code: public static boolean init(JobHistoryObserver jobTracker, JobConf conf,
               String hostname, long jobTrackerStartTime){
      try {
        LOG_DIR = conf.get("hadoop.job.history.location" ,
          "file:///" + new File(
          System.getProperty("hadoop.log.dir", "/tmp")).getAbsolutePath()
          + File.separator + "history");
        JOBTRACKER_UNIQUE_STRING = hostname + "_" + 
                                      String.valueOf(jobTrackerStartTime) + "_";
        jobtrackerHostname = hostname;
        Path logDir = new Path(LOG_DIR);
        LOGDIR_FS = logDir.getFileSystem(conf);
        if (!LOGDIR_FS.exists(logDir)){
          if (!LOGDIR_FS.mkdirs(logDir, new FsPermission(HISTORY_DIR_PERMISSION))) {
            throw new IOException("Mkdirs failed to create " + logDir.toString());
          }
        }
        conf.set("hadoop.job.history.location", LOG_DIR);
        disableHistory = false;
        // set the job history block size (default is 3MB)
        jobHistoryBlockSize = 
          conf.getLong("mapred.jobtracker.job.history.block.size", 
                       3 * 1024 * 1024);
        jtConf = conf;
  
        // initialize the file manager
        fileManager = new JobHistoryFilesManager(conf, jobTracker, logDir);
      } catch(IOException e) {
          LOG.error("Failed to initialize JobHistory log file", e); 
          disableHistory = true;
      }
      fileManager.startIOExecutor();
      return !(disableHistory);
    }
docs: Initialize JobHistory files.
  @param conf Jobconf of the job tracker.
  @param hostname jobtracker's hostname
  @param jobTrackerStartTime jobtracker's start time
  @return true if intialized properly
  false otherwise