code: public void disconnect() throws DBException {
          if (this.session == null) {
              throw new DBException("The Cassandra is null");
          }
          if (this.cluster.isClosed()) {
              throw new DBException("The cluster has been closed");
          }
          this.session.close();
          this.cluster.close();
      }
docs: Disconnect from Cassandra host.
  
  @throws DBException