Spring Boot Kafka JsonSerializer Example

Learn to use JsonSerializer and JsonDeserializer classes for storing and retrieving JSON from Apache Kafka topics and returning Java model objects.

1. Prerequisites

2. Application Configuration

In application.properties file, we have added the following configuration.


spring.kafka.consumer.bootstrap-servers: localhost:9092
spring.kafka.consumer.group-id: group-id
spring.kafka.consumer.auto-offset-reset: earliest
spring.kafka.consumer.key-deserializer: org.apache.kafka.common.serialization.StringDeserializer
spring.kafka.consumer.value-deserializer: org.springframework.kafka.support.serializer.JsonDeserializer

spring.kafka.producer.bootstrap-servers: localhost:9092
spring.kafka.producer.key-serializer: org.apache.kafka.common.serialization.StringSerializer
spring.kafka.producer.value-serializer: org.springframework.kafka.support.serializer.JsonSerializer
  • spring.kafka.consumer.key-deserializer specifies the deserializer class for keys.
  • spring.kafka.consumer.value-deserializer specifies the deserializer class for values.
  • spring.kafka.consumer.properties.spring.json.trusted.packages specifies the comma-delimited list of package patterns allowed for deserialization. '*' means deserializing all the packages.
  • spring.kafka.producer.key-deserializer specifies the serializer class for keys.
  • spring.kafka.producer.value-deserializer specifies the serializer class for values.

3. Model class

We have created User class, which we will send to Kafka. Its instance will be serialized by JsonSerializer to byte array. Kafka finally stores this byte array in the given partition.

During deserialization, JsonDeserializer is used to for receiving JSON from Kafka as byte array and return User object to the application.

public class User
	private long userId;
    private String firstName;
    private String lastName;

	public long getUserId() {
		return userId;
	public void setUserId(long userId) {
		this.userId = userId;
	public String getFirstName() {
		return firstName;
	public void setFirstName(String firstName) {
		this.firstName = firstName;
	public String getLastName() {
		return lastName;
	public void setLastName(String lastName) {
		this.lastName = lastName;

	public String toString() {
		return "User [userId=" + userId + ", firstName="
		        + firstName + ", lastName=" + lastName + "]";

4. Kafka Producer

The producer API simply consumes the user information in an HTTP POST API. It then creates a new User object and send to Kafka using KafkaTemplate.

@PostMapping(value = "/createUser")
public void sendMessageToKafkaTopic(
		@RequestParam("userId") long userId,
		@RequestParam("firstName") String firstName,
		@RequestParam("lastName") String lastName) {

	User user = new User();

private KafkaTemplate<String, Object> kafkaTemplate;
public void saveCreateUserLog(User user) 
  logger.info(String.format("User created -> %s", user));
  this.kafkaTemplate.send(AppConstants.TOPIC_NAME_USER_LOG, user);

5. Kafka Consumer

The consumer is implemented as @KafkaListener which gets notified everytime a new entry is added to the topic.

@KafkaListener(topics = AppConstants.TOPIC_NAME_USER_LOG, 
        groupId = AppConstants.GROUP_ID)
public void consume(User user) 
  logger.info(String.format("User created -> %s", user));

6. Demo

Use any REST API tester and post a few messages to API http://localhost:9000/kafka/createUser as below.

Message post : http://localhost:9000/kafka/createUser?userId=1&firstName=Lokesh&lastName=Gupta

Observe the console logs:

2020-05-24 23:36:47.132  INFO 2092 --- [nio-9000-exec-4] 
2020-05-26 01:03:52.722  INFO 11924 --- [nio-9000-exec-6] c.h.k.demo.service.KafKaProducerService  
: User created -> User [userId=1, firstName=Lokesh, lastName=Gupta]
2020-05-26 01:03:52.729  INFO 11924 --- [ntainer#1-0-C-1] c.h.k.demo.service.KafKaConsumerService  
: User created -> User [userId=1, firstName=Lokesh, lastName=Gupta]

7. Conclusion

In this Spring boot Kafka JsonSerializer example, we learned to use JsonSerializer to serialize and deserialize the Java objects and store in Kafka.

Happy Learning !!

Sourcecode on Github


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