'producer' type is unknown to ReactiveAdapterRegistry (WebFlux handler)

在使用WebFlux做练习的时候，提示'producer' type is unknown to ReactiveAdapterRegistry

//创建Router路由

public RouterFunction<ServerResponse> routerFunction(){

    UserService userService = new UserServiceImpl();
    UserHandler userHandler = new UserHandler(userService);
    //设置路由
    return RouterFunctions.route(
            GET("/users/{id}").and(accept(MediaType.APPLICATION_JSON)), userHandler::getUserById)
            .andRoute(GET("/users").and(accept(MediaType.APPLICATION_JSON)), userHandler::getAllUser)
            .andRoute(POST("/saveusers").and(accept(MediaType.APPLICATION_JSON)),userHandler::addUser);

}

调用getUserById()方法时提示这个错误，该方法如下

public Mono<ServerResponse> getUserById(ServerRequest request){
    Integer id = Integer.valueOf(request.pathVariable("id"));
    //空值处理
    Mono<ServerResponse> responseMono = ServerResponse.notFound().build();
    Mono<User> userMono = userService.getUserById(id);
    //将userMono进行数据处理
    //使用Reactor操作符flatMap
    return userMono.flatMap(user ->
        ServerResponse.ok().contentType(MediaType.APPLICATION_JSON)
                .body(user,User.class)).switchIfEmpty(responseMono);

}

在一篇文章中看到，需要将返回的body方法改为bodyValue即可，如下所示

return userMono.flatMap(user ->
    ServerResponse.ok().contentType(MediaType.APPLICATION_JSON)
            .bodyValue(user)).switchIfEmpty(responseMono);

修改后再访问即可正常访问