When does Hibernate Session.load() throw an exception

问题

try {
   x = session.load(...); 
   y = x; 
 }
 catch(Exception e) {
    //do something
 }

If the key is not present, will load(...) throw an exception

right away
will return a proxy and then throw an exception when the object is trying to be obtained from the db later?

Also, in the code above, if the execution reaches the assignment y = x, is it guaranteed that at that point, x is not null? Are there situations where x can be null at that point?

回答1:

In short:

x will not be null after the load, so neither will y
There may or may not be an exception from load() if the object doesn't exist
If there is no exception from load(), and the object doesn't exist, there will be an exception when accessing x or y.

To elaborate:

Are you trying to determine if the key is present? Or are you assuming it is, but just want to handle anomalies?

See the documentation, specifically:

You should not use this method to determine if an instance exists (use get() instead). Use this only to retrieve an instance that you assume exists, where non-existence would be an actual error.

From the book 'hibernate in action' on using load():

The application may retrieve a valid reference (a proxy) to a persistent instance without hitting the database to retrieve its persistent state. So load() might not throw an exception when it doesn’t find the persistent object in the cache or database; the exception would be thrown later, when the proxy is accessed.

So:

If you are looking to know whether the item exists, use get(), not load().