在mvc层经常会遇到这样的一种情况，是否使用DTO（数据传输对象），还是直接使用model返回？其实这都可以，前者叫封闭领域模型风格，后者叫开放领域模型风格。因为前者可以排除一些不需要返回的字段但是需要进行拷贝，后者就不需要经过处理。另外，都会遭遇到一种情况：如性别在后台是通过0和1，但是需要返回前端男或者女。这种情况可以使用数据字典做一个良好的处理，这个这里不谈。
MapStruct可以很好的处理封闭领域模型风格的model到DTO的拷贝。
https://github.com/mapstruct/mapstruct

public class CycleAvoidingMappingContext {
    private Map<Object, Object> knownInstances = new IdentityHashMap<Object, Object>();

    @BeforeMapping
    public <T> T getMappedInstance(Object source, @TargetType Class<T> targetType) {
        return (T) knownInstances.get( source );
    }

    @BeforeMapping
    public void storeMappedInstance(Object source, @MappingTarget Object target) {
        knownInstances.put( source, target );
    }

public class Car {
 
    private String make;
    private int numberOfSeats;
    private CarType type;
     private List<Car> team;
 
    //constructor, getters, setters etc.
}

public class CarDto {
 
    private String make;
    private int seatCount;
    private String type;
    private List<CarDto> team;
 
    //constructor, getters, setters etc.
}

@Mapper 
public interface CarMapper {
 
    CarMapper INSTANCE = Mappers.getMapper( CarMapper.class ); 
  @Mappings( {
        @Mapping(source = "numberOfSeats", target = "seatCount")
    } )
    
    CarDto carToCarDto(Car car, @Context CycleAvoidingMappingContext context); 
   @InheritInverseConfiguration
    Car fromCar(CarDto carDto, @Context CycleAvoidingMappingContext context);
}

@Test
public void shouldMapCarToDto() {
    //given
    Car car = new Car( "Morris", 5, CarType.SEDAN );
 
    //when
    CarDto carDto = CarMapper.INSTANCE.carToCarDto( car );
 
    //then
    assertThat( carDto ).isNotNull();
    assertThat( carDto.getMake() ).isEqualTo( "Morris" );
    assertThat( carDto.getSeatCount() ).isEqualTo( 5 );
    assertThat( carDto.getType() ).isEqualTo( "SEDAN" );
}