Pro Programmers: [Hibernate] NonUniqueObjectException: a different object with the same identifier value was already associated with the session
今天写程序的时候遇到这个异常，该文章是正解，记下备查。
Problem
When saveOrUpdate() is called on hibernate template, it gives the above exception.
Solution
- use merge() instead of saveOrUpdate()or
- load and set the value before calling saveOrUpdate()
Reason
You have 2 objects: one in session and other you attempt to save both of which have the same identifier.
The object objX1 is already in the session, if you happened to load or find before calling saveOrUpdate().
When saveOrUpdate() is called on objX2, there is already an object, objX1, associated in the session with the same identifier (id=1) and it throws an exception.
1 – merge()
The simples way to sort this out is call merge() instead of saveOrUpdate().
The merge() method copy the state (value=b) of object objX2 to persistent instance, objX1. But it does not attach objX2 into session.
2 – load and set the value()
If you don’t want to or cannot use merge() , load and set the value to call saveOrUpdate().
MyObject objX2 = new MyObject(1);
objX2.setValue(b);
hibernateTemplate.saveOrUpdate(objX2);
The above code may give the NonUniqueObjectException. So we load and set the value as following
MyObject objX2 = hibernateTemplate.get(objX2.class,new Integer(1));
if (objX2 == null){
objX2 = new MyObject(1);
}
objX2.setValue(b);
hibernateTemplate.saveOrUpdate(objX2);
load() method will throw ObjectNotFoundException if no record found in database and thus get() method is used instead.
For the above code, objX2 is actually pointing to objX1 if there is already an object associated with the session.
Automatic State Detection
According to “Automatic state detection”
saveOrUpdate() does the following:
- if the object is already persistent in this session, do nothing
- if another object associated with the session has the same identifier, throw an exception
- if the object has no identifier property,
save()it - if the object’s identifier has the value assigned to a newly instantiated object,
save()it - if the object is versioned (by a
<version>or<timestamp>), and the version property value is the same value assigned to a newly instantiated object,save()it - otherwise
update()the object
and merge() is very different:
- if there is a persistent instance with the same identifier currently associated with the session, copy the state of the given object onto the persistent instance
- if there is no persistent instance currently associated with the session, try to load it from the database, or create a new persistent instance
- the persistent instance is returned
- the given instance does not become associated with the session, it remains detached
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因为业务逻辑的改变，后来我没有用这篇文章的方法。而是在save之前将session清空，也就是用了seesion.clear()方法。成功了。
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