我应该使用哪个注释:@IdClass 或 @EmbeddedId

问题描述

JPA(Java Persistence API)规范有两种不同的方式来指定实体复合键:@IdClass 和 @EmbeddedId.

The JPA (Java Persistence API) specification has 2 different ways to specify entity composite keys: @IdClass and @EmbeddedId.

我在我的映射实体上同时使用了这两个注释，但对于那些不太熟悉 JPA 的人来说，结果是一团糟.

I'm using both annotations on my mapped entities, but it turns out to be a big mess to people who aren't very familiar with JPA.

我只想采用一种方式来指定复合键.哪一个真的是最好的?为什么?

I want to adopt only one way to specify composite keys. Which one is really the best? Why?

推荐答案

我认为 @EmbeddedId 可能更冗长，因为使用 @IdClass 你不能访问整个主要使用任何字段访问运算符的键对象.使用 @EmbeddedId 你可以这样做:

I consider that @EmbeddedId is probably more verbose because with @IdClass you cannot access the entire primary key object using any field access operator. Using the @EmbeddedId you can do like this:

@Embeddable class EmployeeId { name, dataOfBirth }
@Entity class Employee {
  @EmbeddedId EmployeeId employeeId;
  ...
}

这给出了构成复合键的字段的清晰概念，因为它们都聚合在一个通过字段访问运算符访问的类中.

This gives a clear notion of the fields that make the composite key because they are all aggregated in a class that is accessed trough a field access operator.

@IdClass 和 @EmbeddedId 的另一个区别在于编写 HQL 时:

Another difference with @IdClass and @EmbeddedId is when it comes to write HQL :

使用 @IdClass 你写:

select e.name from Employee e

并且使用 @EmbeddedId 你必须写:

and with @EmbeddedId you have to write:

select e.employeeId.name from Employee e

您必须为同一查询编写更多文本.有些人可能会争辩说，这与 IdClass 所提倡的更自然的语言不同.但大多数情况下，直接从查询中理解给定字段是组合键的一部分是非常有用的.

You have to write more text for the same query. Some may argue that this differs from a more natural language like the one promoted by IdClass. But most of the times understanding right from the query that a given field is part of the composite key is of invaluable help.

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