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In section 8.1.1 we mentioned the difficulties in mapping OO inheritance into relational databases. We also alluded to the three strategies used to solve this problem: putting all classes in the OO hierarchy in the same table, using joined tables for the super- and subclasses, or using completely separate tables for each class. We ll explore how each strategy is actually implemented in this section. Recall from section 8.2.7 that we could easily utilize different strategies for generating sequences more or less by changing configuration parameters for the @GeneratedValue annotation. The @Inheritance annotation used to map OO inheritance tries to follow the same philosophy of isolating strategy-specific settings into the configuration. We ll explore inheritance mapping using the three strategies offered through the @Inheritance annotation by implementing a familiar example. As we mentioned earlier, the ActionBazaar system has several user types, including sellers and bidders. We have also introduced the idea of creating a User superclass common to all user types. In this scheme of things, the User entity would hold data and behavior common to all users, while subclasses like Bidder and Seller would hold data and behavior specific to user types. Figure 8.8 shows a simplified class diagram for this OO hierarchy. In this section you ll learn how all the entities in the hierarchy in figure 8.8 can be mapped to database tables using different types of inheritance mapping strategies supported by JPA:

Figure 8.8 The ActionBazaar user hierarchy. Each user type like Bidder and Seller inherit from the common User superclass. The empty arrow signifies there may be some other subclasses of the User class.

The wscompile tool reads the config.xml file, which describes the web service endpoint. The tool uses a .wsdl file for the service, packages everything into an ejb.jar file, and finally packages everything into an .ear file. Here is a sample config.xml file used for this recipe:

In the single-table strategy, all classes in the inheritance hierarchy are mapped to a single table. This means that the single table will contain a superset of all data stored in the class hierarchy. Different objects in the OO hierarchy are identified using a special column called a discriminator column. In effect, the discriminator column contains a value unique to the object type in a given row. The best way to understand this scheme is to see it implemented. For the ActionBazaar schema, assume that all user types, including Bidders and Sellers, are mapped into the USERS table. Figure 8.9 shows how the table might look.

As figure 8.9 depicts, the USERS table contains data common to all users (such as USER_ID and USERNAME), Bidder-specific data (such as BID_FREQUENCY) and Sellerspecific data (such as CREDIT_WORTH). Records 1 and 2 contain Bidder records while record 3 contains a Seller record. This is indicated by the B and S values in the USER_TYPE column. As you can imagine, the USER_TYPE discriminator column can contain values corresponding to other user types, such as A for admin or C for CSR. The persistence provider maps each user type to the table by storing persistent data into relevant mapped columns, setting the USER_TYPE value correctly and leaving the rest of the values NULL. The next step to understanding the single-table strategy is to analyze the actual mapping implementation. Listing 8.12 shows the mapping for the User, Bidder, and Seller entities.

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< xml version="1.0" encoding="UTF-8" > <configuration xmlns="http://java.sun.com/xml/ns/jax-rpc/ri/config"> <service name="EJBWebService" targetNamespace="urn:Name" typeNamespace="urn:Name" packageName="ch4"> <interface name="ch4.EJBWebService"/> </service> </configuration>

@Entity @Table(name="USERS") @Inheritance(strategy=InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name="USER_TYPE", discriminatorType=DiscriminatorType.STRING, length=1) public abstract class User ...

@Entity @DiscriminatorValue(value="S") public class Seller extends User ... @Entity @DiscriminatorValue(value="B") public class Bidder extends User

This recipe only shows how to create the web service endpoint. For information about creating web service clients, go to http://java.sun.com. Creating web services allows you to expose previously developed business logic (that is encapsulated in an EJB) to platform-neutral web service clients. Doing this lets you reuse business logic and expose your application to a variety of clients. In addition, all the web service clients don t need to know the implementation platform or language of your application.