在Java EE和Web项目中使用JPA实体bean时，Netbeans中有两个不同的JPA代码生成器向导。
第一个向导生成“来自enetity beans的JPA控制器”。第二个向导生成“来自实体bean的JSF页面”，它创建的外观类与第一个向导中的控制器完全相同，但使用备用编码。外观类和JPA控制器之间有什么区别？
以下是第一个向导生成的代码的样子：
public class UserimagesJpaController {
public UserimagesJpaController() {
emf = Persistence.createEntityManagerFactory("ArticlesWeb-ejbPU");
}
private EntityManagerFactory emf = null;
public EntityManager getEntityManager() {
return emf.createEntityManager();
}
public void create(Userimages userimages) throws PreexistingEntityException, Exception {
EntityManager em = null;
try {
em = getEntityManager();
em.getTransaction().begin();
Users users = userimages.getUsers();
if (users != null) {
users = em.getReference(users.getClass(), users.getIdUsers());
userimages.setUsers(users);
}
em.persist(userimages);
if (users != null) {
users.getUserimagesList().add(userimages);
users = em.merge(users);
}
em.getTransaction().commit();
} catch (Exception ex) {
if (findUserimages(userimages.getIdUserImages()) != null) {
throw new PreexistingEntityException("Userimages " + userimages + " already exists.", ex);
}
throw ex;
} finally {
if (em != null) {
em.close();
}
}
}
public void edit(Userimages userimages) throws NonexistentEntityException, Exception {
EntityManager em = null;
try {
em = getEntityManager();
em.getTransaction().begin();
Userimages persistentUserimages = em.find(Userimages.class, userimages.getIdUserImages());
Users usersOld = persistentUserimages.getUsers();
Users usersNew = userimages.getUsers();
if (usersNew != null) {
usersNew = em.getReference(usersNew.getClass(), usersNew.getIdUsers());
userimages.setUsers(usersNew);
}
userimages = em.merge(userimages);
if (usersOld != null && !usersOld.equals(usersNew)) {
usersOld.getUserimagesList().remove(userimages);
usersOld = em.merge(usersOld);
}
if (usersNew != null && !usersNew.equals(usersOld)) {
usersNew.getUserimagesList().add(userimages);
usersNew = em.merge(usersNew);
}
em.getTransaction().commit();
} catch (Exception ex) {
String msg = ex.getLocalizedMessage();
if (msg == null || msg.length() == 0) {
Long id = userimages.getIdUserImages();
if (findUserimages(id) == null) {
throw new NonexistentEntityException("The userimages with id " + id + " no longer exists.");
}
}
throw ex;
} finally {
if (em != null) {
em.close();
}
}
}
public void destroy(Long id) throws NonexistentEntityException {
EntityManager em = null;
try {
em = getEntityManager();
em.getTransaction().begin();
Userimages userimages;
try {
userimages = em.getReference(Userimages.class, id);
userimages.getIdUserImages();
} catch (EntityNotFoundException enfe) {
throw new NonexistentEntityException("The userimages with id " + id + " no longer exists.", enfe);
}
Users users = userimages.getUsers();
if (users != null) {
users.getUserimagesList().remove(userimages);
users = em.merge(users);
}
em.remove(userimages);
em.getTransaction().commit();
} finally {
if (em != null) {
em.close();
}
}
}
public List<Userimages> findUserimagesEntities() {
return findUserimagesEntities(true, -1, -1);
}
public List<Userimages> findUserimagesEntities(int maxResults, int firstResult) {
return findUserimagesEntities(false, maxResults, firstResult);
}
