128 Transaction management JDOUserException, the transaction will be
Queries with JDOQL JDO s transparent persistence allows applications to navigate through the persistent fields of an instance that are references to other persistent instances. As the application de-references these objects, those that are not present in the persistence manager s cache are loaded from the data store. This gives the application the impression that the entire inter-connected graph of persistent instances is immediately available in memory. The issue remains as to how an application should obtain the first persistent instance. Three methods are available. 1 The application can use the persistence manager s getObjectById() method if it is able to construct the Object ID instance. The Object ID may have been previously stored by the application for this purpose. Alternatively, an instance of the Object ID for a class with application identity may be constructed based on data input by the user. This is the most efficient way to retrieve a single specific instance. 2 The application could obtain an Iterator from the Extent of a persistence-capable class and iterate through the persistent instances. This approach might be used when it is the application s intention that every instance be processed. However, it is probably inefficient when trying to retrieve a single specific instance or a small group of instances. 3 The application can employ the Query interface and the new JDOQL. This employs a Java-like syntax for query definition, allowing the developer to specify filter criteria. Queries are implemented using the most efficient execution methods available in the target data store. This chapter describes JDOQL in detail. 8.1 Query architecture The JDO specification provides the Query interface and the JDOQL Query definition as an object-oriented and data-store independent means for the definition and execution of queries. By utilizing these features, application query requirements can be met without compromising the portability of the application amongst different implementations. JDOQL is intended to be neutral to the native query language of the underlying data store. Most implementations will map the elements of JDOQL into the data store s native query language. Of course, some data stores do not have their own query language, e.g. file systems and XML documents, and in such cases the implementation will have to provide client-side query execution functionality.
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