Java EE 6 注解继承之谜 [英] The mystery of Java EE 6 annotations inheritance
问题描述
我在一些场景中将继承与 EJB 一起使用，有时在超类中使用注释，例如这个通用 entityDAO:
I'm using inheritance with EJB in a few scenarios, sometimes with annotations in the super class like this generic entityDAO:
public class JpaDAO<T>{
protected Class<T> entityClass;
@PersistenceContext(unitName="CarrierPortalPU")
protected EntityManager em;
protected CriteriaBuilder cb;
@PostConstruct
private void init() {
cb = em.getCriteriaBuilder();
}
public JpaDAO(Class<T> type) {
entityClass = type;
}
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public void create(T entity) {
em.persist(entity);
}
@TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)
public T find(Object id) {
return em.find(entityClass, id);
}
@TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)
public List<T> findAll(){
CriteriaQuery<T> cq = cb.createQuery(entityClass);
Root<T> entity = cq.from(entityClass);
cq.select(entity);
return em.createQuery(cq).getResultList();
}
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public void remove(T entity) {
em.remove(em.merge(entity));
}
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public T edit(T entity) {
return em.merge(entity);
}
}
使用这样实现的示例子类:
With an example subclass implemented like this:
@Stateless
@TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)
public class DepartmentDAO extends JpaDAO<Department> {
public DepartmentDAO() {
super(Department.class);
}
public Department findByName(String name){
CriteriaQuery<Department> cq = cb.createQuery(Department.class);
Root<Department> department = cq.from(Department.class);
cq.where(cb.equal(department.get(Department_.name), name));
cq.select(department);
try{
return em.createQuery(cq).getSingleResult();
}catch(Exception e){
return null;
}
}
}
我最近读到 Java 注释不是继承的 (来源).这应该会导致我的 JpaDAO 在访问其实体管理器或其标准构建器时抛出空指针异常(因为 @PersistanceContext 和 @PostConstruct 都将被忽略)，但事实并非如此.有人可以澄清这实际上是如何工作的吗?我有点担心超类中的 @TransactionAttributes 会发生什么，当子类将 NOT_SUPPORTED 作为类默认值时，我可以信任 REQUIRED 从子类调用时实际使用事务吗?
I recently read that java annotations are NOT inherited (source). This should cause my JpaDAO to throw a null pointer exception when accessing its entitymanager or its criteriabuilder (since both @PersistanceContext and @PostConstruct would be ignored), however this it not the case. Can someone clarify how this really works? I am abit worried about what happens to my @TransactionAttributes in the superclass, can I trust a REQUIRED to actually use transactions when called from the subclass, when the subclass has NOT_SUPPORTED as class default?
推荐答案
Java 注释不是继承的，但 JavaEE 规范更改了规则以允许这些属性按预期工作.请参阅通用注释 1.1 规范.2.1 节甚至以@TransactionAttribute 为例.EJB 3.1 第 13.3.7.1 节还明确规定了@TransactionAttribute 的规则:
Java annotations are not inherited, but the JavaEE specs change the rules to allow these attributes to work as expected. See the common annotations 1.1 spec. Section 2.1 even uses @TransactionAttribute as an example. EJB 3.1 section 13.3.7.1 also explicitly states the rules for @TransactionAttribute:
如果 bean 类具有超类，则适用以下附加规则.
If the bean class has superclasses, the following additional rules apply.
- 在超类 S 上指定的事务属性适用于 S 定义的业务方法.如果在 S 上未指定类级别的事务属性，则相当于在 S 上指定 TransactionAttribute(REQUIRED).
- 可以在类 S 定义的业务方法 M 上指定事务属性，以便为方法 M 覆盖在类 S 上显式或隐式指定的事务属性值.
- 如果类 S 的方法 M 覆盖了由 S 的超类定义的业务方法，则 M 的事务属性由应用于类 S 的上述规则确定.
简而言之，对于大多数 JavaEE 注释，除非子类覆盖该方法，否则方法级注释适用于该方法，并且类级注释仅适用于该类中定义的所有方法.该规则不适用于组件定义"类级注解，例如@Stateless(请参阅 EJB 3.1 规范第 4.9.2.1 节)
In short, for most JavaEE annotations, method-level annotations apply to that method unless a subclass overrides the method, and class-level annotations apply to all methods defined in that class only. The rule does not apply to "component-defining" class-level annotations, such as @Stateless (see the EJB 3.1 specification section 4.9.2.1)
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