title: MapStruct: How to map to existing target?
body: <p>I have a method in a service that updates an entity. It accepts object that has the data to update the entity. Dto object has less fields than entity but the fields have the same names.</p>
  <p>Could it be possible to use mapstruct for that routine job by passing an existing target object?</p>
  <pre><code>class Entity {
    id
    name
    date
    country
    by
    ... //hell of the fields
  }
  class UpdateEntity {
    name
    country
    ... //less but still a lot
  }
  
  class EntityService {
    update(UpdateEntity u) {
      Entity e = // get from storage
      mapstructMapper.mapFromTo(u, e)
    }
  }
  </code></pre>