<xsd:complexType name="Person">
  <xsd:annotation>
    <xsd:appinfo>
      <jxb:property name="car">
        <jxb:baseType name="com.example.Car"/>
      </jxb:property>
    </xsd:appinfo>
  </xsd:annotation>
  <xsd:attribute name="car" type="xsd:IDREF"/>
</xsd:complexType>
<xsd:complexType name="Car">
  <xsd:attribute name="id" type="xsd:ID"/>
</xsd:complexType>

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Person")
public class Person {
    @XmlAttribute(name = "car")
    @XmlIDREF
    @XmlSchemaType(name = "IDREF")
    protected Object car;

    public Object getCar() {
        return car;
    }
    public void setCar(Object value) {
        this.car = value;
    }
}

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Car")
public class Car {
    @XmlAttribute(name = "id")
    @XmlJavaTypeAdapter(CollapsedStringAdapter.class)
    @XmlID
    @XmlSchemaType(name = "ID")
    protected String id;

    public String getId() {
        return id;
    }
    public void setId(String value) {
        this.id = value;
    }
}

<xs:attribute name="idRef" type="xs:IDREF">
      <xs:annotation>
            <xs:appinfo>
                 <jaxb:property>
                    <jaxb:baseType name="PractitionerType"/>
                 </jaxb:property>
            </xs:appinfo>
      </xs:annotation>
</xs:attribute>

@XmlAttribute(name = "idRef")
@XmlIDREF
@XmlSchemaType(name = "IDREF")
protected PractitionerType idRef;