code: private void determineHostsAsProperty() {
          if (hosts.startsWith("${") && hosts.endsWith("}")) {
              String hostsProperty = hosts.substring(2, hosts.length() - 1);
              hosts = PropertyUtil.getProperty(hostsProperty);
              if (hosts == null) {
                  throw new IllegalArgumentException("Could not find property '"
                          + hostsProperty + "'");
              }
          }
  
      }
docs: sets hosts if the _hosts propertey is determined to be a property placeholder
  Throws IllegalArgumentException argument exception when nothing found