Hibernate合並

Question

我正在測試hibernate並給出這個查詢

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

我在命令行中收到這些查詢：

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

我使用merge而不是save，所以為什么hibernate正在更新我的對象，它不應該更新。 是嗎？ 這是什么錯誤？

Answer 1

我將嘗試使用更具體的例子來解釋。 假設您有如下情況：

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();

嘗試重新掛接Detached對象時，發生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.

為了解決上述問題，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

這是一個排序問題。 其實不是問題。 Hibernate完全按照你的要求去做。 正如@TejasArjun所說，merge是關於合並檢測數據的。 這是發生了什么：

...
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

因為會話尚未關閉，並且在持久性條款中， city對象仍然附加到會話。 因此，hibernate會話將監聽該對象中的任何更改，並將調用適當的dml語句。