1. Introduction

1.绪论

In this tutorial, we’ll learn about Composite Primary Keys and the corresponding annotations in JPA.

在本教程中，我们将学习JPA中的复合主键和相应的注释。

2. Composite Primary Keys

2.复合主键

A composite primary key, also called a composite key, is a combination of two or more columns to form a primary key for a table.

复合主键，也被称为复合键，是由两个或多个列组成的表的主键。

In JPA, we have two options to define the composite keys: the @IdClass and @EmbeddedId annotations.

在JPA中，我们有两个选项来定义复合键：@IdClass和@EmbeddedId注释。

In order to define the composite primary keys, we should follow some rules:

为了定义复合主键，我们应该遵循一些规则：。

The composite primary key class must be public.
It must have a no-arg constructor.
It must define the equals() and hashCode() methods.
It must be Serializable.

3. The IdClass Annotation

3.IdClass注释

Let’s say we have a table called Account and it has two columns, accountNumber and accountType, that form the composite key. Now we have to map it in JPA.

假设我们有一个名为Account的表，它有两列，accountNumber和accountType,构成复合键。现在我们必须在JPA中对其进行映射。

As per the JPA specification, let’s create an AccountId class with these primary key fields:

根据JPA规范，让我们创建一个带有这些主键字段的AccountId类。

public class AccountId implements Serializable {
    private String accountNumber;

    private String accountType;

    // default constructor

    public AccountId(String accountNumber, String accountType) {
        this.accountNumber = accountNumber;
        this.accountType = accountType;
    }

    // equals() and hashCode()
}

Next let’s associate the AccountId class with the entity Account.

接下来让我们将AccountId类与实体Account联系起来。

In order to do that, we need to annotate the entity with the @IdClass annotation. We must also declare the fields from the AccountId class in the entity Account and annotate them with @Id:

为了做到这一点，我们需要用@IdClass 注解来注释该实体。我们还必须在实体Account中声明AccountId类中的字段，并用@Id注释它们。

@Entity
@IdClass(AccountId.class)
public class Account {
    @Id
    private String accountNumber;

    @Id
    private String accountType;

    // other fields, getters and setters
}

4. The EmbeddedId Annotation

4.EmbeddedId注释

@EmbeddedId is an alternative to the @IdClass annotation.

@EmbeddedId是@IdClass注解的一个替代。

Let’s consider another example where we have to persist some information of a Book, with title and language as the primary key fields.

让我们考虑另一个例子，我们必须持久化一个书的一些信息，标题和语言是主键字段。

In this case, the primary key class, BookId, must be annotated with @Embeddable:

在这种情况下，主键类，BookId，必须用@Embeddable来注释。

@Embeddable
public class BookId implements Serializable {
    private String title;
    private String language;

    // default constructor

    public BookId(String title, String language) {
        this.title = title;
        this.language = language;
    }

    // getters, equals() and hashCode() methods
}

Then we need to embed this class in the Book entity using @EmbeddedId:

然后我们需要在Book实体中使用@EmbeddedId嵌入这个类。

@Entity
public class Book {
    @EmbeddedId
    private BookId bookId;

    // constructors, other fields, getters and setters
}

5. @IdClass vs @EmbeddedId

5.@IdClass vs @EmbeddedId

As we can see, the difference on the surface between these two is that with @IdClass we had to specify the columns twice, once in AccountId and again in Account; however, with @EmbeddedId we didn’t.

正如我们所看到的，这两者之间表面上的区别是，使用@IdClass时，我们必须指定两次列，一次在AccountId中，另一次在Account中；然而，使用@EmbeddedId时，我们没有。

There are some other trade offs though.

但也有一些其他的交换条件。

For example, these different structures affect the JPQL queries that we write.

例如，这些不同的结构会影响我们编写的JPQL查询。

With @IdClass, the query is a bit simpler:

有了@IdClass，查询就比较简单了。

SELECT account.accountNumber FROM Account account

With @EmbeddedId, we have to do one extra traversal:

使用@EmbeddedId，我们必须做一个额外的遍历。

SELECT book.bookId.title FROM Book book

Also, @IdClass can be quite useful in places where we are using a composite key class that we can’t modify.

另外，@IdClass在我们使用一个不能修改的复合键类的地方，可以相当有用。

If we’re going to access parts of the composite key individually, we can make use of @IdClass, but in places where we frequently use the complete identifier as an object, @EmbeddedId is preferred.

如果我们要单独访问复合键的部分，我们可以使用@IdClass，但是在我们经常使用完整标识符作为对象的地方，@EmbeddedId是首选。

6. Conclusion

6.结语

In this brief article, we explored composite primary keys in JPA.

在这篇简短的文章中，我们探讨了JPA中的复合主键。

As always, the complete code for this article can be found over on Github.

一如既往，本文的完整代码可以在Github上找到。