Clean Architecture & Design Patterns in Practice · Lektion

Ereignisgesteuerte Clean Architecture

Integrieren Sie ereignisgesteuerte Muster in Clean Architecture und nutzen Sie Domain Events, um Entkopplung und Skalierbarkeit zu verbessern.

Ereignisgesteuerte Clean Architecture ist eine kostenlose Clean Architecture & Design Patterns in Practice-Lektion auf CoddyKit. Dies ist Lektion 2 von 4. Du kannst die komplette Lektion unten kostenlos lesen – dann übst du sie direkt im Browser mit einem integrierten Code-Editor und einem KI-Tutor rund um die Uhr. Sie ist Teil des Clean Architecture & Design Patterns in Practice-Lernpfads, und dein Fortschritt wird über Web und CoddyKit-App synchronisiert. Der Clean Architecture & Design Patterns in Practice-Kurs umfasst insgesamt 4 Lektionen.

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Intro to Event-Driven Clean Arch

Welcome to Event-Driven Clean Architecture! In complex systems, parts often need to react to things happening elsewhere without being tightly coupled.

This lesson explores how to integrate event-driven patterns into your Clean Architecture, using domain events to enhance decoupling and scalability, while strictly adhering to the Dependency Rule.

What are Domain Events?

A Domain Event is something that happened in the domain that domain experts care about. It's an immutable fact, a record of an occurrence, like 'Order Placed' or 'User Registered'.

They represent a change in the state of the domain.
They are typically named in the past tense (e.g., OrderPlaced).
They should be simple data structures, containing only relevant information about the event.

Events and the Dependency Rule

In Clean Architecture, the Dependency Rule states that dependencies must flow inwards. Domain events fit perfectly:

Entities can raise events, but don't know who handles them.
Use Cases orchestrate entities and can publish events after an operation.
Interface Adapters (e.g., presenters, external service gateways) can subscribe to and handle events from inner layers.

This maintains strict separation, as inner layers remain unaware of outer layer specifics.

Implementing a Domain Event

A domain event is essentially a data transfer object (DTO) that carries information about what happened. It's good practice to have a common interface for all domain events.

Here's a simple Java interface:

public interface DomainEvent {
  long occurredOn();
}

Code: Concrete Domain Event

Let's create a specific domain event: OrderPlacedEvent. It holds the orderId and the timestamp of when it occurred.

Try running this simple definition:

public class OrderPlacedEvent implements DomainEvent {
  private final String orderId;
  private final long occurredOn;

  public OrderPlacedEvent(String orderId) {
    this.orderId = orderId;
    this.occurredOn = System.currentTimeMillis();
  }

  public String getOrderId() {
    return orderId;
  }

  @Override
  public long occurredOn() {
    return occurredOn;
  }

  public static void main(String[] args) {
    OrderPlacedEvent event = new OrderPlacedEvent("ORD-123");
    System.out.println("Order event for: " + event.getOrderId());
  }
}

The Event Publisher

To publish events, we need an Event Publisher (also known as an Event Dispatcher or Event Bus). This component takes a domain event and dispatches it to all registered handlers.

It acts as a mediator, decoupling the event source from its consumers. The Use Case will depend on this publisher interface, not on specific handlers.

Code: Event Publisher Interface

Here's an interface for our EventPublisher. We'll also need a way for handlers to register themselves.

public interface EventPublisher {
  void publish(DomainEvent event);
  <T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler);
}

Code: In-Memory Event Publisher

A simple in-memory implementation for demonstration purposes. In a real application, this might use a message queue (e.g., Kafka, RabbitMQ) for persistence and distribution.

Run this to see the basic publisher in action:

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public interface DomainEvent { long occurredOn(); }
public interface EventHandler<T extends DomainEvent> { void handle(T event); }

public class OrderPlacedEvent implements DomainEvent {
  private final String orderId; private final long occurredOn;
  public OrderPlacedEvent(String orderId) {
    this.orderId = orderId; this.occurredOn = System.currentTimeMillis();
  }
  public String getOrderId() { return orderId; }
  @Override public long occurredOn() { return occurredOn; }
}

public class SimpleEventPublisher implements EventPublisher {
  private final Map<Class<? extends DomainEvent>, List<EventHandler<?>>> subscribers = new HashMap<>();

  @Override
  public void publish(DomainEvent event) {
    List<EventHandler<?>> handlers = subscribers.get(event.getClass());
    if (handlers != null) {
      for (EventHandler handler : handlers) {
        // Unchecked cast is safe due to type checking during subscription
        ((EventHandler<DomainEvent>) handler).handle(event);
      }
    }
  }

