Custom JSON deserialization only if certain field is present (using Jackson)

Question

When deserializing MyEntity (which is an interface) I either have

• the following input:

  { "id": 123 }
  

  in which case I would like to deserialize it into a

  new MyEntityRef(123)
  

• or I have the following input:

  {
      "id": 123,
      "message": "Hello world",
      "otherEntity": {
          "field": "value",
          ...
      }
  }
  

  in which case I would like to deserialize it as

  new MyEntityImpl(123, "Hello world", otherEntity);
  

  where otherEntity is deserialized the same way as if it was found outside the context of MyEntity.

I've figured out how to register my own custom deserializer through a SimpleModule but I don't know how to

1. Choose a custom deserializer based on the presense of some field (such as message above).
2. Fallback on the "default" serializer for certain fields (such as otherEntity above).

Answer 1

Finally solved it by configuring my ObjectMapper as follows:

ObjectMapper mapper = new ObjectMapper();

SimpleModule idAsRefModule = new SimpleModule("ID-to-ref",
                                              new Version(1, 0, 0, null));

idAsRefModule.addDeserializer(TestEntity.class,
                              new JsonDeserializer<TestEntity>() {
    @Override
    public TestEntity deserialize(JsonParser jp, DeserializationContext dc)
            throws IOException, JsonProcessingException {

        ObjectCodec codec = jp.getCodec();
        JsonNode node = codec.readTree(jp);
        boolean isFullImpl = node.has("message");
        Class<? extends TestEntity> cls = isFullImpl ? TestEntityImpl.class
                                                     : TestEntityRef.class;
        return codec.treeToValue(node, cls);
    }
});

mapper.registerModule(idAsRefModule);

return mapper;