code: public ColumnMapper getMapper(String field) {
          String[] components = field.split("\\.");
          for (int i = components.length - 1; i >= 0; i--) {
              StringBuilder sb = new StringBuilder();
              for (int j = 0; j <= i; j++) {
                  sb.append(components[j]);
                  if (j < i) sb.append('.');
              }
              ColumnMapper columnMapper = columnMappers.get(sb.toString());
              if (columnMapper != null) return columnMapper;
          }
          return null;
      }
docs: Returns the {@link ColumnMapper} identified by the specified field name, or {@code null} if not found.
  
  @param field A field name.
  @return The {@link ColumnMapper} identified by the specified field name, or {@code null} if not found.