code: public B group(EventLoopGroup group) {
          if (group == null) {
              throw new NullPointerException("group");
          }
          if (this.group != null) {
              throw new IllegalStateException("group set already");
          }
          this.group = group;
          return self();
      }
docs: The {@link EventLoopGroup} which is used to handle all the events for the to-be-created
  {@link Channel}