Java Data Objects

Operations on Instances

Now we have a datastore in which we can store instances of our classes. Each application needs to acquire a PersistenceManager to access and update the datastore. Example 1-8 provides the source for the MediaManiaApp class, which serves as the base class for each application in this book. Each application is a concrete subclass of MediaManiaApp that implements its application logic in the execute( ) method.

MediaManiaApp has a constructor that loads the properties from jdo.properties (line [1]). After loading properties from the file, it calls getPropertyOverrides( ) and merges the returned properties into jdoproperties. An application subclass can redefine getPropertyOverrides( ) to provide any additional properties or change properties that are set in the jdo.properties file. The constructor gets a PersistenceManagerFactory (line [2]) and then acquires a PersistenceManager (line [3]). We also provide the getPersistenceManager( ) method to access the PersistenceManager from outside the MediaManiaApp class. The Transaction associated with the PersistenceManager is acquired on line [4].

The application subclasses make a call to executeTransaction( ), defined in the MediaManiaApp class. This method begins a transaction on line [5]. It then calls execute( ) on line [6], which will execute the subclass-specific functionality.

We chose this particular design for application classes to simplify and reduce the amount of redundant code in the examples for establishing ...