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如何选择UserDao类的哪个实现？一个是有状态会话EJB，另一个是带有RequestedScope注释的管理会话bean。RequestedScopre ManagedBean VS有状态Bean
有状态会话的代码似乎更简单。在这个用户欢迎的情况下，两者都应该工作？
另外，有状态EJB的范围是什么？
UserDao.java
public interface UserDao {
User getForUsername(String username);
void createUser(User user);
}
EJBUserDao.java
@Stateful
@Alternative
public class EJBUserDao implements UserDao {
@Inject
private EntityManager entityManager;
public User getForUsername(String username) {
try {
Query query = entityManager.createQuery("select u from User u where u.username = ?");
query.setParameter(1, username);
return (User) query.getSingleResult();
} catch (NoResultException e) {
return null;
}
}
public void createUser(User user) {
entityManager.persist(user);
}
}
ManagedBeanUserDao.java
public class ManagedBeanUserDao implements UserDao {
@Inject
private EntityManager entityManager;
@Inject
private UserTransaction utx;
public User getForUsername(String username) {
try {
User user;
try {
utx.begin();
Query query = entityManager.createQuery("select u from User u where u.username = :username");
query.setParameter("username", username);
user = (User) query.getSingleResult();
} catch (NoResultException e) {
user = null;
}
utx.commit();
return user;
} catch (Exception e) {
try {
utx.rollback();
} catch (SystemException se) {
throw new RuntimeException(se);
}
throw new RuntimeException(e);
}
}
public void createUser(User user) {
try {
try {
utx.begin();
entityManager.persist(user);
} finally {
utx.commit();
}
} catch (Exception e) {
try {
utx.rollback();
} catch (SystemException se) {
throw new RuntimeException(se);
}
throw new RuntimeException(e);
}
}
}