Hibernate Error: org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session

I have had this error many times and it can be quite hard to track down...

Basically, what hibernate is saying is that you have two objects which have the same identifier (same primary key) but they are not the same object.

I would suggest you break down your code, i.e. comment out bits until the error goes away and then put the code back until it comes back and you should find the error.

It most often happens via cascading saves where there is a cascade save between object A and B, but object B has already been associated with the session but is not on the same instance of B as the one on A.

What primary key generator are you using?

The reason I ask is this error is related to how you're telling hibernate to ascertain the persistent state of an object (i.e. whether an object is persistent or not). The error could be happening because hibernate is trying to persist an object that is already persistent. In fact, if you use save hibernate will try and persist that object, and maybe there is already an object with that same primary key associated with the session.

Example

Assuming you have a hibernate class object for a table with 10 rows based on a primary key combination (column 1 and column 2). Now, you have removed 5 rows from the table at some point of time. Now, if you try to add the same 10 rows again, while hibernate tries to persist the objects in database, 5 rows which were already removed will be added without errors. Now the remaining 5 rows which are already existing, will throw this exception.

So the easy approach would be checking if you have updated/removed any value in a table which is part of something and later are you trying to insert the same objects again

This is only one point where hibernate makes more problems than it solves. In my case there are many objects with the same identifier 0, because they are new and don't have one. The db generates them. Somewhere I have read that 0 signals Id not set. The intuitive way to persist them is iterating over them and saying hibernate to save the objects. But You can't do that - "Of course You should know that hibernate works this and that way, therefore You have to.." So now I can try to change Ids to Long instead of long and look if it then works. In the end it's easier to do it with a simple mapper by your own, because hibernate is just an additional intransparent burden. Another example: Trying to read parameters from one database and persist them in another forces you to do nearly all work manually. But if you have to do it anyway, using hibernate is just additional work.

USe session.evict(object); The function of evict() method is used to remove instance from the session cache. So for first time saving the object ,save object by calling session.save(object) method before evicting the object from the cache. In the same way update object by calling session.saveOrUpdate(object) or session.update(object) before calling evict().