Spring Boot | How to consume string messages using Apache Kafka

Apache Kafka is a publish-subscribe messaging queue used for real-time streams of data. A messaging queue lets you send messages between processes, applications, and servers. In this article we will see how to send string messages from apache kafka to the console of a spring boot application. 

Approach: 

Step 1: Go to spring initializr and create a starter project with following dependency: 

• Spring for Apache Kafka

Note: We can also create a maven project and add the following code to pom.xml file. 

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.kafka</groupId>
            <artifactId>spring-kafka</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>org.springframework.kafka</groupId>
            <artifactId>spring-kafka-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

Step 2: Open the project in an IDE and sync the dependencies. Now create a new class Config and add annotations @Configuration and @EnableKafka. 

Step 3: Now create beans ConsumerFactory and ConcurrentKafkaListenerContainerFactory with String object. 

// Java program to consume string
// messages using spring kafka

@EnableKafka
@Configuration
public class Config {

    // Function to establish
    // connection between spring
    // application and kafka server
    @Bean
    public ConsumerFactory<String, String>
    consumerFactory()
    {

        // HashMap to store the configurations
        Map<String, Object> map = new HashMap<>();

        // put the host IP in the map
        map.put(ConsumerConfig
                    .BOOTSTRAP_SERVERS_CONFIG,
                "127.0.0.1:9092");

        // put the group ID in the map
        map.put(ConsumerConfig
                    .GROUP_ID_CONFIG,
                "id");
        map.put(ConsumerConfig
                    .KEY_DESERIALIZER_CLASS_CONFIG,
                StringDeserializer.class);
        map.put(ConsumerConfig
                    .VALUE_DESERIALIZER_CLASS_CONFIG,
                StringDeserializer.class);

        return new DefaultKafkaConsumerFactory<>(map);
    }

    @Bean
    public ConcurrentKafkaListenerContainerFactory<String,
                                                   String>
    kafkaListner()
    {
        ConcurrentKafkaListenerContainerFactory<String,
                                                String>
            obj
            = new ConcurrentKafkaListenerContainerFactory<>();
        obj.setConsumerFactory(consumerFactory());
        return obj;
    }
}

Step 4: Create a class KafkaService with @Service annotation. This class will contain the listener method to publish the message on the console. 

@Service
public class KafkaService {

    // Annotation required to listen the
    // message from kafka server
    @KafkaListener(topics = "StringProducer",
                   groupId = "id")
    public void
    publish(String message)
    {
        System.out.println(
            "You have a new message: "
            + message);
    }
}

Step 5: Start zookeeper and then kafka server using the command below.

For windows: 
.\bin\windows\zookeeper-server-start.bat .\config\zookeeper.properties 
.\bin\windows\kafka-server-start.bat .\config\server.properties 
 

For mac and linux: 
bin/zookeeper-server-start.sh config/zookeeper.properties 
bin/kafka-server-start.sh config/server.properties 
 

Step 6: Now we need to create a new topic with the name StringProducer. To do so, open a new command prompt window and change directory to the kafka directory. Create a new topic using the command given below: 

// For Mac and Linux 
bin/kafka-topics.sh --create --zookeeper localhost:2181 --replication-factor 1 --partitions 1 --topic topic_name
// For Windows 
.\bin\windows\kafka-topics.bat --create --zookeeper localhost:2181 --replication-factor 1 --partitions 1 --topic topic_name 
 

Step 7: Now to run kafka producer console, use the command below: 

// For Mac and Linux 
bin/kafka-console-producer.sh --broker-list localhost:9092 --topic Kafka_Example 
// For Windows 
.\bin\windows\kafka-console-producer.bat --broker-list localhost:9092 --topic Kafka_Example 
 

Step 7: Run the application and type message on kafka producer and press enter. 

Output:  

Here type a message in string format on kafka producer 

>Hello
>Welcome to GeeksForGeeks

Output: