转自：https://www.jianshu.com/p/60ff6d0dae7f

一.原因

• 这两天在写代码的时候遇到一个问题,为什么使用SpringBoot的时候,拦截器中使用@Autowired注入bean会报空指针.如下面代码所示,我们知道,Spring管理的bean发现有这个注解时候，它会直接注入相应的另一个Spring管理的bean.当 Spring 容器启动时，AutowiredAnnotationBeanPostProcessor (继承InstantiationAwareBeanPostProcessorAdapter)将扫描 Spring 容器中所有 Bean，当发现 Bean 中拥有@Autowired 注解时就找到和其匹配（默认按类型匹配）的 Bean，并注入到对应的地方中去。那为什么这里的注解没有生效呢?
• 网上传说:了解SpringBoot的都知道SpringBoot的目录格式有着明确的规定,它减轻编程人员负担的同时,更加要求了编程的规范化,SpringBoot初始化的时候,会加载com.boot.app下的bean,一层一层加载,当注册LoggerInterceptor的时候,发现LoggerInterceptor中有@Autowired注解,就会去另外一个spring管理器中索取另外一个LoggerJpa,而这时候LoggerJpa根本没有初始化.所以就无法注入LoggerJpa的bean类完成相应的操作.
• 自我理解:注册拦截器时直接通过new LoggerInterceptor(),并没有触发Spring去管理bean,所以@Autowired没有生效.

•  /**
   * ========================
   *
   * @author 小龙虾
   * Created with IntelliJ IDEA.
   * Date：2018/1/22
   * Time：20:02
   * ========================
   */
  public class LoggerInterceptor implements HandlerInterceptor {
      public static final String SEND_TIME = "send_time";
      public static final String DATA = "param_data";
      //问题:无法注入loggerDao
      @Autowired
      private LoggerJpa loggerDao;
      /**
      * 进入springMVC的controller之前开始记录日志实体
       * @param httpServletRequest request
       * @param httpServletResponse response
       * @param o 实体类
       * @return  boolean
       * @throws Exception
       */
      @Override
      public boolean preHandle(HttpServletRequest httpServletRequest, HttpServletResponse         httpServletResponse, Object o) throws Exception {
          //创建日记实体类
          LoggerEntity entity = new LoggerEntity();
          //获得sessionId
          String sessionId = httpServletRequest.getRequestedSessionId();
          //请求地址信息
          String requestURI = httpServletRequest.getRequestURI();
          //请求参数信息(利用fastJson转换参数)
          String params = JSON.toJSONString(httpServletRequest.getParameterMap(),
              SerializerFeature.DisableCircularReferenceDetect,
              SerializerFeature.WriteMapNullValue);
          //设置客户端ip
          entity.setClientIp(LoggerUtil.getCliectIp(httpServletRequest));
          //设置请求方法
          entity.setMethod(httpServletRequest.getMethod());
          //设置请求类型
          entity.setType(LoggerUtil.getRequestType(httpServletRequest));
          //设置请求参数
          entity.setParamData(params);
          //设置请求地址
          entity.setUri(requestURI);
          //设置sessionId
          entity.setSessionId(sessionId);
          //设置请求开始时间
          httpServletRequest.setAttribute(SEND_TIME, System.currentTimeMillis());
          //设置请求实体到request内,方便afterCompletion调用
          httpServletRequest.setAttribute(DATA, entity);
          return true;
        }
      @Override
      public void postHandle(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Object o, ModelAndView modelAndView) throws Exception {
      }
  
      @Override
      public void afterCompletion(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Object o, Exception e) throws Exception {
          //请求状态
          int status = httpServletResponse.getStatus();
          //当前时间
          long time = System.currentTimeMillis();
          //上次请求时间
          Long requestTime = Long.valueOf(httpServletRequest.getAttribute(SEND_TIME).toString());
          //获取请求日记的实体
          LoggerEntity entity = (LoggerEntity) httpServletRequest.getAttribute(DATA);
          //设置时间差
          entity.setConsuming(Long.valueOf(time-requestTime).toString());
          //设置错误码
          entity.setStatusCode(status+"");
          //设置返回值
          entity.setReturnData(JSON.toJSONString(httpServletRequest.getAttribute(LoggerUtil.LOGGER_RETURN),
              SerializerFeature.DisableCircularReferenceDetect,
              SerializerFeature.WriteMapNullValue));
          //通过WebApplicationContextUtils获取loggerDao
         //LoggerJpa loggerDao = getDao(LoggerJpa.class,httpServletRequest);
          //将日记写入数据库
          loggerDao.save(entity);
      }
  }
  

二.解决方法

• 了解到原因之后,那么如何解决呢?在这里,有两种常见的解决方法.

①.利用WebApplicationContextUtils去获取WebApplicationContext,然后在通过WebApplicationContext去获取相应的bean.

• public <T> T getDao(Class<T> clazz,HttpServletRequest request){
      //通过该方法获得的applicationContext 已经是初始化之后的applicationContext 容器
      WebApplicationContext applicationContext = WebApplicationContextUtils.getRequiredWebApplicationContext(request.getServletContext());
      return applicationContext.getBean(clazz);
  }
  

②.在初始化LoggerInterceptor 之前就初始化LoggerJpa

• /**
   * ========================
   *
   * @author 小龙虾
   * Created with IntelliJ IDEA.
   * Date：2018/1/23
   * Time：11:51
   * ========================
   */
  @Configuration
  public class LoggerConfiguration extends WebMvcConfigurerAdapter {
  
      @Bean
      public LoggerInterceptor loggerInterceptor(){
          System.out.println("嘻嘻嘻嘻");
          return new LoggerInterceptor();
      }
      /**
       * LoggerInterceptor,形成拦截链
       * @param registry 拦截器注册类
       */
      @Override
      public void addInterceptors(InterceptorRegistry registry){
          //在放入拦截器之前调用loggerInterceptor(),触发LocalContainerEntityManagerFactoryBean使得拦截器的在注册之前所有的bean都持久化
          registry.addInterceptor(loggerInterceptor()).addPathPatterns("/**");
          System.out.println("呵呵呵呵");
      }
  }
  

③.构造器

• public class LoggerInterceptor implements HandlerInterceptor {
      private static final String SEND_TIME = "send_time";
      private static final String DATA = "param_data";
      private LoggerJpa loggerDao;
      public LoggerInterceptor(LoggerJpa loggerDao){
          this.loggerDao = loggerDao;
      }
  //拦截器三个方法略...
  }
  

• /**
   * ========================
   *
   * @author 小龙虾
   * Created with IntelliJ IDEA.
   * Date：2018/1/23
   * Time：11:51
   * ========================
   */
  @Configuration
  public class LoggerConfiguration extends WebMvcConfigurerAdapter {
  
      //@Bean
      //public LoggerInterceptor loggerInterceptor(){
      //    System.out.println("嘻嘻嘻嘻")
      //    return new LoggerInterceptor(loggerDao);
      //}
      @Autowired
      private LoggerJpa loggerDao;
      /**
       * LoggerInterceptor,形成拦截链
       * @param registry 拦截器注册类
       */
      @Override
      public void addInterceptors(InterceptorRegistry registry){
          //利用构造方法注入
         registry.addInterceptor(new LoggerInterceptor(loggerDao)).addPathPatterns("/**");
          System.out.println("呵呵呵呵");
      }
  }