Mapstruct - How can I inject a spring dependency in the Generated Mapper class

As commented by brettanomyces, the service won't be injected if it is not used in mapping operations other than expressions.

The only way I found to this is :

• Transform my mapper interface into an abstract class
• Inject the service in the abstract class
• Make it protected so the "implementation" of the abstract class has access

I'm using CDI but it should be the samel with Spring :

@Mapper(
        unmappedTargetPolicy = org.mapstruct.ReportingPolicy.IGNORE,
        componentModel = "spring",
        uses = {
            // My other mappers...
        })
public abstract class MyMapper {

    @Autowired
    protected MyService myService;

    @Mappings({
        @Mapping(target="x", expression="java(myservice.findById(obj.getId())))")
    })
    public abstract Dto myMappingMethod(Object obj);

}

What's worth to add in addition to the answers above is that there is more clean way to use spring service in mapstruct mapper, that fits more into "separation of concerns" design concept, called "qualifier". Easy re-usability in other mappers as a bonus. For sake of simplicity I prefer named qualifier as noted here http://mapstruct.org/documentation/stable/reference/html/#selection-based-on-qualifiers Example would be:

import org.mapstruct.Mapper;
import org.mapstruct.Named;
import org.springframework.stereotype.Component;

@Component
@Mapper
public class EventTimeQualifier {

    private EventTimeFactory eventTimeFactory; // ---> this is the service you want yo use

    public EventTimeQualifier(EventTimeFactory eventTimeFactory) {
        this.eventTimeFactory = eventTimeFactory;
    }

    @Named("stringToEventTime")
    public EventTime stringToEventTime(String time) {
        return eventTimeFactory.fromString(time);
    }

}

This is how you use it in your mapper:

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;

@Mapper(componentModel = "spring", uses = EventTimeQualifier.class)
public interface EventMapper {

    @Mapping(source = "checkpointTime", target = "eventTime", qualifiedByName = "stringToEventTime")
    Event map(EventDTO eventDTO);

}

It should be possible if you declare Spring as the component model and add a reference to the type of myservice:

@Mapper(componentModel="spring", uses=MyService.class)
public interface MyMapper { ... }

That mechanism is meant for providing access to other mapping methods to be called by generated code, but you should be able to use them in the expression that way, too. Just make sure you use the correct name of the generated field with the service reference.

Since 1.2 this can be solved with a combination of @AfterMapping and @Context.. Like this:

@Mapper(componentModel="spring")
public interface MyMapper { 

   @Mapping(target="x",ignore = true)
   // other mappings
   Target map( Source source, @Context MyService service);

   @AfterMapping
   default void map( @MappingTarget Target.X target, Source.ID source, @Context MyService service) {
        target.set( service.findById( source.getId() ) );
   }
 }

The service can be passed as context.

A nicer solution would be to use an @Context class which wrap MyService in stead of passing MyService directly. An @AfterMapping method can be implemented on this "context" class: void map( @MappingTarget Target.X target, Source.ID source ) keeping the mapping logic clear of lookup logic. Checkout this example in the MapStruct example repository.