JAXB - Unmarshalling of abstract classes (InstantiationException)

Question

I know, that it might seem that this question is a duplicate of this one: InstantiationException during JAXB Unmarshalling (abstract base class, with @XmlSeeAlso concrete sub class)

However, it is slightly different: We are using the maven-jaxb2-plugin to generate our Java-classes from xsd-files. You can find them here.

In our pom we are using the following configuration (we are using the jaxb-api in the version 2.3.1):

  <plugin>
    <groupId>org.jvnet.jaxb2.maven2</groupId>
    <artifactId>maven-jaxb2-plugin</artifactId>
    <executions>
      <execution>
        <id>schema2-generate</id>
        <goals>
          <goal>generate</goal>
        </goals>
        <configuration>
          <args>
            <arg>-Xnamespace-prefix</arg>
          </args>
          <useActiveProxyAsHttpproxy>true</useActiveProxyAsHttpproxy>
          <proxyHost>your.host</proxyHost>
          <proxyPort>your.port</proxyPort>
          <strict>false</strict>
          <encoding>UTF-8</encoding>
          <catalog>src/main/resources/xsd/xbau/v22/catalog.cat</catalog>
          <schemaDirectory>src/main/resources/xsd/xbau/v22</schemaDirectory>
          <schemaIncludes>
            <include>*.xsd</include>
          </schemaIncludes>
          <bindingDirectory>src/main/resources/xsd/xbau/v22/xjb</bindingDirectory>
          <bindingIncludes>
            <bindingInclude>externalBindings.xjb</bindingInclude>
          </bindingIncludes>
          <generatePackage>our.generated.package.xbau.v22</generatePackage>
          <generateDirectory>${project.build.directory}/generated-sources/xbau/v22</generateDirectory>
        </configuration>
      </execution>
    </executions>
  </plugin>

JAXB is generating the following classes:

Nachrichtenkopf.G2G2 (subclass):

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Nachrichtenkopf.G2G")
public class NachrichtenkopfG2G2
    extends NachrichtenkopfG2G
{


}

Nachrichtenkopf.G2G (superclass):

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Nachrichtenkopf.G2G", namespace = "http://www.osci.de/xinneres/basisnachricht/4", propOrder = {
    "identifikationNachricht",
    "leser",
    "autor"
})
@XmlSeeAlso({
    NachrichtenkopfG2G2 .class
})
public abstract class NachrichtenkopfG2G {

    @XmlElement(name = "identifikation.nachricht", namespace = "", required = true)
    protected IdentifikationNachricht2 identifikationNachricht;
    @XmlElement(namespace = "", required = true)
    protected Behoerde leser;
    @XmlElement(namespace = "", required = true)
    protected BehoerdeErreichbar autor;

    ...

}

Now, if we receive a XML without an xsi-type (see below) for the "Nachrichtenkopf" we get an InstantiationException since the unmarshaller is not able to create a concrete instance because of the missing type information:

<?xml version="1.0" encoding="utf-8"?>
<xbau:beteiligung.aufforderung.0300 xmlns="http://www.xleitstelle.de/xbau/2/2/" produkt="Produkt" produkthersteller="Hersteller" produktversion="Version1" standard="XBau" version="2.2" xmlns:xbau="http://www.xleitstelle.de/xbau/2/2">
  <nachrichtenkopf xmlns=""> <!-- Missing Type information: xsi:type="xbau:Nachrichtenkopf.G2G" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"-->
  ...
  </nachrichtenkopf>
  ...

So basically, I have 2 questions:

1. Is there a way to tell the Unmarshaller how to unmarshal the message without the beforementioned type information?
2. Is the xsi-type-information mandatory? Or in other words: is the XML from above valid according to the schemes (xsds)?

