Assessment: This kind of table mapping is often used with a legacy database, which typically has tables with category columns. It requires that the application server provide a way to filter out the rows that are not wanted when running the queries.

Conclusion and Acknowledgments

I didn't invent any of this. Many products exist out there which allow us to use these three techniques (and others) when mapping database tables to hierarchies of classes. I merely applied the three techniques to the special case of entity beans, working around problems as I went.

I specifically want to credit Apple's Enterprise Object Framework (EOF) toolkit for its ingenuity in this field, which inspired me. EOF is a library and tools to map database tables to straight Java (and Objective-C) objects. The maturity of this framework, its flexibility, and its simplicity should be a model for any persistence services provider.

Also worth highlighting, WebLogic Server 7 provided me with all the features needed in order to implement my work-arounds. Kudos to BEA.

Don't get cranky if I didn't mention your favorite persistence services provider tool. I know there are others out there, I just can't explore them all in these few lines.

Next time, we'll see examples of session beans and message-driven beans that use inheritance.

Emmanuel Proulx is an expert in J2EE and Enterprise JavaBeans, and is a certified WebLogic Server 7.0 engineer. He works in the fields of telecommunications and web development.

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