code: @Override // DataNodeMXBean
    public String getNamenodeAddresses() {
      final Map<String, Integer> info = new HashMap<String, Integer>();
      for (NamespaceService ns : namespaceManager.getAllNamenodeThreads()) {
        if (ns != null && ns.initialized()) {
          info.put(ns.getNNSocketAddress().getAddress().getHostAddress(), ns.getNamespaceId());
        }
      }
      return JSON.toString(info);
    }
docs: Returned information is a JSON representation of a map with
  name node host name as the key and block pool Id as the value