Disable non-null check option on @AfterMapping with @MappingTarget

[WalronGabor]

Use case

I use Map class to create XML messages, and I need a check if the created XML element is only an empty element. If empty then return null will do the trick.
This all thing is needed when I map a multi level XML tag, like::
@Mapping(target = "account.equationShortAccountNumber.branchCode", source = "branchCode")
Now the MapStruct only could check at the lowest level if the Map contains the branchCode element, and it will crate empty XML tags (now the Account and EquationShortAccountNumber tag).

Now there could be a good WorkAround with https://mapstruct.org/documentation/dev/reference/html/#customizing-mappings-with-before-and-after Example 110. Mapper with @AfterMapping hook that returns a non-null value if I could turn off the non-null check. I created a common @AfterMapping method which could return null if it's just an empty tag, but because the null check I couldn't use it.

Generated Code that would be great

@Mapper
public abstract class VehicleMapper {

    @PersistenceContext
    private EntityManager entityManager;

    @AfterMapping(nullCheck = false) // Default true
    protected <T> T attachEntity(@MappingTarget T entity) {
        return entityManager.merge(entity);
    }

    public abstract CarDto toCarDto(Car car);
}

// Generates something like this:
public class VehicleMapperImpl extends VehicleMapper {

    public CarDto toCarDto(Car car) {
        if ( car == null ) {
            return null;
        }

        CarDto carDto = new CarDto();
        // attributes mapping ...

        carDto = attachEntity( carDto );

        return carDto ;
    }
}

Possible workarounds at the moment

I created a very complicate method with reflections to handle this, but it slows the interfaces. So I started to create manually a lot of AfterMapping method which are check if the XML tag's children are empty tags if yes the set that tag to null. Like:

        @AfterMapping
        default void removeEmptyTags(@MappingTarget AccountRqType result) {
            if (new EquationShortAccountNumberTypeV2().equals(result.getEquationShortAccountNumber())) {
                result.setEquationShortAccountNumber(null);
            }
        }

MapStruct Version

1.5.5.Final

[filiphr]

@WalronGabor wouldn't something like in #2610 solve your problems? Basically you can check if the source object has something that should be mapped or not, and it it doesn't we just return null.

So the VehicleMapper will look something like

@Mapper
public abstract class VehicleMapper {

    public abstract CarDto toCarDto(Car car);

    @SourceCondition
    protected boolean isPresent(Car car) {
        return car != null; // You can do other more complex checks here as well
    }
}

The generated code then looks like:

// Generates something like this:
public class VehicleMapperImpl extends VehicleMapper {

    public CarDto toCarDto(Car car) {
        if ( !isPresent( car ) ) {
            return null;
        }

        CarDto carDto = new CarDto();
        // attributes mapping ...

        return carDto ;
    }
}