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By default, choose table-per-class-hierarchy only for simple problems. For more complex cases (or when you re overruled by a data modeler insisting on the importance of nullability constraints and normalization), you should consider the table-per-subclass strategy. But at that point, ask yourself whether it may not be better to remodel inheritance as delegation in the object model. Complex inheritance is often best avoided for all sorts of reasons unrelated to persistence or ORM. Hibernate acts as a buffer between the domain and relational models, but that doesn t mean you can ignore persistence concerns when designing your classes. When you start thinking about mixing inheritance strategies, remember that implicit polymorphism in Hibernate is smart enough to handle more exotic cases. For example, consider an additional interface in our application, ElectronicPaymentOption. This is a business interface that doesn t have a persistence aspect except that in our application, a persistent class such as CreditCard will likely implement this interface. No matter how you map the BillingDetails hierarchy, Hibernate can answer a query from ElectronicPaymentOption correctly. This even works if other classes, which aren t part of the BillingDetails hierarchy, are mapped persistent and implement this interface. Hibernate always know what tables to query, which instances to construct, and how to return a polymorphic result. Finally, you can also use <union-subclass>, <subclass>, and <joined-subclass> mapping elements in a separate mapping file (as a top-level element instead of <class>). You then have to declare the class that is extended, such as <subclass name="CreditCard" extends="BillingDetails">, and the superclass mapping must be loaded programmatically before the subclass mapping file (you don t have to worry about this order when you list mapping resources in the XML configuration file). This technique allows you to extend a class hierarchy without modifying the mapping file of the superclass.

on different platforms: some application servers refuse to give a stack trace on failure, for security reasons. Nor is a stack trace directly useful on a production system managed by an operations group; they see Java error and call the software team up. As we move the web application closer to production, we may want to consider writing an error-handling JSP page that emails the stack-trace to operations via email, rather than expose a system failure to normal users. 12.2.2 Integrating tag libraries For many problems, there is no need to write a taglib: reusing an existing library is much easier. As well as Struts, the Apache Jakarta project hosts a complete set of tag libraries in its taglibs project (http://jakarta.apache.org/taglibs/). Jakarta Taglibs is also hosting the reference implementation of the JSP Standard Tag Library (JSTL), which is the official tag library under development with the Java Community Process. Whenever any of these libraries is used, it needs to be included in the WAR file, and in the file templates/taglibs.xml; <webdoclet> will then include its declarations into the web application. To add Struts support, for example, we paste the Struts declarations into this file, below the declaration of our own tag library:

You now know everything you need to know about the mapping of entities, properties, and inheritance hierarchies. You can already map complex domain models. In the second half of this chapter, we discuss another important feature that you should know by heart as a Hibernate user: the Hibernate mapping type system.

<taglib> <taglib-uri>/WEB-INF/antbook.tld</taglib-uri> <taglib-location>/WEB-INF/antbook.tld</taglib-location> </taglib> <taglib> <taglib-uri>/WEB-INF/struts-bean.tld</taglib-uri> <taglib-location>/WEB-INF/struts-bean.tld</taglib-location> </taglib> <taglib> <taglib-uri>/WEB-INF/struts-html.tld</taglib-uri> <taglib-location>/WEB-INF/struts-html.tld</taglib-location> </taglib> <taglib> <taglib-uri>/WEB-INF/struts-logic.tld</taglib-uri> <taglib-location>/WEB-INF/struts-logic.tld</taglib-location> </taglib> <taglib> <taglib-uri>/WEB-INF/struts_template.tld</taglib-uri> <taglib-location>/WEB-INF/struts-template.tld</taglib-location> </taglib>

In chapter 4, we first distinguished between entity and value types a central concept of ORM in Java. We must elaborate on that distinction in order for you to fully understand the Hibernate type system of entities, value types, and mapping types.

With these tag libraries pulled in to WEB-INF/lib as a set of JAR files, our target to build the WAR file is getting more complex, as listing 12.3 demonstrates.

Entities are the coarse-grained classes in your system. You usually define the features of a system in terms of the entities involved. The user places a bid for an item is a typical feature definition; it mentions three entities. Classes of value types often don t even appear in the business requirements they re usually the fine-grained classes representing strings, numbers, and monetary amounts. Occasionally, value types do appear in feature definitions: the user changes billing address is one example, assuming that Address is a value type. More formally, an entity is any class whose instances have their own persistent identity. A value type is a class that doesn t define some kind of persistent identity. In practice, this means that entity types are classes with identifier properties, and value type classes depend on an entity. At runtime, you have a network of entity instances interleaved with value type instances. The entity instances may be in any of the three persistent lifecycle states: transient, detached, or persistent. We don t consider these lifecycle states to apply to the value type instances. (We ll come back to this discussion of object states in chapter 9.) Therefore, entities have their own lifecycle. The save() and delete() methods of the Hibernate Session interface apply to instances of entity classes, never to value type instances. The persistence lifecycle of a value type instance is completely tied to the lifecycle of the owning entity instance. For example, the username becomes persistent when the user is saved; it never becomes persistent independently of the user.