@Nullable
  public PlanCoordinator getCoordinator(long jobId) {
    return mCoordinators.get(jobId);
  }
@Test
  public void testGetCoordinator() throws Exception {
    long jobId = addJob(100);
    assertNull("job id should not exist", mTracker.getCoordinator(-1));
    assertNotNull("job should exist", mTracker.getCoordinator(jobId));
    assertFalse("job should not be finished", mTracker.getCoordinator(jobId).isJobFinished());
    finishAllJobs();
    assertTrue("job should be finished", mTracker.getCoordinator(jobId).isJobFinished());
    assertEquals("finished should be of size 1", 1,
        ((Queue) AlluxioMockUtil.getInternalState(mTracker, "mFinished")).size());
  }