public void setupMap() {
    InputStream inputStream = null;
    LOG.info("Using hostsmap file " + this.hostsMapFile);
    try {
      if (hostsMapFile != null) {
        hostsMap = new Properties();
        inputStream = new FileInputStream(new File(hostsMapFile));
        // load the properties
        hostsMap.load(inputStream);
      }
    } catch (FileNotFoundException fnf) {
      LOG.info("No configuration file " + hostsMapFile + " found in classpath.", fnf);
    } catch (IOException ie) {
      throw new IllegalArgumentException("Can't read configuration file " +
          hostsMapFile, ie);
    } finally {
      if (inputStream != null) {
        try {
          inputStream.close();
        } catch(IOException io) {
          //ignore 
        }
      }
    }
  }
Stores the mapping of hostnames to be used in any configuration on 
the server.
ambari-server/src/main/java/org/apache/ambari/server/controller/HostsMap.java