Question

我是怀念的,请问一下。

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

And I am getting those queries in the command line:

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

我使用的是混杂,不能幸免,因此,藏匿国正在更新我的物体,不应更新。是正确的? 错误是什么?

Answer 1

I will try to explain using a more concrete example. Suppose you have a scenario like below :

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();

更改附后物体的尝试,用户发言。

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.

To get around the above problem, merge() is used, as shown below :

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

它是一个排序问题。实际上不是一个问题。解放者正在做你告诉它的事情。正如“TejasArjun”所说,合并涉及技术化数据中的合并。一、导言

...
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

由于本届会议尚未结束,因此,以持续的措辞,<条码>>>> /条码> 标本仍然附于本届会议。因此,对该目标的任何改动都将通过秘密会议进行聆听,并将援引适当的小发言。

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