private Map<ServerName, ServerMetrics> getOnlineServers() {
    if (serverManager != null) {
      final Map<ServerName, ServerMetrics> map = new HashMap<>();
      serverManager.getOnlineServers().entrySet().forEach(e -> map.put(e.getKey(), e.getValue()));
      return map;
    }
    return null;
  }
Implement to return TableDescriptor after pre-checks
hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java