spring-kafka-impl

Spring Boot 3 + Spring Kafka demo showing:

• String producer/consumer on my-topic
• JSON producer/consumer for a User payload on user-topic

This project exposes REST endpoints that publish to Kafka, and Kafka listeners that log consumed messages.

Tech Stack

• Java 17
• Spring Boot 3.5.7
• Spring Kafka
• Maven

Project Structure

• src/main/java/com/example/springkafka/config
  • KafkaProducerConfig.java: Kafka ProducerFactory and KafkaTemplate beans. Creates topic user-topic.
  • KafkaConsumerConfig.java: Kafka ConsumerFactory and listener container factories.
• src/main/java/com/example/springkafka/controller
  • KafkaController.java: REST endpoints for sending messages/users.
• src/main/java/com/example/springkafka/service
  • KafkaProducerService.java: sends string messages to my-topic.
  • KafkaConsumerService.java: consumes string messages from my-topic.
  • JsonProducer.java: sends User objects to user-topic.
  • JsonConsumer.java: consumes User objects from user-topic.
• src/main/java/com/example/springkafka/model
  • User.java: simple POJO for JSON payloads.
• src/main/resources/application.properties: Kafka and app configuration.

Kafka Topics

• my-topic (String)
  • Producer: KafkaProducerService
  • Consumer: KafkaConsumerService
  • Note: there is no NewTopic bean created for my-topic in the config (commented out).
• user-topic (JSON User)
  • Producer: JsonProducer
  • Consumer: JsonConsumer
  • Created by KafkaProducerConfig#userTopic()

Configuration Notes

There are two places where Kafka settings are defined:

• application.properties (Spring Boot configuration)
• KafkaProducerConfig and KafkaConsumerConfig (hard-coded values)

The config classes currently hard-code:

• bootstrap.servers as localhost:9092
• group IDs (my-consumer-group, user-consumer-group)

If you want centralized configuration, consider moving those values to application.properties and injecting them into the config classes.

Running the App

Prerequisites:

• Java 17
• Kafka broker running on localhost:9092
• Maven (or use the Maven wrapper included in this repo)

Start Kafka (example):

1. Start Zookeeper (if using Kafka in ZK mode)
2. Start Kafka broker on localhost:9092

Run the app:

./mvnw spring-boot:run

Or:

mvn spring-boot:run

The app starts on http://localhost:8080.

REST Endpoints

Send String Message

POST /send?message=hello

Example:

curl -X POST "http://localhost:8080/send?message=hello"

Send User JSON

POST /senduser

Example:

curl -X POST "http://localhost:8080/senduser" \
  -H "Content-Type: application/json" \
  -d "{\"name\":\"Alice\",\"age\":30,\"email\":\"alice@example.com\"}"

Expected Logs

On publish:

• Message sent: <message>
• Sent user: User [name=..., age=..., email=...]

On consume:

• Received message: <message>
• Received user: User [name=..., age=..., email=...]

Tests

./mvnw test