code: public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
          super.group(parentGroup);
          if (childGroup == null) {
              throw new NullPointerException("childGroup");
          }
          if (this.childGroup != null) {
              throw new IllegalStateException("childGroup set already");
          }
          this.childGroup = childGroup;
          return this;
      }
docs: Set the {@link EventLoopGroup} for the parent (acceptor) and the child (client). These
  {@link EventLoopGroup}'s are used to handle all the events and IO for {@link ServerChannel} and
  {@link Channel}'s.