April 2003
Beginner to intermediate
380 pages
12h 25m
English
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Cloning
If you make a clone of a persistent instance, the clone is a separate
transient instance. The clone does not have a JDO identity and it is not
associated with the PersistenceManager of the instance that was
cloned. The clone is a shallow copy of the original instance, without
regard for the persistent fields. Therefore, the fields might not have
been fetched from the datastore yet, causing you to get a null for fields that are references, including
types like Integer and references to
other persistent instances. Normally, the fields in the default fetch
group have been fetched from the datastore, but not always. You should
therefore call retrieve( ) to make sure the field values have been fetched from the
datastore.
Another issue to consider is that the persistent instance may have
references to other persistent instances. For example, a RentalItem has a reference to a MediaItem. If we retrieve all the fields of a
RentalItem instance and then create a
clone of it, the clone will have a reference to the MediaItem, but this clone is transient and
does not really have a relationship with the MediaItem instance. JDO has a well-defined
behavior that allows implementations to create a clone of a persistent
instance properly, but we recommend that you do not clone persistent
instances.
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.