通过一对多关系获取实体

Question

我在程序中有两个实体

@Entity
@Table(name = "users")
public class User {

@OneToMany(mappedBy = "senderUser")
private List<Message> sentMessages;

@OneToMany(mappedBy = "recipientUser")
private List<Message> receivedMessages;

和

@Entity
@Table(name = "messages")
public class Message {

@ManyToOne
@JoinColumn(name = "sender")
private User senderUser;

@ManyToOne
@JoinColumn(name = "recipient")
private User recipientUser;

在我看来，当您在登录过程中下载用户时，您的信息将与这两个列表混合在一起。 但是我想检查一下，只在屏幕上显示这些列表之一的大小，因为我在数据库中有2条消息，而在控制器中显示

System.out.println("Length: " + ((User) session.getAttribute("user")).getReceivedMessages().size());

在浏览器中引发错误

    Exception

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.jonki.Entity.User.receivedMessages, could not initialize proxy - no Session

和

    Root Cause

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.jonki.Entity.User.receivedMessages, could not initialize proxy - no Session

会话似乎有问题，但是怎么办？

Answer 1

假设您的User类应按如下所示进行修改。

User.java

@Entity
@Table(name = "users")
public class User {

@OneToMany(mappedBy = "senderUser",cascade=CascadeType.ALL,fetch=FetchType.LAZY)
private List<Message> sentMessages;

@OneToMany(mappedBy = "recipientUser",cascade=CascadeType.ALL,fetch=FetchType.LAZY)
private List<Message> receivedMessages;

将以下条目添加到您的hibernate.cfg.xml中，如下所示。

 <property name="hibernate.enable_lazy_load_no_trans" value="true"/>

希望这可以帮助。