code: @SuppressWarnings("unchecked")
      public Bootstrap resolver(AddressResolverGroup<?> resolver) {
          this.resolver = (AddressResolverGroup<SocketAddress>) (resolver == null ? DEFAULT_RESOLVER : resolver);
          return this;
      }
docs: Sets the {@link NameResolver} which will resolve the address of the unresolved named address.
  
  @param resolver the {@link NameResolver} for this {@code Bootstrap}; may be {@code null}, in which case a default
  resolver will be used
  
  @see io.netty.resolver.DefaultAddressResolverGroup