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Looking at this descriptor, the tag we have written is declared b, along with its fail c and verbose d attributes exactly what we wanted. We still need to get a reference to this descriptor into the application s web.xml file, which we can do with <webdoclet>. We mentioned in passing that in listing 12.2 we set the mergedir="templates" attribute, which tells the <webdoclet> task to merge XML fragments, in separate files in this directory, into our web.xml file. These files are called merge points. There are many different merge points, clearly documented in XDoclet s distribution. These include setting security roles, mappings for the request filters added in the Servlet 2.3 API, EJB binding information, and even MIME-type bindings. All of these files are optional; the task does not require any of them to generate the web.xml file, but they are the way to customize web applications with <webdoclet>. We create the merge file templates/taglibs.xml and fill it with the declaration of our taglib, binding the URI to its physical location in the file:

... <subclass name="CreditCard" discriminator-value="CC"> <join table="CREDIT_CARD"> <key column="CREDIT_CARD_ID"/> <property name="number" column="CC_NUMBER"/> <property name="expMonth" column="CC_EXP_MONTH"/> <property name="expYear" column="CC_EXP_YEAR"/> ... </join> </subclass> <subclass name="BankAccount" discriminator-value="BA"> <property name=account" column="BA_ACCOUNT"/> ... </subclass> ... </class> </hibernate-mapping>

The <join> element groups some properties and tells Hibernate to get them from a secondary table. This mapping element has many uses, and you ll see it again later in the book. In this example, it separates the CreditCard properties from the table per hierarchy into the CREDIT_CARD table. The CREDIT_CARD_ID column of this table is at the same time the primary key, and it has a foreign key constraint referencing the BILLING_DETAILS_ID of the hierarchy table. The BankAccount subclass is mapped to the hierarchy table. Look at the schema in figure 5.4. At runtime, Hibernate executes an outer join to fetch BillingDetails and all subclass instances polymorphically:

<taglib> <taglib-uri>/WEB-INF/antbook.tld</taglib-uri> <taglib-location>/WEB-INF/antbook.tld</taglib-location> </taglib>

select BILLING_DETAILS_ID, BILLING_DETAILS_TYPE, OWNER, CC.CC_NUMBER, CC.CC_EXP_MONTH, CC.CC_EXP_YEAR, BA_ACCOUNT, BA_BANKNAME, BA_SWIFT from BILLING_DETAILS left outer join CREDIT_CARD CC on BILLING_DETAILS_ID = CC.CREDIT_CARD_ID

We then modify our <war> task to pull in WEB-INF/antbook.tld, and we are ready to test the tag library. First, we create a JSP page, happy.jsp, which contains our test:

You can also use the <join> trick for other subclasses in your class hierarchy. However, if you have an exceptionally wide class hierarchy, the outer join can become a problem. Some database systems (Oracle, for example) limit the number of tables in an outer join operation. For a wide hierarchy, you may want to switch to a different fetching strategy that executes an immediate second select instead of an outer join:

<%@ page contentType="text/plain" %> <%@ taglib uri="/WEB-INF/antbook.tld" prefix="happy" %> <happy:happy/> We are happy

<subclass name="CreditCard" discriminator-value="CC"> <join table="CREDIT_CARD" fetch="select"> <key column="CREDIT_CARD_ID"/> ... </join> </subclass>

We have to build the web application and then deploy the file, which we do using the Tomcat deployment targets of chapter 7. Browsing to the page http://localhost:8080/ antbook/happy.jsp we get the string We are happy a few lines down the page. This shows that we can generate a tag that a JSP page will process and that the servlet version test is succeeding: the container is of an acceptable version. A final test is to verify that the tag works correctly when unhappy, so we write the JSP page unhappy.jsp to force a failure:

Java Persistence also supports this mixed inheritance mapping strategy with annotations. Map the superclass BillingDetails with InheritanceType.SINGLE_ TABLE, as you did before. Now map the subclass you want to break out of the single table to a secondary table.

@Entity @DiscriminatorValue("CC") @SecondaryTable(

<%@ page contentType="text/plain" %> <%@ taglib uri="/WEB-INF/antbook.tld" prefix="happy" %> <happy:happy verbose="true" fail="true"/> We are unhappy

name = "CREDIT_CARD", pkJoinColumns = @PrimaryKeyJoinColumn(name = "CREDIT_CARD_ID") ) public class CreditCard extends BillingDetails { @Column(table = "CREDIT_CARD", name = "CC_NUMBER", nullable = false) private String number; ... }

When we fetch this page, we get a servlet 500 error reported, with an error trace including the error string: