Re: [hibernate-dev] Session and carrying 3rd party state

I took some more time to think about our conversation from 2 IRC meeting ago about offering the ability to carry session bound state not related to ORM per se. Below is a sum and a potential solution. If you are short on time, read Goals, then the SessionSessionEventListener approach and ignore the rest. ## Goals The goal is to be able to carry session bound state for non-ORM projects like search and OGM. We want to avoid ThreadLocal use, in particular when it cannot be protected by a try / catch for proper resource cleaning. We want to avoid a structure that would be shared across threads concurrently i.e. using ConcurrentHashMap with a Weak reference to the session. It needs to be informed of a call to session.clear() It needs to be informed of a call to session.close() The state needs to be accessed from event listener implementations and custom persister / loader implementations i.e. SessionImplementor and maybe EventSource? ## Approaches I'll discuss the approaches we explored in the meeting and then offer an alternative one that I think is pretty interesting and fit better with the current Session model. ### Map This is essentially sticking a map on SessionImpl and use it to carry state. The following is a pseudo implementation /** * interface implemented by SessionImpl and the like */ interface SessionCompanion { Object getCompanion(String key); void addCompanion(String key, Object companion); void removeCompanion(String key); } /** * adds a map to the SessionImpl */ SessionImpl { private Map<String, Object> companions; public Object getCompanion(String key) { return companions.get(key); } public void addCompanion(String key, Object value) { companions.add(key, companion); } public void removeCompanion(String key) { companions.remove(key) } } The persister or event listener would call SessionCompation.*Companion method to put and retrieve its state. There is no clear / close event listener loop and it would need to be added. ### Delegator Gunnar and teve discussed an approach where the delegator would be passed to the underlying session and be accessible via an `unwrap` method. I have not followed the details but this approach has one major flaw: the delegator (OgmSession, FullTextSession etc) is not always created and thus would not be necessarily available. A somewhat similar idea involving passing the session owner has the same drawback. And another one described by Gunnar in https://hibernate.atlassian.net/browse/OGM-469 ### The type-safe map approach This approach is vaguely similar to the Map approach except that the payload is represented and looked up by Class. This has the benefit of not having namespace problems and is generally less String-y. /** * interface implemented by SessionImpl and the like */ interface SessionCompanion { T getCompanion(Class<T> type); void addCompanion(Object companion); void removeCompanion(Class<?> type) } SessionImpl { //could also use an array or an ArrayList private Map<Class<?>, Object> companions; public T getCompanion(Class<T> type) { return (T) companions.get(type); } public void addCompanion(Object companion) { companions.add(companion.getClass(), type); } public void removeCompanion(Class<T> type) { companions.remove(type); } } Like in the Map approach, the persister or custom event listener would interact with SessionCompanion. There are open issues like what should be done when two objects of the same type are added to the same session. Likewise the clear / close hook issues need to be addressed. ### the SessionEventListener approach I did not know but there is a concept of `SessionEventListener` which can be added to a `SessionImplementor`. It has hooks that are addressing most of the goals. //interface already exists interface SessionImplementor { public SessionEventListenerManager getEventListenerManager(); } //interface already exists public interface SessionEventListenerManager extends SessionEventListener { // add this method to be able to retrieve a specific listener holding some state for a 3rd party project List<SessionEventListener> getSessionEventListeners(); } OGM or Search would implement a `SessionEventListener` and attach an instance to a session via `Session.addEventListeners()`. It would require to add a method to retrieve the list of `SessionEventListener`s attached to a `SessionEventListenerManager`. List<SessionEventListeners> listeners = sessionImplementor.getSessionEventListenerManager().getEnlistedListeners(); OgmSessionEventListener ogmListener = findOrAddOgmListener(sessionImplementor, listeners); ogmListener.someStuff(); ## What about clear and close? We have a few ways to react to these. SessionEventListener is already called back when a flush begins / ends as well as when Session closes. We need to either: - add a clear begins / ends callback - have the third party project add a ClearEventListener which would access the SessionEventListeners and do some magic. The first approach has my preference and would do: public interface SessionEventListener { [...] void clearStart(); void clearEnd(); } What do you guys think? The SessionEventListener approach feels more natural.

Emmanuel _______________________________________________ hibernate-dev mailing list hibernate-dev(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/hibernate-dev