Mapping Single Key Relationships

To model non-privately-owned relationships, your "target" objects must have IDs (keys) defined, and your "source" object must use these IDs to map the relationship.

Relationships represented with keys use the @XmlID and @XmlIDREF annotations. Although the JAXB specification requires that the property marked with @XmlID be a String, MOXy JAXB does not enforce this restriction.

In Example 7-1, each Employee has one manager but multiple reports.

Example 7-1 Using the @XmlID and @XmlIDREF Annotations

package example;
 
import javax.xml.bind.annotation.*;
 
@XmlAccessorType(XmlAccessType.FIELD)
public class Employee {
    @XmlAttribute
    @XmlID
    private Integer id;
 
    @XmlAttribute
    private String name;
 
    @XmlIDREF
    private Employee manager;
 
    @XmlElement(name="report")
    @XmlIDREF
    private List<Employee> reports;
 
    ...
}

The following example shows how to define this mapping information in EclipseLink's OXM metadata format.

Example 7-2 Sample XML Mapping

...
<java-type name="Employee">
   <java-attributes>
      <xml-attribute java-attribute="id" type="java.lang.Integer" xml-id="true"/>
      <xml-attribute java-attribute="name" type="java.lang.String"/>
      <xml-element java-attribute="manager" type="mypackage.Employee" xml-idref="true"/>
      <xml-element java-attribute="reports" type="mypackage.Employee" container-type="java.util.ArrayList" xml-idref="true"/>
   </java-attributes>
</java-type>
...

This would produce the following XML:

<company>
    <employee id="1" name="Jane Doe">
        <report>2</report>
        <report>3</report>
    </employee>
    <employee id="2" name="John Smith">
        <manager>1</manager>
    </employee>
    <employee id="3" name="Anne Jones">
        <manager>1</manager>
    </employee>
</company>

The manager and reports elements contain the IDs of the Employee instances they are referencing.

Using @XmlList

Because the @XmlIDREF annotation is also compatible with the @XmlList annotation, the Employee object could be modeled as:

Example 7-3 Using the @XmlList Annotation

package example;
 
import javax.xml.bind.annotation.*;
 
@XmlAccessorType(XmlAccessType.FIELD)
public class Employee {
    @XmlID
    @XmlAttribute
    private Integer id;
 
    @XmlAttribute
    private String name;
 
    @XmlIDREF
    private Employee manager;
 
    @XmlIDREF
    @XmlList
    private List<Employee> reports;
 
    ... 
}

This would produce the following XML:

<company>
   <employee id="1" name="Jane Doe">
      <reports>2 3</reports>
   </employee>
   <employee id="2" name="John Smith">
      <manager>1</manager>
   </employee>
   <employee id="3" name="Anne Jones">
      <manager>1</manager>
   </employee>
</company>