How do Inheritance and JAXB work together?

Question

public abstract class Parent<T> {
    protected List<T> list;

    @XmlTransient   //Question why do we have to give this here?
    public abstract List<T> getList();
    public abstract void setList(List<T> list);
}

@XmlRootElement(name = "child1")
class Child1 extends Parent<ExtendedElement1>{
    @Override
    public void setList(List<ExtendedElement1> list){
        this.list = list;
    }

    @Override
    @XmlElementWrapper(name = "child1-list")
    @XmlElement(name = "child-list-element")
    public List<ExtendedElement1> getList(){
        return this.list;
    }
}

@XmlRootElement(name = "child2")
class Child2 extends Parent<ExtendedElement2>{
    @Override
    public void setList(List<ExtendedElement2> list){
        this.list = list;
    }

    @Override
    @XmlElementWrapper(name = "child1-list")
    @XmlElement(name = "child-list-element")
    public List<ExtendedElement2> getList(){
        return this.list;
    }
}


class Element{
    @XmlElement(name = "integer", type = int.class)
    private int i = 2;
}

class ExtendedElement1 extends Element{
    @XmlElement(name = "extended-element1-str", type = String.class)
    private String str = "hello";
}

class ExtendedElement2 extends Element{
    @XmlElement(name = "extended-element2-str", type = String.class)
    private String str1 = "hello_there";
}

As I have shown in the example when I remove the @XmlTransient from the Parent class getList() method, following xml is marshalled:

<child1>
<!-- List is serialized 2 times -->
    <list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="extendedElement1">
        <integer>2</integer>
        <extended-element1-str>hello</extended-element1-str>
    </list>
    <list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="extendedElement1">
        <integer>2</integer>
        <extended-element1-str>hello</extended-element1-str>
    </list>
    <list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="extendedElement1">
        <integer>2</integer>
        <extended-element1-str>hello</extended-element1-str>
    </list>
    <child1-list>
        <child-list-element>
            <integer>2</integer>
            <extended-element1-str>hello</extended-element1-str>
        </child-list-element>
        <child-list-element>
            <integer>2</integer>
            <extended-element1-str>hello</extended-element1-str>
        </child-list-element>
        <child-list-element>
            <integer>2</integer>
            <extended-element1-str>hello</extended-element1-str>
        </child-list-element>
    </child1-list>
</child1>

But when I add the @XmlTransient annotation, as in the example the xml is serialized with only ONE list as required.

<child1>
    <child1-list>
        <child-list-element>
            <integer>2</integer>
            <extended-element1-str>hello</extended-element1-str>
        </child-list-element>
        <child-list-element>
            <integer>2</integer>
            <extended-element1-str>hello</extended-element1-str>
        </child-list-element>
        <child-list-element>
            <integer>2</integer>
            <extended-element1-str>hello</extended-element1-str>
        </child-list-element>
    </child1-list>
</child1>

So please can someone explain me why is it required to give @XmlTransient in Parent Class getter method? How does inheritance and JAXB inter-relate for these cases?

Answer 1

WHY ITS HAPPENING

A JAXB (JSR-222) implementation will map every domain object that it is aware of to a complex type. This means it believes that the following XML type exists for the Parent class (when list is not @XmlTransient ).

  <xs:complexType name="parent" abstract="true">
    <xs:sequence>
      <xs:element name="list" type="xs:anyType" nillable="true" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

Now Child2 also has a complex type. There are two things JAXB could have done:

• Not allow properties to be overridden, which would be pretty limiting.
• Generate an additional mapping for the overridden property. In the schema below the child2 type extends parent . This means it gets all the elements from the parent type plus its own.

  <xs:complexType name="child2">
    <xs:complexContent>
      <xs:extension base="parent">
        <xs:sequence>
          <xs:element name="child1-list" minOccurs="0">
            <xs:complexType>
              <xs:sequence>
                <xs:element name="child-list-element" type="extendedElement2" minOccurs="0" maxOccurs="unbounded"/>
              </xs:sequence>
            </xs:complexType>
          </xs:element>
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>

HOW TO FIX IT

You could put @XmlTransient on the list property in the Parent class, but instead I would recommend annotating the Parent class with @XmlTransient .

import java.util.List;
import javax.xml.bind.annotation.XmlTransient;

@XmlTransient
public abstract class Parent<T> {
    protected List<T> list;

    public abstract List<T> getList();
    public abstract void setList(List<T> list);

}

This will remove it as a mapped class and the complex type that corresponds to Child2 will become:

  <xs:complexType name="child2">
    <xs:sequence>
      <xs:element name="child1-list" minOccurs="0">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="child-list-element" type="extendedElement2" minOccurs="0" maxOccurs="unbounded"/>
          </xs:sequence>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
  </xs:complexType>