The Documents class, the class we use to store a collection of Document instances, can not be marshalled and unmarshalled with JAXB and JSON over the KIE-Server API (XStream does work). There seem to be multiple problems. The main issue is that Documents contains a List<Document>, where Document is an interface.

JAXB does not like interfaces, and hence won't marshall and/or unmarshall such an object.

The problem with JSON is that the JSON Marshaller won't work unless we register Documents and Document as extra remoteable classes. However, KIE-Server does not allow us to register interfaces as Remoteable classes, so this breaks as well.

What seems to be the solution is to:

• Add JAXB annotations to the List<Document> attribute of the Documents class. To be specific the annotations:

  @XmlElementWrapper
  @XmlAnyElement(lax=true)
  private List<Document> documents = new ArrayList<>();
  

• Add the Documents class automatically to the "extraClasses" of the KieContainerInstance.
• Add the Document interface only to the extra class of the JSONMarshaller (as adding them to the JAXBContext will break JAXB. The idea is to do this via a KIE-Server JSONMarshaller extension.