hibernate是否保留LinkedHashSet的顺序，如果是的话，怎么办？ [英] Does hibernate preserve the order of a LinkedHashSet and if so, how?

问题描述

hibernate是否保留一个LinkedHashSet的顺序，如果是的话，怎么样？如果这取决于数据库的类型，我想知道这是PostgreSQL。

背景：

我知道什么是LinkedHashSet，我之所以问这个问题是因为我将一些执行的函数的名称记录到一个'logError'表中，该表有多个与某些'functionName'表的关系。
我需要这些函数保持与执行它们时的顺序相同，所以首先找到相应的'functionName'对象，将它们放入LinkedHashSet（每个失败的函数之后），然后坚持'logError 'object。

现在，当我再次从数据库中获得'logError'对象时，它仍然会被排序吗？如果是这样的话，我很好奇Hibernate是如何完成这项工作的。

解决方案

第一：我假设你正在谈论两个实体。就像

  @Entity 
 public class A {
 @ManyToMany 
 @JoinTable（name =A_B，joinColumns = {@JoinColumn（name =A_fk）}，inverseJoinColumns = {@JoinColumn（name =B_fk）}）
 private Set< B> bSet = new LinkedHashSet< B>（）; 
 
  

Hibernate不会自行保存订单！

如果您查看从数据库加载实体 A 时使用的类，那么 Set bSet 类型为 PersistentSet ，它是另一个包装设置，这是（在我的情况下）正常 HashSet 。 （ HashSet 不会保留它的元素顺序。）



即使Hibernate使用 List 或 LinkedHashSet ，但仍然不推荐将实现建立在自然数据库（不保证）数据库的顺序上。对于MySQL，它是某种反模式。

但是您可以使用 @Sort 注释（ org.hibernate.annotations.Sort ）来使你的排序清晰。例如：

  @OneToMany（mappedBy =as）
 @Sort（type = SortType.COMPARATOR，comparator = MyBComparator.class）; 
 public SortedSet< C> CS; 
  

@see：

---

$ b $ - 返回子对象在jpa-query b

2012年9月1日由ŁukaszRzeszotarski添加：

但我们必须记住，使用 @Sort 批注会导致内存中的对象（jvm）而不是sql中的对象。相反，我们可以使用 @OrderBy 注释来导致sql服务器端的排序。这两个注释在我的（ŁukaszRzeszotarski）意见中有一个缺点，即默认设置了排序。 I（ŁukaszRzeszotarski）宁愿hibernate使用自己的LinkedHashSet实现，当它'看到'使用了order by子句时。

@see：在sql中定制hibernate

Does hibernate preserve the order of a LinkedHashSet and if so, how? In case this depends on the type of database, I'd like to know this for PostgreSQL.

Background:

I know what a LinkedHashSet is for, and the reason I'm asking this is because I'm logging the names of some functions I execute to a 'logError' table that has a many-to-many relation to some 'functionName' table. I need these functions to remain in the same order as when I executed them, so first I find the corresponding 'functionName' objects, put them in a LinkedHashSet (after each function that failed) and then I persist the 'logError' object.

Now when I get the 'logError' object from the database again, will it still be ordered? And if so, I was curious how this is done by Hibernate.

解决方案

First: I assume you are talking about a relationship between two entities. Something like

@Entity
public class A {
@ManyToMany
    @JoinTable(name = "A_B", joinColumns = { @JoinColumn(name = "A_fk") }, inverseJoinColumns = { @JoinColumn(name = "B_fk") })
    private Set<B> bSet = new LinkedHashSet<B>();
}

Hibernate does not preserve the order by itself!

If you have a look at the classes used when entity A is loaded from the database, then the Set bSet is of type PersistentSet, which is a wrapper around another Set, and this is (in my case) a normal HashSet. (HashSet does not preserve the order of its elements.)

Even if Hibernate used List or LinkedHashSet, it is still inadvisable to base the implementation on the natural (not guaranteed) database order. For MySQL it is some kind of anti-pattern.

But you can use the @Sort annotation (org.hibernate.annotations.Sort) to make your sorting explicit. For example:

@OneToMany(mappedBy = "as")
@Sort(type = SortType.COMPARATOR, comparator = MyBComparator.class);
public SortedSet<C> cs;

@see: Ordering return of Child objects in JPA query

---

Added by Łukasz Rzeszotarski on 1th Sep 2012:

But we have to remember that using @Sort annotation causes sorting objects in memory (jvm) and not in sql. Instead we can use @OrderBy annotation that causes sorting on the sql server side. Both of these annotations have in my (Łukasz Rzeszotarski ) opinion one weakness that set up ordering by default. I (Łukasz Rzeszotarski ) would rather hibernate uses own LinkedHashSet implementation when it 'sees' that order by clause is used.

@see: Hibernate ordering in sql

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