inheritance - Java EE 6 注解继承之谜

Question

我在一些场景中将继承与 EJB 一起使用，有时在父类(super class)中使用注释，例如这个通用 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:

@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 (source) .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？我有点担心父类(super class)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？

Answer 1

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 节还明确说明了 @TransactionAttribute 的规则:

If the bean class has superclasses, the following additional rules apply.

• transaction attribute specified on a superclass S applies to the business methods defined by S. If a class-level transaction attribute is not specified on S, it is equivalent to specification of TransactionAttribute(REQUIRED) on S.
• A transaction attribute may be specified on a business method M defined by class S to override for method M the transaction attribute value explicitly or implicitly specified on the class S.
• If a method M of class S overrides a business method defined by a superclass of S, the transaction attribute of M is determined by the above rules as applied to class S.

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”类级别的注释，例如 @Stateless(请参阅 EJB 3.1 规范第 4.9.2.1 节)