Config.java

@Configuration
public class Config {
  @Bean
  @Autowired
  public RouterFunction<ServerResponse> routerFunction(final CalculatorHandler calculatorHandler) {
    return RouterFunctions.route(RequestPredicates.path("/calculator"), request ->
        request.queryParam("operator").map(operator ->
            Mono.justOrEmpty(
                ReflectionUtils.findMethod(CalculatorHandler.class, operator, ServerRequest.class))
                .flatMap(method -> (Mono<ServerResponse>) ReflectionUtils
                    .invokeMethod(method, calculatorHandler, request))
                .switchIfEmpty(ServerResponse.badRequest().build())
                .onErrorResume(
                    ex -> ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build()))
            .orElse(ServerResponse.badRequest().build()));
  }
}