HAPI FHIR: how to read extension from custom datatype of standard resource? - hapi-fhir

Question

I would like to add custom extension to Patient->telecom class by extending HAPI's ContactPoint class and adding new extension . So this is standard boilerplate code I took from https://hapifhir.io/hapi-fhir/docs/model/custom_structures.html
#DatatypeDef(name="MyContactPoint")
public class MyContactPoint extends ContactPoint implements ICompositeType {
private static final long serialVersionUID = 1L;
#Override
protected MyContactPoint typedCopy() {
MyContactPoint retVal = new MyContactPoint();
super.copyValues(retVal);
return retVal;
}
#Child(name="my-ext-value")
#Extension(definedLocally = false, isModifier = false, url = "http://someurl")
private BooleanType valueMyExtValue;
public Boolean getMyExtValue() {
return valueMyExtValue.getValue();
}
public MyContactPoint setMyExtValue(Boolean theValue) {
this.valueMyExtValue = new BooleanType(theValue);
return this;
}
}
And it works fine when I generate XML:
Patient data = new Patient();
MyContactPoint telecom = new MyContactPoint();
telecom.setMyExtValue(true);
data.addTelecom(telecom);
However, I cannot wrap my head how I can LOAD this data from XML to get strongly-typed MyContactPoint object? I know I can do telecom.getExtensionByUrl() etc and finally load it - but I was hoping there is a way to outsource this legwork to HAPI-FHIR by declaring extensions.
I know I can do it easily in "parent" profile like this:
#ResourceDef()
public class MyTask extends Task {
#Child(name="my-ext-value")
#Extension(definedLocally = false, isModifier = false, url = "http://someurl")
private BooleanType valueMyExtValue;
public Boolean getMyExtValue() {
return valueMyExtValue.getValue();
}
public MyTask setMyExtValue(Boolean theValue) {
this.valueMyExtValue = new BooleanType(theValue);
return this;
}
}
and then I load it as follows:
var fhirContext = FhirContext.forR4Cached();
var fhirParser = fhirContext.newXmlParser();
fhirParser.setPreferTypes(List.of(MyTask.class));
var res = fhirParser.parseResource(xml);
var myTask = (MyTask)res;
var theValue = myTask.getMyExtValue();
So that was easy for IBaseResource->custom extension flow. But how do I do this for IBaseResource->Custom DataType (overriding existing field!)->custom extension?