LazyInitializationException 即使使用@Transactional

Question

当我去获取用户的评论时，很难让 ManyToOne 实体映射的简单 OneToMany 工作。其他答案建议您必须使用 entityManager 自己创建查询，但这看起来太可怕了。如果不硬编码内联 sql 甚至不能做这样简单的事情，那么 ORM 有什么意义？似乎更有可能是我做错了什么。

这似乎与我使用模型从 jsp 访问 user.getComments() 方法这一事实有关。不确定最好的方法是什么。

架构：

CREATE TABLE users (
    id INTEGER AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(255) UNIQUE NOT NULL,
    created_at TIMESTAMP DEFAULT NOW()
);

CREATE TABLE comments (
    id INTEGER AUTO_INCREMENT PRIMARY KEY,
    comment_text VARCHAR(255) NOT NULL,
    photo_id INTEGER NOT NULL,
    user_id INTEGER NOT NULL,
    created_at TIMESTAMP DEFAULT NOW(),
    FOREIGN KEY(user_id) REFERENCES users(id)
);

用户控制器方法：

@RequestMapping("/user")
public ModelAndView getUser(@RequestParam int id) {
    return new ModelAndView("user", "message", userService.getUser(id));
}

用户服务：

@Service
public class UserService {

    private UserDAO userDAO;

    @Autowired
    public UserService(UserDAO userDAO) {
        this.userDAO = userDAO;
    }

    @Transactional
    public User getUser(int id) {
        return userDAO.getUser(id);
    }

}

UserDAO:

@Repository
public class UserDAO {

    @PersistenceContext
    EntityManager entityManager;

    @Nullable
    public User getUser(int id)
    {
        return entityManager.find(User.class, id);
    }
}

用户实体：

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

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="id")
    private int id;

    @Column(name="username")
    private String userName;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="created_at")
    private Date createdAt;

    @OneToMany(mappedBy="user")
    private List<Comment> comments;

    public int getId() {
        return id;
    }

    public String getUserName() {
        return userName;
    }

    public Date getCreatedAt() {
        return createdAt;
    }

    @Transactional
    public List<Comment> getComments() {
        return comments;
    }
}

评论实体：

@Entity
@Table(name="comments")
public class Comment {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="id")
    private int id;

    @Column(name="comment_text")
    private String commentText;

    @Column(name="photo_id")
    private int photoId;

    @ManyToOne
    @JoinColumn(name="user_id")
    private User user;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "created_at")
    private Date createdAt;

    @Override
    public String toString() {
        return "Comment [id=" + id + ", commentText=" + commentText + ", photoId=" + photoId + ", userId=" + user.getId()
                + ", createdAt=" + createdAt + "]";
    }

    public int getId() {
        return id;
    }

    public String getCommentText() {
        return commentText;
    }

    public int getPhotoId() {
        return photoId;
    }

    public User getUser() {
        return user;
    }

    public Date getCreatedAt() {
        return createdAt;
    }
}

堆栈跟踪：

SEVERE: Servlet.service() for servlet [dispatcher] in context with path [/test] threw exception [An exception occurred processing [WEB-INF/jsp/user.jsp] at line [34]

31: User 32: Creation date 33: 34: 35: 36: 37:

Stacktrace:] with root cause org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.instagramviewer.entity.User.comments, could not initialize proxy - no Session at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:602) at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:217) at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:581) at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:148) at org.hibernate.collection.internal.PersistentBag.iterator(PersistentBag.java:303) at org.apache.taglibs.standard.tag.common.core.ForEachSupport.toForEachIterator(ForEachSupport.java:348) at org.apache.taglibs.standard.tag.common.core.ForEachSupport.supportedTypeForEachIterator(ForEachSupport.java:224) at org.apache.taglibs.standard.tag.common.core.ForEachSupport.prepare(ForEachSupport.java:155) at javax.servlet.jsp.jstl.core.LoopTagSupport.doStartTag(LoopTagSupport.java:256) at org.apache.jsp.WEB_002dINF.jsp.user_jsp._jspx_meth_c_005fforEach_005f0(user_jsp.java:285) at org.apache.jsp.WEB_002dINF.jsp.user_jsp._jspService(user_jsp.java:172) at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:476) at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385) at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329) at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:712) at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:459) at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:384) at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:312) at org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:170) at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:316) at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1370) at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1116) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1055) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005) at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:897) at javax.servlet.http.HttpServlet.service(HttpServlet.java:634) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882) at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:200) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:668) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:834) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1415) at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:748)

Answer 1

@Transactional 对您没有帮助，因为当您尝试加载评论时（在 JSP 级别）交易已经关闭（它在 getUser 方法开始时打开并且结束时关闭）。可能的方法是在交易中初始化您的评论。

@Transactional
public User getUser(int id) {
    User user = userDAO.getUser(id);
    user.getComments().size(); //initializing
    return user;
}

这是最原始的方法，但应该适合你。如果有效，您可以稍后阅读有关 Named Entity Graph 的内容。这是初始化所需惰性关联的更高级方法。

Answer 2

@Vlad 已经注意到，您在交易之外引用 Comments。由于您使用的是 jpql，因此您可以急切地获取 User 以及他的评论：

public List<User> getAllUsers()
{
    TypedQuery<User> query = entityManager.createQuery("SELECT e from User e left join fetch e.comments", User.class);
    return (List<User>) query.getResultList();
}

Answer 3

我从一开始就试图避免这样做，但我想保持实体的获取类型惰性的最好方法是在 DAO 层中创建一个使用 join fetch 急切获取的方法。

@Nullable
    public User getUser(int id)
    {
        TypedQuery<User> query = entityManager.createQuery("SELECT e from User e left join fetch e.comments c where e.id = :id", User.class);
        query.setParameter("id", id);
        return query.getSingleResult();
    }

Answer 4

我做错了两件事：

首先：我忘记在我的 spring java 配置中启用事务管理 class

@Configuration
@EnableTransactionManagement
public class AppConfig {

  @Bean
  public LocalContainerEntityManagerFactoryBean factoryBean() {
      LocalContainerEntityManagerFactoryBean factory = new LocalContainerEntityManagerFactoryBean();
      factory.setPersistenceProviderClass(HibernatePersistenceProvider.class);

      return factory;
  }

  @Bean
  public PlatformTransactionManager transactionManager(){
     JpaTransactionManager transactionManager = new JpaTransactionManager();
     transactionManager.setEntityManagerFactory(factoryBean().getObject() );
     return transactionManager;
  }
}

其次：正如 Vlad 所说，我需要在我的服务层中将评论列表初始化为事务的一部分

@Transactional
public User getUser(int id) {
    User user = userDAO.getUser(id);
    Hibernate.initialize(user.getComments());
    return user;
}

现在我可以避免加入获取解决方案，这只会在服务层中我想要检索评论列表的任何位置添加一行 java 代码。

LazyInitializationException 即使使用@Transactional

LazyInitializationException even with @Transactional

java

sql

spring

jpql

kotlin