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
不是最好的解决方案，但对于那些面临LazyInitializationException的人，特别是在序列化上，这将有所帮助。在这里，您将检查惰性初始化属性并将其设置为null。为此，创建以下类
public class RepositoryUtil {
public static final boolean isCollectionInitialized(Collection<?> collection) {
if (collection instanceof PersistentCollection)
return ((PersistentCollection) collection).wasInitialized();
else
return true;
}
}
在你的实体类中，你有一个惰性初始化的属性，添加如下所示的方法。在这个方法中添加所有延迟加载属性。
public void checkLazyIntialzation() {
if (!RepositoryUtil.isCollectionInitialized(yourlazyproperty)) {
yourlazyproperty= null;
}
在加载数据的所有位置之后调用这个checklazyinitialize()方法。
YourEntity obj= entityManager.find(YourEntity.class,1L);
obj.checkLazyIntialzation();
在第二次执行生成JWT令牌的方法后，我得到了这个错误。
line user.getUsersRole().stream().forEachOrdered((ur) -> roles.add(ur. getroleid ()));生成错误。
// MyUserDetails.java
@Service
public class MyUserDetails implements UserDetailsService {
@Override
public UserDetails loadUserByUsername(String email) {
/* ERROR
/* org.hibernate.LazyInitializationException: failed to
/* lazily initialize a collection of role:
/* com.organizator.backend.model.User.usersRole,
/* could not initialize proxy - no Session */
user.getUsersRole().stream().forEachOrdered((ur) ->
roles.add(ur.getRoleId()));
在我的例子中，@Transactional注释解决了它，
// MyUserDetails.java
import org.springframework.transaction.annotation.Transactional;
@Service
public class MyUserDetails implements UserDetailsService {
@Override
@Transactional // <-- added
public UserDetails loadUserByUsername(String email) {
/* No Error */
user.getUsersRole().stream().forEachOrdered((ur) ->
roles.add(ur.getRoleId()));
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.
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。
