Custom deserialization with Jackson: extend default deserializer

Question

I would like to make my own deserializer by extending the default one an setting some more values after it:

simplified code:
public class Dto {
    public String originalJsonString;
}

public MyFooDto extends Dto {
    public String myField;
}



@Bean
public ObjectMapper deserializingObjectMapper() {
    ObjectMapper objectMapper = new ObjectMapper();
    JavaTimeModule javaTimeModule = new JavaTimeModule();
    javaTimeModule.addDeserializer(MyFooDto.class, new JsonDtoDeserializer<>());
    objectMapper.registerModule(javaTimeModule);
    return objectMapper;
}
// or maybe instead of the Beam just @JsonDeserialize(using = JsonDtoDeserializer.class) before MyFooDto?


public class JsonDtoDeserializer<T extends Dto> extends StdDeserializer<T> {
// or maybe extends JsonDeserializer? or UntypedObjectDeserializer? or UntypedObjectDeserializer.Vanilla?

    public T deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        // here I would like:
        T item = super.deserialize"AsUsual"(p, ctxt);
        // the difficulty is to avoid the loop of death, where the deserializer would call itself for the eternity...

        // And then set other Dto-fields depending on the original Json input, for example:
        item.originalJsonString = p.readValueAsTree().toString();
        return item;
    }
}

As you can see, I would also like to reuse this Dto mother class for other DTOs.

• I didn't find any example of it. I am really the first one in the world?
• what should be the deserialize"AsUsual"(p, ctxt)?
• what motherclass should I use? JsonDeserializer / StdDeserializer / UntypedObjectDeserializer?
• Will the deserializer know which class of T it has to instantiate?

Thank you Community!

Answer 1

As Sharon said (based on How do I call the default deserializer from a custom deserializer in Jackson)

@Bean
public ObjectMapper serializingObjectMapper() {

    ObjectMapper objectMapper = new ObjectMapper();
    SimpleModule simpleModule = new SimpleModule();

    simpleModule.setDeserializerModifier(new BeanDeserializerModifier() {
        @Override
        public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
            if (Dto.class.isAssignableFrom(beanDesc.getBeanClass())) {
                return new JsonDtoDeserializer<>(deserializer, beanDesc.getBeanClass());
            }
            return deserializer;
        }
    });

    objectMapper.registerModule(simpleModule);
    return objectMapper;
}



public class JsonDtoDeserializer<T extends Dto> extends StdDeserializer<T> implements ResolvableDeserializer /*StdDeserializer<Dto<T>>*/ /*UntypedObjectDeserializer.Vanilla*/ /*<T>*/ /*implements ResolvableDeserializer*/ {

    private final JsonDeserializer<?> defaultDeserializer;

    public JsonDtoDeserializer(JsonDeserializer<?> defaultDeserializer, Class<?> clazz) {
        super(clazz);
        this.defaultDeserializer = defaultDeserializer;
    }

    @Override
    public T deserialize(JsonParser p, DeserializationContext ctxt)
            throws IOException {

        @SuppressWarnings("unchecked")
        T itemObj = (T) defaultDeserializer.deserialize(p, ctxt);
        return itemObj;
    }

    // for some reason you have to implement ResolvableDeserializer when modifying BeanDeserializer
    // otherwise deserializing throws JsonMappingException??
    @Override public void resolve(DeserializationContext ctxt) throws JsonMappingException
    {
        ((ResolvableDeserializer) defaultDeserializer).resolve(ctxt);
    }
}