[FIXED] Map target property with no source property, while avoiding constant and expression

Issue

I have a personal war going against writing java code in strings. What I'm trying to achieve is 100% doable with an expression. I would, however, like to try and find a way of doing it without.

Situation:

TargetJpa {
    ...
    Instant createdAt;
}

What I want to do:

// using 1.5.2.Final
@Mapper(...)
public interface JpaMapper {

    @Mapping(target = "createdAt", qualifiedByName = "now")
    TargetJpa toJpa(Srouce s);

    @Named("now")
    default Instant now() {
        return Instant.now();
    }

}

This obviously does not work. It complains that I'm missing a source. Which is correct, I don't need a source, I'm generating a value. But this can't be a constant. And I personally wish to use java code in strings as little as humanly possible.

Ofc, there is an obvious workaround, pass a dummy source value.

@Mapper(...)
public interface JpaMapper {

    @Mapping(target = "createdAt", source = "s", qualifiedByName = "now")
    TargetJpa toJpa(Srouce s);

    @Named("now")
    default Instant now(Source s) {
        return Instant.now();
    }

}

But this has a huge disatvantage. Had I not have to name a source, I could remove this "now" from this interface alltogeather, make a "InstantMapper" and reuse the code:

@InstantMapper
public class InstantMapperImpl {

    @Now
    default Instant now() {
        return Instant.now();
    }

}

Then use it on anything:

... uses = { ..., InstantMapperImpl.class }

    ...

    @Mapping(target = "createdAt", qualifiedBy = {InstantMapper.class, Now.class})
    TargetJpa1 toJpa(Srouce1 s);

    @Mapping(target = "createdAt", qualifiedBy = {InstantMapper.class, Now.class})
    TargetJpa2 toJpa(Srouce2 s);

    @Mapping(target = "createdAt", qualifiedBy = {InstantMapper.class, Now.class})
    TargetJpa3 toJpa(Srouce3 s);

Any way to achieve this without tying myself to some source that I simply don't need?

Solution

MapStruct currently doesn't support using a random method to map into a specific property. When using Mapping#target you have few options that you can use for the source of the mapping:

• source - map from a specific source
• expression - map using an expression
• constant - map using some constant value
• ignore - ignore the mapping

All the rest of the properties are in one way or another used for specifying some other things for the mapping.

When none of those 4 options are provided it is assumed that the value that is in target is the value in source.

e.g.

@Mapping(target = "createdAt", qualifiedByName = "now")

actually means

@Mapping(target = "createdAt", source = "createdAt", qualifiedByName = "now")

If you are interested in seeing something like you are requesting in MapStruct I would suggest raising a feature request

Answered By - Filip
Answer Checked By - Candace Johnson (JavaFixing Volunteer)