Question

我有可以包含其他项目的项目，因此我需要与同一实体的@OneToMany关系。我已经像这样配置了我的字段：

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

但是，Java总是抛出以下错误：

SEVERE: Servlet.service() for servlet HelloWorld threw exception
javax.persistence.RollbackException: java.lang.NullPointerException

这是因为有些物品没有孩子吗？

我的JPA实现提供程序是EclipseLink。

编辑：完整的堆栈跟踪是：

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

Answer 1

您的映射绝对不正确首先，它应该是@OneToMany，而不是“@ManyToOne” - 你正在映射一个集合（即很多）。也许这是一个错字，因为问题标题确实说“OneToMany”。

其次，它的映射方式会使关联单向。这不是推荐的方法。您 100％肯定是您想要的吗？最好通过添加“父”属性将其更改为双向映射。

最后，当你选择一个项目（例如项目本身，它的孩子，他们的孩子等等）时，渴望获取这样的关联会拉动整个子树。在某些可能需要的情况下，它们并不常见。你真的想要这种行为吗？也许最好使用不同的模型（比如嵌套集）？

Answer 2

你打算将nullable设为false是什么意思？

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

如果有些物品不能生孩子那是对的吗？

如何与自己的实体建立@OneToMany关系？

2 个答案: