public static SyntheticFileId fromJobConf(JobConf job) {
    String idAsString = job.get(JOBCONF_KEY);
    if (idAsString == null) {
      return null;
    }
    return new SyntheticFileId(idAsString);
  }
Length allows for some backward compatibility wrt field addition.
ql/src/java/org/apache/hadoop/hive/ql/io/SyntheticFileId.java