private List<ServerName> getUnknownServers() {
    if (serverManager != null) {
      final Set<ServerName> serverNames = getAssignmentManager().getRegionStates().getRegionStates()
        .stream().map(RegionState::getServerName).collect(Collectors.toSet());
      final List<ServerName> unknownServerNames = serverNames.stream()
        .filter(sn -> sn != null && serverManager.isServerUnknown(sn)).collect(Collectors.toList());
      return unknownServerNames;
    }
    return null;
  }
Implement to return TableDescriptor after pre-checks
hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java