how to force schema compiled classes to extend specific class outside schema

Need help with following situation: Users can generate their own data structures which are stored as JAXB-ready XSD sources like below:

<xs:schema version="1.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="Group" type="Group"/>
  <xs:element name="Parameter" type="Parameter"/>

  <xs:complexType name="Group">
    <xs:sequence>
      <xs:element name="caption" type="xs:string" minOccurs="0"/>
      <xs:element name="parameters" type="Parameter" nillable="true" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="Parameter">
    <xs:sequence>
      <xs:element name="key" type="xs:string" minOccurs="0"/>
      <xs:element name="group" type="Group" minOccurs="0"/>
      <xs:element name="value" type="xs:string" minOccurs="0"/>
    </xs:sequence>
  </xs:complexType>
</xs:schema>

After new or modified schema appears it is automatically parsed by Schema compiler, java sources being generated, compiled and packaged into user jars:

  SchemaCompiler sc = XJC.createSchemaCompiler();
  // Input source for schema
  InputSource is = new InputSource(new StringInputStream(objectPackage.getObjectSchema()));
  // Parse
  sc.parseSchema(is);
  S2JJAXBModel model = sc.bind();
  // Generate source
  JCodeModel jCodeModel = model.generateCode(null, null);
  jCodeModel.build(packageSourceDirectory);
  // Compile and package 
  // ......

And everything was ok until it was decided that all user-generated classes must extend one specific known class, say UserRootObject:

package user.abc;
public class Group extends com.mycompany.xml.UserRootObject {
  //
}

and

package user.abc;
public class Parameter extends com.mycompany.xml.UserRootObject {
  //
}

Everything is on the fly, I can not force users to modify their schema files but I can transform them prior to code generation. Looks like I have two options to introduce that UserRootObject: somehow via JCodeModel or somehow transforming schema files before building Java sources.

Solution

I do not believe that there is an easy way to do this using JAXB itself. There are a number of customization options available that are not commonly known - read section 7 of JSR222 for details.

If you have some control over the input schemas, then you might want to consider using XSLT to transform the schema. I believe that this can be done by using a javax.xml.transform.dom.DOMResult instance as the target of the transformation and using the output as a DOM tree (e.g., calling getNode() on the result) as the input to parseSchema. A basic transformation would be to replace:

<xs:complexType name="foo">
  <!-- CONTENTS -->
</xs:complexType>

with something like:

<xs:complexType name="foo">
  <xs:complexContent>
    <xs:extension base="UserRootObject">
      <!-- CONTENTS -->
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

Of course this only works for the simple cases. If you already have inheritance in your schema files, then you will have to do some filtering in the XSLT that only applies this transform to types that do not already extend.