Java 如何获取DDL表单'；什么是字段名？

我实现了MVCActionCommand来覆盖ddl表单的add方法。我想从该表单中检索字段的值

@Override
public boolean processAction(ActionRequest actionRequest, ActionResponse actionResponse) 
throws PortletException {      
 try {
        String fieldName = ParamUtil.getString(actionRequest, "field_name");      
        System.out.println(fieldName);    //returned nothing
    } catch (Exception e) {
        System.out.println(e.getMessage());
    }
    return false;
}

也尝试了这个，但给了我一个“null”值

ServiceContext serviceContext = ServiceContextFactory.getInstance(DDLRecord.class.getName(), actionRequest);
serviceContext.getAttribute('field_name');

请检查请检查