How to prevent creation of empty objects

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?

Just tested with 1.2.0.Final and I get Parent having non null Children object with a null children field (Parent.children is non-null, Parent.children.children is null). What would you expect? Parent.children to be null? — jannis
Yes, I'd like exactly that. Because as of now when I map Parent to Parent2 and back, I get different object than what I've started with. — NeplatnyUdaj
hi.. Checkout github.com/mapstruct/mapstruct/issues/1166. Is this what you mean? — Sjaak
Yes, that's exactly it. It looks like it won't be supported in the near future :( — NeplatnyUdaj
It's on my to-do list. But it's a tricky one. I like to have it myself as well. Just need to find time to figure out a decent solution — Sjaak

jannis jannis · Accepted Answer · 2018-12-10T10:21:41

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);
        }
    }
}

How to prevent creation of empty objects

1 Answers

Decorator

AfterMapping