Spring Boot | How to consume JSON messages using Apache Kafka

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Apache Kafka is a stream processing system which lets you send messages between processes, applications, and servers. In this article, we will see how to publish JSON messages on the console of a Spring boot application using Apache Kafka.

In order to learn how to create a Spring boot project, refer to this article.

Working Steps:

Go to Spring initializer and create a starter project with following dependency:
- Spring for Apache Kafka
Open the project in an IDE and sync the dependencies. In this article, we would be creating a student model where we would be posting the student details. Therefore, create a model class Student. Add data members and create constructor and override the toString method to see the messages in JSON format. The following is the implementation of the student class:

Student Model

// Java program to implement a
// student class
 
// Creating a student class
public class Student {
 
    // Data members of the class
    int id;
    String firstName;
    String lastName;
 
    // Constructor of the student
    // class
    public Student()
    {
    }
 
    // Parameterized constructor of
    // the student class
    public Student(int id, String firstName,
                   String lastName)
    {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
    }
 
    @Override
    public String toString()
    {
        return "Student{"
            + "id = " + id
            + ", firstName = '" + firstName + "'"
            + ", lastName = '" + lastName + "'"
            + "}";
    }
}

Create a new class Config and add annotations @Configuration and @EnableKafka. Now create beans ConsumerFactory and ConcurrentKafkaListenerContainerFactory with Student class object.

Config clas

@EnableKafka
@Configuration
public class Config {
 
    // Function to establish a connection
    // between Spring application
    // and Kafka server
    @Bean
    public ConsumerFactory<String, Student>
    studentConsumer()
    {
 
        // 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 of consumer 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,
                JsonDeserializer.class);
 
        // return message in JSON formate
        return new DefaultKafkaConsumerFactory<>(
            map, new StringDeserializer(),
            new JsonDeserializer<>(Student.class));
    }
 
    @Bean
    public ConcurrentKafkaListenerContainerFactory<String,
                                                   Student>
    studentListner()
    {
        ConcurrentKafkaListenerContainerFactory<String,
                                                Student>
            factory
            = new ConcurrentKafkaListenerContainerFactory<>();
        factory.setConsumerFactory(studentConsumer());
        return factory;
    }
}

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

KafkaService Class

@Service
public class KafkaService {
 
    // Annotation required to listen
    // the message from Kafka server
    @KafkaListener(topics = "JsonTopic",
                   groupId = "id", containerFactory
                                   = "studentListner")
    public void
    publish(Student student)
    {
        System.out.println("New Entry: "
                           + student);
    }
}

Start zookeeper and Kafka server. Now we need to create a new topic with the name JsonTopic. To do so, open a new command prompt window and change directory to the Kafka directory.
Now create a new topic using the command given below:

bin/Kafka-topics.sh –create –zookeeper localhost:2181 –replication-factor 1 –partitions 1 –topic topic_name // for mac and linux
.\bin\windows\Kafka-topics.bat –create –zookeeper localhost:2181 –replication-factor 1 –partitions 1 –topic topic_name // for windows

Now to run Kafka producer console, use the command below:

bin/Kafka-console-producer.sh –broker-list localhost:9092 –topic Kafka_Example // for mac and linux
.\bin\windows\Kafka-console-producer.bat –broker-list localhost:9092 –topic Kafka_Example // for windows