Daily Logs for Sep 8, 2025

Tags: letters kafka honda

Written by: Tushar Sharma

Dear Vishi, dear logs for today.

Honda Safety Recall

If you ever got safety recall notice in mail, go to Honda website and enter your VIN to verify.

Kafka Listener in a Reactive Spring Java app

In a traditional servlet-based Spring application, you can use @KafkaListener to consume messages from a topic:

@KafkaListener(topics = "my-topic")
public void listen(String message) {
    // handle message
}

Spring manages this lifecycle automatically.

In a reactive application (WebFlux), there is no direct equivalent to @KafkaListener. Instead, you need to start your Kafka consumer explicitly. To understand where this fits, let’s look at Spring Boot’s lifecycle:

Spring Boot Startup Order

1. ApplicationContext initialization
   • Spring container is created. No beans yet.
2. Bean creation and dependency injection
   • Beans are instantiated and wired.
3. @PostConstruct methods
   • Runs once after bean construction.
   • Good for bean setup (initialize fields, validate config).
   • Not for long-running tasks.
4. SmartLifecycle.start()
   • Invoked on beans implementing SmartLifecycle.
   • Best for long-running processes (e.g., Kafka consumers).
   • Supports clean start/stop hooks.
5. CommandLineRunner / ApplicationRunner
   • Runs after the context is loaded.
   • Good for quick startup tasks (DB init, seed data).
   • Not lifecycle-managed.
6. ApplicationReadyEvent is published
   • Fired when the app is ready.
7. Netty server fully ready
   • Reactive server now accepts requests.
8. @EventListener(ApplicationReadyEvent.class)
   • Runs after the app is fully ready.
   • Safe for startup code.
   • No clean shutdown support.

Using ApplicationReadyEvent

@EventListener(ApplicationReadyEvent.class)
public void consumeKafkaMessage() {
    // start reactive Kafka consumer
}

Pros:

Runs after the application is ready.

Cons: No built-in lifecycle management (no clean shutdown, no running status).

Using SmartLifecycle

@Service
public class KafkaService implements SmartLifecycle {

    private boolean running = false;

    @Override
    public void start() {
        // start Kafka consumer
        running = true;
    }

    @Override
    public void stop() {
        // stop Kafka consumer
        running = false;
    }

    @Override
    public boolean isRunning() {
        return running;
    }

    @Override
    public int getPhase() {
        return 0; // ordering relative to other lifecycle beans
    }
}

Pros:

Integrates with Spring lifecycle

Supports clean startup and shutdown

Exposes running state

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