Spring Cloud Stream Sending Data to Kafka Topics Fails

Question

I have a SCS Sink app. I am trying to send the MessageBuilder message to kafka topics using Spring Kafka

ALso,

I am trying to conditionally forward using Spring Kafka here. Is there a way we can do it , as per this code below, using SCS ( i.e. @SendTo, etc )? Ideally I would like to use a Processor but not a sink. Just not clear on conditional forwarding.

Thank you for your time.

demoSink:

        @EnableBinding(Sink.class)
    public class SampleSink {

        @ServiceActivator(inputChannel = Sink.INPUT)
        public void processor(Message<?> message) {

            @Autowired
            private Sender sender;

            Message<String> message1 = (Message<String>) MessageBuilder.fromMessage(message)
                                        .setHeader("header1", "header1")
                                        .copyHeadersIfAbsent(message.getHeaders())
                                        .build();
    if(someObj.getSomething()==something){
        sender.sendt1(message1);
}
    else{

    sender.sendt2(message)
}

    }

Sender.java

   @Service
    public class Sender {

        private static final Logger LOG = LoggerFactory.getLogger(Sender.class);

        @Autowired
        private KafkaTemplate<String, String> kafkaTemplate;

     @Val

ue("${demo.topic}")
        private String topic1;

    public void sendt1(Message<?> msg){

           Message<String> t1 = (Message<String>) MessageBuilder.fromMessage(msg)
                   .copyHeadersIfAbsent(msg.getHeaders())
                    .setHeader(KafkaHeaders.TOPIC, topic1)
                    .build();

            LOG.info("sending message topic1");
            kafkaTemplate.send(t1);
        }


    public void sendt2(Message<?> msg){

           Message<String> t2 = (Message<String>) MessageBuilder.fromMessage(msg)
                   .copyHeadersIfAbsent(msg.getHeaders())
                    .setHeader(KafkaHeaders.TOPIC, topic2)
                    .build();

            LOG.info("sending message topic2");
            kafkaTemplate.send(t2);
        }
    }

config:

@Configuration
public class SenderConfig {

    @Value("${spring.cloud.stream.kafka.binder.brokers}")
    private String bootstrapServers;

    @Bean
    public Map<String, Object> producerConfigs() {
        Map<String, Object> props = new HashMap<>();
        props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
        props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
        props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
        return props;
    }

    @Bean
    public ProducerFactory<String, String> producerFactory() {
        return new DefaultKafkaProducerFactory<>(producerConfigs());
    }

    @Bean
    public KafkaTemplate<String, String> kafkaTemplate() {
        return new KafkaTemplate<>(producerFactory());
    }

}

I am getting the below Error:-

    Caused by:
 org.apache.kafka.common.errors.SerializationException: Can't convert value of class [B to class org.apache.kafka.common.serialization.StringSerializer
 specified in value.serializer
Caused by: java.lang.ClassCastException: 
[B cannot be cast to java.lang.String

Answer 1

spring is trying to cast ByteArray to String Object, if you are only trying to forward messages, you can do it like this: change to :

Message<?> message1 = MessageBuilder.fromMessage(message)
                                    .setHeader("header1", "header1")
                                    .copyHeadersIfAbsent(message.getHeaders())
                                    .build();

and in your SenderConfig use ByteArraySerializer.class instead for both key & value.