休眠合并

Question

我正在测试休眠并将此查询提供给

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

我在命令行中得到这些查询：

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

我使用的是合并而不是保存，所以为什么休眠更新我的对象，它不应该更新。对吗？什么错误？

Answer 1

我将尝试使用更具体的示例进行解释。假设您有如下场景：

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();

尝试重新附加已分离对象时出现异常，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();

Answer 2

这是一个排序问题。其实不是问题。 Hibernate 正在做你告诉它做的事情。正如@TejasArjun 所说，合并是关于合并去技术数据。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.

Answer 3

因为会话尚未关闭，并且在持久性方面city 对象仍附加到会话。因此，该对象中的任何更改都将被休眠会话监听并调用适当的 dml 语句。