: 20] (through reference chain: papaya.struct.jackson.TypedData["value"])
	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59)
	at com.fasterxml.jackson.databind.DeserializationContext.wrongTokenException(DeserializationContext.java:1913)
	at com.fasterxml.jackson.databind.DeserializationContext.reportWrongTokenException(DeserializationContext.java:1699)
	at com.fasterxml.jackson.databind.jsontype.impl.AsArrayTypeDeserializer._locateTypeId(AsArrayTypeDeserializer.java:141)
	at com.fasterxml.jackson.databind.jsontype.impl.AsArrayTypeDeserializer._deserialize(AsArrayTypeDeserializer.java:96)
	at com.fasterxml.jackson.databind.jsontype.impl.AsArrayTypeDeserializer.deserializeTypedFromObject(AsArrayTypeDeserializer.java:61)
	at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserializeWithType(AbstractDeserializer.java:263)
	at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:542)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeWithErrorWrapping(BeanDeserializer.java:570)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeUsingPropertyBased(BeanDeserializer.java:440)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1493)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:348)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:185)
	at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:342)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4905)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3848)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3816)

Notice that the call chain goes through BeanDeserializer.deserializeFromObjectUsingNonDefault rather than the expected BeanDeserializer.deserializeWithExternalTypeId. This is because _externalTypeIdHandler is null, which is because when BeanDeserializerBase.resolve is called, _beanProperties is empty (!), which is because BeanDeserializerFactory.addBeanProps ignores creator properties when the type is abstract. Changing the value of isConcrete in addBeanProps to true while running in a debugger suffices to fix this problem, though I cannot say if this has any other effects.

[pjfanning]

Have you tried other versions of jackson-databind, eg v2.18.2?

[zhenlin-pay2]

The line in question has not been changed for 8 years, but yes, I confirm that it is broken in 2.18.2 too.

jackson-databind/src/main/java/com/fasterxml/jackson/databind/deser/BeanDeserializerFactory.java

Line 508 in 287af92

final SettableBeanProperty[] creatorProps = isConcrete

[cowtowncoder]

In case of abstract types, perhaps introspection could try to find factory Creator methods -- constructors should not be used as they cannot really be used directly.

[zhenlin-pay2]

Isn't that what the @JsonCreator annotation is for? It is there in the example and does work in basic scenarios (e.g. without polymorphic deserialisation). There are other ways of making a ValueInstantiator available too – the code where I am encountering this problem uses bytecode generation and installs a custom ValueInstantiator.

[cowtowncoder]

@zhenlin-pay2 Yes, @JsonCreator is for indicating which Creator (constructor or factory method) should be used.

But in case of abstract classes, introspection is skipped with assumption that they cannot be created (being abstract). But while true for Constructors (which really cannot be used to actually create instances), Factory methods would be useful (being static and thereby could be called).

What would be useful here would be actual minimized unit test (under tofix package, for branch 2.18 or 2.19) that'd fail. It'd be easier for someone to look into how to fix the problem.

[zhenlin-pay2]

I am saying that @JsonCreator does work (somewhat) for abstract types (both abstract class and interface). In other words, introspection does pick it up. The problem is that there seems to be code that assumes it does not happen. Please look at the line of code I cited and the example I provided.

[zhenlin]

Since I have some time off work, I opened a PR with the proposed fix. Please have a look at #4982.

[cowtowncoder]

@zhenlin Excellent! Added notes.