Nullable String property deserializes null as "null" for JsonTypeInfo.As.EXTERNAL_PROPERTY

[jonatan-bengtsson]

Hi! 👋
I noticed that nullable String propery field for for JsonTypeInfo.As.EXTERNAL_PROPERTY deserializes null as "null" String, while I expected it to return null.

class NullableTypeIdTest {

  companion object {
    val mapper = jacksonObjectMapper()
  }

  interface Payload
  data class UniquePayload(val data: String) : Payload

  data class MyEntity(
    val type: String?,

    @JsonTypeInfo(
      use = JsonTypeInfo.Id.NAME,
      include = JsonTypeInfo.As.EXTERNAL_PROPERTY,
      property = "type"
    )
    @JsonSubTypes(
      JsonSubTypes.Type(value = UniquePayload::class, name = "UniquePayload")
    )
    val payload: Payload?
  )

  @Test
  fun serializeAndDeserializeTypeable() {
    val oldEntity = MyEntity(null, null)
    val json = mapper.writeValueAsString(oldEntity)
    val newEntity = mapper.readValue<MyEntity>(json)

    // Returns false: "null" String instead of null
    assertThat(newEntity.type).isNull()
  }
}

I have also tried using
ObjectMapper().registerModule(KotlinModule(nullIsSameAsDefault = true))
and setting
val type: String? = null
but without any result.

Versions
Kotlin: 1.3.72
Jackson-module-kotlin: 2.11.0
Jackson-databind: 2.11.0

[hkakutalua]

Any update on this?

[dinomite]

I added a failing test for this; haven't looked into where it's happening, though. This could be an issue in jackson-databind, I'm not sure how the internals of Jackson's polymorphic bits work

[dinomite]

@cowtowncoder I can look deeper into this, but do you know if it's possible that this is part of -core or -databind?

[cowtowncoder]

@dinomite Yes, I suspect it could be something wrt jackson-databind (not likely wrt jackson-core as that is just streaming parser / generator... although there is a bit of Type Id reading/writing code in there).
If test could be devised for java-only reproduction, would be happy to include in databind, see what is happening.

[cowtowncoder]

One quick question for @jonatan-bengtsson: does this occur with 2.11.4 / 2.12.0 too?

[jonatan-bengtsson]

The test fails for 2.11.4 and 2.12.0.
I was able to create a failing test in Java also!

public class ExternalPropertyTypeShouldBeNullTest {

    static class Box {
        public String type;
        public Fruit fruit;

        public Box(@JsonProperty("type") String type,

                   @JsonTypeInfo(use = Id.NAME, include = As.EXTERNAL_PROPERTY, property = "type")
                   @JsonSubTypes({@Type(value = Orange.class, name = "orange")})
                   @JsonProperty("fruit") Fruit fruit) {
            this.type = type;
            this.fruit = fruit;
        }
    }

    interface Fruit{}

    static class Orange implements Fruit {
        public String name;
        public String color;

        public Orange(@JsonProperty("name") String name, @JsonProperty("name") String color) {
            this.name = name;
            this.color = color;
        }
    }

    private final ObjectMapper MAPPER = new ObjectMapper();

    @Test
    public void testDeserializationNull() throws Exception {
        MAPPER.disable(DeserializationFeature.FAIL_ON_MISSING_EXTERNAL_TYPE_ID_PROPERTY);
        Box deserOrangeBox = MAPPER.readValue("{\"type\":null,\"fruit\":null}}", Box.class);
        assertNull(deserOrangeBox.fruit);
        assertNull(deserOrangeBox.type); // error: "expected null, but was:<null>"
    }
}

[cowtowncoder]

Ok, I'll create a new issue for databind (could transfer but I think there is probably need to verify Kotlin side if and when databind is fixed).

[cowtowncoder]

Created:

FasterXML/jackson-databind#3008

[dinomite]

This issue was fixed by FasterXML/jackson-databind#3008