@Bean
public HttpMessageConverters fastJsonHttpMessageConverters() {
//1、先定义一个convert转换消息的对象
FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();
//2、添加fastjson的配置信息,比如是否要格式化返回的json数据;
FastJsonConfig fastJsonConfig = new FastJsonConfig();
fastJsonConfig.setSerializerFeatures(SerializerFeature.PrettyFormat);
//附加:处理中文乱码
List<MediaType> fastMedisTypes = new ArrayList<>();
fastMedisTypes.add(MediaType.APPLICATION_JSON_UTF8);
fastConverter.setSupportedMediaTypes(fastMedisTypes);
//3、在convert中添加配置信息
fastConverter.setFastJsonConfig(fastJsonConfig);
HttpMessageConverter<?> converter = fastConverter;
return new HttpMessageConverters(converter);
}

Json和String你通过肉眼能看出来?
也许只是IDE或控制台自动的把JSON对象给友好显示了而已