April 2003
Beginner to intermediate
380 pages
12h 25m
English
Content preview from Java Data ObjectsBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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.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.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.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.
Chapter 9. The JDO Query Language
In Chapter 8 we learned how
to access all the instances of a class by using an Extent. Once we have accessed some instances
from the datastore, we can navigate to other related instances in Java by
traversing references and iterating through collections. This allows us to
access an application-specific closure of related instances to perform the
functionality provided by the application.
But when you iterate an Extent,
you potentially access all the instances of a class. We may only care
about one or a small number of instances of the class that meet certain
criteria. Once these initial instances have been accessed, we typically
then navigate to instances related to those initial instances. However,
getting to the first few persistent instances is a bootstrap issue. JDO
provides a query language, called JDO Query Language (JDOQL), that is used
to access persistent instances based on specified search criteria.
You perform queries in JDO by using the Query interface. The PersistenceManager interface is a factory for
creating Query instances, and queries
are executed in the context of the PersistenceManager instance used to create the
Query instance. JDO queries allow you
to filter out instances from a set of candidate instances specified by
either an Extent or a Collection. A filter consisting of a Boolean
expression is applied to the candidate instances. The query result
includes all of the instances for which the Boolean expression is true.
The JDO ...
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Read now
Unlock full access
More than 5,000 organizations count on O’Reilly
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.