用于自定义鉴别器的Hibernate

Question

我有一个具体的JPA实体超类，它使用鉴别器列与inheritanceType.joined映射，还有两个子类实体，它们用其他属性扩展了这个超类。

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "TYPE")
public class BaseEntity {
    // . . .
}

@Entity
@DiscriminatorValue("SUBTYPE")
public class SubclassEntity extends BaseEntity {
    // . . .
}

在某些情况下，我希望指定额外的鉴别器值，而不必为每个类型显式定义子类（也就是说，并不是每个“baseEntity”都指定支持子类/单独表的额外属性）。这种策略在数据库设计和Java类层次结构中都很有效，但是，Hibernate JPA不允许这样做，并抛出UrtranClassException，因为没有映射到鉴别器的子类：

Caused by: org.hibernate.WrongClassException: Object [id=entity-1] was not of the specified subclass [com.so.jpa.BaseEntity] : Discriminator: custom-1
    at org.hibernate.loader.plan.exec.process.internal.EntityReferenceInitializerImpl.getConcreteEntityTypeName(EntityReferenceInitializerImpl.java:415)
    at org.hibernate.loader.plan.exec.process.internal.EntityReferenceInitializerImpl.hydrateEntityState(EntityReferenceInitializerImpl.java:217)
    at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.readRow(AbstractRowReader.java:90)
    at org.hibernate.loader.plan.exec.internal.EntityLoadQueryDetails$EntityLoaderRowReader.readRow(EntityLoadQueryDetails.java:238)
    at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:112)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:121)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:85)
    at org.hibernate.loader.entity.plan.AbstractLoadPlanBasedEntityLoader.load(AbstractLoadPlanBasedEntityLoader.java:167)
    at org.hibernate.persister.entity.AbstractEntityPersister.load(AbstractEntityPersister.java:3954)
    at org.hibernate.event.internal.DefaultLoadEventListener.loadFromDatasource(DefaultLoadEventListener.java:488)
    at org.hibernate.event.internal.DefaultLoadEventListener.doLoad(DefaultLoadEventListener.java:453)
    at org.hibernate.event.internal.DefaultLoadEventListener.load(DefaultLoadEventListener.java:196)
    at org.hibernate.event.internal.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:258)
    at org.hibernate.event.internal.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:134)
    at org.hibernate.internal.SessionImpl.fireLoad(SessionImpl.java:1071)
    at org.hibernate.internal.SessionImpl.internalLoad(SessionImpl.java:990)
    at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:632)
    at org.hibernate.type.EntityType.resolve(EntityType.java:424)
    at org.hibernate.engine.internal.TwoPhaseLoad.doInitializeEntity(TwoPhaseLoad.java:154)
    at org.hibernate.engine.internal.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:128)
    at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:1132)
    at org.hibernate.loader.Loader.processResultSet(Loader.java:992)
    at org.hibernate.loader.Loader.doQuery(Loader.java:930)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:336)
    at org.hibernate.loader.Loader.doList(Loader.java:2611)
    at org.hibernate.loader.Loader.doList(Loader.java:2594)
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2423)
    at org.hibernate.loader.Loader.list(Loader.java:2418)
    at org.hibernate.loader.hql.QueryLoader.list(QueryLoader.java:501)
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.list(QueryTranslatorImpl.java:371)
    at org.hibernate.engine.query.spi.HQLQueryPlan.performList(HQLQueryPlan.java:220)
    at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1268)
    at org.hibernate.internal.QueryImpl.list(QueryImpl.java:87)
    at org.hibernate.jpa.internal.QueryImpl.list(QueryImpl.java:567)
    at org.hibernate.jpa.internal.QueryImpl.getResultList(QueryImpl.java:436)
    ...

在本例中，我希望Hibernate返回具体的基实体baseEntity，而不是尝试实例化子类。我在JPA规范(JSR338)中没有看到任何东西表明这不应该是可能的（尽管规范也没有明确地调用这个场景）。

Answer 1

不幸的是，Hibernate只期望每个实体类型有一个鉴别器值。我想这与其他JPA提供程序没有区别，因为不能为一个实体类定义多个DiscriminatorValue。

即使没有定义DiscriminatorValue，也会有一个：

如果未指定DiscriminatorValue注释并使用了discriminator列，则将使用特定于提供程序的函数生成表示实体类型的值。如果DiscriminatorType是String，则discriminator值默认值是实体名称。

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorFormula(
    "CASE WHEN TYPE IN ('SUBTYPE', 'SUBTYPE-2', ...) THEN TYPE ELSE 'BaseEntity'")
public class BaseEntity {
    // . . .
}

@Entity
@DiscriminatorValue("SUBTYPE")
public class SubclassEntity extends BaseEntity {
    // ...
}