Java在自定義克隆期間檢測循環引用

Question

我正在爲每個實體編寫自定義克隆方法。對於深層複製，有一種方法可以檢測循環引用，還是必須手動找出並將克隆限制爲單向而不是雙向。

例如，我們使用hibernate，因此User對象具有對Address的引用，Address對User有引用。嘗試查看是否可以進行地址和用戶的深層副本而不會遇到循環參考問題

Answer 1

要實現此目的，您需要一個對已克隆對象的引用的Map。我們實現深克隆是這樣的：

在我們的實體基礎類：

public void deepClone() { 
    Map<EntityBase,EntityBase> alreadyCloned = 
     new IdentityHashMap<EntityBase,EntityBase>(); 
    return deepClone(this,alreadyCloned); 
} 

private static EntityBase deepClone(EntityBase entity, 
            Map<EntityBase,EntityBase> alreadyCloned) { 
    EntityBase clone = alreadyCloned.get(entity); 
    if(clone != null) { 
     return alreadyClonedEntity; 
    } 
    clone = newInstance(entity.getClass); 
    alreadyCloned.put(this,clone); 
    // fill clone's attributes from original entity. Call 
    // deepClone(entity,alreadyCloned) 
    // recursively for each entity valued object. 
    ... 
} 

Answer 2

@Daniel：非常感謝你爲這個偉大的答案！

我剛剛使用你的代碼，並根據我的需要對其進行了一些修改，這使得它更容易與子類一起使用。也許別人有興趣。也因此，這裏是我的基礎類代碼：

/** 
* Perform a deep clone. 
* 
* @return Deep Clone. 
* @throws CloneNotSupportedException 
*/ 
@SuppressWarnings("unchecked") 
public VersionedEntityImpl deepClone() throws CloneNotSupportedException { 
    Map<VersionedEntityImpl, VersionedEntityImpl> alreadyCloned = new IdentityHashMap<VersionedEntityImpl, VersionedEntityImpl>(); 
    return deepClone(this, alreadyCloned); 
} 

/** 
* Perform a deep clone. 
* 
* @param entity 
* @param alreadyCloned 
* @return Deep Clone. 
* @throws CloneNotSupportedException 
*/ 
@SuppressWarnings("unchecked") 
protected VersionedEntityImpl deepClone(VersionedEntityImpl entity, 
     Map<VersionedEntityImpl, VersionedEntityImpl> alreadyCloned) throws CloneNotSupportedException { 
    if (entity != null) { 
     VersionedEntityImpl clone = alreadyCloned.get(entity); 
     if (clone != null) { 
      return clone; 
     } 
     clone = entity.clone(); 
     alreadyCloned.put(entity, clone); 
     return entity.deepCloneEntity(clone, alreadyCloned); 
    } 
    return null; 
} 

/** 
* Method performing a deep clone of an entity (circles are eliminated automatically). Calls 
* deepClone(entity,alreadyCloned) recursively for each entity valued object. 
* 
* @param clone 
* @param alreadyCloned 
* @return clone 
* @throws CloneNotSupportedException 
*/ 
protected abstract VersionedEntityImpl deepCloneEntity(VersionedEntityImpl clone, 
     Map<VersionedEntityImpl, VersionedEntityImpl> alreadyCloned) throws CloneNotSupportedException; 

什麼投入的子類：

@SuppressWarnings("unchecked") 
@Override 
protected VersionedEntityImpl deepCloneEntity(VersionedEntityImpl clone, 
     Map<VersionedEntityImpl, VersionedEntityImpl> alreadyCloned) throws CloneNotSupportedException { 
    // fill clone's attributes from original entity. Call 
    // deepClone(entity,alreadyCloned) 
    // recursively for each entity valued object. 
    if (this.associatedItems != null) { 
     List<SomeClass> listClone = new LinkedList<SomeClass>(); 
     for (SomeClass someClass: this.associatedItems) { 
      listClone.add((SomeClass) super.deepClone(someClass, alreadyCloned)); 
     } 
     ((SomeOtherClass) clone).setAssociatedItems(listClone); 
    } 
    ((SomeOtherClass) clone).setYetAnotherItem((YetAnotherClass) super.deepClone(this.yai, alreadyCloned)); 

    return clone; 
} 

它不是完美的作爲然而，但它得到現在完成的工作很好:)