code: public synchronized void open() throws IOException {
          if (tmap != null) {
              return;
          }
          final RecordManager rm = createRecordManager(indexPath);
          if (valueSerializer == null) {
              tmap = rm.treeMap(IndexerConstants.INDEX_TREE_KEY);
          } else {
              tmap = rm.treeMap(IndexerConstants.INDEX_TREE_KEY, valueSerializer);
          }
          if (tmap.isEmpty()) {
              rm.close();
              throw new IOException("tree map is empty");
          }
          invTmap = tmap.inverseHashView(IndexerConstants.INVERSE_KEY);
      }
docs: Opens the index, for subsequent lookups.
  
  @throws IOException Thrown if an I/O related exception occurs while
  creating or opening the record manager