休眠多对一注释外键 javax.el.E​​LException

Posted 2023-04-14

【中文标题】休眠多对一注释外键 javax.el.E​​LException

【英文标题】：Hibernate many to one annotation foreign key javax.el.ELException

【发布时间】：2016-11-03 07:40:11

【问题描述】：

我有 2 个具有多对一映射的类：

购物车类：

@Entity
@Table(name = "Cart")
public class Cart 

 @Id
 @Column(name = "ID", nullable = false)
 private String id;

 @ManyToOne(fetch = FetchType.LAZY, targetEntity = User.class)
 @JoinColumn(name = "UserID", referencedColumnName = "ID")
 private User user;

用户类别：

@Entity
@Table(name = "User")
public class User 
 @OneToMany(fetch = FetchType.LAZY, mappedBy = "user")
 private Set<Cart> carts = new HashSet<Cart>();

当我从购物车对象中获取外键时，它总是返回错误。像下面的代码：

<c:set var="cart" value='<%=CartDAO.getCart("CT00001")%>' />
<c:out value="$cart.user.id" />

还有我的问题：

SEVERE: Servlet.service() for servlet [jsp] in context with path [/mavenHibernate] threw exception [javax.el.ELException: Error reading 'id' on type model.User_$$_jvst39a_3] with root cause
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:147)
at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:260)
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:73)
at model.User_$$_jvst39a_3.getId(User_$$_jvst39a_3.java)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at javax.el.BeanELResolver.getValue(BeanELResolver.java:94)
at org.apache.jasper.el.JasperELResolver.getValue(JasperELResolver.java:110)
at org.apache.el.parser.AstValue.getValue(AstValue.java:169)
at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:184)
at org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:944)
at org.apache.jsp.test_jsp._jspx_meth_c_005fout_005f0(test_jsp.java:209)
at org.apache.jsp.test_jsp._jspService(test_jsp.java:176)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:443)
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:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:108)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:784)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:802)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1452)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)

当我使用休眠 xml 映射时，它没有发生。请帮我解决这个问题。我找不到任何解决方案。

编辑： 这是xml映射版本：

购物车.hbm.xml：

<hibernate-mapping>
 <class name="model.Cart" table="CART">
    <id name="id" type="java.lang.String">
        <column name="ID" />
        <generator class="assigned" />
    </id>
    <many-to-one name="user" class="model.User" fetch="select">
        <column name="USERID" />
    </many-to-one>
    <set name="cartItems" table="CARTITEM" inverse="true" lazy="true" fetch="select">
        <key>
            <column name="CARTID" />
        </key>
        <one-to-many class="model.CartItem" />
    </set>
 </class>
</hibernate-mapping>

用户.hbm.xml：

<hibernate-mapping>
 <class name="model.User" table="USER">
    <id name="id" type="java.lang.String">
        <column name="ID" />
        <generator class="assigned" />
    </id>
    <set name="carts" table="CART" inverse="true" lazy="true" fetch="select">
        <key>
            <column name="USERID" />
        </key>
        <one-to-many class="model.Cart" />
    </set>
 </class>
</hibernate-mapping>

他们创建相同的查询：

xml查询：

 Hibernate: select cart0_.ID as ID1_2_, cart0_.USERID as USERID2_2_, cart0_.DATEADD as DATEADD3_2_, cart0_.ACTIVE as ACTIVE4_2_ from CART cart0_ where cart0_.ID=?

注解查询：

 Hibernate: select cart0_.ID as ID1_2_, cart0_.Active as Active2_2_, cart0_.DateAdd as DateAdd3_2_, cart0_.UserID as UserID4_2_ from Cart cart0_ where cart0_.ID=?

为什么注解查询会抛出异常而xml查询不会？我仍然不知道是什么导致了这个问题。

已编辑 2：只是想为此找到解决方案。

已编辑 3：仍在寻找解决方案。

【参考方案1】：

您已将@ManyToOne 关系设置为FetchType.LAZY。将其更改为 EAGER，或者更好的是，完全删除 fetch 属性，因为 ManyToOne 默认为 EAGER。

【讨论】：

这就是我想要解决的问题。当我使用 FetchType.Eager 时，它会不必要地加载用户的所有字段。特别是当我只想使用 userid 时，它存储在 UserId 字段中。但我无法使用 cart.user.id 访问它。

用了就没有必要了。但是如果你仍然想保持它 LAZY，你必须在调用 user.getId 之前发出 cart.getUser 以便休眠加载该对象，然后你可以访问它的字段。

我发布了更多信息。希望这可以使它更清楚。感谢您的帮助。