protected void implCloseSelectableChannel() throws IOException {
    synchronized (stateLock) {
      if (state != ST_KILLED) nd.preClose(fd);
      ResourceManager.afterUdpClose();

      // if member of mulitcast group then invalidate all keys
      if (registry != null) registry.invalidateAll();

      long th;
      if ((th = readerThread) != 0) NativeThread.signal(th);
      if ((th = writerThread) != 0) NativeThread.signal(th);
      if (!isRegistered()) kill();
    }
  }
  // package-private
  void drop(MembershipKeyImpl key) {
    assert key.channel() == this;

    synchronized (stateLock) {
      if (!key.isValid()) return;

      try {
        if (key instanceof MembershipKeyImpl.Type6) {
          MembershipKeyImpl.Type6 key6 = (MembershipKeyImpl.Type6) key;
          Net.drop6(fd, key6.groupAddress(), key6.index(), key6.source());
        } else {
          MembershipKeyImpl.Type4 key4 = (MembershipKeyImpl.Type4) key;
          Net.drop4(fd, key4.groupAddress(), key4.interfaceAddress(), key4.source());
        }
      } catch (IOException ioe) {
        // should not happen
        throw new AssertionError(ioe);
      }

      key.invalidate();
      registry.remove(key);
    }
  }
  /** Joins channel's socket to the given group/interface and optional source address. */
  private MembershipKey innerJoin(InetAddress group, NetworkInterface interf, InetAddress source)
      throws IOException {
    if (!group.isMulticastAddress())
      throw new IllegalArgumentException("Group not a multicast address");

    // check multicast address is compatible with this socket
    if (group instanceof Inet4Address) {
      if (family == StandardProtocolFamily.INET6 && !Net.canIPv6SocketJoinIPv4Group())
        throw new IllegalArgumentException("IPv6 socket cannot join IPv4 multicast group");
    } else if (group instanceof Inet6Address) {
      if (family != StandardProtocolFamily.INET6)
        throw new IllegalArgumentException("Only IPv6 sockets can join IPv6 multicast group");
    } else {
      throw new IllegalArgumentException("Address type not supported");
    }

    // check source address
    if (source != null) {
      if (source.isAnyLocalAddress())
        throw new IllegalArgumentException("Source address is a wildcard address");
      if (source.isMulticastAddress())
        throw new IllegalArgumentException("Source address is multicast address");
      if (source.getClass() != group.getClass())
        throw new IllegalArgumentException("Source address is different type to group");
    }

    SecurityManager sm = System.getSecurityManager();
    if (sm != null) sm.checkMulticast(group);

    synchronized (stateLock) {
      if (!isOpen()) throw new ClosedChannelException();

      // check the registry to see if we are already a member of the group
      if (registry == null) {
        registry = new MembershipRegistry();
      } else {
        // return existing membership key
        MembershipKey key = registry.checkMembership(group, interf, source);
        if (key != null) return key;
      }

      MembershipKeyImpl key;
      if ((family == StandardProtocolFamily.INET6)
          && ((group instanceof Inet6Address) || Net.canJoin6WithIPv4Group())) {
        int index = interf.getIndex();
        if (index == -1) throw new IOException("Network interface cannot be identified");

        // need multicast and source address as byte arrays
        byte[] groupAddress = Net.inet6AsByteArray(group);
        byte[] sourceAddress = (source == null) ? null : Net.inet6AsByteArray(source);

        // join the group
        int n = Net.join6(fd, groupAddress, index, sourceAddress);
        if (n == IOStatus.UNAVAILABLE) throw new UnsupportedOperationException();

        key =
            new MembershipKeyImpl.Type6(
                this, group, interf, source, groupAddress, index, sourceAddress);

      } else {
        // need IPv4 address to identify interface
        Inet4Address target = Net.anyInet4Address(interf);
        if (target == null) throw new IOException("Network interface not configured for IPv4");

        int groupAddress = Net.inet4AsInt(group);
        int targetAddress = Net.inet4AsInt(target);
        int sourceAddress = (source == null) ? 0 : Net.inet4AsInt(source);

        // join the group
        int n = Net.join4(fd, groupAddress, targetAddress, sourceAddress);
        if (n == IOStatus.UNAVAILABLE) throw new UnsupportedOperationException();

        key =
            new MembershipKeyImpl.Type4(
                this, group, interf, source, groupAddress, targetAddress, sourceAddress);
      }

      registry.add(key);
      return key;
    }
  }