Reassociated object has dirty collection reference

This is something really weird Hibernate error you may come up in several situations. When I first googled around, I found some sites commenting the source of error as “trying to attach a transient entity with the session, or entity with a collection not in type of PersistentCollection etc.” However, none of those cases maches with mine. Let’s first review how this error occurs in code:

Owner owner = new Owner();
Session session = sessionFactory.openSession();
session = sessionFactory.openSession();

I have two entities, Owner and Pet, which have 1:M association in between. I simply instantiate an Owner with auto generated identity strategy, create an Hibernate Session, and save it. As I use HSQL, save operation causes an immediate INSERT into DB. Then immediately close the session, open a new one and lock detached owner instance. At this point I got the following exception stack trace:

org.hibernate.HibernateException: reassociated object has dirty collection reference
at org.hibernate.event.def.OnLockVisitor.processCollection(OnLockVisitor.java:71)
at org.hibernate.event.def.AbstractVisitor.processValue(AbstractVisitor.java:124)
at org.hibernate.event.def.AbstractVisitor.processValue(AbstractVisitor.java:84)
at org.hibernate.event.def.AbstractVisitor.processEntityPropertyValues(AbstractVisitor.java:78)
at org.hibernate.event.def.AbstractVisitor.process(AbstractVisitor.java:146)
at org.hibernate.event.def.AbstractReassociateEventListener.reassociate(AbstractReassociateEventListener.java:102)
at org.hibernate.event.def.DefaultLockEventListener.onLock(DefaultLockEventListener.java:82)
at org.hibernate.impl.SessionImpl.fireLock(SessionImpl.java:766)
at org.hibernate.impl.SessionImpl.fireLock(SessionImpl.java:758)
at org.hibernate.impl.SessionImpl.access$500(SessionImpl.java:148)
at org.hibernate.impl.SessionImpl$LockRequestImpl.lock(SessionImpl.java:2278)

When I closely inspect the source code in OnLockVisitor.processCollection() metod, I saw that my Owner instance is attached successfully, and Set instance, mapping 1:M association between Owner-Pet is of type PersistentSet. Everything looked fine until coming to a point at which snapshot, corresponding to PersistentSet instance is checked for validity. At this point Hibernate was identifying that snapshot instance was invalid because some of its attributes, key, role for example, were NULL!

It was obvious that at the point of save operation some part of PersistentSet is not initialized fully. When I put a session.flush() statement after save operation, and executed the code block again, the error just disappeared! When I inspected those attributes, which were uninitialized before, were fully initialized this time.

The above code piece is actually only a test case. I actually came up with this error in a project where Spring declerative transaction management is active, and HibernateTransactionManager is used to manage transactions. As the save logic executed inside a transactional method, I had expected an immediate flush at the point of TX commit. However, when I checked log messages, there was no log messages indicating flush operation after TX commit. When I returned back to place where declerative transaction is configured for this operation, and noticed that there was a readOnly=true attribute in the TX definition!

As this attribute changes FlushMode to MANUAL in Hibernate, as long as we don’t call session.flush() explicitly, there will be no flush. As a result, code which first saves an entity in one session, and then tries to attach it with another session will face with the above error.

readOnly=true attribute is useful when we try to implement conversations together with Spring and Hibernate, however in our case Session’s lifetime was bound till the end of HTTP request. Obviously, it was mistakenly placed in this case. However, this error warns us that if we employ long running Sessions and somehow evict an instance after saving it and then try to reattach it before flush, we will have the same error. Just keep in your mind!

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