用thymeleaf的th：each双重循环遍历不出来

Question

@Controller
@RequestMapping("/admin")
public class AdminController {

    @Autowired
    private VideoServiceImpl videoServiceimpl;

    @GetMapping(value = {"","/index"})
    public String index(HttpServletRequest request){
        List

public class Video {
    private int id;
    private String topic;
    private String path;
    private Date created;
    private int typeId;
    private List<Type> types=new ArrayList<Type>();

public class Type {
    private int id;
    private String name;
    private String photo;

            <div th:each="video:${videos}">
                <span th:text="${video.topic}">视频标题</span>
                <span th:text="${video.created}">视频创建时间</span>
                
                <div th:each="type:${video.types}"><span th:text="${type.name}">视频的分类</span></div>
            </div>

为什么页面显示不出来