class TypeConverterDelegate {
  private final PropertyEditorRegistrySupport propertyEditorRegistry;
  @Nullable
  private final Object targetObject;
  public <T> T convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue, @Nullable Object newValue,
      @Nullable Class<T> requiredType, @Nullable TypeDescriptor typeDescriptor) throws IllegalArgumentException {
    // 查看是否为当前这个类型配置了定制的PropertyEditor
    PropertyEditor editor = this.propertyEditorRegistry.findCustomEditor(requiredType, propertyName);
    ConversionFailedException conversionAttemptEx = null;
    // 获取当前容器中的类型转换业务类
    ConversionService conversionService = this.propertyEditorRegistry.getConversionService();
        // 在这里可以看出，Spring底层在进行类型转换时有两套机制
        // 1.首选的是采用PropertyEditor
        // 2.在没有配置PropertyEditor的情况下，会采用conversionService
    if (editor == null && conversionService != null && newValue != null && typeDescriptor != null) {
      TypeDescriptor sourceTypeDesc = TypeDescriptor.forObject(newValue);
      if (conversionService.canConvert(sourceTypeDesc, typeDescriptor)) {
        try {
                    // 通过conversionService进行类型转换
          return (T) conversionService.convert(newValue, sourceTypeDesc, typeDescriptor);
        }
        catch (ConversionFailedException ex) {
          // fallback to default conversion logic below
          conversionAttemptEx = ex;
        }
      }
    }
    Object convertedValue = newValue;
    // 配置了定制的属性编辑器，采用PropertyEditor进行属性转换
    if (editor != null || (requiredType != null && !ClassUtils.isAssignableValue(requiredType, convertedValue))) {
      if (typeDescriptor != null && requiredType != null && Collection.class.isAssignableFrom(requiredType) &&
          convertedValue instanceof String) {
        TypeDescriptor elementTypeDesc = typeDescriptor.getElementTypeDescriptor();
        if (elementTypeDesc != null) {
          Class<?> elementType = elementTypeDesc.getType();
          if (Class.class == elementType || Enum.class.isAssignableFrom(elementType)) {
            convertedValue = StringUtils.commaDelimitedListToStringArray((String) convertedValue);
          }
        }
      }
      if (editor == null) {
                // 没有配置定制的属性编辑器，采用默认的属性编辑器
        editor = findDefaultEditor(requiredType);
      }
            // 采用属性编辑器进行转换，需要注意的是，默认情况下PropertyEditor只会对String类型的值进行类型转换
      convertedValue = doConvertValue(oldValue, convertedValue, requiredType, editor);
    }
        // .....
    return (T) convertedValue;
  }
}
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@FunctionalInterface
public interface Converter<S, T> {
  // 从 S 转换到 T 
   @Nullable
   T convert(S source);
}
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