By Sameer Tyagi, Michael Vorburger, Keiron McCammon, Heiko Bobzin
Center Java info gadgets supplies: * functional examples exhibiting how JDO may be utilized to current purposes * robust insights for utilizing JDO to architect new structures extra successfully * step by step information for integrating JDO with different J2EE applied sciences * top practices for utilizing JDO in real-world company environments
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Extra resources for Core Java Data Objects
Extents An Extent represents a collection of all instances in the datastore of a particular persistence-capable class and can consist of just instances of the class or instances of all subclasses. An application can get an Extent for a class from a PersistenceManager and then use an Iterator to iterate through all instances, or it can use the Extent as input to a query. By default, it is possible to get an Extent for any persistence-capable class. If an Extent is not required, then as a potential optimization, it is possible to specify explicitly that an extent is not required in the JDO metadata for the class.
If implemented, the close() method on PersistenceManager may not physically close anything, but may instead return the PersistenceManager instance to the PersistenceManagerFactory to be reused. close(); } } Of course, before this example can be used, the "XXX" strings would need to be replaced with values appropriate to the JDO implementation being used. As an alternative to hard coding the properties as in the previous example, it is generally better to specify them as system properties or read them from a file.
This familiar Java feature is known as garbage collection. The days of C and other low-level programming languages with the problem called "dangling pointers" (references to non-existing objects previously de-allocated memory) that led to screaming developers are long gone when using Java. However, persistent objects can usually be deleted explicitly. , foreign-key references to non-existent primary keys. Again, marrying these two different views is an issue that a mapping layer needs to address, be it by explicitly supporting relational referential integrity constraints and performing correct statement ordering if required, maybe by supporting cascaded delete options, or by explicitly not supporting relational referential integrity.