我有这样一个问题:

org.hibernate.LazyInitializationException:惰性初始化role: mvc3.model.Topic.comments集合失败,没有会话或会话已关闭

下面是模型:

@Entity
@Table(name = "T_TOPIC")
public class Topic {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int id;

    @ManyToOne
    @JoinColumn(name="USER_ID")
    private User author;

    @Enumerated(EnumType.STRING)    
    private Tag topicTag;

    private String name;
    private String text;

    @OneToMany(mappedBy = "topic", cascade = CascadeType.ALL)
    private Collection<Comment> comments = new LinkedHashSet<Comment>();

    ...

    public Collection<Comment> getComments() {
           return comments;
    }

}

调用model的控制器如下所示:

@Controller
@RequestMapping(value = "/topic")
public class TopicController {

    @Autowired
    private TopicService service;

    private static final Logger logger = LoggerFactory.getLogger(TopicController.class);


    @RequestMapping(value = "/details/{topicId}", method = RequestMethod.GET)
    public ModelAndView details(@PathVariable(value="topicId") int id)
    {

            Topic topicById = service.findTopicByID(id);
            Collection<Comment> commentList = topicById.getComments();

            Hashtable modelData = new Hashtable();
            modelData.put("topic", topicById);
            modelData.put("commentList", commentList);

            return new ModelAndView("/topic/details", modelData);

     }

}

jsp页面看起来如下所示:

<%@page import="com.epam.mvc3.helpers.Utils"%>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page session="false" %>
<html>
<head>
      <title>View Topic</title>
</head>
<body>

<ul>
<c:forEach items="${commentList}" var="item">
<jsp:useBean id="item" type="mvc3.model.Comment"/>
<li>${item.getText()}</li>

</c:forEach>
</ul>
</body>
</html>

在查看jsp时,将引发异常。在c:forEach循环的行中


当前回答

控制器上缺少@Transactional注释

@Controller
@RequestMapping("/")
@Transactional
public class UserController {
}

其他回答

根据我的经验,我有以下方法来解决著名的LazyInitializationException:

(1)使用Hibernate.initialize

Hibernate.initialize(topics.getComments());

(2)使用JOIN FETCH

您可以在JPQL中使用JOIN FETCH语法显式地取出子集合。这有点像EAGER取回。

3)使用OpenSessionInViewFilter

LazyInitializationException经常发生在视图层。如果你使用Spring框架,你可以使用OpenSessionInViewFilter。但是,我不建议你这样做。如果使用不当,可能会导致性能问题。

原因是你试图在关闭服务内的会话后在你的控制器上获得commentList。

topicById.getComments();

只有当您的休眠会话处于活动状态时,上面才会加载commentList,我猜您在服务中关闭了休眠会话。

因此,您必须在关闭会话之前获得commentList。

如果你知道你想在每次检索主题时看到所有的注释,那么将注释的字段映射更改为:

@OneToMany(fetch = FetchType.EAGER, mappedBy = "topic", cascade = CascadeType.ALL)
private Collection<Comment> comments = new LinkedHashSet<Comment>();

默认情况下,集合是惰性加载的,如果你想了解更多,可以看看这个。

为了摆脱惰性初始化异常,在操作分离对象时不应该调用惰性收集。

在我看来,最好的方法是使用DTO,而不是实体。在这种情况下,您可以显式地设置您想要使用的字段。像往常一样,这就足够了。无需担心由Lombok生成的jackson ObjectMapper或hashCode之类的东西会隐式调用您的方法。

对于某些特定的情况,您可以使用@EntityGrpaph注释,它允许您即使在实体中有fetchType=lazy也可以进行即时加载。

此问题是由于在关闭hibernate会话的情况下访问属性造成的。控制器中没有hibernate事务。

可能的解决方式:

Do all this logic, in the service layer, (with the @Transactional), not in the controller. There should be the right place to do this, it is part of the logic of the app, not in the controller (in this case, an interface to load the model). All the operations in the service layer should be transactional. i.e.: Move this line to the TopicService.findTopicByID method: Collection commentList = topicById.getComments(); Use 'eager' instead of 'lazy'. Now you are not using 'lazy' .. it is not a real solution, if you want to use lazy, works like a temporary (very temporary) workaround. use @Transactional in the Controller. It should not be used here, you are mixing service layer with presentation, it is not a good design. use OpenSessionInViewFilter, many disadvantages reported, possible instability.

一般来说,最佳解是1。