如果从数据库加载两次，hibernate 是否加载同一个实例的两个单独副本？

Question

我知道关于延迟加载的 SO 有很多不同的问题，但我的问题有点不同。

假设我有一个实体 A，其中我有实体 B 的集合。同样，在实体 B 中，我有 A 的集合。在这两种情况下，都使用了 lazy="true" 选项。

实体 A的实例 aA 具有 --> Set<B>===（此集合包含实体 B 的实例 bB）

实体 B的实例 bB 具有 --> Set<A>===（此集合包含实体 A 的实例 aA）

现在，如果我加载实体 A 的集合（即Set<B>）。它现在已初始化，即完成 A 的 aA 实例，包括集合。我现在期望的是实体 B 的实例 bB 也已完全初始化但不，它没有，当我引用具有实体 A 实例 aA 的实体 B 的集合时，我得到了延迟初始化异常。

如果从数据库加载两次，hibernate 是否加载同一个实例的两个单独副本？如果是这样，有没有办法同步会话中所有副本的更改？

希望我足够清楚并且没有用乱七八糟的信息把事情搞砸:)

Answer 1

如果从数据库加载两次，hibernate 是否加载同一个实例的两个单独副本？

不，它不会在同一个会话中两次加载同一个对象（为了我们的理智）。

我创建了一个简单的Spring Boot 项目来查看这一点。

EntityA#setOfB(B1,B2)
EntityB#setOfA(A1)

在通过 ID 将 EntityA 1 和 EntityB 加载到 Session 后，我们强制 A1.setOfB 初始化。在下面的日志中，我们可以看到，尽管它必须查询获取两行（B1 和 B2）的集合，但它只会隐藏一个对象（B2），因为 B1 是在会话缓存中找到的。参见测试 1 (mvn spring-boot:run -Drun.arguments="1")

