如何與自己的實體建立@OneToMany關係？

Question

我有可以包含更多項目的項目，因此我需要@OneToMany與同一個實體的關係。我已經配置了這樣的我的領域：

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

但是，Java總是拋出以下錯誤：

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

是不是因爲有些項目沒有孩子嗎？

我的JPA實現提供者是EclipseLink。

編輯：完整的堆棧跟蹤是：

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

你的映射是絕對不正確的。
首先，它應該是@OneToMany，而不是`@ManyToOne' - 您正在映射集合（即MANY）。但是，也許這是一個錯字，因爲問題標題確實會說「OneToMany」。

其次，它的映射方式將使聯想單向。這不是一個推薦的方法。你是100％積極這就是你想要的？通過添加「父」屬性將其更改爲雙向映射可能會更好。最後，當你選擇一個項目（例如項目本身，它的孩子，他們的孩子等......）時，對這樣的一個關聯的提取將會拉動整個子樹。儘管在某些情況下，可能是可取的，但它們並不常見。你真的想要這樣的行爲嗎？也許最好使用不同的模型（如嵌套）？

Answer 2

你說的可空設置爲false打算？

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

如果有些項目不能生孩子是嗎？