在获取用户评论时,我很难让简单的OneToMany映射到ManyToOne实体正常工作。其他答案建议您使用entityManager自己创建查询,但那似乎太可怕了。如果您甚至不能做一些简单的事情而不硬编码内联SQL,那么ORM有什么意义呢?看起来更可能是我做错了什么。
似乎与我使用模型从jsp访问user.getComments()方法有关。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:
严重: 在上下文中为servlet [dispatcher]的Servlet.service()处理抛出异常[/test]路径下的[WEB-INF/jsp/user.jsp]处理异常发生了异常,行号[34]。堆栈跟踪: org.hibernate.LazyInitializationException:无法延迟初始化角色的集合:com.instagramviewer.entity.User.comments,无法初始化代理 - 没有会话 在org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:602)处 在org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:217)处 在org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:581)处 在org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:148)处 在org.hibernate.collection.internal.PersistentBag.iterator(PersistentBag.java:303)处 在org.apache.taglibs.standard.tag.common.core.ForEachSupport.toForEachIterator(ForEachSupport.java:348)处 在org.apache.taglibs.standard.tag.common.core.ForEachSupport.supportedTypeForEachIterator(ForEachSupport.java:224)处 在org.apache.taglibs.standard.tag.common.core.ForEachSupport.prepare(ForEachSupport.java:155)处 在javax.servlet.jsp.jstl.core.LoopTagSupport.doStartTag(LoopTagSupport.java:256)处 在org.apache.jsp.WEB_002dINF.jsp.user_jsp._jspx_meth_c_005fforEach_005f0(user_jsp.java:285)处 在org.apache.jsp.WEB_002dINF.jsp.user_jsp._jspService(user_jsp.java:172)处 at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) 在javax.servlet.http.HttpServlet.service(HttpServlet.java:741)处 在org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:476)处 在org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)处 at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329) 在javax.servlet.http.HttpServlet.service(HttpServlet.java:741)处 在org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)处 在org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)处 在org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)处 在org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)处 在org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)处 在org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:712)处 在org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:459)处 在org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:384)处 在org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:312)处 在org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:170)处 at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:316) 在org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1370)处 在org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1116)处 在org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1055)处 在org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942)处 在org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005)处 在org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:897)处 在javax.servlet.http.HttpServlet.service(HttpServlet.java:634)处 在org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882)处 在javax.servlet.http.HttpServlet.service(HttpServlet.java:741)处 在org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)处 在org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)处 在org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)处 在org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)处 在org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)处 在org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:200)处 在org.apache.catalina.core.StandardContextVal
@OneToMany(mappedBy="user",fetch = FetchType.LAZY
)和
@ManyToOne(fetch = FetchType.LAZY)
中使用。 - SRy