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Advanced Spring Boot 4: Event-Driven Architecture (Kafka) · Lesson

Sending Messages with KafkaTemplate

Utilize Spring's KafkaTemplate to programmatically send messages to Kafka topics, including synchronous and asynchronous approaches.

Sending Messages with KafkaTemplate is a free Advanced Spring Boot 4: Event-Driven Architecture (Kafka) lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Advanced Spring Boot 4: Event-Driven Architecture (Kafka) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Meet Spring's KafkaTemplate

Welcome to sending messages with Spring Boot and Kafka! At the heart of sending messages is Spring's KafkaTemplate.

  • It simplifies interacting with Kafka.
  • It handles connection management and serialization.
  • It lets you send messages to any Kafka topic easily.

Think of it as your primary tool for producing events.

Injecting KafkaTemplate in Spring

To use KafkaTemplate, you simply inject it into your Spring component (like a service or controller). Spring Boot auto-configures it for you, provided you have the spring-kafka dependency.

You just need to declare it, and Spring handles the rest!

import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.stereotype.Service;

@Service
public class MyProducerService {
  private final KafkaTemplate<String, String> kafkaTemplate;

  public MyProducerService(KafkaTemplate<String, String> kafkaTemplate) {
    this.kafkaTemplate = kafkaTemplate;
  }
}

Sending Your First Message

The simplest way to send a message is using the send() method. You specify the topic name and the message payload.

A topic is a category or feed name where records are stored and published. The payload is the actual data you want to send.

Basic KafkaTemplate Send Example

Here's a complete, runnable Spring Boot application that sends a simple string message to a topic named my-topic. Make sure a Kafka broker is running (e.g., on localhost:9092) for this to work.

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.stereotype.Component;

@SpringBootApplication
public class KafkaProducerApp {

  public static void main(String[] args) {
    SpringApplication.run(KafkaProducerApp.class, args);
  }

  @Component
  public class MyMessageSender implements CommandLineRunner {
    private final KafkaTemplate<String, String> kafkaTemplate;

    public MyMessageSender(KafkaTemplate<String, String> kafkaTemplate) {
      this.kafkaTemplate = kafkaTemplate;
    }

    @Override
    public void run(String... args) throws Exception {
      String topic = "my-topic";
      String message = "Hello from CoddyKit!";
      kafkaTemplate.send(topic, message);
      System.out.println("Sent message: " + message + " to topic: " + topic);
    }
  }
}

Synchronous Message Sending

By default, kafkaTemplate.send() is asynchronous. However, you can make it synchronous by calling .get() on the returned ListenableFuture.

  • This blocks the current thread until the message is sent and acknowledged by Kafka.
  • Useful when you need immediate confirmation that a message was processed.
  • Can impact performance due to blocking, so use wisely.
import org.springframework.kafka.support.SendResult;
import java.util.concurrent.ExecutionException;

// ... in a service method
try {
  SendResult<String, String> result = 
    kafkaTemplate.send("sync-topic", "Sync message").get();
  System.out.println("Message sent synchronously: " + 
    result.getProducerRecord().value());
} catch (InterruptedException | ExecutionException e) {
  System.err.println("Failed to send message: " + e.getMessage());
}

Asynchronous Sending: The Preferred Way

For most applications, asynchronous sending is preferred. It allows your application to continue processing without waiting for Kafka's acknowledgment, improving throughput.

  • send() returns a ListenableFuture (or CompletableFuture in newer Spring versions).
  • You attach callbacks to this future to handle success or failure.
  • This non-blocking approach is key for scalable microservices.

Handling Asynchronous Success

To process the result of an asynchronous send, you use callbacks. The success callback receives a SendResult object, which contains details about the sent record.

This is where you'd log successful sends or update application state.

kafkaTemplate.send("async-topic", "Async message")
  .addCallback(
    result -> System.out.println("Sent successfully: " + 
      result.getProducerRecord().value()),
    ex -> System.err.println("Failed to send: " + 
      ex.getMessage())
  );

Handling Asynchronous Failure

The failure callback is crucial for robust applications. It's invoked if the message cannot be sent after retries, or if an immediate error occurs.

In this callback, you should:

  • Log the error details.
  • Implement retry logic (if not handled by Kafka config).
  • Move the message to a Dead Letter Topic (DLT) for later inspection.

Async Send with Callbacks Example

Let's update our previous example to use asynchronous sending with success and failure callbacks. This demonstrates a more robust way to send messages.

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.stereotype.Component;

@SpringBootApplication
public class KafkaAsyncProducerApp {

  public static void main(String[] args) {
    SpringApplication.run(KafkaAsyncProducerApp.class, args);
  }

  @Component
  public class MyAsyncMessageSender implements CommandLineRunner {
    private final KafkaTemplate<String, String> kafkaTemplate;

    public MyAsyncMessageSender(KafkaTemplate<String, String> kafkaTemplate) {
      this.kafkaTemplate = kafkaTemplate;
    }

    @Override
    public void run(String... args) throws Exception {
      String topic = "my-async-topic";
      String message = "Hello async from CoddyKit!";

      kafkaTemplate.send(topic, message)
        .addCallback(
          result -> System.out.println("Async success: " + result.getProducerRecord().value()),
          ex -> System.err.println("Async failure: " + ex.getMessage())
        );
      System.out.println("Attempted to send async message.");
    }
  }
}

Sending with Keys for Ordering

Kafka allows you to send messages with a key. The key is used to determine which partition a message goes to.

  • Messages with the same key always go to the same partition.
  • This ensures ordering for related messages (e.g., all updates for a specific user).
  • Use kafkaTemplate.send(topic, key, message).
kafkaTemplate.send("user-events", "user-123", "User 123 updated profile");
kafkaTemplate.send("user-events", "user-456", "User 456 logged in");

KafkaTemplate Question

Which of the following statements about KafkaTemplate.send() and its return type is TRUE?

Recap: Sending Messages

Great job! You've learned the essentials of sending messages with Spring Boot's KafkaTemplate:

  • Injection: How to get KafkaTemplate in your services.
  • Basic Send: Using send(topic, message).
  • Synchronous: Blocking with .get() for immediate confirmation.
  • Asynchronous: The preferred method using ListenableFuture and callbacks for efficiency.
  • Keys: How to use message keys for ordering and partitioning.

Next, we'll dive into customizing producer configurations for optimized performance and reliability!

Frequently asked questions

Is the “Sending Messages with KafkaTemplate” lesson free?

Yes — the full text of “Sending Messages with KafkaTemplate” is free to read here on the web, and the Advanced Spring Boot 4: Event-Driven Architecture (Kafka) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Advanced Spring Boot 4: Event-Driven Architecture (Kafka) course, upgrade to CoddyKit PRO.

What will I learn in “Sending Messages with KafkaTemplate”?

Utilize Spring's KafkaTemplate to programmatically send messages to Kafka topics, including synchronous and asynchronous approaches. You practise Advanced Spring Boot 4: Event-Driven Architecture (Kafka) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Advanced Spring Boot 4: Event-Driven Architecture (Kafka)?

No prior experience is required. Advanced Spring Boot 4: Event-Driven Architecture (Kafka) on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Sending Messages with KafkaTemplate” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Advanced Spring Boot 4: Event-Driven Architecture (Kafka) lesson?

Yes. Every Advanced Spring Boot 4: Event-Driven Architecture (Kafka) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Integrating Spring Kafka Starter
  2. Sending Messages with KafkaTemplate
  3. Customizing Producer Configurations
  4. Handling Producer Send Callbacks and Acknowledgments
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