JPA：与惰性初始化集合合并的行为是什么？

The semantics of the merge operation
applied to an entity X are as follows:

• If X is a detached entity,the state
  of X is copied onto a pre-existing
  managed entity instance X’ of the same
  identity or a new managed copy X’ of X
  is created.

• If X is a new entity
  instance,a new managed entity
  instance X’ is created and the state
  of X is copied into the new managed
  entity instance X’.

• If X is a
  removed entity instance,an
  IllegalArgumentException will be
  thrown by the merge operation (or the
  transaction commit will fail).

• If X
  is a managed entity,it is ignored by
  the merge operation,however,the
  merge operation is cascaded to
  entities referenced by relationships
  from X if these relationships have
  been annotated with the cascade
  element value cascade=MERGE or
  cascade=ALL annotation.

• For all
  entities Y referenced by relationships
  from X having the cascade element
  value cascade=MERGE or cascade=ALL,Y
  is merged recursively as Y’. For all
  such Y referenced by X,X’ is set to
  reference Y’. (Note that if X is
  managed then X is the same object as
  X’.)

• If X is an entity merged to X’,
  with a reference to another entity Y,
  where cascade=MERGE or cascade=ALL is
  not specified,then navigation of the
  same association from X’ yields a
  reference to a managed object Y’ with
  the same persistent identity as Y.

如你所见,这里没有魔力.已分离实例的状态将复制到新创建的托管实例中.由于您的分离实例有一个空列表,因此托管实例也会拥有它.

进一步的行为取决于关系的所有权,因为数据库中的表示反映了关系的拥有方：

>如果Team是拥有者,那么团队和玩家之间的关系将在刷新期间被销毁(但是除非你的关系中有orphanRemoval = true,否则玩家本身会存活).
>否则在Team中拥有空列表不会影响数据库.

如果在刷新上下文之前刷新团队,则Team的所有属性都将由数据库中的值重写,因此将恢复播放器列表(因为尚未刷新播放器的空列表).

如果在调用refresh()之前调用flush(),并且Team是拥有者,则list将为空,因为在flush()期间,关系的破坏会传播到数据库.