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:
-
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>
-
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.
-
Create a Java class named
OrderCreatedEvent
and define its properties:public class OrderCreatedEvent { private String orderId; private String customerName; // Getter and Setter }
-
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.
-
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(); } }
-
Create a A Java class named
OrderAggregateIdentifierResolver
and implements theAggregateIdentifierResolver
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.
-
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.
-
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 }
-
Create a A Java interface named
OrderQueryModelRepository
and inheritingCrudRepository
:@Repository public interface OrderQueryModelRepository extends CrudRepository<OrderQueryModel, String> { }
-
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.
-
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); } }
-
Create a class named
CreateOrderDTO
's Java class and define its properties:public class CreateOrderDTO { private String customerName; // Getter and Setter }
-
Create a Java class named
GetOrderQuery
and define its properties :public class GetOrderQuery { private String orderId; // Getter and Setter }
8. Start the application
-
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); } }
- Run the
main
method of theApplication
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.
-
Send a POST request to create an order:
URL: http://localhost:8080/orders Method: POST Body: {"customerName": "John Doe"}
-
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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