我使用的繼承與EJB在少數情況下,有時在這樣的通用entityDAO超類的註解奧祕:Java EE 6個的註解繼承
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);
}
}
像這樣實施的示例子類:
@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;
}
}
}
我最近讀到,Java註釋不是繼承(source)。這應該會導致我的JpaDAO在訪問其實體管理器或其標準構建器時拋出空指針異常(因爲@PersistanceContext和@PostConstruct都將被忽略),但事實並非如此。有人可以澄清這是如何工作的嗎?我高調擔心超類中的@TransactionAttributes發生了什麼,如果子類的NOT_SUPPORTED爲類的默認值,我可以信任REQUIRED實際使用從子類調用的事務嗎?