@Data
class ParentDto {
    ChildDto child;
}

@Data
class ChildDto {
    ParentDto parent;
}

@Data
class ParentEntity {
    ChildEntity child;
}

public interface ParentRelation<T> {
    void setChildToParent(T parent);
}

@Data
class ChildEntity implements ParentRelation<ParentEntity> {
    ParentEntity childToParent;
}

class JpaContext<T> {
    T parentEntity;

    @BeforeMapping
    void setEntity(@MappingTarget T parentEntity) {
        this.parentEntity = parentEntity;
    }

    @AfterMapping
    void establishRelation(@MappingTarget ParentRelation<T> childEntity) {
        childEntity.setChildToParent(parentEntity);
    }
}

interface BaseMapper<T, E> {
    default E toEntity(T s) {
        return toEntity(s, new JpaContext<>());
    }

    E toEntity(T s, @Context JpaContext<E> ctx);
}

@Mapper
interface SourceTargetMapper extends BaseMapper<ParentDto, ParentEntity> {
}

@Generated(
    value = "org.mapstruct.ap.MappingProcessor"
)
class SourceTargetMapperImpl implements SourceTargetMapper {

    @Override
    public ParentEntity toEntity(ParentDto s, Ctx ctx) {
        if ( s == null ) {
            return null;
        }

        ParentEntity parentEntity = new ParentEntity();

        ctx.setEntity( parentEntity );

        parentEntity.setChild( childDtoToChildEntity( s.getChild(), ctx ) );

        return parentEntity;
    }

    protected ChildEntity childDtoToChildEntity(ChildDto childDto, Ctx ctx) {
        if ( childDto == null ) {
            return null;
        }

        ChildEntity childEntity = new ChildEntity();

        ctx.establishRelation( childEntity );

        return childEntity;
    }
}

interface BaseMapper<T, E, C extends JpaContext<E>> {
    default E toEntity(T s) {
        return toEntity(s, getCtx());
    }

    E toEntity(T s, @Context C ctx);

    C getCtx();
}

@Mapper
interface SourceTargetMapper extends BaseMapper<ParentDto, ParentEntity, SourceTargetMapper.Ctx> {
    default Ctx getCtx() {
        return new Ctx();
    }

    class Ctx extends JpaContext<ParentEntity> {
    }
}