休眠:比较电流&之前的纪录 [英] Hibernate: comparing current & previous record

问题描述

我想将内存中的Hibernate实体的当前值与数据库中的值进行比较:

I want to compare the current value of an in-memory Hibernate entity with the value in the database:

HibernateSession sess = HibernateSessionFactory.getSession();
MyEntity newEntity = (MyEntity)sess.load(MyEntity.class, id);
newEntity.setProperty("new value");
MyEntity oldEntity = (MyEntity)sess.load(MyEntity.class, id);
// CODEBLOCK#1 evaluate differences between newEntity and oldEntity
sess.update(newEntity);    

在 CODEBLOCK#1 中，我得到了 newEntity.getProperty()=新值" 和 oldEntity.getProperty()=新值" (当然，我希望 oldEntity.getProperty()=旧值" ).实际上，这两个对象在内存中完全相同.

In CODEBLOCK#1 I get that newEntity.getProperty()="new value" AND oldEntity.getProperty()="new value" (while I expected oldEntity.getProperty()="old value", of course). In fact the two objects are exactly the same in memory.

我弄乱了 HibernateSessionFactory.getSession().evict(newEntity)并尝试设置 oldEntity = null 摆脱它(我只需要它比较):

I messed around with HibernateSessionFactory.getSession().evict(newEntity) and attempted to set oldEntity=null to get rid of it (I need it only for the comparison):

HibernateSession sess = HibernateSessionFactory.getSession();
MyEntity newEntity = (MyEntity)sess.load(MyEntity.class, id);
newEntity.setProperty("new value");
HibernateSessionFactory.getSession().evict(newEntity);
MyEntity oldEntity = (MyEntity)sess.load(MyEntity.class, id);
// CODEBLOCK#1 evaluate differences between newEntity and oldEntity
oldEntity = null;
sess.update(newEntity);

现在这两个实体是不同的，但是我当然得到了可怕的 org.hibernate.NonUniqueObjectException:具有相同标识符值的另一个对象已经与该会话关联了.

and now the two entities are distinct, but of course I get the dreaded org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session.

有什么主意吗?

编辑:我尝试了两次会话策略；我修改了 HibernateSessionFactory 以实现会话映射，然后...

I tried the double session strategy; I modified my HibernateSessionFactory to implement a map of session and then...

Session session1 = HibernateSessionFactory.getSession(SessionKeys.DEFAULT);
Session session2 = HibernateSessionFactory.getSession(SessionKeys.ALTERNATE);
Entity newEntity = (Entity)entity;
newEntity.setNote("edited note");
Entity oldEntity = (Entity)session1.load(Entity.class, id);

System.out.println("NEW:" + newEntity.getNote());
System.out.println("OLD: " + oldEntity.getNote()); // HANGS HERE!!!

HibernateSessionFactory.closeSession(SessionKeys.ALTERNATE);

尝试打印oldEntity注释时，我的单元测试挂起...:-(

My unit test hangs while attempting to print the oldEntity note... :-(

推荐答案

想到两个简单的选择:

1. 保存newEntity之前先驱逐oldEntity
2. 在oldEntity上使用session.merge()将会话缓存(newEntity)中的版本替换为原始(oldEntity)

编辑:稍微详细一点，这里的问题是Hibernate保留了一个持久性上下文，这是作为每个会话的一部分监视的对象.当上下文中有一个附加的对象时，您不能对一个分离的对象(不在上下文中的对象)执行update().这应该起作用:

EDIT: to elaborate a little, the problem here is that Hibernate keeps a persistence context, which is the objects being monitored as part of each session. You can't do update() on a detached object (one not in the context) while there's an attached object in the context. This should work:

HibernateSession sess = ...;
MyEntity oldEntity = (MyEntity) sess.load(...);
sess.evict(oldEntity); // old is now not in the session's persistence context
MyEntity newEntity = (MyEntity) sess.load(...); // new is the only one in the context now
newEntity.setProperty("new value");
// Evaluate differences
sess.update(newEntity); // saving the one that's in the context anyway = fine

这也应该如此:

HibernateSession sess = ...;
MyEntity newEntity = (MyEntity) sess.load(...);
newEntity.setProperty("new value");
sess.evict(newEntity); // otherwise load() will return the same object again from the context
MyEntity oldEntity = (MyEntity) sess.load(...); // fresh copy into the context
sess.merge(newEntity); // replaces old in the context with this one

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