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本文主要研究一下ddd-by-examples的DomainEvent
DomainEvent
代码语言：javascript复制public interface DomainEvent {
UUID getEventId();
UUID getAggregateId();
Instant getWhen();
}
DomainEvent接口定义了getEventId、getAggregateId、getWhen方法
DomainEvents
代码语言：javascript复制public interface DomainEvents {
void publish(DomainEvent event);
default void publish(List<DomainEvent> events) {
events.forEach(this::publish);
}
}
DomainEvents接口定义了publish方法，同时提供一个批量的publish方法
JustForwardDomainEventPublisher
代码语言：javascript复制@AllArgsConstructor
public class JustForwardDomainEventPublisher implements DomainEvents {
private final ApplicationEventPublisher applicationEventPublisher;
@Override
public void publish(DomainEvent event) {
applicationEventPublisher.publishEvent(event);
}
}
JustForwardDomainEventPublisher实现了DomainEvents接口，其publish方法通过spring的ApplicationEventPublisher来发布
MeteredDomainEventPublisher
代码语言：javascript复制@AllArgsConstructor
public class MeteredDomainEventPublisher implements DomainEvents {
private static final String DOMAIN_EVENTS = "domain_events";
private static final String TAG_NAME = "name";
private final DomainEvents delegate;
private final MeterRegistry metricsRegistry;
@Override
public void publish(DomainEvent event) {
delegate.publish(event);
metricsRegistry.counter(DOMAIN_EVENTS, TAG_NAME, event.getClass().getSimpleName()).increment();
}
}
MeteredDomainEventPublisher实现了DomainEvents接口，其publish先执行delegate.publish(event)，然后递增metrics的counter计数
StoreAndForwardDomainEventPublisher
代码语言：javascript复制@AllArgsConstructor
public class StoreAndForwardDomainEventPublisher implements DomainEvents {
private final DomainEvents eventsPublisher;
private final EventsStorage eventsStorage;
@Override
public void publish(DomainEvent event) {
eventsStorage.save(event);
}
@Scheduled(fixedRate = 3000L)
@Transactional
public void publishAllPeriodically() {
List<DomainEvent> domainEvents = eventsStorage.toPublish();
domainEvents.forEach(eventsPublisher::publish);
eventsStorage.published(domainEvents);
}
}
StoreAndForwardDomainEventPublisher实现了DomainEvents接口，其publish方法先通过eventsStorage.save(event)保存事件，之后有一个定时任务不断执行eventsStorage.toPublish()获取events，发布事件，最后执行eventsStorage.published(domainEvents)
EventsStorage
代码语言：javascript复制public interface EventsStorage {
void save(DomainEvent event);
List<DomainEvent> toPublish();
void published(List<DomainEvent> events);
}
EventsStorage接口定义了save、toPublish、published方法
InMemoryEventsStorage
代码语言：javascript复制public class InMemoryEventsStorage implements EventsStorage {
//it's not thread safe, enough for testing
private final java.util.List<DomainEvent> eventList = Collections.synchronizedList(new ArrayList<>());
@Override
synchronized public void save(DomainEvent event) {
eventList.add(event);
}
@Override
synchronized public List<DomainEvent> toPublish() {
return List.ofAll(eventList);
}
@Override
synchronized public void published(List<DomainEvent> events) {
eventList.removeAll(events.asJava());
}
}
InMemoryEventsStorage实现了EventsStorage接口，其save方法添加event到eventList，其toPublish方法返回当时的eventList，其published方法从eventList移除指定的events
小结
ddd-by-examples定义了DomainEvent、DomainEvents接口，其中JustForwardDomainEventPublisher、MeteredDomainEventPublisher、StoreAndForwardDomainEventPublisher实现了DomainEvents接口；StoreAndForwardDomainEventPublisher借助了EventsStorage，EventsStorage接口定义了save、toPublish、published方法。
doc
- ddd-by-examples/library
