ssm java.lang.NullPointerException 在service.impl下面

java.lang.NullPointerException
com.lightning.service.impl.CategoryServiceImpl.listCategories(CategoryServiceImpl.java:18)
com.lightning.controller.CategoryController.listCategories(CategoryController.java:20)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:498)
org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:215)
org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:743)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:672)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:82)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:933)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:867)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:951)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:842)
javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:827)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

百度了一下，发现很多人都犯过这个错误。原因是service.impl.XXServiceImpl下面，忘记给mapper/dao 写@Autowired了。
自己写代码总是能踩到很多坑。所以光看不行，还要多写。