code: @Override
      public void setApplicationContext(ApplicationContext applicationContext) {
          if (context == null) {
              synchronized (ApplicationContextHolder.class) {
                  if (context == null) {
                      ApplicationContextHolder.context = applicationContext;
                  }
              }
          }
      }
docs: 实现 ApplicationContextAware 接口的回调方法，设置上下文环境
  
  @param applicationContext ApplicationContext