title: mapstruct using mapper inside another return null in test
body: <p>I m using mapstruct to generate mappers. when I see in the generated code it is autowired the <code>Mymapper</code> in <code>MainMapper</code> but  when debug it is  showing that myMapper is null. When I change in the generated code  </p>
  
  <pre><code>@Autowired MyMapper myMapper
  </code></pre>
  
  <p>with : </p>
  
  <pre><code>MyMapper myMapper=Mappers.getMaper(MyMapper.class)
  </code></pre>
  
  <p>, it is working but this is not what I want. I want it to be auto-generated like expected from the framwork.
  nb : <code>MainMapper</code> uses <code>MyMapper</code> , and <code>MyMapper</code> uses <code>Mapper2</code></p>
  
  <p>mainMapper:</p>
  
  <pre><code>@Mapper(componentModel = "spring", 
      unmappedTargetPolicy = ReportingPolicy.IGNORE, 
      uses = MyMapper.class ,
      nullValueMappingStrategy = NullValueMappingStrategy.RETURN_NULL)
  public interface MainMapper {
  
      @Mappings({ @Mapping(target = "year", source = "Period.year") })
      Resource toResource(MyObject myObject);
  
      @Mapping(target = "Period.year", source = "year")
      MyObject toModel(Resource resource);
  }
  </code></pre>
  
  <p>MyMapper :</p>
  
  <pre><code>@Mapper(componentModel = "spring", 
      unmappedTargetPolicy = ReportingPolicy.IGNORE,
      uses = Mapper2.class,
      nullValueMappingStrategy = NullValueMappingStrategy.RETURN_NULL)
  public interface MyMapper {
  
      RequestResource toResource(Request request);
  
      Request toModel(RequestResource requestResource);
  }
  </code></pre>
  
  <p>any help is appreciated .</p>