Object-XML Descriptor Concepts

The following section describes the use of default root elements in Object-XML descriptors.

Default Root Element

You configure XML descriptors with a default root element so that the EclipseLink runtime knows the data source data type associated with the class the descriptor describes.

Note:

The undefined document root element of a referenced object is ignored during marshalling with an any collection mapping and object mapping.

This section describes what a default root element is and how EclipseLink uses it.

Consider the Customer and Address classes and their mappings, shown in Example 6-2.

Example 6-2 Customer and Address Classes

Class: Customer
Default Root: customer
Attributes and Mappings:
    name:String                Direct Mapping                   name/text()
    billingAddress:Address     Composite Object Mapping to     billing-address
    shippingAddress:Address    Composite Object Mapping to     shipping-address

Class: Address
Default Root: address
Attributes and Mappings:
    street:String              Direct Mapping to               street/text()
    city:String                Direct Mapping to               city/text()

These classes correspond to the XML schema, shown in Example 6-3.

Example 6-3 Customer and Address Schema

<xsd:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xsd:complexType name="address-type">
        <xsd:sequence>
            <element name="street" type="xsd:string"/>
            <element name="city" type="xsd:string"/>
        </xsd:sequence>
    </xsd:complexType>
    <xsd:element name="customer" type="customer-type"/>
    <xsd:complexType name="customer-type">
        <xsd:sequence>
            <xsd:element name="name" type="xsd:string"/>
            <xsd:element name="billing-address" type="address-type"/>
            <xsd:element name="shipping-address" type="address-type"/>
        </xsd:sequence>
    </xsd:complexType>
</xsd:schema>

When an instance of the Customer class is persisted to XML, the EclipseLink runtime performs the following:

Gets the default root element.

The Customer class instance corresponds to the root of the XML document. The EclipseLink runtime uses the default root element specified on the descriptor (customer) to start the XML document. EclipseLink then uses the mappings on the descriptor to marshal the object's attributes:
```
<customer>
    <name>…</name>
</customer>
```
When the EclipseLink runtime encounters an object attribute such as billingAddress, it checks the mapping associated with it to determine with what element (billing-address) to continue:
```
<customer>
    <name>…</name>
    <billing-address/>
</customer>
```
The EclipseLink runtime checks the mapping's reference descriptor (Address) to determine what attributes to persist:
```
<customer>
    <name>…</name>
    <billing-address>
        <street>…</street>
        <city>…</city>
    </billing-address>
</customer>
```