JAXB unmarshalling of “generic” real world documents

Question

We have a large set of configuration documents of the style

  
    ...more content
  
  

        

Answer 1

You could do something like the following with JAXB (JSR-222) and StAX (JSR-173):

Demo

A StAX XMLStreamReader can be used to parse the XML document. You can use it to advance to a bar element, then you can read the class attribute and load the appropriate Java class from a ClassLoader. Then you can leverage one of the unmarshal methods that takes a class parameter.

package forum12402215;

import javax.xml.bind.*;
import javax.xml.stream.*;
import javax.xml.transform.stream.StreamSource;

public class Demo {

    public static void main(String[] args) throws Exception {
        ClassLoader classLoader = Bork.class.getClassLoader();
        JAXBContext jc = JAXBContext.newInstance(Bork.class, Gnarf.class);
        Unmarshaller unmarshaller = jc.createUnmarshaller();

        StreamSource source = new StreamSource("src/forum12402215/input.xml");
        XMLInputFactory xif = XMLInputFactory.newFactory();
        XMLStreamReader xsr = xif.createXMLStreamReader(source);
        xsr.nextTag(); // Advance to "foo" element
        xsr.nextTag(); // Advance to "bar" element

        while(xsr.getLocalName().equals("bar")) {
             String className = xsr.getAttributeValue("", "class");
             Class clazz = classLoader.loadClass(className);
             Object object = unmarshaller.unmarshal(xsr, clazz).getValue();
             System.out.println(object);
             xsr.nextTag();
        }
    }

}

Bork

Below is a sample Bork class.

package forum12402215;

public class Bork {

    private String b;

    public String getB() {
        return b;
    }

    public void setB(String b) {
        this.b = b;
    }

    @Override
    public String toString() {
        return "Bork(b=" + b + ")";
    }

}

Gnarf

Below is a sample Gnarf class:

package forum12402215;

public class Gnarf {

    private int a;

    public int getA() {
        return a;
    }

    public void setA(int a) {
        this.a = a;
    }

    @Override
    public String toString() {
        return "Gnarf(a=" + a + ")";
    }

}

input.xml

Below is he sample document I used used for this example based on the one from your question. I changed the package names.

    
        123
    
    
        Hello World
    

Output

Below is the output from running the demo code.

Gnarf(a=123)
Bork(b=Hello World)