How to create Custom serializer in kafka?

Here you have an example to use your own serializer/deserializer for the Kafka message value. For Kafka message key is the same thing.

We want to send a serialized version of MyMessage as Kafka value and deserialize it again into a MyMessage object at consumer side.

Serializing MyMessage in producer side.

You should create a serializer class that implements org.apache.kafka.common.serialization.Serializer

serialize() method do the work, receiving your object and returning a serialized version as bytes array.

public class MyValueSerializer implements Serializer<MyMessage>
{
    private boolean isKey;

    @Override
    public void configure(Map<String, ?> configs, boolean isKey)
    {
        this.isKey = isKey;
    }

    @Override
    public byte[] serialize(String topic, MyMessage message)
    {
        if (message == null) {
            return null;
        }

        try {

            (serialize your MyMessage object into bytes)

            return bytes;

        } catch (IOException | RuntimeException e) {
            throw new SerializationException("Error serializing value", e);
        }
    }

    @Override
    public void close()
    {

    }
}

final IntegerSerializer keySerializer = new IntegerSerializer();
final MyValueSerializer myValueSerializer = new MyValueSerializer();
final KafkaProducer<Integer, MyMessage> producer = new KafkaProducer<>(props, keySerializer, myValueSerializer);

int messageNo = 1;
int kafkaKey = messageNo;
MyMessage kafkaValue = new MyMessage();
ProducerRecord producerRecord = new ProducerRecord<>(topic, kafkaKey, kafkaValue);
producer.send(producerRecord, new DemoCallBack(logTag, startTime, messageNo, strValue));

Deserializing MyMessage in consumer side.

You should create a deserializer class that implements org.apache.kafka.common.serialization.Deserializer

deserialize() method do the work, receiving serialized value as bytes array and returning your object.

public class MyValueDeserializer implements Deserializer<MyMessage>
{
    private boolean isKey;

    @Override
    public void configure(Map<String, ?> configs, boolean isKey)
    {
        this.isKey = isKey;
    }

    @Override
    public MyMessage deserialize(String s, byte[] value)
    {
        if (value == null) {
            return null;
        }

        try {

            (deserialize value into your MyMessage object)

            MyMessage message = new MyMessage();
            return message;

        } catch (IOException | RuntimeException e) {
            throw new SerializationException("Error deserializing value", e);
        }
    }

    @Override
    public void close()
    {

    }
}

Then use it like this:

final IntegerDeserializer keyDeserializer = new IntegerDeserializer();
final MyValueDeserializer myValueDeserializer = new MyValueDeserializer();
final KafkaConsumer<Integer, MyMessage> consumer = new KafkaConsumer<>(props, keyDeserializer, myValueDeserializer);

ConsumerRecords<Integer, MyMessage> records = consumer.poll(1000);
for (ConsumerRecord<Integer, MyMessage> record : records) {

    int kafkaKey = record.key();
    MyMessage kafkaValue = record.value();

    ...
}

No words, only code

1. Some object, which you sent to Kafka

   import lombok.AllArgsConstructor;
   import lombok.Data;
   import lombok.NoArgsConstructor;
   import lombok.ToString;
   
   @Data
   @AllArgsConstructor
   @NoArgsConstructor
   @ToString
   public class TestDto {
   
       private String name;
       private String version;
   
   }
   

2. Create Serializer, which will be used by Producer

   @Slf4j
   public class KafkaValueSerializer implements Serializer<TestDto> {
   
       private ObjectMapper objectMapper = new ObjectMapper();
   
       @Override
       public void configure(Map<String, ?> configs, boolean isKey) {
       }
   
       @Override
       public byte[] serialize(String topic, TestDto data) {
           try {
               return objectMapper.writeValueAsBytes(data);
           } catch (JsonProcessingException e) {
               log.error("Unable to serialize object {}", data, e);
               return null;
           }
       }
   
       @Override
       public void close() {
       }
   }
   

3. Of couser, don't foget about Deserialiser for Consumer

   @Slf4j
   public class KafkaValueDeserializer implements Deserializer<TestDto> {
   
       private ObjectMapper objectMapper = new ObjectMapper();
   
       @Override
       public void configure(Map<String, ?> configs, boolean isKey) {
       }
   
       @Override
       public TestDto deserialize(String topic, byte[] data) {
           try {
               return objectMapper.readValue(new String(data, "UTF-8"), TestDto.class);
           } catch (Exception e) {
               log.error("Unable to deserialize message {}", data, e);
               return null;
           }
       }
   
       @Override
       public void close() {
       }
   }
   

4. Last moment, add serializer/deserializer to application.yml

   spring:
       kafka:
         bootstrap-servers:  192.168.192.168:9092
         producer:
             value-serializer: com.package.service.kafka.KafkaValueSerializer
         consumer:
             group-id: groupId
             value-deserializer: com.package.service.kafka.KafkaValueDeserializer
   

That's all. It's not necessary any configuration file or dancing with a tamboirine :)

5. Send

   KafkaTemplate<String, TestDto> kafkaTemplate;
   
   TestDto test = new TestDto("test name", "test-version");
   kafkaTemplate.send(topic, testDto);
   

6. Listen

   @KafkaListener(topics = "${ktp-agent.kafka.request-topic}", groupId = "${spring.kafka.consumer.group-id}")
   public void listen(TestDto message) {
   
       log.info("Received message '{}' from Kafka.", message.toString());
   }