CollectionMappingStrategy with option not to clean target collection prior to adding all elements

[typekpb]

Use case

Having map in a mapping target, I want to have option to keep it's existing elements, just call putAll() on addiing new ones (or replacing existing with the same key)

class Test {
Map<String,String> customFields;
}

@Mapper
TestMapper {

  @Mapping(target = "customFields", source = "customFields)
  Test update(Test source, @MappingTarget Test target);
}

Generated Code

if ( target.getCustomFields() != null ) {
            Map<String, String> map = source.getCustomFields();
            if ( map != null ) {
                // the following line would not be present
                // target.getCustomFields().clear();
                target.getCustomFields().putAll( map );
            }
            else {
                target.setCustomFields( new java.util.HashMap<>() );
            }
        }

Possible workarounds

custom mapping implementation needed

MapStruct Version

1.5.5.Final

[filiphr]

@typekpb have you perhaps tried using an adder and CollectionMappingStrategy#ADDER_PREFERRED? I am not sure if we are clearing the collection in this case.

[aprevost]

@typekpb have you perhaps tried using an adder and CollectionMappingStrategy#ADDER_PREFERRED? I am not sure if we are clearing the collection in this case.

I also have a need for this feature... or at least, something similar. In my case, it's a bit simpler, where both the source and the target have a property that is a List, and I want to just add any values in the source list proprety to any pre-existing values in the target list property.

I don't own the target model so can't easily add an "add" method to it as you suggest. We just really need a way to modify the current generated code:

        if ( target.getListProperty() != null ) {
            target.getListProperty.clear();
            List<String> list = source.getListProperty();
            if ( list != null ) {
                target.getListProperty().addAll( list );
            }
        }

so that, it doesn't do the clear first:

       if ( target.getListProperty() != null ) {
            List<String> list = source.getListProperty();
            if ( list != null ) {
                target.getListProperty().addAll( list );
            }
        }

AND also handles the case where the target list is null but the sourceList is not null:

       else { //target.getListProperty() == null
            List<String> list = source.getListProperty();
            if ( list != null ) {
                target.setListProperty(new ArrayList<>(list));
            }
        }