My entity had a version property:

    @Version
    @Column(name = "OBJ_VERSION", nullable = false)
    protected Integer version;

In a very involved ajax reRender/conversation situation I sometimes got the following exception from HibernatePersistenceProvider:

   public static void checkVersion(Object value, Session session, Object oldVersion, Object version)
   {
      ClassMetadata classMetadata = getClassMetadata(value, session);
      VersionType versionType = (VersionType) classMetadata.getPropertyTypes()[ classMetadata.getVersionProperty() ];
      if ( !versionType.isEqual(oldVersion, version) )
      {
         throw new StaleStateException("current database version number does not match passivated version number");
      }
   }

This failed because oldVersion was 'null' and version was '0'.

Hibernate returned '0' from ClassMetaData.getVersion(), it looks like it can't return null, from Javadoc: Get the version number (or timestamp) from the object's version property (or return null if not versioned).

We should do a null check before asking Hibernate.

Workaround: Use primitive for version field in entity or initialize it to '0'.