JAXB - XML Schema Types, Defining Subtypes

Although object orientation isn't a key feature of XML or the XML Schema language, it's still possible to apply the fundamental OO paradigm when designing a schema: inheritance. This is based on the schema element xsd:extension which lets you add both child elements and attributes to some elsewhere defined type acting as the base type. The example given below presents the components for defining a simple menu (this time it's for a graphical user interface) where menu entries may come in several flavours: simple items, check boxes, radio buttons and sub-menus.

<xsd:complexType name="EntryType">
    <xsd:attribute name="Text" type="xsd:string"/>
</xsd:complexType>

<xsd:complexType name="ItemType">
    <xsd:complexContent>
        <xsd:extension base="EntryType">
            <xsd:sequence>
                <xsd:element name="Command" type="xsd:string"/>
            </xsd:sequence>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

<xsd:complexType name="CheckBoxType">
    <xsd:complexContent>
        <xsd:extension base="ItemType">
            <xsd:attribute name="State" type="xsd:boolean"/>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

<xsd:complexType name="RadioButtonType">
    <xsd:complexContent>
        <xsd:extension base="ItemType">
            <xsd:attribute name="Group" type="xsd:string"/>            
            <xsd:attribute name="State" type="xsd:boolean"/>
            <xsd:attribute name="Value" type="xsd:string"/>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

<xsd:complexType name="MenuType">
    <xsd:complexContent>
        <xsd:extension base="EntryType">
            <xsd:choice maxOccurs="unbounded">
                <xsd:element name="Item" type="ItemType"/>
                <xsd:element name="CheckBox" type="CheckBoxType"/>
                <xsd:element name="RadioButton" type="RadioButtonType"/>
                <xsd:element name="Menu" type="MenuType"/>
            </xsd:choice>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

The base class EntryType is extended in several ways:

• ItemType adds a command definition to the base type.
• CheckBoxType extends ItemType, inheriting the command and adding an attribute for the initial state of the check box.
• RadioButtonType is another extension of ItemType, again adding some attributes. Group is the button group's identification, and Value defines the string to be used for indicating the selection.
• MenuType reflects the recursive structure of menus by being both another subclass of ItemType (so that it may represent cascades) as well as a container for all kinds of menu entries, including itself.

Before we look at the generated Java code, we should note that the definition of MenuType isn't quite what an OO aficionado would expect. After all the pains taken to establish this little class hierarchy, one still must explicitly put all the subclasses into the choice list. Using just the supertype EntryType in an xsd:sequence would result in very dull menus.

The JAXB compiler, however, rewards you with a set of class definitions that uses extends wherever we have xsd:extension in the schema. A look at the (much abbreviated) code shows the expected inheritance structure.

public class EntryType {
    protected String text;

    // ...(getText, setText)
}

public class ItemType extends EntryType {
    protected String command;

    // ...(getCommand, setCommand)
}

public class CheckBoxType extends ItemType {
    protected Boolean state;

    // ...(isState, setState)
}

public class RadioButtonType extends ItemType {
    protected String group;
    protected Boolean state;
    protected String value;

    // ...(getters and setters)
}

Finally there is MenuType, which contains a java.util.List<EntryType>. JAXB has briefly reflected upon the element types bunched into the choice and has, literally, reverse engineered the common superclass. A reminder that the list is a mixture is embedded in the name of the getter which is made up from the first three tags.

public class MenuType extends EntryType {
    protected List<EntryType> itemOrCheckBoxOrRadioButton;

    public List<EntryType> getItemOrCheckBoxOrRadioButton() {
        if (itemOrCheckBoxOrRadioButton == null) {
            itemOrCheckBoxOrRadioButton = new ArrayList<EntryType>();
        }
        return this.itemOrCheckBoxOrRadioButton;
    }
}