java - 将哈希函数委托(delegate)给 hibernate 中未初始化的委托(delegate)会导致更改哈希代码

Question

我对使用 hibernate 委托(delegate)给未初始化对象的 hashCode() 有问题。

我的数据模型如下所示(以下代码经过高度修剪以强调问题并因此损坏，请勿复制!):

class Compound {
  @FetchType.EAGER
  Set<Part> parts = new HashSet<Part>();

  String someUniqueName;

  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((getSomeUniqueName() == null) ? 0 : getSomeUniqueName().hashCode());
    return result;
  }
}

class Part {
  Compound compound;

  String someUniqueName;

  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((getCompound() == null) ? 0 : getCompound().hashCode());
    result = prime * result + ((getSomeUniqueName() == null) ? 0 : getSomeUniqueName().hashCode());
    return result;
  }
}

请注意，hashCode() 的实现完全遵循给定的建议 in the hibernate documentation .

现在，如果我加载类型为 Compound 的对象，它会急切地加载带有部件的 HasSet。这会调用部件上的 hashCode()，部件又会调用复合上的 hashCode()。然而，问题是此时，并非所有考虑用于创建复合的 hashCode 的值都可用。因此，初始化完成后各部分的哈希码发生变化，从而破坏HashSet的契约并导致各种难以追踪的错误(如例如，在零件组中设置相同的对象两次)。

所以我的问题是:避免这个问题的最简单解决方案是什么(我想避免为自定义加载/初始化编写类)？我在这里做错了什么吗？

编辑:我在这里遗漏了什么吗？这似乎是一个基本问题，为什么我在任何地方都找不到相关信息？

Instead of using the database identifier for the equality comparison, you should use a set of properties for equals() that identify your individual objects. [...] No need to use the persistent identifier, the so called "business key" is much better. It's a natural key, but this time there is nothing wrong in using it! (article from hibernate)

和

It is recommended that you implement equals() and hashCode() using Business key equality. Business key equality means that the equals() method compares only the properties that form the business key. It is a key that would identify our instance in the real world (a natural candidate key). (hibernate documentation)

编辑:这是加载发生时的堆栈跟踪(以防万一)。此时，属性someUniqueName为null，因此hashCode计算错误。

Compound.getSomeUniqueName() line: 263  
Compound.hashCode() line: 286   
Part.hashCode() line: 123   
HashMap<K,V>.put(K, V) line: 372    
HashSet<E>.add(E) line: 200 
HashSet<E>(AbstractCollection<E>).addAll(Collection<? extends E>) line: 305 
PersistentSet.endRead() line: 352   
CollectionLoadContext.endLoadingCollection(LoadingCollectionEntry, CollectionPersister) line: 261   
CollectionLoadContext.endLoadingCollections(CollectionPersister, List) line: 246    
CollectionLoadContext.endLoadingCollections(CollectionPersister) line: 219  
EntityLoader(Loader).endCollectionLoad(Object, SessionImplementor, CollectionPersister) line: 1005  
EntityLoader(Loader).initializeEntitiesAndCollections(List, Object, SessionImplementor, boolean) line: 993  
EntityLoader(Loader).doQuery(SessionImplementor, QueryParameters, boolean) line: 857    
EntityLoader(Loader).doQueryAndInitializeNonLazyCollections(SessionImplementor, QueryParameters, boolean) line: 274 
EntityLoader(Loader).loadEntity(SessionImplementor, Object, Type, Object, String, Serializable, EntityPersister, LockOptions) line: 2037    
EntityLoader(AbstractEntityLoader).load(SessionImplementor, Object, Object, Serializable, LockOptions) line: 86 
EntityLoader(AbstractEntityLoader).load(Serializable, Object, SessionImplementor, LockOptions) line: 76 
SingleTableEntityPersister(AbstractEntityPersister).load(Serializable, Object, LockOptions, SessionImplementor) line: 3293  
DefaultLoadEventListener.loadFromDatasource(LoadEvent, EntityPersister, EntityKey, LoadEventListener$LoadType) line: 496    
DefaultLoadEventListener.doLoad(LoadEvent, EntityPersister, EntityKey, LoadEventListener$LoadType) line: 477    
DefaultLoadEventListener.load(LoadEvent, EntityPersister, EntityKey, LoadEventListener$LoadType) line: 227  
DefaultLoadEventListener.proxyOrLoad(LoadEvent, EntityPersister, EntityKey, LoadEventListener$LoadType) line: 269   
DefaultLoadEventListener.onLoad(LoadEvent, LoadEventListener$LoadType) line: 152    
SessionImpl.fireLoad(LoadEvent, LoadEventListener$LoadType) line: 1090  
SessionImpl.internalLoad(String, Serializable, boolean, boolean) line: 1038 
ManyToOneType(EntityType).resolveIdentifier(Serializable, SessionImplementor) line: 630 
ManyToOneType(EntityType).resolve(Object, SessionImplementor, Object) line: 438 
TwoPhaseLoad.initializeEntity(Object, boolean, SessionImplementor, PreLoadEvent, PostLoadEvent) line: 139   
QueryLoader(Loader).initializeEntitiesAndCollections(List, Object, SessionImplementor, boolean) line: 982   
QueryLoader(Loader).doQuery(SessionImplementor, QueryParameters, boolean) line: 857 
QueryLoader(Loader).doQueryAndInitializeNonLazyCollections(SessionImplementor, QueryParameters, boolean) line: 274  
QueryLoader(Loader).doList(SessionImplementor, QueryParameters) line: 2542  
QueryLoader(Loader).listIgnoreQueryCache(SessionImplementor, QueryParameters) line: 2276    
QueryLoader(Loader).list(SessionImplementor, QueryParameters, Set, Type[]) line: 2271   
QueryLoader.list(SessionImplementor, QueryParameters) line: 459 
QueryTranslatorImpl.list(SessionImplementor, QueryParameters) line: 365 
HQLQueryPlan.performList(QueryParameters, SessionImplementor) line: 196 
SessionImpl.list(String, QueryParameters) line: 1268    
QueryImpl.list() line: 102  
<my code where the query is executed>

Answer 1

您有一个完美的合法用例，它确实应该有效。但是，如果在设置“someUniqueName”之前设置复合对象的“部分”，那么在常规 Java 中也会遇到同样的问题。

因此，如果您可以说服 Hibernate 在“部分”属性之前设置“someUniqueName”属性。您是否尝试过在 java 类中对它们重新排序？或者将“零件”重命名为“zparts”？ hibernate 文档只是说不能保证顺序。我会在 hibernate 中提交一个错误以允许执行此命令...

另一个可能更简单的解决方案:

class Part {
  public int hashCode() {
    //don't include getCompound().hashCode()
    return getSomeUniqueName() == null ? 0 : getSomeUniqueName().hashCode();
  }

  public boolean equals(Object o)
  {
    if (this == o) return true;
    if (!o instanceof Part) return false;

    Part part = (Part) o;

    if (getCompound() != null ? !getCompound().equals(part.getCompound()) : part.getCompound()!= null) 
       return false;
    if (getSomeUniqueName()!= null ? !getSomeUniqueName().equals(part.getSomeUniqueName()) : part.getSomeUniqueName()!= null)
        return false;

    return true;
  }
}

在 Compound.equals() 中确保它也以

开头

public boolean equals(Object o)
{
    if (this == o) return true;

这应该可以避免您现在遇到的问题。

hashCode() 方法中的每个属性都应该在 equals() 方法中，但不一定相反。