what kind of error messages should I display to users when non-recoverable exceptions are thrown from Session most methods? (subclasses of HibernateException, list below)

CacheException, CallbackException, IdentifierGenerationException, InstantiationException, JDBCException, MappingException, ObjectDeletedException, ObjectNotFoundException, PersistentObjectException, PropertyAccessException, QueryException, SerializationException, StaleObjectStateException, TransactionException, TransientObjectException, ValidationFailure, WrongClassException


some exceptions like ObjectNotFound, I catch and send messages back to the user but what about the rest?

Generally, I would not show any of these to the user (depending on who the user is). You want to have exceptions be appropriate for the level of abstraction. If there is no way that the caller will no how to handle the low-level Hibernate exceptions, then nest those exceptions in a higher-level PersistenceException of your own. For a more thorough treatise on exception handling; check out this JavaWorld series.

the exceptions being thrown by hibernate would be wrapped by an upper-level in my case so I would not directly show the hibernate exception, something like this:

try {
UserManager.saveUser(user);
} catch(ManagerDuplicateIdException mdie) {
// user name already taken show somekind of error telling user to re-input a // different username
} catch(ManagerException me) {
// some other error, could be a JDBCException, StaleObjectException, I dont //know but what kind of error or message should I give the user?
}


in this case I have ManagerDuplicateIdException a subclass of ManagerException, the UserManager.saveUser() method throws just ManagerException

maybe im doing something else worng here and just have a bad design to start off with, maybe someone could help me out here with my approach

What you may want to do is create something like

ManagerServiceException extends ManagerException to handle this case. For your user, all he needs to know is that the service failed in a way that he has no recourse for (as opposed to the duplicate ID where you can display an appropriate message to try a different idea). For the ManagerServiceException you may want to provide ability to view detail (the wrapped HibernateException), e-mail this error to support, etc.

ok, ill take your advice, I was already logging the exceptions the app wasnt able to recover from, and i'll just send a message to the user telling them "some unknown error occured" or something along those lines, but that isnt very helpfull to the user.

is there anything I can do to prevent any of these un-recoverable exceptions from being thrown at all? or are most of these exceptions caused programming errors?

Well, you could change these "unrecoverable" exceptions into RuntimeExceptions -- that way you don't have to declare them. Regardless, you should log the errors in either case.

It kind of depends on your application -- if its a web app, then throwing a RuntimeException will result in a 500 (server) error -- you can create an error page to show a "prettier" page to the user on a server error.

If you are using something like Struts -- this is a good opportunity to investigate the use of declarative exception handling. You could define a global exception handler for your base exception that could declare a "prettier" page (or even a whole different workflow) and provide as much or as little detail as you desire.