private List<Userimages> findUserimagesEntities(boolean all, int maxResults, int firstResult) {
EntityManager em = getEntityManager();
try {
CriteriaQuery cq = em.getCriteriaBuilder().createQuery();
cq.select(cq.from(Userimages.class));
Query q = em.createQuery(cq);
if (!all) {
q.setMaxResults(maxResults);
q.setFirstResult(firstResult);
}
return q.getResultList();
} finally {
em.close();
}
}
public Userimages findUserimages(Long id) {
EntityManager em = getEntityManager();
try {
return em.find(Userimages.class, id);
} finally {
em.close();
}
}
public int getUserimagesCount() {
EntityManager em = getEntityManager();
try {
CriteriaQuery cq = em.getCriteriaBuilder().createQuery();
Root<Userimages> rt = cq.from(Userimages.class);
cq.select(em.getCriteriaBuilder().count(rt));
Query q = em.createQuery(cq);
return ((Long) q.getSingleResult()).intValue();
} finally {
em.close();
}
}
}
这是来自第二个向导：
@Stateless
public class UserimagesFacade extends AbstractFacade<Userimages> {
@PersistenceContext(unitName = "ArticlesWeb-ejbPU")
private EntityManager em;
protected EntityManager getEntityManager() {
return em;
}
public UserimagesFacade() {
super(Userimages.class);
}
}
public abstract class AbstractFacade<T> {
private Class<T> entityClass;
public AbstractFacade(Class<T> entityClass) {
this.entityClass = entityClass;
}
protected abstract EntityManager getEntityManager();
public void create(T entity) {
getEntityManager().persist(entity);
}
public void edit(T entity) {
getEntityManager().merge(entity);
}
public void remove(T entity) {
getEntityManager().remove(getEntityManager().merge(entity));
}
public T find(Object id) {
return getEntityManager().find(entityClass, id);
}
public List<T> findAll() {
javax.persistence.criteria.CriteriaQuery cq = getEntityManager().getCriteriaBuilder().createQuery();
cq.select(cq.from(entityClass));
return getEntityManager().createQuery(cq).getResultList();
}
public List<T> findRange(int[] range) {
javax.persistence.criteria.CriteriaQuery cq = getEntityManager().getCriteriaBuilder().createQuery();
cq.select(cq.from(entityClass));
javax.persistence.Query q = getEntityManager().createQuery(cq);
q.setMaxResults(range[1] - range[0]);
q.setFirstResult(range[0]);
return q.getResultList();
}
public int count() {
javax.persistence.criteria.CriteriaQuery cq = getEntityManager().getCriteriaBuilder().createQuery();
javax.persistence.criteria.Root<T> rt = cq.from(entityClass);
cq.select(getEntityManager().getCriteriaBuilder().count(rt));
javax.persistence.Query q = getEntityManager().createQuery(cq);
return ((Long) q.getSingleResult()).intValue();
}
}
这是一个设计问题。哪个更好？我更喜欢使用第二个向导并仅实现Facade以揭示JPA控制器的功能。这是从设计角度来看的“正确”方式吗？现在，如果它们的含义完全相同，为什么JPA控制器包含如此多的编码？
答案 0 :(得分：7)
我不使用Netbeans，所以我不会直接说话，我只是在诊断生成的代码。
“来自enetity beans的JPA控制器”生成Java SE兼容的DAO类。任何地方都没有容器管理依赖注入的方法。手动创建实体管理器并手动管理事务。这些类在“普通的”Java SE桌面应用程序和Java EE Web应用程序中都可以正常工作。
“来自实体bean的JSF页面”生成了与无状态EJB相似的Java EE兼容DAO类。实体管理器由容器注入，并且事务由容器完全透明地管理（通过规范，单个无状态EJB方法调用计为单个完整事务）。它们需要运行Java EE容器，并且不能在Java SE桌面应用程序中使用它们。由于Java EE容器透明地管理依赖注入和事务，这解释了为什么它们的代码少得多。
因此，“JPA控制器”也可以在Java EE Web应用程序中原样使用，但您迟早可能会遇到事务性问题。想象一下，单个HTTP请求（单个Web表单提交）意图以特定顺序通过多个这些类执行多个DB操作，并且只要后一个操作之一失败，就应该回滚所有除了。当您使用“JPA Controller”方法而不重构大量代码以便在外部创建事务时，这将无法工作，但这将完全透明地使用您在使用EJB时的预期方式。