Java 如何与自己的实体建立@OneToMany关系？

我有可以包含更多项目的项目，因此我需要与同一实体建立@OneToMany关系。我已将我的字段配置为：

@ManyToOne(targetEntity=PersistentItem.class, fetch=FetchType.EAGER)
@JoinColumn(name="ID_CHILDREN", nullable=false)
HashSet<PersistentItem> children=new HashSet<PersistentItem>();

这是因为有些物品没有子项吗

我的JPA实现提供者是EclipseLink

编辑：完整堆栈跟踪为：

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

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通过将nullable设置为false，您打算做什么

@JoinColumn(name="ID_CHILDREN", nullable=false)

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如果某些项目不能有子项，对吗？

您的映射肯定是不正确的。
首先，它应该是

@OneToMany

，而不是“manytone”-您正在映射一个集合（即多个）。也许这是一个打字错误，尽管，正如问题的标题所说的“OneToMany”

其次，它的映射方式将使关联成为单向的。这不是推荐的方法。你是否100%肯定这就是你想要的？最好通过添加“父”属性将其更改为双向映射

---

最后，当您选择一个项目（例如，项目本身、它的子项、它们的子项等）时，对这样的关联进行快速抓取将拉动整个子树。虽然在某些情况下这可能是可取的，但它们非常罕见。你真的想要那种行为吗？也许最好使用不同的模型（如嵌套集）？

可以获得完整的堆栈跟踪吗？

@JoinColumn(name="ID_CHILDREN", nullable=false)