如题，在spring拦截器中注入了一个service，结果运行时报了空指针：
2020-01-03 09:26:33.646 |-DEBUG [http-nio-9988-exec-3] org.springframework.web.servlet.DispatcherServlet [1100] -| Failed to complete request: java.lang.NullPointerException 2020-01-03 09:26:33.648 |-ERROR [http-nio-9988-exec-3] org.apache.catalina.core.ContainerBase.[Tomcat].[localhost].[/].[dispatcherServlet] [175] -| 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 at com.wlf.order.prize.aop.RequestInterceptor.postHandle(RequestInterceptor.java:118) at org.springframework.web.servlet.HandlerExecutionChain.applyPostHandle(HandlerExecutionChain.java:156) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1046) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005) at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:908) at javax.servlet.http.HttpServlet.service(HttpServlet.java:660) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882) at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:118) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:853) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1587) at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:745) 2020-01-03 09:26:33.657 |-DEBUG [http-nio-9988-exec-3] org.springframework.web.servlet.DispatcherServlet [91] -| "INCLUDE" dispatch for POST "/error", parameters={} 2020-01-03 09:26:33.661 |-DEBUG [http-nio-9988-exec-3] org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping [420] -| Mapped to public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest) 2020-01-03 09:26:33.662 |-DEBUG [http-nio-9988-exec-3] org.springframework.orm.jpa.support.OpenEntityManagerInViewInterceptor [86] -| Opening JPA EntityManager in OpenEntityManagerInViewInterceptor 2020-01-03 09:26:33.664 |-DEBUG [http-nio-9988-exec-3] org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor [223] -| Found 'Content-Type:application/json;charset=UTF-8' in response 2020-01-03 09:26:33.669 |-DEBUG [http-nio-9988-exec-3] org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor [91] -| Writing [{timestamp=Fri Jan 03 09:26:33 CST 2020, status=200, error=OK, message=No message available, path=/j (truncated)...] 2020-01-03 09:26:33.685 |-DEBUG [http-nio-9988-exec-3] org.springframework.orm.jpa.support.OpenEntityManagerInViewInterceptor [111] -| Closing JPA EntityManager in OpenEntityManagerInViewInterceptor
按日志提示，到RequestInterceptor.java的118行看一下：
发现我们通过@Autowired注入进来的mailService并未实例化：
而mailService本身是被spring容器管理的
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看起来都在spring容器的管控范围内，为啥没有实例化呢？还得看看拦截器是怎么实例化的
很明显，这里对拦截器的实例化并不在spring容器的管控之内，它是new出来的。由此可以推断，我们只需要把拦截器的实例化让spring容器来做即可，也就是放开上面注掉的代码
再次调用接口，拦截器的实例化交给了spring容器，容器自会管理起mailService的注入，再无空指针异常。