It seems JAXB can't read what it writes. Consider the following code:
interface IFoo {
    void jump();
}
@XmlRootElement
class Bar implements IFoo {
    @XmlElement
    public String y;
    public Bar() {
        y = "";
    }
    public Bar(String y) {
        this.y = y;
    }
    @Override
    public void jump() {
        System.out.println(y);
    }
}
@XmlRootElement
class Baz implements IFoo {
    @XmlElement
    public int x;
    public Baz() {
        x = 0;
    }
    public Baz(int x) {
        this.x = x;
    }
    @Override
    public void jump() {
        System.out.println(x);
    }
}
@XmlRootElement
public class Holder {
    private List<IFoo> things;
    public Holder() {
        things = new ArrayList<>();
    }
    @XmlElementWrapper
    @XmlAnyElement
    public List<IFoo> getThings() {
        return things;
    }
    public void addThing(IFoo thing) {
        things.add(thing);
    }
}
// ...
try {
    JAXBContext context = JAXBContext.newInstance(Holder.class, Bar.class, Baz.class);
    Holder holder = new Holder();
    holder.addThing(new Bar("1"));
    holder.addThing(new Baz(2));
    holder.addThing(new Baz(3));
    for (IFoo thing : holder.getThings()) {
        thing.jump();
    }
    StringWriter s = new StringWriter();
    context.createMarshaller().marshal(holder, s);
    String data = s.toString();
    System.out.println(data);
    StringReader t = new StringReader(data);
    Holder holder2 = (Holder)context.createUnmarshaller().unmarshal(t);
    for (IFoo thing : holder2.getThings()) {
        thing.jump();
    }
}
catch (Exception e) {
    System.err.println(e.getMessage());
}
It's a simplified example, of course. The point is that I have to store two very differently implemented classes, Bar and Baz, in one collection. Well, I observed that they have pretty similar public interface, so I created an interface IFoo and made them two to implement it. Now, I want to have tools to save and load this collection to/from XML. Unfortunately, this code doesn't quite work: the collection is saved, but then it cannot be loaded! The intended output is 
1
2
3
some xml
1
2
3
But unfortunately, the actual output is
1
2
3
some xml
com.sun.org.apache.xerces.internal.dom.ElementNSImpl cannot be cast to testapplication1.IFoo
Apparently, I need to use the annotations in a different way? Or to give up on JAXB and look for something else? I, well, can write "XMLNode toXML()" method for all classes I wan't to (de)marshal, but...