Spring Boot | How to publish JSON messages on Apache Kafka

Apache Kafka is a publish-subscribe messaging system. A messaging queue lets you send messages between processes, applications, and servers. In this article, we will see how to send JSON messages to Apache Kafka in a spring boot application.

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

The full-form of JSON is JavaScript Object Notation. JSON is a lightweight data format for data interchange which can be easily read and written by humans, easily parsed and generated by machines. Though it is derived from a subset of JavaScript, yet it is Language independent. It is a complete language-independent text format. The following steps can be followed in order to publish JSON messages to Apache Kafka:

  1. Go to spring initializr and create a starter project with following dependencies:
    • Spring Web
    • Spring for Apache Kafka
  2. 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 create getters and setters. The following is the implementation of the student class:
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    // Java program to implement a
    // student class
      
    // Creating a student class
    public class Student {
      
        // Data members of the
        // student class
        int id;
        String firstName;
        String lastName;
      
        // Constructor of the student
        // class
        public Student(int id, String firstName,
                       String lastName)
        {
            this.id = id;
            this.firstName = firstName;
            this.lastName = lastName;
        }
      
        // Implementing the getters
        // and setters
        public int getId()
        {
            return id;
        }
      
        public void setId(int id)
        {
            this.id = id;
        }
      
        public String getFirstName()
        {
            return firstName;
        }
      
        public void setFirstName(String firstName)
        {
            this.firstName = firstName;
        }
      
        public String getLastName()
        {
            return lastName;
        }
      
        public void setLastName(String lastName)
        {
            this.lastName = lastName;
        }
    }

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  3. Now, create a new class Controller with the annotation @RestController. Create a GET API and initialize KafkaTemplate with parameter as string and model class object. The following is the implementation of the controller:
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    // Java program to implement a
    // controller
      
    @RestController
    @RequestMapping("gfg")
    public class UserResource {
      
        @Autowired
        private KafkaTemplate<String, Student>
            kafkaTemplate;
      
        private static final String TOPIC
            = "StudentExample";
      
        @GetMapping("/publish/{id}/"
                    + "{firstName}/{lastName}")
      
        public String post(
            @PathVariable("id") final int id,
            @PathVariable("firstName") final
                String firstName,
            @PathVariable("lastName") final
                String lastName)
        {
      
            kafkaTemplate.send(
                TOPIC,
                new Student(
                    id, firstName,
                    lastName));
      
            return "Published successfully";
        }
    }

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  4. Create a class StudentConfig with the annotation @Configuration. In this class we will serialize the object of the model class.
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    // Java program to serialize the
    // object of the model class
      
    @Configuration
    public class StudentConfig {
      
        @Bean
        public ProducerFactory<String, Student>
        producerFactory()
        {
            // Create a map of a string
            // and object
            Map<String, Object> config
                = new HashMap<>();
      
            config.put(
                ProducerConfig.BOOTSTRAP_SERVERS_CONFIG,
                "127.0.0.1:9092");
      
            config.put(
                ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG,
                StringSerializer.class);
      
            config.put(
                ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG,
                JsonSerializer.class);
      
            return new DefaultKafkaProducerFactory<>(config);
        }
      
        @Bean
        public KafkaTemplate<String, Student>
        kafkaTemplate()
        {
            return new KafkaTemplate<>(
                producerFactory());
        }
    }

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  5. Now, start zookeeper and Kafka server. We need to create a new topic with the name StudentExample. To do so, open a new command prompt window and change directory to the Kafka folder.
  6. Now, 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



  7. Now to see the messages on the Kafka server in the real-time, use the command below:

    For Mac and Linux: bin/kafka-console-consumer.sh –bootstrap-server localhost:9092 –topic topic_name –from-beginning

    For Windows: .\bin\windows\kafka-console-consumer.bat –bootstrap-server localhost:9092 –topic topic_name –from-beginning

  8. Run the application and call the API as:

    localhost:8080/gfg/publish/{id}/{first name}/{last name}

    Note: If a different port has been used, then replace the port with 8080.

Output:

  • Calling the API:
  • Checking the message in real time:



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