Advanced Spring Boot 4: Event-Driven Architecture (Kafka) · レッスン

KafkaTemplateでメッセージを送信する

SpringのKafkaTemplateを使って、同期・非同期の方式でKafkaトピックにプログラムからメッセージを送信します。

「KafkaTemplateでメッセージを送信する」はCoddyKit上の無料Advanced Spring Boot 4: Event-Driven Architecture (Kafka)レッスンです。これはレッスン2/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはAdvanced Spring Boot 4: Event-Driven Architecture (Kafka)学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Advanced Spring Boot 4: Event-Driven Architecture (Kafka)コースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

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!

よくある質問

「KafkaTemplateでメッセージを送信する」レッスンは無料ですか？

はい。「KafkaTemplateでメッセージを送信する」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Advanced Spring Boot 4: Event-Driven Architecture (Kafka)コースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Advanced Spring Boot 4: Event-Driven Architecture (Kafka)コースには全4レッスンが含まれています。

「KafkaTemplateでメッセージを送信する」で何を学びますか？

SpringのKafkaTemplateを使って、同期・非同期の方式でKafkaトピックにプログラムからメッセージを送信します。ブラウザで直接実行するハンズオンコードでAdvanced Spring Boot 4: Event-Driven Architecture (Kafka)を演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Advanced Spring Boot 4: Event-Driven Architecture (Kafka)を始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのAdvanced Spring Boot 4: Event-Driven Architecture (Kafka)は初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン2/4です。

「KafkaTemplateでメッセージを送信する」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このAdvanced Spring Boot 4: Event-Driven Architecture (Kafka)レッスンでコードを書いて実行できますか？

はい。すべてのAdvanced Spring Boot 4: Event-Driven Architecture (Kafka)レッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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