按照课程来写，一开始运行报 500，服务器出错，看控制台如如下输出：

Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: 

Could not write JSON: No serializer found for class com.imooc.dto.User and no properties discovered to create BeanSerializer 

(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS); 

nested exception is com.fasterxml.jackson.databind.JsonMappingException:

No serializer found for class com.imooc.dto.User and no properties discovered to create BeanSerializer

(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.ArrayList[0])

感觉应该是User类的内容都是null造成的，然后给User类的属性都赋好值（加了有参数的构造器），再放到list中就好用了，然后想再重现一下问题，把User类的属性值都设为空（就是使用无参构造器产生类），结果问题竟不能重现了，这是一种什么情况？