We have a class Beans used to serialize a list of generic objects into an xml. This is done like this:

public class Call
{
  public Beans input;
  public Beans output;
  ...
}

@XmlJavaTypeAdapter(value = BeanAdapter.class)
public class Beans
{
  public List<Object> bean;
}

Thanks to @XmlJavaTypeAdapter, we're able to write xml in whatever form we want.

When we're serializing a Call instance:

Call call = ...
Beans beans = ...;

call.setInput(beans);

JAXBContext context = ...;
Marshaller marshaler = context.createMarshaller();
ObjectFactory factory = ...;

marshaler.marshal(factory.createCall(call), result);

things work as expected, meaning that BeanAdapter is used during xml serialization. But if it's happened that you want to serialize a Beans instance itself, you start getting problems with the serialization of unknown objects. That's because JAXB does not use BeanAdapter.

We have found a similar case "How to assign an adapter to the root element?", unfortunately with no satisfactory explanation.

That is strange.