Inheritance: Table-Per-Class Hierarchy in Java

Creating PDF-417 2d barcode in Java Inheritance: Table-Per-Class Hierarchy

Now it s time to put some of the concepts about mapping inheritance into practice. In the first example from the TechConf system, we will map the User, Attendee, and Presenter class hierarchy. As you can deduce from Figure 4-14, an Attendee and a Presenter are types of User.

Figure 4-14. User, Attendee, and Presenter class hierarchy Using the Table-Per-Class-Hierarchy strategy, we will use a single table named USER to hold the data for Users, Attendees, and Presenters. The SQL required to create this table is shown in Listing 4-24. Listing 4-24. SQL to Create the USER Table CREATE TABLE USER ( PK_ID INTEGER NOT NULL PRIMARY KEY AUTO_INCREMENT, -- DISCRIMINATOR COLUMN USERTYPE VARCHAR(3) NOT NULL, PASSWORD VARCHAR(16) NOT NULL, FIRSTNAME VARCHAR(32) NOT NULL, LASTNAME VARCHAR(32) NOT NULL, EMAIL VARCHAR(64) NOT NULL, HOMEPHONE VARCHAR(12), WORKPHONE VARCHAR(12), FAX VARCHAR(12), FK_ADDRESS_ID INTEGER NOT NULL,

-- ATTENDEE FK_ATTENDEEGROUP_ID INTEGER, ALUMNUS BOOLEAN, -- PRESENTER BIO VARCHAR(255), COMPANY VARCHAR(64), COMPANYURL VARCHAR(64), FK_BLOGLINK_ID VARCHAR(64), CONSTRAINT USER_ADDRESS_FK FOREIGN KEY(FK_ADDRESS_ID) REFERENCES ADDRESS(PK_ID), CONSTRAINT UNIQUE_USER_EMAIL UNIQUE(EMAIL) ); Notice that the table has all of the fields required to store every element of the User-AttendeePresenter class hierarchy. Also notice that all of the fields for the subclasses are nullable. The column USERTYPE a VARCHAR(3) will serve as the discriminator column that Hibernate will use to determine the type of object that s stored in a given row. Now let s take a look at the HBM mapping file for the user class hierarchy shown in Listing 4-25. Listing 4-25. Table-Per-Class Hierarchy Mapping for User-Attendee-Presenter < xml version="1.0" > <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.integrallis.techconf.domain"> <class name="User" table="user" discriminator-value="USR"> <id name="Id" column="PK_ID" type="integer"> <generator class="identity" /> </id> <discriminator column="USERTYPE" length="3"/> <!-- User fields --> <property name="HomePhone" not-null="false" /> ... <many-to-one class="Address" name="Address" not-null="true" cascade="all" lazy="false" > <column name="FK_ADDRESS_ID" /> </many-to-one> ... <!-- Attendee Subclass --> <subclass name="Attendee" discriminator-value="ATD"> <property name="Alumnus" type="boolean" /> <many-to-one name="Group" class="AttendeeGroup">

<column name="FK_ATTENDEEGROUP_ID" /> </many-to-one> ... </subclass> <!-- Presenter Subclass --> <subclass name="Presenter" discriminator-value="PST"> <property name="Bio" /> <property name="Company" /> <property name="CompanyURL" /> ... </subclass> </class> </hibernate-mapping> As you can see, the subclass element is used to map the subclasses Attendee and Presenter. For User objects the discriminator column value is USR , for Attendee objects it is ATD , and for Presenter objects it is PST . Also, from this mapping we can see that Users can have an Address. I have omitted some sections of the mapping for clarity s sake. Now we can write a simple test to create some User, Attendee, and Presenter objects and persist them to the database. Listing 4-26 shows some example code to create the objects. Notice that I am creating the required Address objects for each of the User, Attendee, and Presenter objects. Listing 4-26. Creating and Persisting Some User, Attendee, and Presenter Objects File configFile = new File("hibernate.cfg.xml"); Configuration configuration = new Configuration().configure(configFile); SessionFactory factory = configuration.buildSessionFactory(); // create some addresses Address address1 = new Address("123 Main Street", "N/A", "Columbus", "OH", "43081"); Address address2 = new Address("456 Vine Street", "N/A", "Cincinnati", "OH", "45202"); Address address3 = new Address("1 Easy Street", "N/A", "Columbus", "GA", "31901"); // create a user User user = new User("Jim", "Smith", "jim@smith.com", "jimspassword"); user.setAddress(address1); // create an attendee Attendee attendee = new Attendee("Bob", "Smithers", "bsmithers@acme.com", "bobspassword"); attendee.setAlumnus(true); attendee.setAddress(address2);

// create a presenter Presenter presenter = new Presenter("Jackie", "Roberts", "jroberts@acme.com", "jackiespassword"); presenter.setBio("Jackie loves Java"); presenter.setCompany("Acme Java Corporation"); presenter.setCompanyURL("www.acme.com"); presenter.setAddress(address3); Session session = null; Transaction tx = null; try { session = factory.openSession(); tx = session.beginTransaction(); // persist the objects session.persist(user); session.persist(attendee); session.persist(presenter); tx.commit(); } catch (Exception e) { if (tx != null) { tx.rollback(); } } finally { session.close(); } Running the example shown in Listing 4-26 would produce the following SQL statements: Hibernate: insert into Address (StreetAddress, State, ZipCode, City, AptNumber) values ( , , , , ) Hibernate: insert into user (HOMEPHONE, PASSWORD, FAX, WORKPHONE, LASTNAME, FIRSTNAME, EMAIL, FK_ADDRESS_ID, USERTYPE) values ( , , , , , , , , 'USR') Hibernate: insert into Address (StreetAddress, State, ZipCode, City, AptNumber) values ( , , , , ) Hibernate: insert into user (HOMEPHONE, PASSWORD, FAX, WORKPHONE, LASTNAME, FIRSTNAME, EMAIL, FK_ADDRESS_ID, ALUMNUS, FK_ATTENDEEGROUP_ID, USERTYPE) values ( , , , , , , , , , , 'ATD') Hibernate: insert into Address (StreetAddress, State, ZipCode, City, AptNumber) values ( , , , , ) Hibernate: insert into user (HOMEPHONE, PASSWORD, FAX, WORKPHONE, LASTNAME, FIRSTNAME, EMAIL, FK_ADDRESS_ID, BIO, COMPANY, COMPANYURL, FK_BLOGLINK_ID, USERTYPE) values ( , , , , , , , , , , , , 'PST')