Detailed explanation of the usage of Spring Data's Domain Event

1.DDD and Spring Data

DDD: domain-driven design is the implementation method of software development under complex requirements. I will explain DDD specifically when I have time.

Spring Data is designed in many places according to the DDD principle (such as Repository). Here Spring Data mainly implements DDD's aggregate and domain event:

1. aggregate: A combination of a series of domain objects that can be regarded as a single unit. For example, orders and shopping lists are separate objects, but they are treated as separate units (aggregate). Each aggregate has an aggregate root. Any external interaction should only be through aggregate root, so that aggregate root can ensure the integrity of aggregate. An aggregate has transaction (data consistency) boundaries inside.
2. domain event: To ensure data consistency between aggregates, event-Driven Architecture is used to achieve the ultimate consistency of data. And these events are domain events published through aggregate root.

Below we demonstrate a statistical number of male and female numbers when Person's aggregate root is saved successfully, through Person's gender attribute, the statistical entity GenderStat is updated.

2. Demo

2.1 Person Aggregate Root

As an Aggregate Root, Person has the ability to publish domain event. There are two ways to implement it under Spring Data:

1. Inherit AbstractAggregateRoot and register publishing events using its registerEvent() method
2. Publish events using @DomainEvents annotation method

like:

 @Entity@Data@AllArgsConstructor@NoArgsConstructor@ToString(exclude = "domainEvents")public class Person { @Id @GeneratedValue private Long id; private String name; private Integer gender;//1:male;2:female @DomainEvents Collection<Object> domainEvents() { List<Object> events= new ArrayList<Object>(); events.add(new PersonSavedEvent(this.id,this.gender)); return events; } @AfterDomainEventPublication void callbackMethod() { // }}

or

 @Entity@Data@AllArgsConstructor@NoArgsConstructor@ToString(exclude = "domainEvents")public class Person extends AbstractAggregateRoot{ @Id @GeneratedValue private Long id; private String name; private Integer gender;//1:male;2:female public Person afterPersonSavedCompleted(){ registerEvent(new PersonSavedEvent(this.id, this.gender)); return this; }}

2.2 Listening event handling

 @Componentpublic class GenderStatProcessor { @Autowired GenderRepository genderRepository; @Async @TransactionalEventListener public void handleAfterPersonSavedComplete(PersonSavedEvent event){ GenderStat genderStat = genderRepository.findOne(1l); if(event.getGender()==1){ genderStat.setMaleCount(genderStat.getMaleCount()+1); }else { genderStat.setFemaleCount(genderStat.getFemaleCount()+1); } genderRepository.save(genderStat); }}

3 Source code address : https://github.com/wiselyman/spring-data-domain-event

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