Java如何解决“无法延迟初始化角色集合”的Hibernate异常

小编典典

Java如何解决“无法延迟初始化角色集合”的Hibernate异常

java

我有这个问题：

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: mvc3.model.Topic.comments, no session or session was closed

这是模型：

@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;
    }

}

调用模型的控制器如下所示：

@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>

查看jsp时会引发异常。与c：forEach循环一致

阅读 515

2020-03-06

共1个答案

小编典典

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

你可以在JPQL中使用JOIN FETCH语法来显式提取子集合。这有点像EAGER提取。

（3）使用OpenSessionInViewFilter

LazyInitializationException经常在视图层中发生。如果使用Spring框架，则可以使用OpenSessionInViewFilter。但是，我不建议你这样做。如果使用不正确，可能会导致性能问题。

2020-03-06