Java 执行FreeMarker时，以下内容的计算结果为null或缺失

我正在使用FreeMarker（一个基于Java的免费模板引擎，最初专注于使用MVC软件架构生成动态网页）来生成一些HTML页面 我有一个freemarker模板，其中包含以下代码：

 Dear ${user.firstname} ${user.surname},</h3>

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public static void main(String[] args) throws IOException, TemplateException {


    class User {

        String firstname;

        public String getFirstname() {
            return firstname;
        }

        public void setFirstname(String firstname) {
            this.firstname = firstname;
        }





    }


    String templateName = "tdk_reminder_ext_body.ftl";

     final Map<String, Object> finalModel = new HashMap<>();

     User user = new User();

     user.setFirstname("firstname");

     finalModel.put("serverName", "local");
     finalModel.put("user", user);



    Configuration cfg = new Configuration(Configuration.getVersion());

    cfg.setDirectoryForTemplateLoading(new File("C:/Work/eclipse-tdk/templates/engine/"));
    cfg.setDefaultEncoding("UTF-8");
    cfg.setTemplateExceptionHandler(TemplateExceptionHandler.RETHROW_HANDLER);

    final freemarker.template.Template freemarkerTemplate = cfg.getTemplate(templateName);

    System.out.println (FreeMarkerTemplateUtils.processTemplateIntoString(freemarkerTemplate, finalModel));


}

FTL stack trace ("~" means nesting-related):
    - Failed at: ${user.firstname}  [in template "tdk_reminder_ext_body.ftl" at line 34, column 38]
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public class User {

    String firstname;

    public String getFirstname() {
        return firstname;
    }

    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }

}