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
为了延迟加载集合，必须有一个活动会话。在web应用中，有两种方法可以做到这一点。您可以使用Open Session In View模式，其中使用拦截器在请求开始时打开会话，并在结束时关闭会话。风险在于你必须有可靠的异常处理，否则你可能会绑定所有的会话，你的应用程序可能会挂起。
The other way to handle this is to collect all the data you need in your controller, close your session, and then stuff the data into your model. I personally prefer this approach, as it seems a little closer to the spirit of the MVC pattern. Also if you get an error from the database this way you can handle it a lot better than if it happens in your view renderer. Your friend in this scenario is Hibernate.initialize(myTopic.getComments()). You will also have to reattach the object to the session, since you're creating a new transaction with every request. Use session.lock(myTopic,LockMode.NONE) for that.
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。
模型类Topic中的集合注释是惰性加载的，如果你不使用fetch = FetchType注释它，这是默认的行为。热切的特别。
findTopicByID服务很可能正在使用无状态Hibernate会话。无状态会话没有第一级缓存，即没有持久性上下文。稍后，当您尝试迭代注释时，Hibernate将抛出异常。
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: mvc3.model.Topic.comments, no session or session was closed
解决方案可以是:
使用fetch = FetchType注释注释。急切的
@OneToMany(fetch = FetchType。mappedBy = "topic"， cascade = CascadeType.ALL)
private Collection<Comment> comments = new LinkedHashSet<Comment>();
如果您仍然希望延迟加载注释，请使用Hibernate的有状态会话，这样您就可以在需要时获取注释。
