How to Listen to a Kafka Consumer in Spring Boot (Reactive & Non-Reactive)

Written by: Tushar Sharma

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In a traditional servlet based Spring boot, you can use KafkaListener annotation

            @Service
public class MyService {
    @KafkaListener(topics = "myTopic") 
    public void listen(ConsumerRecord<String, String> record) {
        // process the record
    }
}

          

✅ Pros:

Simple, declarative, and well-documented.
Supports message deserialization, error handling, retries, and DLQ out of the box.
Integrates seamlessly with Spring’s dependency injection.

❌ Cons:

Blocking I/O: Each message is processed synchronously.
Not suitable for high-throughput reactive pipelines.
Threads are tied up during processing (limited scalability under load).

Reactive Kafka Consumer with Project Reactor

ApplicationEventListener

            @Service
public class MyService {
    @Autowired
    private ReactiveKakfaConsumer<String, String> ReactiveKakfaConsumer;


    @EventListener(ApplicationReadyEvent.class)
    public void listen() {
        ReactiveKakfaConsumer
          .reactive()
            .subscribe( record -> {
                // process the message
            }, error -> {

            });
    }
}

          

PostConstruct

            @Service
public class MyService {
    @Autowired
    private ReactiveKakfaConsumer<String, String> ReactiveKakfaConsumer;


    @PostConstruct
    public void listen() {
        ReactiveKakfaConsumer
          .reactive()
            .subscribe( record -> {
                // process the message
            }, error -> {

            });
    }
}

          

CommanLineRunner

            @Service
public class MyService implements CommanLineRunner {
    @Autowired
    private ReactiveKakfaConsumer<String, String> ReactiveKakfaConsumer;


    @Override
    public void run(String... args) {
        ReactiveKakfaConsumer
          .reactive()
            .subscribe( record -> {
                // process the message
            }, error -> {

            });
    }
}

          

When to use Each Approach

PostConstruct	When you need initialization right after bean creation	Triggered immediately after bean construction and dependency injection
CommnadLineRunner	When you need to run code after the application context is fully started	Executes after the application context is ready but before the application start
ApplicationReadyEvent	When you need to ensure all aspects of application are ready	Trigger after the application is started and ready to server the request