Starting with Spring Data Ingalls release publishing domain events by aggregate roots becomes easier. Instead of leveraging Spring’s ApplicationEventPublisher you can use @DomainEvents annotation on a method of your aggregate root.
Let’s look at an example.
class BankTransfer {
@DomainEvents
Collection<Object> domainEvents() {
// … return events you want to get published here
}
@AfterDomainEventsPublication
void callbackMethod() {
// … potentially clean up domain events list
}
}
The method annotated with @DomainEvents will be called if one of the save() methods of the Spring Data repository is called. The method can return a single event instance or a collection of events and cannot have any arguments. After all events have been published a method annotated with @AfterDomainEventsPublication is called.
Spring Data Commons provides a convenient base class (AbstractAggregateRoot) to help to register domain events and is using the publication mechanism implied by @DomainEvents and @AfterDomainEventsPublication
public class AbstractAggregateRoot {
/**
* All domain events currently captured by the aggregate.
*/
@Getter(onMethod = @__(@DomainEvents)) //
private transient final List<Object> domainEvents = new ArrayList<Object>();
/**
* Registers the given event object for publication on a call to a Spring Data repository's save method.
*
* @param event must not be {@literal null}.
* @return
*/
protected <T> T registerEvent(T event) {
Assert.notNull(event, "Domain event must not be null!");
this.domainEvents.add(event);
return event;
}
/**
* Clears all domain events currently held. Usually invoked by the infrastructure in place in Spring Data
* repositories.
*/
@AfterDomainEventPublication
public void clearDomainEvents() {
this.domainEvents.clear();
}
}
The @Getter(onMethod = @__(@DomainEvents)) Lombok construct makes sure that the @DomainEvents annotation is put on the generated getter method. Let’s modify the example to extend from AbstractAggregateRoot base class.
public class BankTransfer extends AbstractAggregateRoot {
...
public BankTransfer complete() {
id = UUID.randomUUID().toString();
registerEvent(new BankTransferCompletedEvent(id));
return this;
}
...
}
In the example the BankTransfer aggregate root registers a BankTransferCompletedEvent when its complete method is called.
The client calls this method in a transactional context saving the BankTransfer also via Spring Data Repository abstraction, which triggers the publication of BankTransferCompletedEvent event.
@Service
public class BankTransferService {
...
@Transactional
public String completeTransfer(BankTransfer bankTransfer) {
return repository.save(bankTransfer.complete()).getId();
}
...
}
On the event listener side by using TransactionalEventListener the event handler can be bound to a phase of the transaction which published the event. The typical use case is to handle the event when the transaction completed successfully, which is the default setting for @TransactionalEventListener. It can be further customized via the phase attribute.
@Service
public class BankTransferProcessor {
...
@Async
@TransactionalEventListener
public void handleBankTransferCompletedEvent(BankTransferCompletedEvent event) {
BankTransfer bankTransfer = repository.findById(event.getBankTransferId());
bankTransfer.markStarted();
bankTransfer = repository.save(bankTransfer);
log.info("Starting to process bank transfer {}.", bankTransfer);
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
if (new Random().nextBoolean()) {
bankTransfer.markCompleted();
} else {
bankTransfer.markFailed();
}
repository.save(bankTransfer);
log.info("Finished processing bank transfer {}.", bankTransfer);
}
}
In the example the @Async is used to return immediately the client without waiting for the processing of the bank transfer.
$ echo '{
"from":"DE89 3704 0044 0532 0130 00",
"to":"HU42 1177 3016 1111 1018 0000 0000",
"amount":100.25
}' | \
http post :8080/bank-transfers
HTTP/1.1 201
Content-Type: application/json;charset=UTF-8
Date: Thu, 04 May 2017 13:54:12 GMT
{
"bankTransferId": "783b9b13-8424-4004-b59f-eef400d8a52c"
}
Retrieving the details of the bank transfer:
$ http :8080/bank-transfers/783b9b13-8424-4004-b59f-eef400d8a52c
HTTP/1.1 200
Content-Type: application/json;charset=UTF-8
Date: Thu, 04 May 2017 13:55:33 GMT
Transfer-Encoding: chunked
{
"bankTransferId": "783b9b13-8424-4004-b59f-eef400d8a52c",
"from": "DE89 3704 0044 0532 0130 00",
"to": "HU42 1177 3016 1111 1018 0000 0000",
"amount": 100.25,
"status": "COMPLETED"
}