@Test
 public void testGetOnlyNameServiceIdOrNull() {
   HdfsConfiguration conf = new HdfsConfiguration();
   conf.set(DFS_NAMESERVICES, "ns1,ns2");
   assertNull(DFSUtil.getOnlyNameServiceIdOrNull(conf));
   conf.set(DFS_NAMESERVICES, "");
   assertNull(DFSUtil.getOnlyNameServiceIdOrNull(conf));
   conf.set(DFS_NAMESERVICES, "ns1");
   assertEquals("ns1", DFSUtil.getOnlyNameServiceIdOrNull(conf));
 }