The mystery of Java EE 6 annotations inheritance

问题

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);
    }

}

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;
        }
    }
}

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?

回答1:

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:

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.

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)

来源：https://stackoverflow.com/questions/5536583/the-mystery-of-java-ee-6-annotations-inheritance