Calling the default deserializer in a custom deserializer don't effect instance

Question

I'm trying to deserialize a Json into an existing instance in my process. So it only creates a new instance if none exists. Alls objects contains an id to Identifiy them.

I used this answer: https://stackoverflow.com/a/18405958/11584969 and tried to create a custon Deserializer for this.

So far I have managed to create a custon Deserializer which checks for existing instances, but I was not able to fill the new instance or change the existing one.

My deserialize function is:

public T deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
  JsonNode node = jp.getCodec().readTree(jp);

  if (node instanceof NullNode) {
    return null;
  }

  // get id from node
  String strId = node.get("id").asText();
  UUID id = UUID.fromString(strId);

  // search for existing instance or create it
  T mObject = ...

  // fill/change instance
  return (T) defaultDeserializer.deserialize(jp, ctxt, mObject);
}

The object mapper creation:

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.enableDefaultTyping();
objectMapper.registerModule(new Jdk8Module());
objectMapper.registerModule(new JavaTimeModule());

SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new BeanDeserializerModifier() {
  @Override
  public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
    if (beanDesc.getBeanClass() == Table.class)
      return new ModelObjectDeserializer<>(Table.class, (JsonDeserializer<Table>) deserializer);

    return deserializer;
  }
});
objectMapper.registerModule(module);

The code above runs without any error or exception but the instance from mObject is not filled by defaultDeserializer.deserialize(jp, ctxt, mObject);

If I don't use my custom deserializer, the created instances are filled as expected.

score 0 · Accepted Answer · answered Jun 03 '19 at 02:32

It is not quite an answer to the question, but my initial goal was:

'm trying to deserialize a Json into an existing instance in my process. So it only creates a new instance if none exists. Alls objects contains an id to Identifiy them.

For everyone who tries to accomplish the same, here is how I implemented it:

public class ModelInstantiator extends StdValueInstantiator {

  private static final long serialVersionUID = -7760885448565898117L;

  private Class<? extends ModelObject> clazz;

  /**
   * @param config
   * @param valueType
   */
  public ModelInstantiator(DeserializationConfig config, Class<? extends ModelObject> clazz) {
    super(config, config.constructType(clazz));

    this.clazz = clazz;
  }

  @Override
  public boolean canCreateFromObjectWith() {
    return true;
  }

  @Override
  public Object createFromObjectWith(DeserializationContext ctxt, Object[] args) throws IOException, JsonProcessingException {
    UUID id = (UUID) args[0];

    // get local object
    ModelObject object = ...

    // if id was not found => create and add
    if (object == null) {
      try {
        object = clazz.newInstance();
      } catch (InstantiationException | IllegalAccessException e) {
        throw new IOException(e);
      }

      object.setId(id);
      // add to local list
      ...
    }

    return object;
  }

  @Override
  public SettableBeanProperty[] getFromObjectArguments(DeserializationConfig config) {
    CreatorProperty idProp = new CreatorProperty(new PropertyName("id"), config.constructType(UUID.class), null, null, null, null,
        0, null, PropertyMetadata.STD_REQUIRED);

    return new SettableBeanProperty[] { idProp };
  }

}

I had to split the local and json id. Ohterwise the id in the array is null.

Calling the default deserializer in a custom deserializer don't effect instance

1 Answers1