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Creating Mappings with Hibernate XML Files CHAPTER 7 n n n Creating Mappings with Hibernate XML Files CHAPTER 7 n n n In the simple example programs in Chapters 1 and 3, we demonstrated how a mapping file could be used to establish the relationship between the object model and the database schema. A mapping file can map a single class or multiple classes to the database. The mapping can also describe standard queries (in HQL and SQL) and filters. Hibernate Types Although we have referred to the Hibernate types in passing, we have not discussed the terminology in any depth. In order to express the behavior of the mapping file elements, we need to make these fine distinctions explicit. Hibernate types fall into three broad categories: entities, components, and values. Entities Generally, an entity is a POJO class that has been mapped into the database using the or elements. An entity can also be a dynamic map (actually a Map of Maps). These are mapped against the database in the same way as a POJO, but with the default entity mode of the SessionFactory set to dynamic-map. The advantage of POJOs over the dynamic-map approach is that compile-time type safety is retained. Conversely, dynamic maps are a quick way to get up and running when building prototypes. It is also possible to represent your entities as Dom4J Document objects. This is a useful feature when importing and exporting data from a preexisting Hibernate database, but it is not really central to the everyday use of Hibernate. We recommend that you use the standard entity mode unless you need to sacrifice accuracy for timeliness, so the alternate approaches are not discussed in this chapter however, we give some simple examples of the Dom4J- and Map-based mappings in Appendix A.
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