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領(lǐng)域事件發(fā)布是一個(gè)領(lǐng)域?qū)ο鬄榱俗屍渌鼘?duì)象知道自己已經(jīng)處理完成某個(gè)操作時(shí)發(fā)出的一個(gè)通知,事件發(fā)布力求從代碼層面讓自身對(duì)象與外部對(duì)象解耦,并減少技術(shù)代碼入侵。
一、 手動(dòng)發(fā)布事件
// 實(shí)體定義 @Entity public class Department implements Serializable { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer departmentId; @Enumerated(EnumType.STRING) private State state; } // 事件定義 public class DepartmentEvent { private Department department; private State state; public DepartmentEvent(Department department) { this.department = department; state = department.getState(); } } // 領(lǐng)域服務(wù) @Service public class ApplicationService { @Autowired private ApplicationEventPublisher applicationEventPublisher; @Autowired private DepartmentRepository departmentRepository; @Transactional(rollbackFor = Exception.class) public void departmentAdd(Department department) { departmentRepository.save(department); // 事件發(fā)布 applicationEventPublisher.publishEvent(new DepartmentEvent(department)); } }
使用applicationEventPublisher.publishEvent
在領(lǐng)域服務(wù)處理完成后發(fā)布領(lǐng)域事件,此方法需要在業(yè)務(wù)代碼中顯式發(fā)布事件,并在領(lǐng)域服務(wù)里引入ApplicationEventPublisher類,但對(duì)領(lǐng)域服務(wù)本身有一定的入侵性,但靈活性較高。
二、 自動(dòng)發(fā)布事件
// 實(shí)體定義 @Entity public class SaleOrder implements Serializable { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer orderId; @Enumerated(EnumType.STRING) private State state; // 返回類型定義 @DomainEvents public List<Object> domainEvents(){ return Stream.of(new SaleOrderEvent(this)).collect(Collectors.toList()); } // 事件發(fā)布后callback @AfterDomainEventPublication void callback() { System.err.println("ok"); } } // 事件定義 public class SaleOrderEvent { private SaleOrder saleOrder; private State state; public SaleOrderEvent(SaleOrder saleOrder) { this.saleOrder = saleOrder; state = saleOrder.getState(); } } // 領(lǐng)域服務(wù) @Service public class ApplicationService { @Autowired private OrderRepository orderRepository; @Transactional(rollbackFor = Exception.class) public void saleOrderAdd(SaleOrder saleOrder) { orderRepository.save(saleOrder); } }
使用@DomainEvents
定義事件返回的類型,必須是一個(gè)集合,使用@AfterDomainEventPublication
定義事件發(fā)布后的回調(diào)。
此方法實(shí)事件類型定義在實(shí)體中,與領(lǐng)域服務(wù)完全解耦,沒(méi)有入侵。系統(tǒng)會(huì)在orderRepository.save(saleOrder)后自動(dòng)調(diào)用事件發(fā)布,另delete方法不會(huì)調(diào)用事件發(fā)布。
三、 事件監(jiān)聽(tīng)
@Component public class ApplicationEventProcessor { @EventListener(condition = "#departmentEvent.getState().toString() == 'SUCCEED'") public void departmentCreated(DepartmentEvent departmentEvent) { System.err.println("dept-event1:" + departmentEvent); } @Async @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT, condition = "#saleOrderEvent.getState().toString() == 'SUCCEED'") public void saleOrderCreated(SaleOrderEvent saleOrderEvent) { System.err.println("sale-event succeed1:" + saleOrderEvent); } @TransactionalEventListener(phase = TransactionPhase.BEFORE_COMMIT, condition = "#saleOrderEvent.getState().toString() == 'SUCCEED'") public void saleOrderCreatedBefore(SaleOrderEvent saleOrderEvent) { System.err.println("sale-event succeed2:" + saleOrderEvent); } @Async @TransactionalEventListener(phase = TransactionPhase.AFTER_ROLLBACK) public void saleOrderCreatedFailed(SaleOrderEvent saleOrderEvent) { System.out.println("sale-event failed:" + saleOrderEvent); } }
1. 使用@EventListener監(jiān)聽(tīng)事件
@EventListener沒(méi)有事務(wù)支持,只要事件發(fā)出就可監(jiān)控到
@Transactional(rollbackFor = Exception.class) public void departmentAdd(Department department) { departmentRepository.save(department); applicationEventPublisher.publishEvent(new DepartmentEvent(department)); throw new RuntimeException("failed"); }
上述情況會(huì)造成事務(wù)失敗回滾,但事件監(jiān)控端已經(jīng)執(zhí)行,可能導(dǎo)致數(shù)據(jù)不一致的情況發(fā)生
2. 使用@TransactionalEventListener監(jiān)聽(tīng)事件
使用TransactionPhase.AFTER_COMMIT
可在事務(wù)完成后,再執(zhí)行事件監(jiān)聽(tīng)方法,從而保證數(shù)據(jù)的一致性
3. TransactionPhase.AFTER_ROLLBACK回滾事務(wù)問(wèn)題
@Async @TransactionalEventListener(phase = TransactionPhase.AFTER_ROLLBACK, condition = "#departmentEvent.getState().toString() == 'SUCCEED'") public void departmentCreatedFailed(DepartmentEvent departmentEvent) { System.err.println("dept-event3:" + departmentEvent); }
由于@DomainEvents作用在實(shí)體上的,只有剛orderRepository.save(saleOrder)執(zhí)行成功后才會(huì)發(fā)送事件,故AFTER_ROLLBACK方法只會(huì)在同一事務(wù)中其它語(yǔ)句執(zhí)行失敗或顯式rollback時(shí)才會(huì)執(zhí)行,如果save方法執(zhí)行失敗,將不會(huì)監(jiān)聽(tīng)到回滾事件。
4. @Async異步事件監(jiān)聽(tīng)
四、 總結(jié)
通過(guò)對(duì) @DomainEvents、@TransactionalEventListener的使用,在有效的解決領(lǐng)域事件發(fā)布的情況下,減少了對(duì)業(yè)務(wù)代碼的入侵,同時(shí)盡一步解決了數(shù)據(jù)一致性問(wèn)題。
在分布式結(jié)構(gòu)下,通過(guò)MQ發(fā)送事件通知給其它服務(wù),為解決一致性問(wèn)題,防止對(duì)方服務(wù)處理失敗可先將事件保久化到數(shù)據(jù)庫(kù)后,再重試。
五、 源碼
https://gitee.com/hypier/barry-jpa/tree/master/jpa-section-5
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