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<target name="do-common"> <ant dir="common" target="${target}" inheritAll="false"> <property file="masterbuild.properties"/> </ant> </target>

@Entity @Inheritance(strategy = InheritanceType.SINGLE_TABLE) @org.hibernate.annotations.DiscriminatorFormula( "case when CC_NUMBER is not null then 'CC' else 'BA' end" ) public abstract class BillingDetails { ... }

Let us assume that the file masterbuild.properties includes the following property declarations:

The disadvantages of the table per class hierarchy strategy may be too serious for your design after all, denormalized schemas can become a major burden in the long run. Your DBA may not like it at all. The next inheritance mapping strategy doesn t expose you to this problem.

The fourth option is to represent inheritance relationships as relational foreign key associations. Every class/subclass that declares persistent properties including abstract classes and even interfaces has its own table. Unlike the table per concrete class strategy we mapped first, the table here contains columns only for each noninherited property (each property declared by the subclass itself) along with a primary key that is also a foreign key of the superclass table. This approach is shown in figure 5.3. If an instance of the CreditCard subclass is made persistent, the values of properties declared by the BillingDetails superclass are persisted to a new row of the BILLING_DETAILS table. Only the values of properties declared by the subclass are persisted to a new row of the CREDIT_CARD table. The two rows are linked together

These properties would all trickle down to the subprojects, controlling their build options. One of the problems with this approach is that it does not work for relative file references. All properties loaded from a file are treated as simple values, rather than relative file locations, which need to be immediately resolved. This limits the value of this technique. Setting references requires a declaration of the reference earlier in the build file; a <reference> tag must then point to the reference:

by their shared primary key value. Later, the subclass instance may be retrieved from the database by joining the subclass table with the superclass table. The primary advantage of this strategy is that the SQL schema is normalized. Schema evolution and integrity constraint definition are straightforward. A polymorphic association to a particular subclass may be represented as a foreign key referencing the table of that particular subclass. In Hibernate, you use the <joined-subclass> element to create a table per subclass mapping. See listing 5.3.

If you want to rename a reference, then you must supply the name of the reference ID by which the path will be known in the destination:

<hibernate-mapping> <class name="BillingDetails" table="BILLING_DETAILS"> <id name="id" column="BILLING_DETAILS_ID" type="long"> <generator class="native"/>

The value of setting such references increases in complex projects, especially with library Ant projects, which we shall cover in section 9.6. 9.5.4 Sharing properties via XML file fragments Although we have demonstrated the different ways of passing information to child projects, astute readers will have noticed from our master build example that we use of none of these. We use a slightly different technique, which is much more powerful but harder to use. The problem is that we want to make each child project stand-alone, so that you can call it without having to go via the master build file. Yet we do not want any duplicate definitions of properties or the locations of the library files we use in our projects. Because some of the projects depend on the work of other projects, we also need references to the output files of all our projects again, with no duplication. How do we solve this We use XML file fragments. XML supports the ability to import fragments from other files as entities, inserting these fragments into the local XML file wherever these entities appear. This is roughly equivalent to the #include feature in C and C++, which inserts a named text file into the source code. The difference between XML and C or C++ is that the insertion is done in two phases: declaration and then importation.

<property name="number" column="NUMBER"/> <property name="expMonth" column="EXP_MONTH"/> <property name="expYear" column="EXP_YEAR"/> </joined-subclass> <joined-subclass name="BankAccount" table="BANK_ACCOUNT"> ... </class> </hibernate-mapping>

You first declare the fragment at the beginning of the file, after the < xml > header and before the XML data itself: