Spring kafka JsonSerializer to payload with map with null key throws exception

Question

I have created a rest endpoint to push message to kafka, the details as follows

• Data or message payload, used for example

package com.learn.kafka.model;

import com.fasterxml.jackson.annotation.JsonTypeInfo;
import lombok.Data;

import java.util.Map;

@Data
@JsonTypeInfo(
        use = JsonTypeInfo.Id.NAME,
        property = "type")
public class SpecialData {

    Map<String, Object> messageInfo;
}

• consumer service with kafka listener

package com.learn.kafka.service;

import org.springframework.kafka.annotation.KafkaListener;
import org.springframework.stereotype.Component;
import lombok.extern.slf4j.Slf4j;

@Component
@Slf4j
public class ConsumerService {

    @KafkaListener(topics={"#{'${spring.kafka.topic}'}"},groupId="#{'${spring.kafka.consumer.group-id}'}")
    public void consumeMessage(String message){
        log.info("Consumed message - {}",message);
    }
}

• producer service, that contains kafka template

package com.learn.kafka.service;

import java.text.MessageFormat;

import com.learn.kafka.model.SpecialData;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.kafka.core.KafkaTemplate;
import lombok.extern.slf4j.Slf4j;
import org.springframework.kafka.support.KafkaHeaders;
import org.springframework.messaging.Message;
import org.springframework.messaging.support.MessageBuilder;
import org.springframework.stereotype.Service;

@Service
@Slf4j
public class ProducerService{

    @Value("${spring.kafka.topic:demo-topic}")
    String topicName;
    @Autowired
    KafkaTemplate<String,Object> kafkaTemplate;

     public String sendMessage(SpecialData messageModel){
        log.info("Sending message from producer - {}",messageModel);
        Message message = constructMessage(messageModel);
        kafkaTemplate.send(message);
        return MessageFormat.format("Message Sent from Producer - {0}",message);
    }

    private Message constructMessage(SpecialData messageModel) {

        return MessageBuilder.withPayload(messageModel)
                .setHeader(KafkaHeaders.TOPIC,topicName)
                .setHeader("reason","for-Local-validation")
                .build();
    }
}

• sample controller to send same message

package com.learn.kafka.controller;
import com.learn.kafka.model.SpecialData;
import com.learn.kafka.service.ProducerService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import lombok.extern.slf4j.Slf4j;

import java.util.HashMap;
import java.util.Map;

@RestController
@RequestMapping("/api")
@Slf4j
public class MessageController {

    @Autowired
    private ProducerService producerService;

    @GetMapping("/send")
    public void sendMessage(){
        SpecialData messageData = new SpecialData();
        Map<String,Object> input = new HashMap<>();
        input.put(null,"the key is null explicitly");
        input.put("1","the key is one non-null");

        messageData.setMessageInfo(input);

        producerService.sendMessage(messageData);
    }
}

• custom serializer

package com.learn.kafka;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;
import com.fasterxml.jackson.databind.util.StdDateFormat;
import org.apache.kafka.common.errors.SerializationException;
import org.apache.kafka.common.serialization.Serializer;

import java.io.IOException;
import java.util.Map;

public class CustomSerializer implements Serializer<Object> {
    private static final ObjectMapper MAPPER = new ObjectMapper();

    static {
        MAPPER.findAndRegisterModules();
        MAPPER.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
        MAPPER.setDateFormat(new StdDateFormat().withColonInTimeZone(true));
        MAPPER.getSerializerProvider().setNullKeySerializer(new NullKeyKeySerializer());
    }

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

    @Override
    public byte[] serialize(String topic, Object data) {
        try {
            if (data == null){
                System.out.println("Null received at serializing");
                return null;
            }
            System.out.println("Serializing...");
            return MAPPER.writeValueAsBytes(data);
        } catch (Exception e) {
            e.printStackTrace();
            throw new SerializationException("Error when serializing MessageDto to byte[]");
        }
    }

    @Override
    public void close() {
    }

    static class NullKeyKeySerializer extends StdSerializer<Object> {

        public NullKeyKeySerializer() {
            this(null);
        }

        public NullKeyKeySerializer(Class<Object> t) {
            super(t);
        }
        @Override
        public void serialize(Object obj, JsonGenerator gen, SerializerProvider provider) throws IOException {
            gen.writeFieldName("null");
        }

    }
}

• application.yaml

spring:
  kafka:
     topic: input-topic
     consumer:
        bootstrap-servers: localhost:9092
        group-id: input-group-id
        auto-offset-reset: earliest
        key-deserializer: org.apache.kafka.common.serialization.StringDeserializer
        value-deserializer: org.apache.kafka.common.serialization.StringDeserializer
     producer:
         bootstrap-servers: localhost:9092
         key-serializer: org.apache.kafka.common.serialization.StringSerializer
         value-serializer: com.learn.kafka.CustomSerializer
         properties:
             spring.json.add.type.headers: false

• Above code works. I was able to serialize the SpecialData with null key in map and send to broker and receive the message. The consumer uses the String Deserializer, so it printed is as expected. But I think there will be issue when using simply the JsonDeSerializer.

• Is there different approaches like,

  • Extend the existing spring JsonSerializer, just to add the NullKeySerializer to the ObjectMapper?
  • Simple configuration in the application.yaml?

Reference for null key serializer implementation

Answer 1

You don't need to extend the deserializer, it already has a constructor that takes a custom ObjectMapper; simply create one in Java and add it to the consumer factory using setValueDeserializer(). (There are similar setters for serializers on the producer factory).

However, extending the class will allow you to configure it in the yaml, if that is what you prefer.