使用Spring Kafka模板时如何指定类类型？

Question

My current setup has the following configuration:

@Bean
public ConcurrentKafkaListenerContainerFactory<String, String> myKafkaListenerContainerFactory(
        ConsumerFactory<String, String> consumerFactory) {

    ConcurrentKafkaListenerContainerFactory<String, String> factory = new
            ConcurrentKafkaListenerContainerFactory<>();
    factory.setConsumerFactory(consumerFactory);
    factory.setMessageConverter(stringJsonMessageConverter());

    return factory;
}

where stringJsonMessageConverter has

@Bean
public StringJsonMessageConverter stringJsonMessageConverter() {
    return new StringJsonMessageConverter(objectMapper());
}

to make use of my object mapper

@Bean
public ObjectMapper objectMapper() {
    return new ObjectMapper()
            .registerModule(new JavaTimeModule())
            .registerModule(myCustomJacksonModules())
            .configure(FAIL_ON_UNKNOWN_PROPERTIES, false)
            .configure(ACCEPT_SINGLE_VALUE_AS_ARRAY, true)
            .configure(WRITE_DATES_AS_TIMESTAMPS, false);
}

With this configuration, i am able to publish as:

...
headers = new MessageHeaders(singletonMap(TOPIC, topic));
Foo foo = ....
Message<?> message = new GenericMessage<>(foo, headers);
kafkaTemplate.send(message);

And consume as:

@KafkaListener(topics = "myTopic",
        groupId = "g1",
        containerFactory = "myKafkaListenerContainerFactory")
public void onMessageReceived(Foo foo) {
    ... works with foo here
}

This works fine in this example if Foo is concrete class. If however Foo is an abstract and what is send as message are it's subclasses

F1 extends Foo; F2 extends Foo ...

... // publisher

headers = new MessageHeaders(singletonMap(TOPIC, topic));
F1 f1 = ....
Message<?> message = new GenericMessage<>(f1, headers);
kafkaTemplate.send(message);

... // Listener

@KafkaListener(topics = "myTopic",
        groupId = "g1",
        containerFactory = "myKafkaListenerContainerFactory")
public void onMessageReceived(Foo foo) {
    ... wont work
}

Declaring Foo in my consumer as type would fail but this would work:

@KafkaListener(topics = "myTopic",
        groupId = "g1",
        containerFactory = "myKafkaListenerContainerFactory")
public void onMessageReceived(Map<String,Object> fooAsMap) {
    ... this works too
}

Aside from the option mentioned above which is to use Map, ive also tried the following:

- class level kafka listener ([from here)][1]; it still requires method with param of type Map
- custom deserializer registered in object mapper?

Is there a way to specify the target type when i send the message?

Answer 1

Configure the StringJsonMessageConverter with a custom DefaultJackson2JavaTypeMapper where it has the setTypePrecedence(TypePrecedence.TYPE_ID) .

Also set the type mappings to map to the desired class, based on the sending type id header (only needed if the producer's class is different to the consumer's class).