如何有自己的实体@OneToMany关系?
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11-07-2019 - |
题
我有可以包含进一步的物品的物品,因此,我需要与同一实体A @OneToMany关系。我已经配置我的字段是这样的:
@ManyToOne(targetEntity=PersistentItem.class, fetch=FetchType.EAGER)
@JoinColumn(name="ID_CHILDREN", nullable=false)
HashSet<PersistentItem> children=new HashSet<PersistentItem>();
然而,Java总是引发以下错误:
SEVERE: Servlet.service() for servlet HelloWorld threw exception
javax.persistence.RollbackException: java.lang.NullPointerException
是因为有些项目没有孩子吗?
我JPA实现提供者是的EclipseLink。
编辑:全堆栈跟踪是:
03-Aug-2009 19:22:09 org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet HelloWorld threw exception
javax.persistence.RollbackException: java.lang.NullPointerException
at org.eclipse.persistence.internal.jpa.transaction.EntityTransactionImpl.commitInternal(EntityTransactionImpl.java:102)
at org.eclipse.persistence.internal.jpa.transaction.EntityTransactionImpl.commit(EntityTransactionImpl.java:63)
at info.lenni.gate.servlet.HelloWorld.doGet(HelloWorld.java:52)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:637)
Caused by: java.lang.NullPointerException
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.checkForUnregisteredExistingObject(UnitOfWorkImpl.java:678)
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.discoverAndPersistUnregisteredNewObjects(UnitOfWorkImpl.java:4007)
at org.eclipse.persistence.mappings.ObjectReferenceMapping.cascadeDiscoverAndPersistUnregisteredNewObjects(ObjectReferenceMapping.java:743)
at org.eclipse.persistence.internal.descriptors.ObjectBuilder.cascadeDiscoverAndPersistUnregisteredNewObjects(ObjectBuilder.java:1511)
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.discoverAndPersistUnregisteredNewObjects(UnitOfWorkImpl.java:4019)
at org.eclipse.persistence.internal.sessions.RepeatableWriteUnitOfWork.discoverUnregisteredNewObjects(RepeatableWriteUnitOfWork.java:182)
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.calculateChanges(UnitOfWorkImpl.java:628)
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.commitToDatabaseWithChangeSet(UnitOfWorkImpl.java:1387)
at org.eclipse.persistence.internal.sessions.RepeatableWriteUnitOfWork.commitRootUnitOfWork(RepeatableWriteUnitOfWork.java:160)
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.commitAndResume(UnitOfWorkImpl.java:1060)
at org.eclipse.persistence.internal.jpa.transaction.EntityTransactionImpl.commitInternal(EntityTransactionImpl.java:84)
... 16 more
解决方案
您映射是最肯定不正确。结果
首先,应该@OneToMany
,不`@ManyToOne” - 你映射集合(也就是多)。也许这是一个错字,不过,作为问题的标题不说“一对多”。
其次,它的映射方式将关联的单向即可。这不是一个推荐的方法。您的 100%正这就是你想要什么?它可能是更好通过添加一个“父”属性此更改为双向映射。
最后,渴望获取有关关联这样会拉的全部子树,当你选择一个项目(如项目本身,它的孩子,他们的孩子,等...)虽然有某些情况下,在那里,可能是可取的,他们是相当罕见的。你真的希望这样的行为?也许这将是最好使用不同的模型(如嵌套集合)?
其他提示
你说的可空设置为false打算?
@JoinColumn(name="ID_CHILDREN", nullable=false)
如果某些项目不能生孩子是这样吗?
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