如何毫无问题地使用 JPA 2.0 @ManyToMany

【问题标题】：How to use JPA 2.0 @ManyToMany without issues如何毫无问题地使用 JPA 2.0 @ManyToMany
【发布时间】：2012-06-01 22:27:33
【问题描述】：

我在开发中使用JPA 2.0 和Spring。我的实体类包含两个 @ManyToMany 关系。 

@Entity("payment")
 public class PaymentData implements Serializable
{
    private Long pk;

    private Collection<PaymentItemData> paymentItem;
    /**
     *  minorPaymentItem
     *
     */
    private Collection<MinorPayItemData> minorPaymentItem;

    @ManyToMany(fetch=FetchType.EAGER)
    @JoinTable(name = "payitem_m_assig",
    joinColumns =
    @JoinColumn(name = "pay_item_id", nullable = false),
    inverseJoinColumns =
    @JoinColumn(name = "minor_pay_item_id", nullable = false))
    public Collection<MinorPayItemData> getMinorPaymentItem()
    {
        return minorPaymentItem;
    }

    /**
     * @param minorPaymentItem the minorPaymentItem to set
     */
    public void setMinorPaymentItem(final Collection<MinorPayItemData> value)
    {
        this.minorPaymentItem = value;
    }

    @ManyToMany(fetch=FetchType.EAGER)
    @JoinTable(name = "payitem_assigned",
    joinColumns =
    @JoinColumn(name = "pay_item_id", nullable = false),
    inverseJoinColumns =
    @JoinColumn(name = "pay_item_id", nullable = false))
    public Collection<PaymentItemData> getPaymentItem()
    {
        return paymentItem;
    }

    /**
     * Set the property paymentItem
     *
     * @param value -paymentItem
     *
     */
    public void setPaymentItem(final Collection<PaymentItemData> value)
    {
        this.paymentItem = value;
    }    

}

当我运行查询以从数据库中的支付表中检索记录时，例如

Query q = manager.createQuery("select a from PaymentData a");
q.getResultList();

如果我在@ManyToMany 上允许fetch=FetchType.EAGER，我会收到以下错误

Caused by: org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags
    at org.hibernate.loader.BasicLoader.postInstantiate(BasicLoader.java:94)
    at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:119)
    at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:71)
    at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:54)
    at org.hibernate.loader.entity.BatchingEntityLoader.createBatchingEntityLoader(BatchingEntityLoader.java:133)
    at org.hibernate.persister.entity.AbstractEntityPersister.createEntityLoader(AbstractEntityPersister.java:1914)
    at org.hibernate.persister.entity.AbstractEntityPersister.createEntityLoader(AbstractEntityPersister.java:1937)
    at org.hibernate.persister.entity.AbstractEntityPersister.createLoaders(AbstractEntityPersister.java:3205)
    at org.hibernate.persister.entity.AbstractEntityPersister.postInstantiate(AbstractEntityPersister.java:3191)
    at org.hibernate.persister.entity.SingleTableEntityPersister.postInstantiate(SingleTableEntityPersister.java:728)
    at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:348)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1845)
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:906)
    ... 39 more

