code: private void handleArgPropertyFile(final String arg, final Deque<String> args) {
          final Map.Entry<String, String> entry = parse(arg.substring(2), args);
          if (entry.getValue() == null) {
              throw new BuildException("You must specify a property filename when using the --propertyfile argument");
          }
          propertyFiles.addElement(entry.getValue());
      }
docs: Handle the --propertyfile argument.