Using Abstract base class type to traverse entire JAXB Object tree

Question

The system hardware I write software for is physically connect via hardware in a tree structure. The data model in our application is a Tree. For our new rewrite, we're using JAXB to create the data model.

We have three types of Devices, and they all share some properties, so I made an Abtract DeviceType in the XSD schema. My three devices (Pushers, Switchers, Receivers) are all extended from the DeviceType in the XSD like this:

<xs:complexType name="DeviceType" abstract="true">
  <xs:sequence>
    <xs:element name="atrr1" type="xs:int"></xs:element>
    <xs:element name="attr2" type="xs:int"></xs:element>
  </xs:sequence>
</xs:complexType>

<xs:complexType name="PusherType">
  <xs:complexContent>
    <xs:extension base="pts:DeviceType">
      <xs:sequence>
        <xs:element name="Switcher" type="pts:SwitcherType" minOccurs="1"></xs:element>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

<xs:complexType name="SwitcherType">
  <xs:complexContent>
    <xs:extension base="pts:DeviceType">
      <xs:sequence>
        <xs:element name="switcher" type="pts:SwitcherType" minOccurs="1"></xs:element>
        <xs:element name="receiver" type="pts:ReceiverType" minOccurs="1"></xs:element>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

Pushers have only switcher child elements, and switchers can have both switcher or receiver children. Receivers are the end of the line (leaf nodes). xjc builds the classes. I got the Unmarshaller to construct the object tree, but I can't figure out how to get a getter method for getDevice(). For tree traversal, I was hoping JAXB would provide something like "getChildren", but I'm not seeing in in the API. If I get a switcher object, I have the methods for getSwitcher() and getReceiver(), but no getDevice() method. But I'm trying to avoid using instanceof when I do a full tree traversal. The Java code that xjc builds, does extend from the Device class, but I just haven't learned how to get a generic getter method for all devices. I just started with Jaxb two days ago and I have a ton to learn about the Jaxb API.

Yesterday was my first day playing with JAXB, I think this tool suits our system incredible well. Our hardware is literally a tree,we have multiple deployments, and using XML as our site config file to build a state model would be ideal.

Any suggestions for a JAXB novice here?

Answer 1

JAXB (JSR-222) is the Java standard for mapping objects to XML. There are several implementations including: Metro, EclipseLink MOXy (I'm the tech lead), Apache JaxMe, etc.

JAXB is designed to map existing object structures to XML. In your example you are leveraging JAXB's ability to generate a class model from an XML schema. This produces something like a "typed DOM". There will be a Java class matching each complex type (with the necessary inheritance relationships), and a property matching each attribute/element. For example the following class corresponds to the SwitcherType complex type:

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlType;

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "SwitcherType", propOrder = {
    "switcher",
    "receiver"
})
public class SwitcherType extends DeviceType
{

    @XmlElement(required = true)
    protected SwitcherType switcher;

    @XmlElement(required = true)
    protected ReceiverType receiver;

    public SwitcherType getSwitcher() {
        return switcher;
    }

    public void setSwitcher(SwitcherType value) {
        this.switcher = value;
    }

    public ReceiverType getReceiver() {
        return receiver;
    }

    public void setReceiver(ReceiverType value) {
        this.receiver = value;
    }

}

Unlike a DOM there are no generic getChildren() methods. Although you could implement these yourself by modifying the object model.

For more information on JAXB:

• http://blog.bdoughan.com/