code: void connectionCloses(final Session connection) {
          synchronized (connections) {
              final ExecutorService executorService = connectionThreads.get(connection);
              if (executorService != null) {
                  executorService.shutdownNow();
              }
              connectionThreads.remove(connection);
              connections.remove(connection);
          }
      }
docs: Informs this {@link CommandTransferServer} that a client connection got closed.
  
  @param connection The connection that was closed