MOXy un/marshal an object belonging to two different lists - jaxb

Question

This is my scenario.
I have a NodeElement class with two lists, inLinks and outLinks.
the Link class has two refrences to NodeElement, one for source and the other for target.
#XmlAccessorType(XmlAccessType.FIELD)
class NodeLement{
#XmlElement(name="link")
#XmlElementWrapper
private List<Link> inLinks = new ArrayList<Link>();
#XmlElement(name="link")
#XmlElementWrapper
private List<Link> outLinks = new ArrayList<Link>();
...
}
#XmlAccessorType(XmlAccessType.FIELD)
class Link{
#XmlInverseReference(mappedBy="inLinks")
private NodeElement source;
#XmlInverseReference(mappedBy="outLinks")
private NodeElement target;
public NodeElement getSource() {
return source;
}
public void setSource(NodeElement source) {
this.source = source;
this.source.getInLink().add(this);
}
public NodeElement getTarget() {
return target;
}
public void setTarget(NodeElement target) {
this.target = target;
this.target.getOutLink().add(this);
}
}
The test class
class test {
public static void main(String args[]) throws JAXBException, FileNotFoundException {
//rootelement
ContainerElement c = new ContainerElement();
Link link = new Link();
NodeElement target = new NodeElement();
NodeElement source = new NodeElement();
link.setSource(source);
link.setTarget(target);
c.addChild(target);
c.addChild(source);
JAXBContext jc = JAXBContext.newInstance(ContainerElement.class);
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(c, new File(XML_MODEL));
}
}
The result :
<?xml version="1.0" encoding="UTF-8"?>
<containerElement>
<ID>427485825424142</ID>
<inLinks/>
<outLinks/>
<children>
<child xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="NodeElement">
<ID>427520478709873</ID>
<inLinks/>
<outLinks>
<link xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="Link">
<ID>427582226289815</ID>
</link>
</outLinks>
</child>
<child xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="NodeElement">
<ID>427548409900894</ID>
<inLinks>
<link xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="Link">
<ID>427582226289815</ID>
</link>
</inLinks>
<outLinks/>
</child>
</children>
</containerElement>
Obviously, i get two instances of Link when unmarshaling, each one has either the source or the target as null and the other target/source correctly set.
How could i fix this so the unmarshaling provides one instance of Link correctly initialized ?
I was thinking of using before and after marshal to resolve the relation between the source and the target but I am just learning JAXB, so any help would be appreciated

Answer 1

There are two kinds of way to represent relationships in XML:
Containment (i.e. #XmlElement) the referenced object is represented as child elements.
Reference (i.e. #XmlIDREF) the referenced object is represented with a foreign key (see: http://blog.bdoughan.com/2010/10/jaxb-and-shared-references-xmlid-and.html).
Every object (other than the root object) must be reachable through exacttly one containment relationship. Optionally it may be referenced by one or more reference relationships.
If you are looking to mix these concepts where the first time an object is encountered it is represented via containment and then subsequent times it is represented by reference then check out the following answer that contains a full example.
Can JAXB marshal by containment at first then marshal by #XmlIDREF for subsequent references?