Is it possible for MapStruct to automatically detect mappings for Collections containing generic types as both source and target?

Question

I recently picked up MapStruct, and I really like the way it works. Since I'm new to MapStruct, I apologize in advance if this question is silly or makes little sense.

Due to server version, I'm bound to Java 6.

I have 3 types of entities, A, B, and C, as well as their DTO counterparts.

public class A{
    //...
}
public class B extends A{
    //...
}
public class C extends A{
    //...
}

public class ADto{
    //...
}
public class BDto extends ADto{
    //...
}
public class CDto extends ADto{
    //...
}

My mapper defines:

public abstract ADto mapAToADto(A source);

public abstract A mapADtoToA(ADto source);

public abstract BDto mapBToBDto(B source);

public abstract B mapBDtoToB(BDto source);

public abstract CDto mapCToCDto(C source);

public abstract C mapCDtoToC(CDto source);

Which works fine for now.

In my application, I work with List<A> and List<ADto> that contains both subtypes.

My current implementation was to implement my own mapping method that iterates over source list and checks types with instanceof , then calls matching mapping method listed above.

Current implementation:

public <T extends ADto, S extends A> List<T> toList(List<S> source) {
if (source == null) {
    return null;
}

List<T> list = new ArrayList<T>();
for (S entity : source) {
    if (entity instanceof B) {
        list.add((T) mapBToBDto((B) entity));
    } else if (entity instanceof C) {
        list.add((T) mapCToCDto((C) entity));
    } else {
        list.add((T) mapADtoToA((A) entity));
    }
}

return list;
};

I was wondering if there is a way to direct MapStruct to figure it out automatically. Am I missing something?

Edit: My optimistic approach was with:

@IterableMapping(nullValueMappingStrategy = NullValueMappingStrategy.RETURN_NULL)
public abstract <T extends ADto, S extends A> List<T> listAToADto(List<S> source);

Which results in:

Can't generate mapping method for a generic type variable target

Thank you

Answer 1

Your implementation is correct. MapStruct is an Annotation Processor that generates code during compilation. We don't generate any reflection checks (All the types that implement and interface or extend a class are not known during compilation). Therefore your approach to the problem is correct.