DEBUG 6452 --- [           main] o.g.hiplay.app.HibernateService          : ########## Retrieving A1's set of B ##########
DEBUG 6452 --- [           main] org.hibernate.SQL                        : select setofb0_.id_a as id_a1_0_0_, setofb0_.id_b as id_b2_2_0_, entityb1_.id as id1_1_1_, entityb1_.description as descript2_1_1_ from relations setofb0_ inner join entityb entityb1_ on setofb0_.id_b=entityb1_.id where setofb0_.id_a=?
TRACE 6452 --- [           main] o.h.l.p.e.i.AbstractLoadPlanBasedLoader  : Bound [2] parameters total
DEBUG 6452 --- [           main] o.h.l.p.e.p.i.ResultSetProcessorImpl     : Preparing collection intializer : [oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Constructing collection load context for result set [rs3: org.h2.result.LocalResult@5c99abd7 columns: 4 rows: 2 pos: -1]
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Starting attempt to find loading collection [[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]]
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Collection not yet initialized; initializing
TRACE 6452 --- [           main] o.h.l.p.e.p.i.ResultSetProcessorImpl     : Processing result set
DEBUG 6452 --- [           main] o.h.l.p.e.p.i.ResultSetProcessorImpl     : Starting ResultSet row #0
DEBUG 6452 --- [           main] e.p.i.CollectionReferenceInitializerImpl : Found row of collection: [oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Starting attempt to find loading collection [[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]]
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Attempting to locate loading collection entry [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]] in any result-set context
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Collection [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]] located in load context
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Found loading collection bound to current result set processing; reading row
TRACE 6452 --- [           main] o.h.e.internal.DefaultLoadEventListener  : Loading entity: [oss.gabrielgiussi.hiplay.entities.EntityB#1]
TRACE 6452 --- [           main] o.h.e.internal.DefaultLoadEventListener  : Attempting to resolve: [oss.gabrielgiussi.hiplay.entities.EntityB#1]
TRACE 6452 --- [           main] o.h.e.internal.DefaultLoadEventListener  : Resolved object in session cache: [oss.gabrielgiussi.hiplay.entities.EntityB#1]
DEBUG 6452 --- [           main] o.h.l.p.e.p.i.ResultSetProcessorImpl     : Starting ResultSet row #1
TRACE 6452 --- [           main] l.p.e.p.i.EntityReferenceInitializerImpl : hydrating entity state
TRACE 6452 --- [           main] l.p.e.p.i.EntityReferenceInitializerImpl : Initializing object from ResultSet: [oss.gabrielgiussi.hiplay.entities.EntityB#2]
TRACE 6452 --- [           main] o.h.p.entity.AbstractEntityPersister     : Hydrating entity: [oss.gabrielgiussi.hiplay.entities.EntityB#2]
DEBUG 6452 --- [           main] e.p.i.CollectionReferenceInitializerImpl : Found row of collection: [oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Starting attempt to find loading collection [[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]]
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Attempting to locate loading collection entry [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]] in any result-set context
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Collection [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]] located in load context
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Found loading collection bound to current result set processing; reading row
TRACE 6452 --- [           main] o.h.e.internal.DefaultLoadEventListener  : Loading entity: [oss.gabrielgiussi.hiplay.entities.EntityB#2]
TRACE 6452 --- [           main] o.h.e.internal.DefaultLoadEventListener  : Attempting to resolve: [oss.gabrielgiussi.hiplay.entities.EntityB#2]
TRACE 6452 --- [           main] o.h.e.internal.DefaultLoadEventListener  : Resolved object in session cache: [oss.gabrielgiussi.hiplay.entities.EntityB#2]
TRACE 6452 --- [           main] o.h.l.p.e.p.i.ResultSetProcessorImpl     : Done processing result set (2 rows)
TRACE 6452 --- [           main] o.h.l.p.e.p.internal.AbstractRowReader   : Total objects hydrated: 1
DEBUG 6452 --- [           main] o.h.engine.internal.TwoPhaseLoad         : Resolving associations for [oss.gabrielgiussi.hiplay.entities.EntityB#2]
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Attempting to locate loading collection entry [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityB.setOfA#2]] in any result-set context
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Collection [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityB.setOfA#2]] not located in load context
TRACE 6452 --- [           main] org.hibernate.type.CollectionType        : Created collection wrapper: [oss.gabrielgiussi.hiplay.entities.EntityB.setOfA#2]
DEBUG 6452 --- [           main] o.h.engine.internal.TwoPhaseLoad         : Done materializing entity [oss.gabrielgiussi.hiplay.entities.EntityB#2]
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Attempting to locate loading collection entry [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]] in any result-set context
TRACE 6452 --- [           main] o.h.e.loading.internal.LoadContexts      : Collection [CollectionKey[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]] located in load context
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Removing collection load entry [org.hibernate.engine.loading.internal.LoadingCollectionEntry<rs=rs3: org.h2.result.LocalResult@5c99abd7 columns: 4 rows: 2 pos: 2, coll=[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]>@131c8e88]
DEBUG 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : 1 collections were found in result set for role: oss.gabrielgiussi.hiplay.entities.EntityA.setOfB
TRACE 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Ending loading collection [org.hibernate.engine.loading.internal.LoadingCollectionEntry<rs=rs3: org.h2.result.LocalResult@5c99abd7 columns: 4 rows: 2 pos: 2, coll=[oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]>@131c8e88]
DEBUG 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : Collection fully initialized: [oss.gabrielgiussi.hiplay.entities.EntityA.setOfB#1]
DEBUG 6452 --- [           main] o.h.e.l.internal.CollectionLoadContext   : 1 collections initialized for role: oss.gabrielgiussi.hiplay.entities.EntityA.setOfB
TRACE 6452 --- [           main] o.h.e.i.StatefulPersistenceContext       : Initializing non-lazy collections
DEBUG 6452 --- [           main] o.g.hiplay.app.HibernateService          : ########## The object hasn't been loaded twice ##########

Hibernate 检查对象是否已经在会话缓存中，询问 StatefulPersistenceContext

我现在期望的是实体 B 的实例 bB 也已完全初始化，但不，它没有，当我引用具有实体 A 实例 aA 的实体 B 的集合时，我得到了延迟初始化异常。

真正未初始化的是 bB 的 A 集。在执行以下操作后：

EntityA aA = session.get("A",a) aA.setOfB.size() // 强制初始化惰性集合。此时 bB 是 hydratate 并放入内存中，但他的 A 集合未初始化。

如果您尝试访问集合的元素，Hibernate 需要初始化集合（到目前为止，它所拥有的只是一个代理，具有在需要时从数据库加载集合的智能，例如请求元素或集合的大小） . 参见测试 3 (mvn spring-boot:run -Drun.arguments="2")

// outside of the transaction
EntityB bB = aA.setOfB().get(0)
bB.setOfA().size() // LazyInitializationExample

如果您在事务内部，则集合将被初始化，并且与 aA 对应的行将再次从基数中检索，但不会被水合。参见测试 3 (mvn spring-boot:run -Drun.arguments="3")