为什么hibernate持久性上下文在事务之外不可用？

Question

Vlad关于如何修复MultipleBagsException的示例将是我们的起点：如何修复MultipleBagsException-Vlad Mihalcea

utx.begin();
List<Post> posts = entityManager.createQuery("""
    select distinct p
    from Post p
    left join fetch p.comments
    where p.id between :minId and :maxId""", Post.class)
.setParameter("minId", 1L)
.setParameter("maxId", 50L)
.setHint(QueryHints.PASS_DISTINCT_THROUGH, false)
.getResultList();
 
posts = entityManager.createQuery("""
    select distinct p
    from Post p
    left join fetch p.tags t
    where p in :posts""", Post.class)
.setParameter("posts", posts)
.setHint(QueryHints.PASS_DISTINCT_THROUGH, false)
.getResultList();
utx.rollback();

<persistence-unit name="some-p-unit-name">
    <jta-data-source>java:/jdbc/someDB</jta-data-source>
</persistence-unit>  

EntityManager的定义如下：

    @PersistenceContext(type = PersistenceContextType.EXTENDED, unitName = "some-p-unit-name")
private EntityManager em;

@Produces
@RequestScoped
@SomeResource // this is an annotation to differentiate it from another entity manager that can also be injectable
public EntityManager getEm() {
    return em;
}

那么，为什么我们需要启动事务以便启用PersistenceContext，即使我们将其设置为使用扩展上下文呢？

Answer 1

谢谢@smutje给我找到了正确的答案。在我们的例子中，解决方案是用@statefult注释定义EntityManager的类，根据文档，PersistenceContext仅适用于容器管理场景中的有状态EJB。

下面的示例代码。请注意有状态注释和持久化上下文type=extended。

@Stateful
public class Resources {
    @PersistenceContext(type = PersistenceContextType.EXTENDED, unitName = "some-p-unit-name")
    private EntityManager em;

    @Produces
    @RequestScoped
    public EntityManager getEm() {
        return em;
    }
}