Jackson ContextualDeserializer cannot get the contextualType when decoding a generic Kotlin class - kotlin

Question

I have a custom implementation of a JsonDeserializer which implements a ContextualDeserializer in order to deserialize a generic class. Everything works fine until the generic class itself has a property which is also a generic class.
For demo purposes, I simplified the code to the minimum
My class is
data class MyObject<out Content>(
val content: Content
)
Deserializing MyObject<String> works:
{
"content": "Hello"
}
but deserializing MyObject<MyObject<String>>> does not:
{
"content": {
"content": "Hello"
}
}
The custom deserializer is the following. The issue seems to be that contextualType.containedType is returning null.
I believe this issue is in the line
val content = ctxt.readTreeAsValue(contentNode, contentType!!.rawClass)
because the .rawClass does not have any additional information about the generic type. So the content is deserialized as just MyObject instead of MyObject<String>.
class MyObjectDeserializer : JsonDeserializer<MyObject<*>?>(), ContextualDeserializer {
private var contentType: JavaType? = null
override fun createContextual(ctxt: DeserializationContext?, property: BeanProperty?): JsonDeserializer<*> {
contentType = if (property == null)
ctxt!!.contextualType.containedType(0)
else
property.type.containedType(0)
if (contentType != null) {
println("${ctxt!!.contextualType} has contained type $contentType")
} else {
// Here is where the issue occurs
println("${ctxt!!.contextualType} does not have any contained types")
}
return this
}
override fun deserialize(p: JsonParser?, ctxt: DeserializationContext): MyObject<*> {
val codec = p?.codec ?: throw NullPointerException()
val node = codec.readTree<JsonNode>(p)
val contentNode = node.get("content")
val content = ctxt.readTreeAsValue(contentNode, contentType!!.rawClass)
return MyObject(content)
}
}
and I register the deserialzers like so
fun ObjectMapper.registerMyDeserializers(): ObjectMapper {
val module = SimpleModule().also {
it.addDeserializer(MyObject::class.java, MyObjectDeserializer())
}
return this.registerModule(module)
}
The unit test the demonstrates the issue is the follow. The first test works fine, the second fails.
class ResponseParsingTests {
private val objectMapper = jacksonObjectMapper().registerMyDeserializers()
#Test
fun `parses String as content`() {
val json = """
{
"content": "Hello"
}
""".trimIndent()
val result = objectMapper.readValue<MyObject<String>>(json)
assertEquals("Hello", result.content)
}
#Test
fun `parses MyObject as content`() {
val json = """
{
"content": {
"content": "Hello"
}
}
""".trimIndent()
val result = objectMapper.readValue<MyObject<MyObject<String>>>(json)
assertEquals("Hello", result.content.content)
}
}
Please note that I'm aware the this example class does not require any custom deserialzer at all. However my real use case is a bit more complex and I need to use a custom deserializer because I'm publishing my code as part of a library which suppoorts multiple serialization frameworks (gson, jackson, kotlinx). So the serialization cannot be part of my actual class but rather in a separate one.