JPA Hibernate集合不会被延迟加载 [英] JPA Hibernate collections not lazily loaded

问题描述

我有一个JPA设置，如果我不使用延迟加载，几乎整个数据库都会被加载。我也直接在模型上使用序列化，所以有时我需要初始化代理。

我只想对集合使用延迟加载。事实上，一些奇异的实体被热切地提取工作得很好。但是，无论我如何设置集合，我都从未得到代理集合，我总是得到完全加载的集合。

这是一些示例代码：

  @Entity 
公共类Thread实现Externalizable {
 @OneToMany（mappedBy =parentThread，fetch = FetchType。 LAZY）
 public List< Reply> getReplies（）{
返回答案; 
 
 
 
 
 
 $ b 
所以这里的问题是当我检查调试器时，persistantBag-list的答复总是充满信息，并且是实际的答复对象，而不是我想要的空代理。 
 
 
 
我使用entityManager.find（Thread.class，ID），当我想要线程和像这样的所有集合总是完全加载，无论我有fetch = FetchType.LAZY或不。
 
 
 
据我所知，设置fetchtype懒惰应该按我的意愿工作。另外，我希望在使用thread.getReplies（）时加载实体，以便序列化并将它们发送到客户端。我不知道getReplies是否可以与代理实体一起工作，因为我从来没有收集到任何延迟加载的集合。
 
 
 
一边不使用Intellij并用它来设置JPA与休眠。 
我也问过一个类似的问题，我希望这个集合完全是空的，但我不确定这是否可行，我用它来询问这个问题。
 
 
 
  JPA Hibernate希望延迟加载返回空集合
 a> 
 
 
 
我还没有看到有关stackoverflow这个看似基本问题的好答案。请仅回复一个链接，如果问题是以教学方式真正回答的，因为我是JPA / Hibernate和数据库的新手。
 
 
 
非常感谢提前！
解决方案
一个懒集合不包含代理。该集合本身是一个延迟加载的集合。这意味着当你从数据库中获得一个线程时，它的回复列表将不会被初始化。  $ b 调用 getReplies（）会简单的返回这个未初始化的列表。只有在列表本身调用一个方法时（比如 size（）或 iterator（）），Hibernate才会初始化列表通过执行一个SQL查询加载线程所有回复的状态。
 
 
使用调试器来检查集合包含的内容不是一个好主意，因为调试器通常在列表后面的列表中调用方法，这会导致列表自行初始化。您可以使用  Hibernate.isInitialized（thread.getReplies（）） 方法 
 
 
关于你的其他问题：Hibernate用于将数据库行映射到对象。如果一个线程有回复，Hibernate永远不会通过返回一个空列表来告诉你它没有任何回应。这将是一个严重的错误。如果您不想加载回复，则不要在回复列表中调用任何方法。
 
I have a JPA setup in such a way that if I do not use lazy load, almost the entire database will be loaded. I also use serializing directly on the models so sometimes I need to initialize the proxies.

I only want to use lazy load on the collections. The fact that some singular entities are fetched eagerly works just fine. But no matter how I try to setup the collections I never get a collection of proxies, I always get the fully loaded collection.

This is some example code:
@Entity
public class Thread implements Externalizable {
    @OneToMany(mappedBy = "parentThread", fetch = FetchType.LAZY)
    public List<Reply> getReplies() {
        return replies;
    }
So the problem here is that when I check the debugger, the persistantBag-list of replies are always filled with info and are the actual Reply objects instead of empty proxies which I want. 

I use entityManager.find(Thread.class, "ID") when I want the thread and all collections like these are always fully loaded no matter if I have fetch = FetchType.LAZY or not.

As far as I understand, setting fetchtype lazy should work as I want it to. Also I would like the entities to be loaded when using the thread.getReplies() so that I can serialize and send them to the client. I do not know if getReplies will work with proxied entities since I never got any collection to be lazily loaded.

On a side not I use Intellij and used it to setup JPA with Hibernate.
I have also asked a similar question where I want the collection to be completely empty but I am not sure if that is possible and I am therefor asking this question instead.

JPA Hibernate want lazy load to return empty collection

I have not yet seen a good answer on this seemingly basic question on stackoverflow. Please only reply with a link if the question is really answered in a pedagogical way since I am new to JPA/Hibernate and really databases as well.

Thank you very much in advance!
解决方案
A lazy collection doesn't contain proxies. The collection itself is a lazy-loaded collection. This means that when you get a thread from the database, its list of replies won't be initialized. 

Calling getReplies() will simply return this not-initialized list. Only when calling a method on the list itself (like size() or iterator()) will Hibernate initialize the list by executing a SQL query loading the state of all the replies of the thread.

Using a debugger to check what the collection contains is not a good idea, because the debugger usually calls methods on the list behind your back, which causes the list to initialize itself. You can check if a collection is initialized using the Hibernate.isInitialized(thread.getReplies()) method.

Regarding your other question: Hibernate is used to map database rows to objects. If a thread has replies, Hibernate will never tell you that it doesn't have any by returning you an empty list. That would be a serious bug. If you don't want to load the replies, simply don't call any method on the list of replies.

                        
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