For the sake of completeness, the content of the externalBindings.xjb

<?xml version="1.0" encoding="UTF-8"?>
<jaxb:bindings
    xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:annox="http://annox.dev.java.net"
    xmlns:namespace="http://jaxb2-commons.dev.java.net/namespace-prefix"
    jaxb:version="2.1">
    
    
    <!-- Bemerkung: Die Schemagenerierung hat hier dazu geführt, dass es in der Factory
    zwei gleichnamige Methoden gibt, daher werden die Methoden hier explizit benannt -->
    
    <jaxb:globalBindings fixedAttributeAsConstantProperty="true"/>

    
    <jaxb:bindings schemaLocation="http://www.w3.org/1999/xlink.xsd">
        
        <jaxb:bindings node="xs:group[@name='arcModel']">
            <jaxb:bindings node=".//xs:element[@ref='xlink:title']">
                <jaxb:property name="arcModelTitle"/>
            </jaxb:bindings>
        </jaxb:bindings>
        
        <jaxb:bindings node="xs:group[@name='locatorModel']">
            <jaxb:bindings node=".//xs:element[@ref='xlink:title']">
                <jaxb:property name="locatorModelTitle"/>
            </jaxb:bindings>
        </jaxb:bindings>
        
        <jaxb:bindings node="xs:element[@name='arc']">
            <jaxb:factoryMethod name="arc42"/>
        </jaxb:bindings>
        
    </jaxb:bindings>
    
    <jaxb:bindings schemaLocation="../xbau-baukasten.xsd">

        <jaxb:bindings node="xs:complexType[@name='Nachrichtenkopf.G2G']">
            <jaxb:class name="NachrichtenkopfG2G2"/>
        </jaxb:bindings>
        
        <jaxb:bindings>
            <namespace:prefix name="xbau" />
        </jaxb:bindings>
                
    </jaxb:bindings>
    
    <jaxb:bindings schemaLocation="http://www.osci.de/xinneres/basisnachricht/4/xinneres-basisnachricht.xsd">

        <jaxb:bindings node="xs:complexType[@name='Nachricht.G2G']">
            <jaxb:class name="NachrichtG2G2"/>
        </jaxb:bindings>
                
    </jaxb:bindings>
    
    <jaxb:bindings schemaLocation="http://www.osci.de/xinneres/basisnachricht/4/xinneres-basisnachricht.xsd">

        <jaxb:bindings node="xs:complexType[@name='Identifikation.Nachricht']">
            <jaxb:class name="IdentifikationNachricht2"/>
        </jaxb:bindings>
                
    </jaxb:bindings>
    
    <jaxb:bindings schemaLocation="http://www.osci.de/xinneres/kommunikation/3/xinneres-kommunikation.xsd">

        <jaxb:bindings node="xs:complexType[@name='Code.Erreichbarkeit']">
            <jaxb:class name="CodeErreichbarkeit2"/>
        </jaxb:bindings>
                
    </jaxb:bindings>

    <jaxb:bindings schemaLocation="http://www.osci.de/xinneres/kommunikation/3/xinneres-kommunikation.xsd">

        <jaxb:bindings node="xs:complexType[@name='Kommunikation']">
            <jaxb:class name="Kommunikation2"/>
        </jaxb:bindings>
                
    </jaxb:bindings>

    <jaxb:bindings schemaLocation="http://www.w3.org/1999/xlink.xsd">

        <jaxb:bindings node="xs:complexType[@name='arcType']">
            <jaxb:class name="ArcType2"/>
        </jaxb:bindings>
                
    </jaxb:bindings>
    
</jaxb:bindings>

Answer 1

Solved the issue by using a catalog-file. The catalog.cat-file (have a look at the first code-block in my original question) contains an entry, to advise the schema parser to look for the scheme that causes the problem in my local file-system rather than looking it up online. You can have a look at this post, if you are interested in rewriting online resources to local resources in the context of xsd-parsing (jaxb - how to map xsd files to URL to find them).

This way I can change the scheme which now resides in my src/main/resources folder. The scheme in question is http://www.osci.de/xinneres/basisnachricht/4/xinneres-basisnachricht.xsd. I changed name="Nachrichtenkopf.G2G" abstract="true" to name="Nachrichtenkopf.G2G" abstract="false" so that the class Nachrichtenkopf.G2G is no longer abstract and therefore, can be instantiated.