Question

我刚刚开始使用JPA和hibernate，我已经能够在一个表上插入查询和更新，但我似乎无法删除一个实体。即使在我使用合并函数附加实体之后，程序仍然坚持将其分离。以下是相关代码：

public User getUserByEmail(String email) throws DAOException{
    User user = null; 
    EntityManager em = null; //declare here for use in finally block
    try{                        
        //get em for use
        EntityManagerFactory factory = JPAUtil.getEntityManagerFactory();
        em = factory.createEntityManager();
        //setup return type as user
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<User> criteriaQuery = cb.createQuery(User.class);
        Root<User> userRoot = criteriaQuery.from(User.class);
        //populate the where clause
        criteriaQuery.where(cb.equal(userRoot.get("email"), cb.parameter(String.class, "email")));          
        Query query = em.createQuery(criteriaQuery);
        query.setParameter("email", email);
        //actually run the query on db
        user = (User) query.getSingleResult();
    }catch(Exception ex){
        DAOException dE = new DAOException(2, "getUserByEmail failed", ex);
        //TODO log
        throw dE;
    }       
    //cleanup
    finally{
        if(em != null && em.isOpen()){
            em.close();
        }
    }
    return user; //if not found, will still be null
}



@Override
/*
 * (non-Javadoc)
 * @see dao.IUserDAO#deleteUser(java.lang.String)
 */
public void deleteUser(String email) throws DAOException {
    EntityManagerFactory entityManagerFactory;
    EntityManager em = null;
    EntityTransaction trans = null;
    try{            
        //search user on email
        User user = getUserByEmail(email);          
        //check we found user with specified email
        if(user != null){
            //setup for interaction with database
            entityManagerFactory = JPAUtil.getEntityManagerFactory();
            em = entityManagerFactory.createEntityManager();
            //alterations of db must occur in scope of transaction
            trans = em.getTransaction();
            trans.begin();
            em.merge(user); //TODO update find to take transaction as parameter
            em.remove(user);
            trans.commit();
            //explicitly flush here
            em.flush();             
        }else{
            //TODO we didn't find the user
        }

    }catch(Exception ex){
        DAOException dE = new DAOException(6, "delete failed", ex);
        //TODO log
        trans.rollback();
        throw dE;
    }finally{
        if(em != null && em.isOpen()){
            em.close();
        }
    }       
}

以下是eclipse控制台的输出： “删除用户＃1的已删除实例”。

有人可以解释为什么在我明确调用merge之后这个bean仍然被释放了吗？另外，我该如何解决这个问题呢？感谢。

Answer 1

指定的分离实体对象的内容被复制到具有相同标识的现有管理实体对象（即相同类型和主键）。如果EntityManager不管理这样的实体对象还构造了一个新的托管实体对象。超然然而，对象本身保持不变和分离。

即使在合并后，分离的对象本身仍然分离，合并将返回新创建的合并实体

user = em.merge(user); //TODO update find to take transaction as parameter
em.remove(user);

JPA删除分离的实体

1 个答案: