code: def _get_jar(self, command, alts=None, allow_missing=False):
          
          dirs = []
          for bdir in [self._gatk_dir, self._picard_ref]:
              dirs.extend([bdir,
                           os.path.join(bdir, os.pardir, "gatk")])
          if alts is None: alts = []
          for check_cmd in [command] + alts:
              for dir_check in dirs:
                  try:
                      check_file = config_utils.get_jar(check_cmd, dir_check)
                      return check_file
                  except ValueError as msg:
                      if str(msg).find("multiple") > 0:
                          raise
                      else:
                          pass
          if allow_missing:
              return None
          else:
              raise ValueError("Could not find jar %s in %s:%s" % (command, self._picard_ref, self._gatk_dir))
docs: Retrieve the jar for running the specified command.