Hi,

I get the following exception when I call em.persist(myObj):

java.lang.NullPointerException
at org.hibernate.type.descriptor.java.AbstractTypeDescriptor.extractHashCode(AbstractTypeDescriptor.java:88)
at org.hibernate.type.AbstractStandardBasicType.getHashCode(AbstractStandardBasicType.java:210)
at org.hibernate.type.AbstractStandardBasicType.getHashCode(AbstractStandardBasicType.java:214)
at org.hibernate.type.EntityType.getHashCode(EntityType.java:352)
at org.hibernate.type.ComponentType.getHashCode(ComponentType.java:263)
at org.hibernate.engine.spi.EntityKey.generateHashCode(EntityKey.java:114)
at org.hibernate.engine.spi.EntityKey.<init>(EntityKey.java:79)
at org.hibernate.internal.AbstractSessionImpl.generateEntityKey(AbstractSessionImpl.java:247)
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:172)
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:135)
at org.hibernate.ejb.event.EJB3PersistEventListener.saveWithGeneratedId(EJB3PersistEventListener.java:78)
at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:208)
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:151)
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:78)
at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:853)
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:827)
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:831)
at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:875)
at...

There are 2 entities have a relationship one to one by an id.
I am using hibernate 4.2.1 with JPA 2 on JBoss 7.1.

Any idea?

You did not post enough information.

Does the answer here help: http://stackoverflow.com/questions/6744451/hibernate-nm-extracthashcode-throws-nullpointerexception

Thanks you hit the point. I quite new with hibernate and JPA and the object structure is much more complex like in the examples I have seen. I have to persist the references entities in a strict order and then it is working. I would have expected that em.persist(rootObject) would be enough and the JPA annotation are sufficient for the automatic storage.