Java: Mapping DTOs hierarchy using MapStruct - mapstruct

Question

I have Pet, Dog and Cat entity classes. Dog and Cat classes extend Pet.
Also I have PetDTO, DogDTO and CatDTO annotated with #JsonSubtype so Jackson resolves well the class of the dtos.
I want to write a mapper using MapStruct that takes a PetDTO entity (can be a DogDTO or a CatDTO) and returns a Dog or a Cat.
For me in this case, the main goal of using a mapping library is to avoid awful code using instanceof.
Any idea? Thanks!

Answer 1

Not currently possible out-of-the-box - see this ticket in mapstruct's GitHub: #366 Support for abstract class mapping or classes with base class. You can try to push it there or maybe contribute this feature yourself. Looks like a reasonable feature to ask for.
I guess that with the current state of affairs this is your best option:
#Mapper
public interface PetMapper {
default PetDTO toPetDto(Pet pet) {
if (pet instanceof Dog) {
return toDogDTO((Dog) pet);
}
if (pet instanceof Cat) {
return toCatDTO((Cat) pet);
}
throw new IllegalArgumentException("Unknown subtype of Pet");
}
default Pet toPetEntity(PetDTO petDTO) {
if (petDTO instanceof DogDTO) {
return toDogEntity((DogDTO) petDTO);
}
if (petDTO instanceof CatDTO) {
return toCatEntity((CatDTO) petDTO);
}
throw new IllegalArgumentException("Unknown subtype of PetDTO");
}
DogDTO toDogDTO(Dog dog);
Dog toDogEntity(DogDTO dogDTO);
CatDTO toCatDTO(Cat cat);
Cat toCatEntity(CatDTO catDTO);
}

Answer 2

The way I ended up implementing a Mapper for a similar case as above was using a combination of a switch-type, with MapStruct Update Existing and creation Mappers.
In my case a property on the source object dictated the subclass we had to generate.
I initially had different mappers for each subtype, but the duplication of the common mapped properties just seemed wrong. So I came up with the following, leveraging the ability of MapStruct to use updating mappers in order to tackle the common parent type properties:
import org.mapstruct.*;
#Mapper
#Named("QualifierPetMapper")
public interface PetMapper {
#Named("DelegatingPetMapper")
#BeanMapping(ignoreByDefault = true)
default PetTarget mapPet(PetSource petSource) {
switch (petSource.getPetType()) {
case "DOG":
DogTarget dogTarget = mapDog(petSource);
updatePet(dogTarget, petSource);
return (dogTarget);
case "CAT":
CatTarget catTarget = mapCat(petSource);
updatePet(catTarget, petSource);
return (catTarget);
default:
throw new CustomException("Unsupported Pet type: "+ petSource.getPetType());
}
}
#BeanMapping(ignoreByDefault = true)
// Specific mappings for Dog
#Mapping(target = "dogfood.name", source = "dogfoodName")
DogTarget mapDog(PetSource petSource);
#BeanMapping(ignoreByDefault = true)
// Specific mappings for Cat
#Mapping(target = "fish.name", source = "favoriteFish")
CatTarget mapCat(PetSource petSource);
#Named("RootPetMapper")
#BeanMapping(ignoreByDefault = true)
// Common properties for Pet
#Mapping(target = "weight.value", source = "weightValue")
#Mapping(target = "name.value", source = "petName")
#Mapping(target = "color", source = "mainColor")
void updatePet(#MappingTarget PetTarget petTarget, PetSource petSource);
}