code: static JobContext copy() {
          JobContext current = current_.get();
          //JobContext ctxt = new JobContext(keepParent ? current : null);
          JobContext ctxt = new JobContext(null);
          if (null != current) {
              ctxt.bag_.putAll(current.bag_);
          }
          return ctxt;
      }
docs: Make a copy of JobContext of current thread
  @return the copy of current job context or an empty job context