I am trying to use an optimistic locking strategy. So I have the following steps:
1. Load an object from DB.
2. Get its ID.
3. Send ID to the client.
4. Client sends back ID and some new data.
5. Server site gets the object from the cache by its ID without hitting DB.
6. Get the old timestamp of the object.
7. Reload the object to get its new timestamp.
8. Compare the timestamps.
9. Update the object.

and the problem is I can't figure out how to implement steps 5-7. I don't even know if it's possible.
I can't send timestamps to the client and I don't hold references to the loaded objects on the server side, but I have the same Session object all the time (so the object must be in the cache, right?).

Could anyone enlight me on this issue?

Hibernate has a mapping tag to map a field as the optimistic locking field. In this case its <timestamp>. When you do the update hibernate will check the value against the database for you and throw an exception if the lock guard has changed. All automatically.

But what happen if I have an ID of the object but not the object itself? To update it I need to get it first from somewhere. The question is, if I have an ID of the object (but don't have a timestamp or version) and I loaded it before (and didn't evict it) is there any way to get it from the cache rather than from DB even if the data is stale?

What exactly is your problem? If you do a Session.load(primaryKey), you will of course get the data from the cache. If you then try to save or update it and get a timestamp violation, you will get a StaleObjectException.

Does the loaded object always stay in cache and the only way to remove it from there is to evict it?

Yes, either evict the object or close the session

Thank you!