code: private void setTypeRef(Configuration conf) {
      String className = conf.get(CLASS_CONF_KEY);
  
      if (className == null) {
        throw new RuntimeException(CLASS_CONF_KEY + " is not set");
      }
  
      Class<?> clazz = null;
      try {
        clazz = conf.getClassByName(className);
      } catch (ClassNotFoundException e) {
        throw new RuntimeException("failed to instantiate class '" + className + "'", e);
      }
  
      typeRef = new TypeRef<M>(clazz){};
    }
docs: set typeRef from conf