使用泛型实现对象转换器设计模式

Question

我正在尝试为我的 dto 实体类创建一个通用对象转换器。我创建了一个抽象类，其中包含两个代表两种转换的函数，然后在我的具体转换器类中对其进行了扩展。

但是我想要一个通用的转换服务，我可以在启动期间注册我所有的转换器，然后方便地调用一个方法来处理彼此之间的转换。

这是我迄今为止提出的：

转换器抽象类

public abstract class Converter<D, E> {

  private final Function<D, E> fromFirstToSecond;
  private final Function<E, D> fromSecondToFirst;

  public Converter(final Function<D, E> fromFirstToSecond, final Function<E, D> fromSecondToFirst) {
    this.fromFirstToSecond = fromFirstToSecond;
    this.fromSecondToFirst = fromSecondToFirst;
  }

  public final E convertFromFirstToSecond(final D first) {
    return fromFirstToSecond.apply(first);
  }

  public final D convertFromSecondToFirst(final E second) {
    return fromSecondToFirst.apply(second);
  }

}

转换器具体类

public class MyConverter extends Converter<MyDTO, MyEntity> {

    public OrderConverter() {
        super(
            MyConverter::fromDTOToEntity,
            MyConverter::fromEntityToDTO
        );
    }

    private static MyEntity fromDTOToEntity(MyDTO dto) {
        return MyEntity.builder()
            .field1(dto.getField1())
            .field2(dto.getField2())
            .build();
    }

    private static MyDTO fromEntityToDTO(MyEntity entity) {
        return MyDTO.builder()
            .field1(entity.getField1())
            .field2(entity.getField2())
            .build();
    }

}

我想实现这样的目标（这是我需要帮助的地方）：

@Configuration
public class ConverterConfiguration {

  @Bean
  public ConverterService converterService() {
    ConverterService converterService = new ConverterService();
    converterService.registerConverter(new MyConverter());
    // Register any other converter...
    return converterService;
  }

}

服务看起来像这样：

public class ConverterService {

  private Map<Key, Converter<?, ?>> converters = new ConcurrentReferenceHashMap<>();

  public void registerConverter(Converter<?, ?> converter) {
    this.converterCache.put(key, converter);
  }

  protected <I, O> I mapBetweenTypes(final O from, final Class<I> clazz) {
    // Based on the parameters I should be able to figure out which function to get from the map and then call (convertFromFirstToSecond or convertFromSecondToFirst)
    // return this.converters.get(key).convertFromFirstToSecond(from);
    return this.converters.get(key).convertFromSecondToFirst(from);
  }

}

Answer 1

解决该问题的一种方法是在Converter 类中显式存储属于转换一部分的类型。我会考虑的另一个简化是将转换函数直接存储在地图中，而不是转换器。这样你就不必弄清楚你需要走哪条路。

第一步，存储类型，并为转换函数添加getter：

public abstract class Converter<D, E> {

  private final Function<D, E> fromFirstToSecond;
  private final Function<E, D> fromSecondToFirst;
  private final Class<D> classOfFirst;
  private final Class<E> classOfSecond;

  public Class<D> getClassOfFirst() {
      return classOfFirst;
  }

  public Class<E> getClassOfSecond() {
      return classOfSecond;
  }

  public Converter(Class<D> first, Class<E> second, 
      Function<D, E> fromFirstToSecond, Function<E, D> fromSecondToFirst) {
    this.fromFirstToSecond = fromFirstToSecond;
    this.fromSecondToFirst = fromSecondToFirst;
    classOfFirst = first;
    classOfSecond = second;
  }

  public Function<D, E> getForward() {
      return fromFirstToSecond;
  }

  public Function<E, D> getBackward() {
      return fromSecondToFirst;
  }

那么ConverterService可以存储两个方向的转换函数，转换相当于在两级映射中查找。

public class ConverterService {

    private Map<Class<?>, Map<Class<?>, Function<?,?>>> converters = createMap();

    public void registerConverter(Converter<?, ?> converter) {
        if( !converters.containsKey(converter.getClassOfFirst())) {
            converters.put(converter.getClassOfFirst(), createInnerMap());
        }
        if( !converters.containsKey(converter.getClassOfSecond())) {
            converters.put(converter.getClassOfSecond(), createInnerMap());
        }
        converters.get(converter.getClassOfFirst()).put(
            converter.getClassOfSecond(), converter.getForward());
        converters.get(converter.getClassOfSecond()).put(
            converter.getClassOfFirst(), converter.getBackward());
    }

    @SuppressWarnings("unchecked") // Also needs input validation
    protected <D,E> E mapBetweenTypes(D from, E to) {
        return ((Function<D,E>) converters.get(
            from.getClass()).get(to.getClass())).apply(from);
    }

    private static Map<Class<?>, Map<Class<?>, Function<?,?>>> createMap() { 
        return /* Whatever impl you need */ 
    }

    private static Map<Class<?>, Function<?,?>> createInnerMap() { 
        return /* Whatever impl you need */ 
    }
}