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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: Abstract Class Persistence
Date Wed, 01 Aug 2007 19:17:29 GMT
Ok, I've built JPOX and have looked at the enhancer code base.

I looked on the JPOX site for a suitable wiki page to discuss the  
design and couldn't see where to put it.

So if you could put the following on the wiki and send me a link, we  
can discuss the design there.

An abstract class has two purposes in JDO:

1. It can be a base class for user-written persistent classes, and  
declare its own persistent fields (and properties). This use requires  
that the class be enhanced. The property methods are not abstract in  
this case, but are implemented in the class to use internal (non- 
persistent) fields to store the property values.

2. It can be a base class for implementation-generated persistent  
classes. In this case, the class behaves very much like a persistent  
interface. Abstract persistent properties are declared using  
metadata; the class is not directly usable by an application; the  
abstract properties must be implemented by the implementation- 
generated subclass.

As a sub-case of purpose 2, an abstract persistent class can be a  
base class for other abstract classes that define their own  
persistent fields, persistent properties, and abstract persistent  
properties. The implementation is responsible for implementing all  
the persistent abstract properties of all the abstract classes.

A concrete persistent class can extend an abstract persistent class  
and provide a concrete implementation for all of the abstract  
properties.

An abstract persistent class can extend an abstract persistent class  
and provide a concrete implementation for some of the abstract  
properties.

Purpose 1 is already handled by JDO 1 and no further design  
discussion is needed. The rest of this discussion is for purpose 2.

A persistent abstract class might also inherit a persistent abstract  
or concrete class that itself defines persistent fields and  
properties. The implementation-generated subclass is responsible for  
generating implementations for the persistent properties defined by  
the abstract classes that are either directly or indirectly inherited.

During enhancement, an abstract class needs to have the "regular" jdo  
fields and methods added to it for implementing its concrete  
persistent fields and properties. None of the abstract persistent  
members need to be included in the enhanced class, as the  
implementation of these are only in the concrete implementation- 
generated class.

During runtime, an implementation class for the parameter of  
pm.newInstance(Class PAClass) is generated. The generated class  
(PAImpl) is identical to the generated class for a persistent  
interface except for the following:

A. The PAImpl is not a "least-derived (topmost) persistence-capable  
class", so it doesn't implement the basic jdo methods.

B. The PAImpl extends PAClass.


Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!


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