休眠合并

Question

我测试休眠，并给予该查询

transaction = session.beginTransaction(); 
city = new City("A"); 
city = (City)session.merge(city); 
city.setName("B"); 
transaction.commit(); 

，我在命令行中获得这些查询：

Hibernate: insert into CITY (name) values (?) 
Hibernate: update CITY set name=? where CITY_ID=? 

我使用合并不救，为什么Hibernate是更新我的对象，它不应该更新。它是正确的吗？什么是错误？

Answer 1

我会尽量用一个更具体的例子来解释。假设您的场景如下所示：

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userA时发生异常。

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，合并（）时，如下图所示：

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(); 

Answer 2

因为会话尚未关闭，并且以持久性术语city对象仍然附加到会话。因此，该对象的任何更改都将被hibernate会话监听，并且适当的dml语句将被调用。

Answer 3

这是一个测序问题。其实不是问题。 Hibernate完全按照你所说的去做。正如@TejasArjun所说，合并是关于合并检测到的数据。这里是怎么回事：

... 
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.