How to use Java to develop an event-driven application based on Axon Framework

How to use Java to develop an event-driven application based on Axon Framework

1. Background introduction
Axon Framework is a Java program used to build event-driven applications frame. It provides core functions and tools for implementing CQRS (Command Query Responsibility Segregation) and event-driven architecture (EDA). Axon Framework has good scalability and flexibility, allowing developers to easily build and maintain complex applications.

2. Environment setup
Before starting development, we need to set up the environment. First, confirm that the Java SDK and Maven build tools have been installed. Next, introduce the necessary dependencies by following these steps:

1. Add the following dependencies in the project's pom.xml file:

   <dependencies>
    <dependency>
        <groupId>org.axonframework</groupId>
        <artifactId>axon-spring-boot-starter</artifactId>
        <version>4.1.3</version>
    </dependency>
    <dependency>
        <groupId>org.axonframework</groupId>
        <artifactId>axon-test</artifactId>
        <version>4.1.3</version>
        <scope>test</scope>
    </dependency>
   </dependencies>

2. In Add the following configuration to the application.properties file:

   spring.datasource.driverClassName=org.h2.Driver
   spring.datasource.url=jdbc:h2:mem:test
   spring.datasource.username=sa
   spring.datasource.password=
   spring.jpa.database-platform=org.hibernate.dialect.H2Dialect

3. Define events and commands
In Axon Framework, events and commands are the core concepts in applications. Events are facts that occur in the system, while commands are actions used to change the state of the system.

1. Create a Java class named OrderCreatedEvent and define its properties:

   public class OrderCreatedEvent {
   
    private String orderId;
    private String customerName;
   
    // Getter and Setter
   }

2. Create a class named CreateOrderCommand's Java class and define its properties:

   public class CreateOrderCommand {
   
    private String orderId;
    private String customerName;
   
    // Getter and Setter
   }

4. Create an aggregate root
The aggregate root is a domain object with a unique identity. Responsible for processing external commands and generating corresponding events.

1. Create a Java class named OrderAggregate and define its fields and methods:

   @Aggregate
   public class OrderAggregate {
   
    @AggregateIdentifier
    private String orderId;
    private String customerName;
   
    public OrderAggregate() {
    }
   
    @CommandHandler
    public OrderAggregate(CreateOrderCommand command) {
        AggregateLifecycle.apply(new OrderCreatedEvent(command.getOrderId(), command.getCustomerName()));
    }
   
    @EventSourcingHandler
    public void on(OrderCreatedEvent event) {
        this.orderId = event.getOrderId();
        this.customerName = event.getCustomerName();
    }
   }

2. Create a A Java class named OrderAggregateIdentifierResolver and implements the AggregateIdentifierResolver interface:

   @Component
   public class OrderAggregateIdentifierResolver implements AggregateIdentifierResolver {
   
    @Override
    public String resolveId(Object command) {
        if (command instanceof CreateOrderCommand) {
            return ((CreateOrderCommand) command).getOrderId();
        }
        return null;
    }
   }

5. Create a command processor
The command processor is responsible Processes external commands and dispatches them to the appropriate aggregate root.

1. Create a Java class named OrderCommandHandler and define the methods in it:

   @Component
   public class OrderCommandHandler {
   
    private final CommandGateway commandGateway;
   
    public OrderCommandHandler(CommandGateway commandGateway) {
        this.commandGateway = commandGateway;
    }
   
    @CommandHandler
    public void handle(CreateOrderCommand command) {
        commandGateway.send(new CreateOrderCommand(command.getOrderId(), command.getCustomerName()));
    }
   }

6. Create query Model
The query model is responsible for processing external queries and returning appropriate results.

1. Create a Java class named OrderQueryModel and define its fields and methods:

   @Entity
   public class OrderQueryModel {
   
    @Id
    private String orderId;
    private String customerName;
   
    // Getter and Setter
   }

2. Create a A Java interface named OrderQueryModelRepository and inheriting CrudRepository:

   @Repository
   public interface OrderQueryModelRepository extends CrudRepository<OrderQueryModel, String> {
   }

3. Create a Java class named OrderQueryHandler , and define the methods:

   @Component
   public class OrderQueryHandler {
   
    private final OrderQueryModelRepository orderQueryModelRepository;
   
    public OrderQueryHandler(OrderQueryModelRepository orderQueryModelRepository) {
        this.orderQueryModelRepository = orderQueryModelRepository;
    }
   
    @QueryHandler
    public OrderQueryModel handle(GetOrderQuery query) {
        return orderQueryModelRepository.findById(query.getOrderId()).orElse(null);
    }
   }

7. Create REST API
Create REST API for external calls.

1. Create a Java class named OrderController and define the methods in it:

   @RestController
   @RequestMapping("/orders")
   public class OrderController {
   
    private final CommandGateway commandGateway;
    private final QueryGateway queryGateway;
   
    public OrderController(CommandGateway commandGateway, QueryGateway queryGateway) {
        this.commandGateway = commandGateway;
        this.queryGateway = queryGateway;
    }
   
    @PostMapping
    public ResponseEntity<String> create(@RequestBody CreateOrderDTO createOrderDTO) {
        String orderId = UUID.randomUUID().toString();
        commandGateway.send(new CreateOrderCommand(orderId, createOrderDTO.getCustomerName()));
        return ResponseEntity.ok(orderId);
    }
   
    @GetMapping("/{orderId}")
    public ResponseEntity<OrderQueryModel> get(@PathVariable String orderId) throws ExecutionException, InterruptedException {
        OrderQueryModel order = queryGateway.query(new GetOrderQuery(orderId), ResponseTypes.instanceOf(OrderQueryModel.class)).get();
        if (order == null) {
            return ResponseEntity.notFound().build();
        }
        return ResponseEntity.ok(order);
    }
   }

2. Create a class named CreateOrderDTO's Java class and define its properties:

   public class CreateOrderDTO {
   
    private String customerName;
   
    // Getter and Setter
   }

3. Create a Java class named GetOrderQuery and define its properties ：

   public class GetOrderQuery {
   
    private String orderId;
   
    // Getter and Setter
   }

8. Start the application

1. Create a Java class named Application and add @SpringBootApplication Note:

   @SpringBootApplication
   public class Application {
   
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
   }

2. Run the main method of the Application class to start the application.

9. Test the application
Use Postman or a similar tool to send HTTP requests to test the functionality of the application.

1. Send a POST request to create an order:

   URL: http://localhost:8080/orders
   Method: POST
   Body: {"customerName": "John Doe"}

2. Send a GET request to get order information:

   URL: http://localhost:8080/orders/{orderId}
   Method: GET

10. Summary
This article introduces how to use Java to develop an event-driven application based on Axon Framework. By defining events and commands, creating aggregate roots, command handlers, query models, and REST APIs, we can use the Axon Framework to build efficient and scalable event-driven applications.

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