我有一个数据库表，其中包含一个简单的自我父/子关系（类别），如下所示：
+------------+----------+--------------------+
| categoryid | parentid | category_name |
+------------+----------+--------------------+
| 1 | 0 | Java |
| 2 | 0 | SKM |
| 3 | 0 | Neuigkeiten |
| 4 | 0 | Neue Versionen |
| 5 | 0 | PlugIn |
..
| 9 | 2 | Subversion |
| 10 | 2 | DVCS |
| 11 | 2 | SVK |
| 12 | 2 | Bazaar |
| 13 | 2 | CVS |
| 14 | 2 | SpectrumSCM |
| 15 | 2 | Plastic SCM |
| 16 | 2 | Monotone |
| 17 | 2 | Mercurial |
| 18 | 2 | ClearCase |
现在我已经使用上面的表创建了一个Hibernate映射类CategoryBO（AbstractBaseBO只包含两个公共方法getId（）和setId（）..）：
@Entity
@Table(name = TabellenNamen.CATEGORY)
public class CategoryBO extends AbstractBaseBO {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "categoryid", nullable = false, unique = true)
private Long Id;
@Column(name = "category_name")
private String categoryName;
@ManyToOne( cascade = { CascadeType.ALL } )
@JoinColumn(name = "parentid")
private CategoryBO parent;
@OneToMany(mappedBy = "parent")
private ArrayList<CategoryBO> subCategories = new ArrayList<CetegoryBO>();
...getter/setters
}
但是，如果我尝试从数据库中获取类别条目列表，则会收到以下错误消息：
2013-05-30 16:58:57,329|DEBUG|main|hibernate.cfg.AnnotationBinder||Processing annotations of com.soebes.casestudy.bo.CategoryBO.subCategories
2013-05-30 16:58:57,329|DEBUG|main|hibernate.cfg.Ejb3Column||Binding column null. Unique false. Nullable true.
2013-05-30 16:58:57,329|DEBUG|main|hibernate.cfg.Ejb3Column||Binding column subCategories. Unique false. Nullable true.
2013-05-30 16:58:57,331|DEBUG|main|hibernate.cfg.Ejb3Column||Binding column null. Unique false. Nullable true.
FAILED: testGet
org.hibernate.AnnotationException: Illegal attempt to map a non collection as a @OneToMany, @ManyToMany or @CollectionOfElements: com.soebes.casestudy.bo.CategoryBO.subCategories
at org.hibernate.cfg.annotations.CollectionBinder.getCollectionBinder(CollectionBinder.java:266)
at org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:1448)
at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:754)
at org.hibernate.cfg.AnnotationConfiguration.processArtifactsOfType(AnnotationConfiguration.java:546)
at org.hibernate.cfg.AnnotationConfiguration.secondPassCompile(AnnotationConfiguration.java:291)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1319)
at org.hibernate.cfg.AnnotationConfiguration.buildSessionFactory(AnnotationConfiguration.java:867)
at com.soebes.casestudy.hibernate.HibernateUtil.getSession(HibernateUtil.java:36)
at com.soebes.casestudy.hibernate.HibernateUtil.beginTransaction(HibernateUtil.java:48)
我不明白。
那么我的误解或问题在哪里（我假设一些非常愚蠢的事情）......
更新：
在我根据Jimmy T的回答中相应地更新了代码之后：
@OneToMany(mappedBy = "parent")
private List<CategoryBO> subCategories;
我收到以下错误消息：
2013-05-30 19:08:33,363|DEBUG|main|hibernate.loader.Loader||done entity load
2013-05-30 19:08:33,364|INFO|main|event.def.DefaultLoadEventListener||Error performing load command
org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.soebes.casestudy.bo.CategoryBO#0]
at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:449)
at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:233)
at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:285)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152)
at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1080)
at org.hibernate.impl.SessionImpl.internalLoad(SessionImpl.java:1028)
at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:623)
at org.hibernate.type.EntityType.resolve(EntityType.java:431)
at org.hibernate.engine.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:140)
at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:898)
at org.hibernate.loader.Loader.doQuery(Loader.java:773)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:270)
at org.hibernate.loader.Loader.doList(Loader.java:2449)
at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2192)
at org.hibernate.loader.Loader.list(Loader.java:2187)
at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:119)
at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1706)
at org.hibernate.impl.CriteriaImpl.list(CriteriaImpl.java:347)
at com.soebes.casestudy.dao.HibernateDAO.findAll(HibernateDAO.java:68)
at com.soebes.casestudy.dao.HibernateDAO.get(HibernateDAO.java:75)
at com.soebes.casestudy.dao.IdDAO.get(IdDAO.java:23)
at com.soebes.casestudy.CategoryBOTest.testGet(CategoryBOTest.java:26)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:80)
at org.testng.internal.Invoker.invokeMethod(Invoker.java:714)
at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:901)
at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1231)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:128)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:111)
at org.testng.TestRunner.privateRun(TestRunner.java:767)
at org.testng.TestRunner.run(TestRunner.java:617)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:334)
我假设是由parentId为0的行引起的。这是有道理的，因为不存在categoryid为0的类别。但问题是：我如何将其映射到没有父类别的含义？
更新
所以在用这样的代码更新我的定义之后：
@ManyToOne( cascade = { CascadeType.ALL } )
@JoinColumn(name = "parentid")
@NotFound(action = NotFoundAction.IGNORE)
private CategoryBO parent;
@OneToMany(mappedBy = "parent")
@NotFound(action = NotFoundAction.IGNORE)
private List<CategoryBO> subCategories;
现在我得到一个类别列表。
答案 0 :(得分：4)
您应该使用List代替ArrayList。
Hibernate可能会放入另一个List实现来支持延迟加载。
编辑：您还必须使用NULL而不是0，否则Hibernate会搜索ID为0的对象。