但如果我删除 fetch=FetchType.EAGER 并将其保留为 @ManyToMany，我将遇到异常

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.data.PaymentData.paymentItem, no session or session was closed
    at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:383)
    at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:375)
    at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:368)
    at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPersistentCollection.java:111)
    at org.hibernate.collection.PersistentBag.toString(PersistentBag.java:506)
    at java.lang.String.valueOf(String.java:2826)
    at java.lang.StringBuilder.append(StringBuilder.java:115)
    at com.niu.util.Util.toString(Util.java:131)
    at com.niu.util.data.BaseData.toString(BaseData.java:107)
    at java.lang.String.valueOf(String.java:2826)
    at java.lang.StringBuilder.append(StringBuilder.java:115)
    at java.util.AbstractCollection.toString(AbstractCollection.java:422)
    at java.lang.String.valueOf(String.java:2826)
    at java.io.PrintStream.println(PrintStream.java:771)
    at org.apache.tomcat.util.log.SystemLogHandler.println(SystemLogHandler.java:269)
    at sun.reflect.GeneratedMethodAccessor1861.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:452)
    at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:291)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:254)
    at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:133)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.niu.web.common.interceptor.ApplicationModelDrivenInterceptor.intercept(ApplicationModelDrivenInterceptor.java:31)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at org.apache.struts2.interceptor.ProfilingActivationInterceptor.intercept(ProfilingActivationInterceptor.java:104)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:270)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:190)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)
    at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:498)
    at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
    at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:563)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:399)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:317)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:204)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:311)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:662)

我做错了什么

【问题讨论】：

    标签： hibernate jpa struts2 jpa-2.0


    【解决方案1】：

    要避免使用MultipleBagFetchException，而不是使用FetchType.EAGER，请尝试使用@LazyCollection(LazyCollectionOption.FALSE)，如下例所示：

    @ManyToMany
@LazyCollection(LazyCollectionOption.FALSE)
@JoinTable(name = "payitem_m_assig", joinColumns = @JoinColumn(name = "pay_item_id", nullable = e), inverseJoinColumns = @JoinColumn(name = "minor_pay_item_id", nullable = false))
public Collection<MinorPayItemData> getMinorPaymentItem()
{
    return minorPaymentItem;
}

    这里是来自docs的小描述：

    • @LazyCollection：定义@ManyToMany 和 @OneToMany 关联。 LazyCollectionOption 可以是 TRUE（ 集合是惰性的，将在访问其状态时加载）， EXTRA（集合是惰性的，所有操作都会尽量避免 集合加载，这对于大型集合特别有用 当不需要加载所有元素时）和 FALSE（关联 不懒）

    • @Fetch：定义用于加载关联的获取策略。 FetchMode可以是SELECT（关联时触发一个select 需要加载），SUBSELECT（仅适用于集合，使用 子选择策略 - 请参考 Hibernate Reference 有关更多信息的文档）或 JOIN（使用 SQL JOIN 加载 加载所有者实体时的关联）。 JOIN 覆盖任何 惰性属性（通过 JOIN 策略加载的关联不能 懒惰）。

    希望对你有帮助

    【讨论】：

    • 它运行顺畅。如果我更改为 Eclipse 链接 JPA 实现，会不会破坏代码。
    • @Uchenna 确实在 JPA 实现中存在差异。出于便携性原因，您应该避免使用特定功能。据我所知，这是一个特定于 Hibernate 的问题，但一个实现中存在的问题可能不会发生在另一个实现中。所以我认为最好的办法是处理手头的问题。
    • @RodrigoS.Bento：因为 LazyCollection 的默认获取模式是 SELECT 而不是 JOIN，所以它可以工作吗？
    【解决方案2】：

    最好的方法是使 XxxtoMany 关联变得惰性（默认）。这样，集合只会在需要时加载，或者当您告诉 Hibernate 使用带有 join fetch 子句的查询急切地获取它们时。使它们 EAGER 强制 Hibernate始终加载它们，即使您不需要它们。

    当然，如果您需要它们并且它们被配置为 LAZY，则只有在会话仍然打开时才能加载它们。一旦会话关闭，实体就会分离，并且不能再延迟加载集合。因此，您需要在关闭会话之前显式初始化它们（通过调用集合的方法或调用Hibernate.initialize(collection)）。

    如果你真的想让它们保持热切加载，那么其中只有一个应该是袋子（即类型为 Collection 或 List）。其他的应该声明为Set。

    旁注：第二个关联的映射是错误的：您使用了相同的连接列两次。

    【讨论】：

    • 感谢您的观察和回复。我将同时更改为 Set 并保持热切加载，看看它是否有效。就目前的情况而言，我希望使代码具有可移植性。
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