1.添加有参构造,忘记添加无参构造
## 错误提示
(no Creators, like default construct, exist ): cannot deserialize from Object value (no delegate- or property-based Creator)
2.忘记注入@Autowired
## 错误提示
Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException: null