Option field deserialization failure

[ypoluektovich]

Given:

package test
trait T
case class A(a:Int) extends T
case class X(x: Option[T])

When:

    val om = new ObjectMapper with ScalaObjectMapper {
      registerModule(DefaultScalaModule)
      enableDefaultTypingAsProperty(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE, "_t")
    }
    val v = X(Some(A(1)))
    val str = om.writeValueAsString(v)
    println(str) // 1
    val d = om.readValue[X](str) // 2
    println(d)

Line 1 prints {"x":{"_t":"test.A","a":1}} and line 2 crashes:

com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Could not resolve type id 'test.A' as a subtype of [reference type, class scala.Option<test.T<[simple type, class test.T]>]: Not a subtype
 at [Source: (String)"{"x":{"_t":"test.A","a":1}}"; line: 1, column: 12] (through reference chain: test.X["x"])
	at com.fasterxml.jackson.databind.exc.InvalidTypeIdException.from(InvalidTypeIdException.java:43)
	at com.fasterxml.jackson.databind.DeserializationContext.invalidTypeIdException(DeserializationContext.java:1628)
	at com.fasterxml.jackson.databind.DatabindContext.resolveSubType(DatabindContext.java:200)
	at com.fasterxml.jackson.databind.jsontype.impl.ClassNameIdResolver._typeFromId(ClassNameIdResolver.java:49)
	at com.fasterxml.jackson.databind.jsontype.impl.ClassNameIdResolver.typeFromId(ClassNameIdResolver.java:44)
	at com.fasterxml.jackson.databind.jsontype.impl.TypeDeserializerBase._findDeserializer(TypeDeserializerBase.java:156)
	at com.fasterxml.jackson.databind.jsontype.impl.AsPropertyTypeDeserializer._deserializeTypedForId(AsPropertyTypeDeserializer.java:113)
	at com.fasterxml.jackson.databind.jsontype.impl.AsPropertyTypeDeserializer.deserializeTypedFromObject(AsPropertyTypeDeserializer.java:97)
	at com.fasterxml.jackson.databind.jsontype.impl.AsPropertyTypeDeserializer.deserializeTypedFromAny(AsPropertyTypeDeserializer.java:193)
	at com.fasterxml.jackson.module.scala.deser.OptionDeserializer.deserializeWithType(OptionDeserializerModule.scala:71)
	at com.fasterxml.jackson.module.scala.deser.OptionDeserializer.deserializeWithType(OptionDeserializerModule.scala:11)
	at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:527)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeWithErrorWrapping(BeanDeserializer.java:528)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeUsingPropertyBased(BeanDeserializer.java:417)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1280)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4001)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3030)
	at com.fasterxml.jackson.module.scala.experimental.ScalaObjectMapper$class.readValue(ScalaObjectMapper.scala:190)

[ypoluektovich]

I've traced the error to this line: https://github.com/FasterXML/jackson-module-scala/blob/master/src/main/scala/com/fasterxml/jackson/module/scala/deser/OptionDeserializerModule.scala#L71
In it, typeDeserializer is a AsPropertyTypeDeserializer that references the Option-typed property. It then goes on to detect the type hint from the actual JSON, and the mismatch between the expected Option[T] and the actual A crashes the program.

[cowtowncoder]

I don't have any information to add, but wanted to make sure this occurs with the latest release (2.9.6); or if not, that version it occurs with is included here. That helps verify the problem.

[ypoluektovich]

Yes, it does occur with 2.9.6.

[ypoluektovich]

A quick note: instead of a quick-and-hacky fix like in my patch linked above, it might be a better idea to rewrite OptionDeserializer to extend ReferenceTypeDeserializer (similar to OptionSerializer that extends ReferenceTypeSerializer), which seems to include some handling for this kind of issue.

[nbauernfeind]

Ah, interesting idea. I am going to get to this before the next jackson release or the scala-lang 2.13.0-M5 release (whichever comes first). Thanks for the patch and extra thanks for the test. I want to look a little more closely at the before/after effects before merging (I just want to be certain this only affects the default typing feature or to see if there are other tests that should be added).

[cowtowncoder]

Yes, ReferenceType[De]Serializer is designed to be used as the skeletal implementation here; it is the basis for Java 8 Optional, Guava Optional, JDK AtomicReference handling, and I think Kotlin's equivalent.
It was added relatively recently (2.7 or 2.8) but does implement things according to current understanding of how things should resolve, including handling of polymorphic types.

[pjfanning]

@cowtowncoder this is still an issue - would you have a suggestion about how com.fasterxml.jackson.databind.jsontype.impl.AsPropertyTypeDeserializer could be told about the Option wrapper class as well as the inner class (Int)?

[cowtowncoder]

@pjfanning I think the way databind at least sees reference types is to consider them mostly invisibile, so that type info only ever relates to value contained. Or put another way: serialization of contents (polymorphic typing or no) should be same as serialization of one wrapped in Option.

But maybe the problem here is due to Type Erasure: it is generally bad idea to use generic types as root values -- that can be made to work (but user MUST provide type via ObjectWriter w = mapper.writerFor(TypeReferenceOrJavaType)), but add to problem.

[pjfanning]

@milenkovicm that link you made is clearly merged to the 2.13 branch