InferredType determination not working if using @KafkaHandler annotation and StringJsonMessageConverter

[stefluhh]

Hi,

I noticed a strange behaviour.
When using a StringJsonMessageConverter the Payload Class Type cannot determined because the passed Method constructor param is null in MessagingMessageListenerAdapter. Therefore the field inferredType stays null and the fallback java.lang.Object is used which yields to erronous HashMap conversion of my JSON object.

This is only related to usage of @KafkaListener class annotation with @KafkaHandler method annotation. Another test where I use a @KafkaListener annotation directly the Payload is converted to TestEvent correctly:

@KafkaListener(topics = "product", groupId = KafkaEventConsumerRegistry.KAFKA_GROUP_ID)
public void handleEvent(@Payload TestEvent event) {
    LOGGER.error(LogMarker.KAFKA, "Event: " + event.getTest()); // Works
}

@KafkaListener(topics = KafkaEventPublisher.ORDER_TOPIC, groupId = KafkaEventConsumerRegistry.KAFKA_GROUP_ID)
public class KafkaEventListener {

    @KafkaHandler
    public void listenTo(@Payload KafkaEvent event) throws Exception {
        LOGGER.error(LogMarker.KAFKA, "Event: " + event); // ClassCastException, cannot convert HashMap to KafkaEvent
    }
}

I guess this should work also for the combined annotation variant.
For now I will change my code to use only @KafkaListener annotation.

[artembilan]

The inferredType is for the conversion the incoming payload to the target argument type.

The @KafkaHandler approach is exactly for opposite situation: when we have a payload already, but would like to select the proper method:

 * <p><b>It is important to understand that when a message arrives, the method selection
 * depends on the payload type. The type is matched with a single non-annotated parameter,
 * or one that is annotated with {@code @Payload}.
 * There must be no ambiguity - the system
 * must be able to select exactly one method based on the payload type.</b>

If we would use inferring algorithm here as well, we would end up with some conflicts.
We need exact argument type here to select the target method before invoking it.

[artembilan]

Closed as Works as Designed.

[garyrussell]

We have the following note in the Rabbit docs...

This type inference can only be achieved when the @RabbitListener annotation is declared at the method level. With class-level @RabbitListener, the converted type is used to select which @RabbitHandler method to invoke. For this reason, the infrastructure provides the targetObject message property which can be used by a custom converter to determine the type.

We should add a similar note in the kafka docs.