How to prevent creation of empty objects - mapstruct

Question

I'm trying to map a model of a webservice where every List is inside nested object to something more simple.
Model 1
public class Parent {
private Children children;
}
public class Children {
private List<Child> children;
}
public class Child {
}
Model 2(simplified)
public class Parent2 {
private List<Child2> children;
}
public class Child {
}
The mapping is pretty straightforward:
#Mappings({#Mapping(source = "entity.children.children", target = "children")})
Parent2 parentToParent2(Parent entity);
#InheritInverseConfiguration
Parent parent2ToParent(Parent2 entity);
Mapping works fine except for one problem. When I map Parent with null children to Parent2 and back to Parent, the Children object is created with empty list. Is there some way to prevent that?

Answer 1

You can achieve that with both a mapper decorator or an AfterMapping hook.
Decorator
Decorator:
public abstract class MapperDecorator implements Mapper {
private final Mapper delegate;
public MapperDecorator(Mapper delegate) {
this.delegate = delegate;
}
#Override
public Parent parent2ToParent(Parent2 entity) {
Parent parent = delegate.parent2ToParent(entity);
if (entity.getChildren() == null) {
parent.setChildren(null);
}
return parent;
}
}
Mapper:
#org.mapstruct.Mapper
#DecoratedWith(MapperDecorator.class)
public interface Mapper {
#Mapping(source = "entity.children.children", target = "children")
Parent2 parentToParent2(Parent entity);
#InheritInverseConfiguration
Parent parent2ToParent(Parent2 entity);
Child2 childToChild2(Child entity);
Child child2ToChild(Child2 entity);
}
AfterMapping
Mapper:
#org.mapstruct.Mapper
public abstract class Mapper {
#Mapping(source = "entity.children.children", target = "children")
public abstract Parent2 parentToParent2(Parent entity);
#InheritInverseConfiguration
public abstract Parent parent2ToParent(Parent2 entity);
public abstract Child2 childToChild2(Child entity);
public abstract Child child2ToChild(Child2 entity);
#AfterMapping
public void afterParent2ToParent(Parent2 source,
#MappingTarget Parent target) {
if (source.getChildren() == null) {
target.setChildren(null);
}
}
}