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 ...
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