Hibernate合并
我正在测试hibernate并给出这个查询
transaction = session.beginTransaction(); city = new City("A"); city = (City)session.merge(city); city.setName("B"); transaction.commit(); 我在命令行中收到这些查询：
Hibernate: insert into CITY (name) values (?) Hibernate: update CITY set name=? where CITY_ID=?
我使用merge not save，所以为什么hibernate正在更新我的对象，它不应该更新。 是吗？ 这是什么错误？
我将尝试使用更具体的例子来解释。 假设您有如下情况：
Session session = sessionFactory.openSession(); Transaction transaction = session.beginTransaction(); User userA = (User)session.get(User.class, 1101); transaction.commit(); session.close(); // Since session is closed, userA is detached. session = sessionFactory.openSession(); transaction = session.beginTransaction(); User userB = (User)session.get(User.class, 1101); //Now here, userB represents the same persistent row as userA. //When an attempt to reattach userA occurs, an exception is thrown session.update(userA); transaction.commit(); session.close();
尝试重新挂接Detached对象时，发生userA。
Exception in thread "main" org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: This is because Hibernate is enforcing that only a single instance of a Persistent object exists in memory.
为了解决上述问题，使用了merge（），如下所示：
Session session = sessionFactory.openSession(); Transaction transaction = session.beginTransaction(); User userA = (User)session.get(User.class, 1101); transaction.commit(); session.close(); //userA is now detached as session is closed. session = sessionFactory.openSession(); transaction = session.beginTransaction(); User userB = (User)session.get(User.class, 1101); User userC = (User)session.merge(userA); if (userB == userC) { System.out.println("Reattched user is equal"); } transaction.commit(); session.close();
这是一个排序问题。 其实不是问题。 Hibernate完全按照你的要求去做。 正如@TejasArjun所说，merge是关于合并检测数据的。 这是发生了什么：
... city = (City)session.merge(city); // No different to save(). Hibernate schedules an insert to the // database to store the current record. city.setName("B"); // The object is now flagged as dirty and needing to be saved. // Hiberate automatically tracks properties on objects and knows when they change. transaction.commit(); // Hibernate sees that 'city' has been changed since it was saved, // so it schedules an update to store the new data.
由于会话尚未关闭，并且在持久性条款中， city对象仍然附加到会话。 因此，hibernate会话将监听该对象中的任何更改，并将调用适当的dml语句。