Hibernate合并 [英] Hibernate merge

问题描述

我正在测试hibernate并将此查询提供给

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


我在命令行中获取这些查询：

  Hibernate：插入CITY（名称）值（？）
 Hibernate：更新CITY set name =？ CITY_ID =？ 
  

我正在使用merge not save，所以为什么hibernate正在更新我的对象，它不应该更新。它是正确的吗？什么是错误？

解决方案

我会尝试用更具体的例子来解释。假设你有如下的场景：

  Session session = sessionFactory.openSession（）; 
 Transaction transaction = session.beginTransaction（）; 
 User userA =（User）session.get（User.class，1101）; 
 transaction.commit（）; 
 session.close（）; 
 //由于会话关闭，userA被分离。 
 
 session = sessionFactory.openSession（）; 
 transaction = session.beginTransaction（）; 
 User userB =（User）session.get（User.class，1101）; 
 //现在在这里，userB代表与userA相同的持久行。 
 //当发生重新连接userA的尝试时，抛出异常
 session.update（userA）; 
 transaction.commit（）; 
 session.close（）; 
  

尝试重新附加Detached对象userA时发生异常。

 线程main中的异常org.hibernate.NonUniqueObjectException：具有相同标识符值的
不同对象已经与
关联session：
 
这是因为Hibernate强制在内存中只存在一个Persistent对象的实例。 
  

为了解决上述问题，使用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现在在会话关闭时分离。 
 
 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（）; 
  

I am testing hibernate and giving this query to

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=?

I am using merge not save, so why hibernate is updating my object, It should not update. am it right? What is the mistake?

解决方案

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();

Exception when an attempt to reattach a Detached object, userA is made.

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();

这篇关于Hibernate合并的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！