[英]Hibernate merge
我正在测试hibernate并给出这个查询
transaction = session.beginTransaction();
city = new City("A");
city = (City)session.merge(city);
city.setName("B");
transaction.commit();
我在命令行中收到这些查询:
Hibernate: insert into CITY (name) values (?)
Hibernate: update CITY set name=? where CITY_ID=?
我使用merge而不是save,所以为什么hibernate正在更新我的对象,它不应该更新。 是吗? 这是什么错误?
我将尝试使用更具体的例子来解释。 假设您有如下情况:
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();
尝试重新挂接Detached对象时,发生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.
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();
这是一个排序问题。 其实不是问题。 Hibernate完全按照你的要求去做。 正如@TejasArjun所说,merge是关于合并检测数据的。 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.
因为会话尚未关闭,并且在持久性条款中, city
对象仍然附加到会话。 因此,hibernate会话将监听该对象中的任何更改,并将调用适当的dml语句。
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