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“合并不保存”，所以为什么休眠要更新我的对象，所以不应该更新。对吗 怎么了

Answer 1

我将尝试使用一个更具体的示例进行解释。假设您有以下情况：

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();

尝试重新附加分离的对象时发生异常，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();