To store objects in a database, JPOX persistence module was added. Now persistent classes (all but Cart.java) will be declared in package.jdo file:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE jdo PUBLIC

    "-//Sun Microsystems, Inc.//DTD Java Data Objects Metadata 2.0//EN"

    "jdo.dtd">

<jdo>

    <package name="example.bookshop.domain">

        <class name="Author" detachable="true"></class>

        <class name="Book" detachable="true" persistence-capable-superclass="Product">

            <field name="author" default-fetch-group="true"/>

        </class>

        <class name="Magazine" detachable="true" persistence-capable-superclass="Product">

        </class>

        <class name="Order" detachable="true">

            <field name="products" default-fetch-group="true"/>

        </class>

        <class name="Product" detachable="true"></class>

    </package>

</jdo>

Notice how Java inheritance is indicated for classes Product, Book and Magazine. Also, notice how associations are defined for classes Book and Order. For example, by including <field name="author" default-fetch-group="true"/> in the Book class, JPOX is told to fetch the associated author everytime a Book object is fetched from database.