code: static String getRequiredParameter(Map<String, String> parameters,
                                         String parameterName)
              throws JournalException {
          String value = parameters.get(parameterName);
          if (value == null) {
              throw new JournalException("'" + parameterName + "' is required.");
          }
          return value;
      }
docs: Get the requested parameter, or throw an exception if it is not found.