Send a json payload to Apache Kafka topic in Spring

Question

I need to send (Post) Json payload to Apache Kafka topic but I am receiving the following error :- "message": "Can't convert value of class com.xyz.User to class org.apache.kafka.common.serialization.StringSerializer specified in value.serializer"

Also Spring shows class cast exception :- java.lang.ClassCastException: com.xyz.User cannot be cast to java.lang.String

Following is my modal,kafka config and controller

public class User {
    private String firstname;
    private String email;


    public User() {}
    public User(String firstname, String email) {
        super();
        this.firstname = firstname;
        this.email = email;
    }
    public String getFirstname() {
        return firstname;
    }
    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    @Override
    public String toString() {
        return "UserModel [firstname=" + firstname + ", email=" + email + "]";
    }


}

Kafka Configuration

 package config;

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

import org.apache.kafka.clients.producer.ProducerConfig;
import org.apache.kafka.common.serialization.StringSerializer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.kafka.core.DefaultKafkaProducerFactory;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.kafka.core.ProducerFactory;
import org.springframework.kafka.support.serializer.JsonSerializer;

import com.xyz.User;

@Configuration
public class KafkaConfiguration {

    @Bean
    public ProducerFactory<String, User> producerFactory(){
        Map<String, Object> config = new HashMap<>();

        config.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "127.0.0.1:9092");
        config.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
        config.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, JsonSerializer.class);

        return new DefaultKafkaProducerFactory<>(config);

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

}

Controller

@RestController
@RequestMapping("/kafka")
public class UserController {
    @Autowired
    private KafkaTemplate<String, User> kafkaTemplate;

    private static String TOPIC = "kafka-producer";
    @PostMapping("/publish")
    public void getUserId(@RequestBody User user) {

        kafkaTemplate.send(TOPIC, user);

Json Payload send from postman

{
    "firstname" : "xyz",
    "email" : "xyz@gmail.com.com"

}

Answer 1

Able to reproduce your error at my local setup.

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

If we look at log then it says value.serializer value still referring to default StringSerializer rather than expected is JsonSerializer which in turn says your producer configuration are not getting into effect. In short your KafkaCOnfiguration class is not being referred.

Your KafkaConfiguration class is in some config package and User class in some com.xyz package. So solution would be to make sure that it gets picks up your configuration. Most probably that package may not getting scanned for configuration/beans definition. If you move KafkaConfiguration to root package of your application then your original code should work fine.

If you say that your KafkaTemplate object getting injected then it's actually not. The one which is getting injected is defined by Spring kafka Autoconfiguration.