Hibernate中的空集合与空集合 [英] Null vs empty collections in Hibernate

问题描述

  public class ClassA {
 @OneToMany（fetch = EAGER）
私人列表< ClassB> bList; 
 
  

当我读取 ClassA 从一个Hibernate会话中， bList 字段用 PersistentList 对象进行初始化， p>

我发现自己需要在列表为空的情况下，我需要Hibernate将 bList 字段初始化为 null ，而不是空的 PersistentList 。从理论上讲，Hibernate有它需要的信息来做这件事，因为列表上的提取是渴望的。问题在于，根据 Hibernate文档的第6.1节：
$ b

由于底层关系
模型，集合值属性
不支持null价值语义。
Hibernate不区分
a空集合引用和
空集合。

这使得完美的感觉，但我希望有人能想出一个狡猾的诡计来克服这个限制。我在想，也许一些监听/回调机制可能允许我用空引用替换空列表。

你试过检查getbList（）方法？你可以这样做：

  if（bList.isEmpty（））
 return null; 
返回bList; 
  

Hibernate将始终为您的引用创建一个对象，但您可以控制吸气剂和吸附剂。如果列表中有0个元素，则可以始终返回null。

Say I have the following Hibernate-mapped class:

public class ClassA {       
   @OneToMany(fetch=EAGER)
   private List<ClassB> bList;
}

When I read an object of ClassA from a Hibernate session, the bList field is initialized with a PersistentList object, as expected.

I find myself with a requirement where in situations where the list is empty, I need Hibernate to initialize the bList field to null, rather than with an empty PersistentList. In theory, Hibernate has the information it needs to do this, since the fetch on the list is eager. The problem is that according to section 6.1 of the Hibernate docs:

Due to the underlying relational model, collection-valued properties do not support null value semantics. Hibernate does not distinguish between a null collection reference and an empty collection.

This makes perfect sense, but I'm hoping someone can come up with a cunning ruse to overcome this limitation. I'm thinking perhaps some listener/callback mechanism might allow me to replace empty lists with null references.

解决方案

Have you tried to check in the getbList() method? You could do:

if(bList.isEmpty()) 
    return null;
return bList;

Hibernate will always create an object for your references, but you are allowed to control the data inside of the getter and setters. If the list has 0 elements you can always return null.

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