code: public int registerDirectory(String dir, boolean scanSubdirectories) {
          int count = 0;
          try {
              File file = new File(dir);
              if (!file.exists() || !file.isDirectory())
                  return 0;
              String files[] = file.list();
              if (files == null)
                  return 0;
              for (int k = 0; k < files.length; ++k) {
                  try {
                      file = new File(dir, files[k]);
                      if (file.isDirectory()) {
                          if (scanSubdirectories) {
                              count += registerDirectory(file.getAbsolutePath(), true);
                          }
                      } else {
                          String name = file.getPath();
                          String suffix = name.length() < 4 ? null : name.substring(name.length() - 4).toLowerCase();
                          if (".afm".equals(suffix) || ".pfm".equals(suffix)) {
                              /* Only register Type 1 fonts with matching .pfb files */
                              File pfb = new File(name.substring(0, name.length() - 4) + ".pfb");
                              if (pfb.exists()) {
                                  register(name, null);
                                  ++count;
                              }
                          } else if (".ttf".equals(suffix) || ".otf".equals(suffix) || ".ttc".equals(suffix)) {
                              register(name, null);
                              ++count;
                          }
                      }
                  }
                  catch (Exception e) {
                      //empty on purpose
                  }
              }
          }
          catch (Exception e) {
              //empty on purpose
          }
          return count;
      }
docs: Register all the fonts in a directory and possibly its subdirectories.
  @param dir the directory
  @param scanSubdirectories recursively scan subdirectories if <code>true</true>
  @return the number of fonts registered
  @since 2.1.2