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Object/relational mapping metadata ignored, but all other annotations on the Item class are still applied. Also note that you didn t specify an access strategy in this mapping, so field or accessor method access is used depending on the position of the @Id annotation in Item. (We ll get back to this detail in the next chapter.) An obvious problem with XML deployment descriptors in Java Persistence is their compatibility with native Hibernate XML mapping files. The two formats aren t compatible at all, and you should make a decision to use one or the other. The syntax of the JPA XML descriptor is much closer to the actual JPA annotations than to the native Hibernate XML mapping files. You also need to consider vendor extensions when you make a decision for an XML metadata format. The Hibernate XML format supports all possible Hibernate mappings, so if something can t be mapped in JPA/Hibernate annotations, it can be mapped with native Hibernate XML files. The same isn t true with JPA XML descriptors they only provide convenient externalized metadata that covers the specification. Sun does not allow vendor extensions with an additional namespace. On the other hand, you can t override annotations with Hibernate XML mapping files; you have to define a complete entity class mapping in XML. For these reasons, we don t show all possible mappings in all three formats; we focus on native Hibernate XML metadata and JPA/Hibernate annotations. How ever, you ll learn enough about the JPA XML descriptor to use it if you want to. Consider JPA/Hibernate annotations the primary choice if you re using JDK 5.0. Fall back to native Hibernate XML mapping files if you want to externalize a particular class mapping or utilize a Hibernate extension that isn t available as an annotation. Consider JPA XML descriptors only if you aren t planning to use any vendor extension (which is, in practice, unlikely), or if you want to only override a few annotations, or if you require complete portability that even includes deployment descriptors. But what if you re stuck with JDK 1.4 (or even 1.3) and still want to benefit from the better refactoring capabilities and reduced lines of code of inline metadata? 3.3.3 Using XDoclet The XDoclet project has brought the notion of attribute-oriented programming to Java. XDoclet leverages the Javadoc tag format (@attribute) to specify class-, field-, or method-level metadata attributes. There is even a book about XDoclet from Manning Publications, XDoclet in Action (Walls and Richards, 2004). XDoclet is implemented as an Ant task that generates Hibernate XML metadata (or something else, depending on the plug-in) as part of the build process.
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