Question

I would like to build some kind of object generation engine for my domain objects.
For example, lets assume, I’m working with graphs. The models are represented by xml and I should be able to load them and build a java representation at runtime.

Lets say, graph has vertices and edges
So it will look like this:

<graph>
   <vertex id="n1" color="red", thickness="2">
   <vertex id="n2">
   <edge end1="${n1}", end2="${n2}"/>
</graph>

For this I would like to get the objects that can be described by the following java classes:

class Graph {
     List<Vertex> vertexList
     List<Edge> edgeList
}

class Vertex {
   String id
    ... various properties ... 
}

class Edge {
   Vertex end1
   Vertex end2
}

Another requirement is to be able to define vertices in loop like this:

<graph>
  ...
    <for var = i, min = 1, max = 10, step = 1>
      <vertex id=$i.../> 
    </for>
  ... 
</graph>

I thought about using Apache Jelly but it seems to be a ‘dead’ project, JaxB doesn’t allow such a level of dynamic behavior AFAIK…

My question is – what framework can you recommend for implementing such a task?

If there is something that works like Apache Jelly but still maintained, it could be great also 🙂

Thanks a lot in advance

Answer 1

JAXB (JSR-222) implementations can easily handle references within a document using a combination of @XmlID and @XmlIDREF. I will demonstrate below with an example.

JAVA MODEL

Graph

package forum13404583;

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

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class Graph {

    @XmlElement(name = "vertex")
    List<Vertex> vertexList;

    @XmlElement(name = "edge")
    List<Edge> edgeList;

}

Vertex

In the Vertex class you need to use the @XmlID annotation to indicate that the id field is the id.

package forum13404583;

import javax.xml.bind.annotation.*;

@XmlAccessorType(XmlAccessType.FIELD)
class Vertex {

    @XmlAttribute
    @XmlID
    String id;

    @XmlAttribute
    String color;

    @XmlAttribute
    Integer thickness;

}

Edge

In the Edge class the @XmlIDREF annotation is used to indicate that the XML value contains a foreign key that references the real value.

package forum13404583;

import javax.xml.bind.annotation.*;

@XmlAccessorType(XmlAccessType.FIELD)
class Edge {

    @XmlAttribute
    @XmlIDREF
    Vertex end1;

    @XmlAttribute
    @XmlIDREF
    Vertex end2;

}

DEMO CODE

package forum13404583;

import java.io.File;
import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Graph.class);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/forum13404583/input.xml");
        Graph graph = (Graph) unmarshaller.unmarshal(xml);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(graph, System.out);
    }

}

INPUT (input.xml)/OUTPUT

Below is the input to and output from running the demo code.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<graph>
    <vertex id="n1" color="red" thickness="2"/>
    <vertex id="n2"/>
    <edge end1="n1" end2="n2"/>
</graph>

For More Information

• http://blog.bdoughan.com/2010/10/jaxb-and-shared-references-xmlid-and.html