code: @Override
      public void close() throws IOException, IllegalStateException {
          if (this.open.compareAndSet(true, false)) {
              try {
                  this.eventGroup.shutdownGracefully(0L, 5L, TimeUnit.SECONDS).sync();
              } catch (Exception e) {
                  throw new IOException("Could not be gracefully closed.", e);
              }
          } else {
              throw new IllegalStateException("Network Manager is already closed");
          }
      }
docs: {@inheritDoc}
  
  @throws IllegalStateException If manager is already closed.