  @Override
  public <T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler) {
    subscribers.computeIfAbsent(eventType, k -> new ArrayList<>()).add(handler);
  }

  public static void main(String[] args) {
    SimpleEventPublisher publisher = new SimpleEventPublisher();
    publisher.subscribe(OrderPlacedEvent.class, event -> {
      System.out.println("Handler 1: Order " + event.getOrderId() + " placed!");
    });
    publisher.subscribe(OrderPlacedEvent.class, event -> {
      System.out.println("Handler 2: Notifying ops for order " + event.getOrderId());
    });

    OrderPlacedEvent event = new OrderPlacedEvent("DEMO-456");
    publisher.publish(event);
  }
}

Integrating Use Cases & Handlers

Now, let's see how a Use Case publishes an event and how an Event Handler (residing in, for example, the Interface Adapters layer) reacts to it. The Use Case remains decoupled from the handler.

This design allows new handlers to be added without modifying the Use Case.

Code: Full Event-Driven Flow

This example demonstrates a PlaceOrderUseCase publishing an OrderPlacedEvent, which is then handled by an OrderEmailNotifier.

Notice how PlaceOrderUseCase only depends on EventPublisher, not the specific notifier.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

// Domain Layer Interfaces & Classes
interface DomainEvent { long occurredOn(); }
class OrderPlacedEvent implements DomainEvent {
  private final String orderId; private final long occurredOn;
  public OrderPlacedEvent(String orderId) {
    this.orderId = orderId; this.occurredOn = System.currentTimeMillis();
  }
  public String getOrderId() { return orderId; }
  @Override public long occurredOn() { return occurredOn; }
}

// Application Layer Interfaces & Classes
interface EventPublisher {
  void publish(DomainEvent event);
  <T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler);
}
interface EventHandler<T extends DomainEvent> { void handle(T event); }

// Use Case (Application Layer)
class PlaceOrderUseCase {
  private final EventPublisher eventPublisher;

  public PlaceOrderUseCase(EventPublisher eventPublisher) {
    this.eventPublisher = eventPublisher;
  }

  public void execute(String orderDetails) {
    // Simulate order creation logic
    String newOrderId = "ORD-" + System.nanoTime();
    System.out.println("Order " + newOrderId + " created with details: " + orderDetails);

    // Publish domain event
    eventPublisher.publish(new OrderPlacedEvent(newOrderId));
  }
}

// Infrastructure/Interface Adapters Layer Implementation
class SimpleEventPublisher implements EventPublisher {
  private final Map<Class<? extends DomainEvent>, List<EventHandler<?>>> subscribers = new HashMap<>();

  @Override
  public void publish(DomainEvent event) {
    List<EventHandler<?>> handlers = subscribers.get(event.getClass());
    if (handlers != null) {
      for (EventHandler handler : handlers) {
        ((EventHandler<DomainEvent>) handler).handle(event);
      }
    }
  }

  @Override
  public <T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler) {
    subscribers.computeIfAbsent(eventType, k -> new ArrayList<>()).add(handler);
  }
}

class OrderEmailNotifier implements EventHandler<OrderPlacedEvent> {
  @Override
  public void handle(OrderPlacedEvent event) {
    System.out.println("Email Notifier: Sending email for order " + event.getOrderId());
  }
}

class InventoryUpdater implements EventHandler<OrderPlacedEvent> {
  @Override
  public void handle(OrderPlacedEvent event) {
    System.out.println("Inventory Updater: Updating inventory for order " + event.getOrderId());
  }
}

public class Main {
  public static void main(String[] args) {
    SimpleEventPublisher publisher = new SimpleEventPublisher();

    // Register handlers
    publisher.subscribe(OrderPlacedEvent.class, new OrderEmailNotifier());
    publisher.subscribe(OrderPlacedEvent.class, new InventoryUpdater());

    // Create use case with publisher dependency
    PlaceOrderUseCase placeOrderUseCase = new PlaceOrderUseCase(publisher);

    // Execute use case, which publishes the event
    placeOrderUseCase.execute("Laptop, Quantity: 1");
    placeOrderUseCase.execute("Keyboard, Quantity: 2");
  }
}

Quick Check: Domain Events

When integrating domain events into Clean Architecture, what is the primary benefit of having Use Cases publish events rather than directly calling other services?

Recap: Event-Driven Clean Arch

In this lesson, we explored Event-Driven Clean Architecture. You learned:

Domain Events are immutable facts representing significant occurrences.
They enable decoupling, allowing Use Cases to publish events without knowing their subscribers.
Event Publishers mediate event dispatch to Event Handlers.
This pattern supports scalability and extensibility, making systems easier to evolve while adhering to the Dependency Rule.

By leveraging domain events, your Clean Architecture can become even more robust and adaptable.

Häufig gestellte Fragen

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