Persistence Pays Offs: Advanced Mapping with JPA

he Java Persistence API is a new addition to the Java specification that updates Java's method of performing object/relational mapping. In my first article on JPA, I explained the basics of JPA, how to map a one-to-one relationship, and how to query simple objects. In this article I'll move on to discuss some advanced features. Unlike with Entity Beans 2.x, JPA allows inheritance and can implement it in various ways (SINGLE_TABLE, TABLE_PER_CLASS, and JOINED). Because JPA supports coding-by-exception, developers needn't use additional technologies to accomplish advanced ORM. However, if you want, you can always use Java's new annotations to customize mappings and relationships. I'll also show you how to query complex object graphs with inheritance and collections using JQPL.

In this article I'm going to build on the example application that I created for the first article: a customer address book for a fictional music company called Watermelon. To begin, I'll imagine that Watermelon has some new requirements. There are now two types of customers: individuals and companies. Individuals are customers who have a first name, a last name, and a date of birth; companies have a name, a contact name, and a number of employees. This will give me the opportunity to map inheritance with JPA.