用列表填充表时JSP中出现异常

Question

我有这个jsp代码。

<c:forEach var="item" items="${UsersList}">
 <tr>
 <td><c:out value="${item.userId}" /></td>
 <td><c:out value="${item.userName}" /></td>
 <td><c:out value="${item.car}" /></td>  //Here , I am getting exception
 </tr>
 </c:forEach>

UserList由数据类型为“Car”的名为“car”的属性组成

用户的Pojo类：

@Entity
@Table(name = "Users")
public class Users implements java.io.Serializable {

private int userId;
private Car car;
private Groups groupId;
private UserType userType;
private String userName;

//getters and setters
}

我使用的是Spring MVC框架。在页面加载时，我的应用程序应该显示一个包含用户列表的表格。在controller中，我查询用户列表，并将其添加到ModelAndView对象中

控制器代码：

List<Users> userList = service.getUserList();  //this will get list of users
//System.out.println("Userlist ==" +userList.size()); 
mv.addObject("UsersList" , usersList);
//skipped remaining code

当我尝试不使用car（tabledata标记）时，JSP正在毫无例外地运行，并在表中显示用户列表。如果我尝试添加car（tabledata标签），那么我会得到一个异常

HTTP Status 500 - An exception occurred processing JSP page /WEB-INF/pages/CreateUser.jsp at line 199 
org.apache.jasper.JasperException: An exception occurred processing JSP page /WEB-INF/pages/CreateUser.jsp at line 199

196:<tr>
197:<td><c:outvalue="${item.userId}" /></td>
198:<td><c:out value="${item.userName}" /></td>
199:<!-- <td><c:out value="${item.car}" /></td> --> 
200:<td><c:out value="${item.modifiedDate}" /></td>
201:</tr>
202:</c:forEach>                                            


Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWra    pper.java:568)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:4    70)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
org.springframework.web.servlet.view.InternalResourceView.renderMergedOutput    Model(InternalResourceView.java:168)
org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:3    03)
org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.j    ava:1244)
org.springframework.web.servlet.DispatcherServlet.processDispatchResult(Disp    atcherServlet.java:1027)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServl    et.java:971)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServle    t.java:893)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkSer    vlet.java:970)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java    :861)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.ja    va:846)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
root cause

org.hibernate.LazyInitializationException: could not initialize proxy - no  Session
org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:149)
org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:195)
org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:185)
com.bdp.pojo.Role_$$_javassist_11.toString(Role_$$_javassist_11.java)
org.apache.taglibs.standard.tag.common.core.OutSupport.out(OutSupport.java:178)
org.apache.taglibs.standard.tag.common.core.OutSupport.doStartTag(OutSupport.java:99)
org.apache.jsp.WEB_002dINF.pages.CreateUser_jsp._jspx_meth_c_005fout_005f2(CreateUser_jsp.java:667)
org.apache.jsp.WEB_002dINF.pages.CreateUser_jsp._jspx_meth_c_005fforEach_005f0(CreateUser_jsp.java:591)
org.apache.jsp.WEB_002dINF.pages.CreateUser_jsp._jspService(CreateUser_jsp.java:264)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:168)
org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:303)
org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1244)
org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1027)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:971)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
org.springframeweb.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)

Answer 1

您收到错误LazyInitializationException<代码>用户懒惰汽车中的对象对象<代码>服务。getuserlist（）方法可以通过获取汽车对象引入。

抓取策略

获取类型。懒惰是随需应变的

获取类型。迫不及待

@OneToOne（fetch=FetchType.EAGER）当你想一直把用户对象作为汽车的对象时。

service.getUserList（）请写。

编辑：

示例getUserList方法：

public List<Users> getUserList(){
    String sql = " SELECT user from Users user left join fetch user.car ";
    Query query = entityManager.createQuery(sql);
    return query.getResultList();
}

Answer 2

因为您使用的是hibernate惰性初始化，所以每当您将主对象返回到JSP时，它都会尝试获取子类。这不会发生，因为会议将结束。

因此，将您的获取策略更改为Eager：

关于汽车的getter方法：

@OneToOne(fetch=FetchType.EAGER)

这将导致在获取主类时立即获取car。

在查询时制作：

 List<Users> items = session.createCriteria(Users.class)
                    .setFetchMode("car", FetchMode.JOIN).
                    .createAlias("car", "car").
                    .list();