NullNode injected when instantiating data class with nullable property

Following tests fails:

class Test {

    data class MyData(val value: JsonNode?)

    @Test
    fun testNullValue() {
        val mapper = ObjectMapper()
        mapper.registerModule(KotlinModule())

        val json = "{\"value\": null}"

        val actual: MyData = mapper.readValue(json, MyData::class.java)

        val expected = MyData(null)
        Assert.assertEquals(expected, actual)
    }

    @Test
    fun testMissingProperty() {
        val mapper = ObjectMapper()
        mapper.registerModule(KotlinModule())

        val json = "{}"

        val actual: MyData = mapper.readValue(json, MyData::class.java)

        val expected = MyData(null)
        Assert.assertEquals(expected, actual)
    }
}

[cowtowncoder]

I will move this to jackson-databind since there is Java-only reproduction.

I can not say off-hand if I’d considered it a bug or not: problem is that “null value” for type of JsonNode makes sense to be NullNode in many use cases. But I will see if it is possible to distinguish these cases from “empty value” handling.

[cowtowncoder]

Agreed. Documentation is a big challenge. But there may also be semantic component, as JsonNode is not (or I don’t think of it as) quite same “reference type” as Optional. I can see this going either way.

On documentation, I suppose this is in area of “null handling”, in a way. But more generally there probably should be some sort of table for various Features for different Java types, their general handling. My biggest problem is usually that of getting started, creating structure for documentation: it is easier to fill in structure with more information, incrementally, than come up with a good structure to start.

Another related challenge is that much of this is controlled in decentralized manner, basically by JsonSerializer/JsonDeserializer implementing certain handlers.