项目中如何优雅的消除if-else

消除if-else的方法有很多种，本文结合spring中ArgumentResolver的应用，提供一种优雅地方式。

先看ArgumentResolver的应用方式

public class UserArgumentResolver implements HandlerMethodArgumentResolver {
  private static final String TOKEN_KEY = "token";

  @Override
  public boolean supportsParameter(MethodParameter parameter) {
      return parameter.hasParameterAnnotation(LoginUser.class);
  }

  @Override
  public Object resolveArgument(MethodParameter parameter,
                                ModelAndViewContainer mavContainer,
                                NativeWebRequest webRequest,
                                WebDataBinderFactory binderFactory) throws Exception {
      // 通过session取userid
      String token = webRequest.getHeader(TOKEN_KEY);
      User user = null;
      // 从redis中获取user json
      // user  = JSON.parse(redis.get(token),User.class);

      return user;
  }
}

controller 需要注入参数的地方

@RequestMapping(value = "/doSth", method = RequestMethod.POST)
public String feed(@LoginUser User user) {
  System.out.println("userId: " + JSON.toJsonString(user));
  return "success";
}

声明注解LoginUser

@Target(ElementType.TYPE,ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RestController {

 String value() default "";

}

配置类添加

@Configuration
public class WebConfiguration implements WebMvcConfigurer {

  @Override
  public void addArgumentResolvers (List<HandlerMethodArgumentResolver> resolvers) {
    resolvers.add(new UserArgumentResolver());
  }
}

有多个ArgumentResolver在WebConfiguration中配置即可。

if-else消除

常用代码 如交通方式有 bike, bus,subway,train,plan

if (bike) {
    
} else if (bus) {
    
} else if (subway) {
    
} else if (train) {
    
} else if (plan) {
    
}else {
    
}

编写不同的reslover

定义统一的接口

public interface TrafficResolver {

    boolean support(String type);

    Object resolver(Object argument);
}

定义实现类BusTrafficResolver，其他省略

@Compoment
public class BusTrafficResolver implements TrafficResolver{

  @Override
  public boolean support (String type) {
    return "BUS".equals(type);
  }

  @Override
  public Object resolver (Object argument) {
    // doSth
    return "bus resolver";
  }
}

真正消除if-else的服务类

List 若没有bean则需要required 比提供默认值new ArrayList<>() ,有bean 可以去掉。

@Service
public class TrafficService {

  // required = false
  @Autowired(required = false)
  List<TrafficResolver> trafficResolverList = new ArrayList<>();

  public Object traffic(String type,Object argument){
    System.out.printf("trafficResolverList "+trafficResolverList.size());
    TrafficResolver resolver = getFirstResolver(type);
    if(resolver!=null){
      return resolver.resolver(argument);
    }

    return "traffic not find. type: "+type;
  }

  private TrafficResolver getFirstResolver(String type){
    TrafficResolver resolver = trafficResolverList.stream()
        .filter(item -> item.support(type))
        .findFirst()
        .orElse(null);
    return resolver;
  }

}

通过此种方式完成了对if-else调用类的解耦，实现了高扩展性。遵循了面向修改关闭，面向扩展开放的原则。

消除if-else完成，当然可以用注解的方式，通过注解类型判断是否支持。

其实if-else挺好用，除非考虑扩展性很强的地方，否则不要过度设计。