spring - 如何自定义@FeignClient Expander 来转换参数？

Question

Feign 默认扩展器转换参数:

final class ToStringExpander implements Expander {

    @Override
    public String expand(Object value) {
      return value.toString();
    }
  }

我想自定义它以将用户转换为支持 GET 参数，就像这样

@FeignClient("xx")
interface UserService{

   @RequestMapping(value="/users",method=GET)
   public List<User> findBy(@ModelAttribute User user);
}

userService.findBy(user);

我能做什么？

Answer 1

首先，你必须写一个像 ToJsonExpander 这样的扩展器:

public class ToJsonExpander implements Param.Expander {

  private static ObjectMapper objectMapper = new ObjectMapper();

  public String expand(Object value) {
    try {
      return objectMapper.writeValueAsString(value);
    } catch (JsonProcessingException e) {
      throw new ExpanderException(e);
    }
  }
}

其次，编写一个像 JsonArgumentParameterProcessor 这样的 AnnotatedParameterProcessor 来为你的处理器添加扩展器。

public class JsonArgumentParameterProcessor implements AnnotatedParameterProcessor {

  private static final Class<JsonArgument> ANNOTATION = JsonArgument.class;

  public Class<? extends Annotation> getAnnotationType() {
    return ANNOTATION;
  }

  public boolean processArgument(AnnotatedParameterContext context, Annotation annotation) {
    MethodMetadata data = context.getMethodMetadata();
    String name = ANNOTATION.cast(annotation).value();
    String method = data.template().method();

    Util.checkState(Util.emptyToNull(name) != null,
        "JsonArgument.value() was empty on parameter %s", context.getParameterIndex());

    context.setParameterName(name);

    if (method != null && (HttpMethod.POST.matches(method) || HttpMethod.PUT.matches(method) || HttpMethod.DELETE.matches(method))) {
      data.formParams().add(name);
    } else {
      `data.indexToExpanderClass().put(context.getParameterIndex(), ToJsonExpander.class);`
      Collection<String> query = context.setTemplateParameter(name, data.template().queries().get(name));
      data.template().query(name, query);
    }

    return true;
  }
}

第三，将其添加到 Feign 配置中。

  @Bean
  public Contract feignContract(){
    List<AnnotatedParameterProcessor> processors = new ArrayList<>();
    processors.add(new JsonArgumentParameterProcessor());
    processors.add(new PathVariableParameterProcessor());
    processors.add(new RequestHeaderParameterProcessor());
    processors.add(new RequestParamParameterProcessor());
    return new SpringMvcContract(processors);
  }

现在，您可以使用 @JsonArgument 发送模型参数，例如:

public void saveV10(@JsonArgument("session") Session session);