java – 在hibernate中将散列函数委托给未初始化的委托会导致更

发布时间：2020-12-14 06:04:19 所属栏目：Java 来源：网络整理

导读：我有一个hashCode()的问题,它使用hibernate委托给未初始化的对象. 我的数据模型看起来如下(以下代码经过高度修剪以强调问题因此破坏,不要复制！)： class Compound { @FetchType.EAGER SetPart parts = new HashSetPart(); String someUniqueName; public in

我有一个hashCode()的问题,它使用hibernate委托给未初始化的对象.

我的数据模型看起来如下(以下代码经过高度修剪以强调问题因此破坏,不要复制！)：

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in the hibernate documentation给出的建议.

现在,如果我加载一个Compound类型的对象,它会急切地加载带有部件的HasSet.这会调用部件上的hashCode(),然后调用复合体上的hashCode().但问题是,此时并非所有考虑用于创建复合的hashCode的值都可用.因此,部件的hashCode在初始化完成后改变,从而制动HashSet的合同并导致各种难以跟踪的错误(例如,在部件组中设置两次相同的对象).

所以我的问题是：避免这个问题的最简单的解决方案是什么(我想避免为自定义加载/初始化编写类)？我完全做错了吗？

编辑：我错过了什么吗？这似乎是一个基本问题,为什么我在任何地方都找不到任何关于它的东西？

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! (07001)

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). (07002)

编辑：这是加载发生时的堆栈跟踪(如果这有帮助).在那个时间点,属性someUniqueName为null,因此hashCode被错误地计算.

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,boolean) line: 993  
EntityLoader(Loader).doQuery(SessionImplementor,QueryParameters,boolean) line: 857    
EntityLoader(Loader).doQueryAndInitializeNonLazyCollections(SessionImplementor,boolean) line: 274 
EntityLoader(Loader).loadEntity(SessionImplementor,Type,String,Serializable,EntityPersister,LockOptions) line: 2037    
EntityLoader(AbstractEntityLoader).load(SessionImplementor,LockOptions) line: 86 
EntityLoader(AbstractEntityLoader).load(Serializable,LockOptions) line: 76 
SingleTableEntityPersister(AbstractEntityPersister).load(Serializable,LockOptions,SessionImplementor) line: 3293  
DefaultLoadEventListener.loadFromDatasource(LoadEvent,EntityKey,LoadEventListener$LoadType) line: 496    
DefaultLoadEventListener.doLoad(LoadEvent,LoadEventListener$LoadType) line: 477    
DefaultLoadEventListener.load(LoadEvent,LoadEventListener$LoadType) line: 227  
DefaultLoadEventListener.proxyOrLoad(LoadEvent,LoadEventListener$LoadType) line: 269   
DefaultLoadEventListener.onLoad(LoadEvent,LoadEventListener$LoadType) line: 152    
SessionImpl.fireLoad(LoadEvent,LoadEventListener$LoadType) line: 1090  
SessionImpl.internalLoad(String,boolean,boolean) line: 1038 
ManyToOneType(EntityType).resolveIdentifier(Serializable,SessionImplementor) line: 630 
ManyToOneType(EntityType).resolve(Object,Object) line: 438 
TwoPhaseLoad.initializeEntity(Object,PreLoadEvent,PostLoadEvent) line: 139   
QueryLoader(Loader).initializeEntitiesAndCollections(List,boolean) line: 982   
QueryLoader(Loader).doQuery(SessionImplementor,boolean) line: 857 
QueryLoader(Loader).doQueryAndInitializeNonLazyCollections(SessionImplementor,boolean) line: 274  
QueryLoader(Loader).doList(SessionImplementor,QueryParameters) line: 2542  
QueryLoader(Loader).listIgnoreQueryCache(SessionImplementor,QueryParameters) line: 2276    
QueryLoader(Loader).list(SessionImplementor,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>

解决方法

你有一个完美的合法用例,事实上它应该有效.但是,如果在设置’someUniqueName’之前设置Compound对象的’parts’,那么在常规Java中会遇到同样的问题.

因此,如果你能说服hibernate在’parts’属性之前设置’someUniqueName’属性.您是否尝试过在java类中重新排序它们？或者将“部分”重命名为“zparts”？ hibernate文档只是说订单不能保证.我在hibernate中提交了一个错误,允许强制执行此命令…

另一种可能更容易的解决方案：

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

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

这应该避免你现在遇到的问题.

hashCode()方法中的每个属性都应该在equals()方法中,但不一定是相反的方式.

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!