code: private synchronized RunningJobImpl get(final String jobIdentifier) {
      final RunningJobImpl result = this.jobs.get(jobIdentifier);
      if (null == result) {
        throw new RuntimeException("Trying to get a RunningJob that is unknown: " + jobIdentifier);
      }
      return result;
    }
docs: A guarded get() that throws an exception if the RunningJob isn't known.
  
  @param jobIdentifier
  @return