PageManager getById can return a Page.class for a blogpost or other non-page

As mentioned in CONF-16393, CONF-16735 and CONF-16694, calling the getById method on the PageManager with the id of a blog, or any other contententity object, will always return a Page object. The returned Page is horribly broken and can end up in the cache.

Since PageManager implements ContentEntityManager, and getById returns a ContentEntityObject, this it is not reasonable to assume that getById will only get called with a pageId. There is an expectation that if there is no matching page, then null will be returned. It can be extremely confusing to debug this problem when a PageManager is passed into some other method as a ContentEntityManager.

It is also quite inobvious how this bug occurs, so I'll explain it.
The getById method is implemented in only the super class, DefaultContentEntityManager. It is implemented like this:

    public ContentEntityObject getById(long id)
    {
        return contentEntityObjectDao.getById(id);
    }

The weirdness comes down to what contentEntityObjectDao is, and that depends on the applicationContext.xml. In the case of a PageManager the contentEntityObjectDao is currently a PageDao, which returns Page.class from getPersistentClass.

This gets passed into this method in our HibernateObjectDao

    protected EntityObject getByClassId(final long id)
    {
        EntityObject obj = (EntityObject) getHibernateTemplate().execute(new HibernateCallback()
        {
            public Object doInHibernate(Session session) throws HibernateException
            {
                return session.get(getPersistentClass(), new Long(id));
            }
        });

        return obj;
    }

The javadoc for session.get might lead you to think this would only return pages:

• Return the persistent instance of the given entity class with the given identifier,
• or null if there is no such persistent instance. (If the instance, or a proxy for the
• instance, is already associated with the session, return that instance or proxy.)

But it doesn't do that until Hibernate 3.2.1. See http://opensource.atlassian.com/projects/hibernate/browse/HHH-1460 and http://opensource.atlassian.com/projects/hibernate/browse/HHH-416.