docs: private static Boolean getBoolean(ServletConfig config, String key, boolean dflt) {
          String value;
  
          value = getString(config, key, null);
          if (value == null) {
              return dflt;
          }
          if ("true".equals(value)) {
              return true;
          } else if ("false".equals(value)) {
              return false;
          } else {
              throw new IllegalArgumentException(key + ": 'true' or 'false' expected, got '" + value + "'");
          }
      }