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Domain models and metadata
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node="@seller-id"/> </class> <class name="User" table="USERS" node="user"> <id name="id" type="long" column="USER_ID" node="@id"> <generator class="native"/> </id> <property name="username" type="string" column="USERNAME" node="@username"/> <bag name="itemsForSale" inverse="true" cascade="all" embed-xml="true" node="items-for-sale"> <key column="USER_ID"/> <one-to-many class="Item"/> </bag> </class> </hibernate-mapping>
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Each node attribute defines the XML representation:
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A node="name" attribute on a <class> mapping defines the name of the XML element for that entity. A node="name" attribute on any property mapping specifies that the property content should be represented as the text of an XML element of the given name. A node="@name" attribute on any property mapping specifies that the property content should be represented as an XML attribute value of the given name. A node="name/@attname" attribute on any property mapping specifies that the property content should be represented as an XML attribute value of the given name, on a child element of the given name.
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The embed-xml option is used to trigger embedding or referencing of associated entity data. The updated mapping results in the following XML representation of the same data you ve seen before:
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<user id="1" username="johndoe"> <items-for-sale> <item id="2" seller-id="1"> <item-details initial-price="99" description="An item for auction"/> </item>
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Alternative entity representation
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<item id="3" seller-id="1"> <item-details initial-price="123" description="Another item for auction"/> </item> </items-for-sale> </user>
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Be careful with the embed-xml option you can easily create circular references that result in an endless loop! Finally, data in an XML representation is transactional and persistent, so you can modify queried XML elements and let Hibernate take care of updating the underlying tables:
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Element itemXML = (Element) dom4jSession.get(Item.class, storedItemId); itemXML.element("item-details") .attribute("initial-price") .setValue("100"); session.flush(); // Hibernate executes UPDATEs Element userXML = (Element) dom4jSession.get(User.class, storedUserId); Element newItem = DocumentHelper.createElement("item"); Element newItemDetails = newItem.addElement("item-details"); newItem.addAttribute("seller-id", userXml.attribute("id").getValue() ); newItemDetails.addAttribute("initial-price", "123"); newItemDetails.addAttribute("description", "A third item"); dom4jSession.save(Item.class.getName(), newItem); dom4jSession.flush(); // Hibernate executes INSERTs
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There is no limit to what you can do with the XML that is returned by Hibernate. You can display, export, and transform it in any way you like. See the dom4j documentation for more information. Finally, note that you can use all three built-in entity modes simultaneously, if you like. You can map a static POJO implementation of your domain model, switch to dynamic maps for your generic user interface, and export data into XML. Or, you can write an application that doesn t have any domain classes, only dynamic maps and XML. We have to warn you, though, that prototyping in the software industry often means that customers end up with the prototype that nobody wanted to throw away would you buy a prototype car We highly recommend that you rely on static domain models if you want to create a maintainable system.
Domain models and metadata
We won t consider dynamic models or XML representation again in this book. Instead, we ll focus on static persistent classes and how they are mapped.
Summary
In this chapter, we focused on the design and implementation of a rich domain model in Java. You now understand that persistent classes in a domain model should to be free of crosscutting concerns, such as transactions and security. Even persistencerelated concerns should not leak into the domain model implementation. You also know how important transparent persistence is if you want to execute and test your business objects independently and easily. You have learned the best practices and requirements for the POJO and JPA entity programming model, and what concepts they have in common with the old JavaBean specification. We had a closer look at the implementation of persistent classes, and how attributes and relationships are best represented. To be prepared for the next part of the book, and to learn all the object/relational mapping options, you needed to make an educated decision to use either XML mapping files or JDK 5.0 annotations, or possibly a combination of both. You re now ready to write more complex mappings in both formats. For convenience, table 3.1 summarizes the differences between Hibernate and Java Persistence related to concepts discussed in this chapter.
Table 3.1 Hibernate and JPA comparison chart for chapter 3 Hibernate Core Persistent classes require a no-argument constructor with public or protected visibility if proxybased lazy loading is used. Persistent collections must be typed to interfaces. Hibernate supports all JDK interfaces. Java Persistence and EJB 3.0 The JPA specification mandates a no-argument constructor with public or protected visibility for all entity classes. Persistent collections must be typed to interfaces. Only a subset of all interfaces (no sorted collections, for example) is considered fully portable. Persistent properties of an entity class are accessed through fields or accessor methods, but not both if full portability is required.
Persistent properties can be accessed through fields or accessor methods at runtime, or a completely customizable strategy can be applied.
Summary
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