*
 *   public MyServer(...) {
 *     ...
 *     this.serverSocket = new ServerSocket(...);
 *     this.cleanable = cleaner.register(this, closeServerSocketRunnable(serverSocket));
 *     ...
 *   }
 *
 *   private static Runnable closeServerSocketRunnable(ServerSocket serverSocket) {
 *     return () -> {
 *       if (!serverSocket.isClosed()) {
 *         ...log a message about how nobody called close()...