JPA关系映射之one-to-one、one-to-many、many-to-one和many-to-many

SpringDataJPA是Spring Data的一个子项目，通过提供基于JPA的Repository极大的减少了JPA作为数据访问方案的代码量，你仅仅需要编写一个接口集成下SpringDataJPA内部定义的接口即可完成简单的CRUD操作。

前言

本篇文章引导你通过Spring Boot，Spring Data JPA和MySQL 映射一对一外键、一对一主键、一对多，多对一，多对多、多对多额外的列的关系。

准备

• JDK 1.8 或更高版本
• Maven 3 或更高版本
• MySQL Server 5.6

技术栈

• Spring Data JPA
• Spring Boot
• MySQL

目录结构

父pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0modelVersion>

    <groupId>cn.merryyougroupId>
    <artifactId>jpa-exampleartifactId>
    <version>1.0-SNAPSHOTversion>
    <modules>
        <module>one-to-one-foreignkeymodule>
        <module>one-to-one-primarykeymodule>
        <module>one-to-manymodule>
        <module>many-to-manymodule>
        <module>many-to-many-extra-columnsmodule>
    modules>
    <packaging>pompackaging>


    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>io.spring.platformgroupId>
                <artifactId>platform-bomartifactId>
                <version>Brussels-SR6version>
                <type>pomtype>
                <scope>importscope>
            dependency>
        dependencies>
    dependencyManagement>
project>

一对一外键

目录结构

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>jpa-exampleartifactId>
        <groupId>cn.merryyougroupId>
        <version>1.0-SNAPSHOTversion>
    parent>
    <modelVersion>4.0.0modelVersion>

    <artifactId>one-to-one-foreignkeyartifactId>

    <properties>
        <project.build.sourceEncoding>UTF-8project.build.sourceEncoding>
        <java.version>1.8java.version>
    properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.bootgroupId>
            <artifactId>spring-boot-starter-data-jpaartifactId>
        dependency>

        <dependency>
            <groupId>mysqlgroupId>
            <artifactId>mysql-connector-javaartifactId>
            <scope>runtimescope>
        dependency>

        <dependency>
            <groupId>org.projectlombokgroupId>
            <artifactId>lombokartifactId>
        dependency>

        <dependency>
            <groupId>org.springframework.bootgroupId>
            <artifactId>spring-boot-starter-testartifactId>
            <scope>testscope>
        dependency>
    dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.bootgroupId>
                <artifactId>spring-boot-maven-pluginartifactId>
            plugin>
            <plugin>
                <groupId>org.apache.maven.pluginsgroupId>
                <artifactId>maven-compiler-pluginartifactId>
                <version>3.6.1version>
                <configuration>
                    <source>1.8source>
                    <target>1.8target>
                configuration>
            plugin>
        plugins>
    build>
project>

一对一关系

book.book_detail_id 和 book_detail.id

db.sql

CREATE DATABASE  IF NOT EXISTS `jpa_onetoone_foreignkey`;
USE `jpa_onetoone_foreignkey`;

--
-- Table structure for table `book_detail`
--

DROP TABLE IF EXISTS `book_detail`;
CREATE TABLE `book_detail` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`number_of_pages` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

--
-- Table structure for table `book`
--

DROP TABLE IF EXISTS `book`;
CREATE TABLE `book` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`book_detail_id` int(11) unsigned DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fk_book_bookdetail` (`book_detail_id`),
CONSTRAINT `fk_book_bookdetail` FOREIGN KEY (`book_detail_id`) REFERENCES `book_detail` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

实体类

Book

@Entity
@Data
@Table(name = "book")
public class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private int id;

    @Column(name = "name")
    private String name;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "book_detail_id")
//    @Lazy(false)
    private BookDetail bookDetail;

    public Book() {
    }

    public Book(String name, BookDetail bookDetail) {
        this.name = name;
        this.bookDetail = bookDetail;
    }
}

BookDetail

@Entity
@Table(name = "book_detail")
@Data
public class BookDetail {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Integer id;

    @Column(name = "number_of_pages")
    private Integer numberOfPages;

    @OneToOne(mappedBy = "bookDetail")
    private Book book;

    public BookDetail() {
    }

    public BookDetail(Integer numberOfPages) {
        this.numberOfPages = numberOfPages;
    }
}

• @Table声明此对象映射到数据库的数据表，通过它可以为实体指定表(talbe),目录(Catalog)和schema的名字。该注释不是必须的，如果没有则系统使用默认值(实体的短类名)。
• @Id 声明此属性为主键。该属性值可以通过应该自身创建，但是Hibernate推荐通过Hibernate生成

• @GeneratedValue 指定主键的生成策略。

  1. TABLE：使用表保存id值
  2. IDENTITY：identitycolumn
  3. SEQUENCR ：sequence
  4. AUTO：根据数据库的不同使用上面三个

• @Column 声明该属性与数据库字段的映射关系。

• @OneToOne 一对一关联关系
• @JoinColumn 指定关联的字段

Spring Data JPA Repository

public interface BookRepository extends JpaRepository<Book, Integer> {
}

Spring Data JPA包含了一些内置的Repository，实现了一些常用的方法：findone，findall，save等。

application.yml

spring:
  datasource:
    url: jdbc:mysql://localhost/jpa_onetoone_foreignkey
    username: root
    password: admin
    driver-class-name: com.mysql.jdbc.Driver
  jpa:
    show-sql: true

BookRepositoryTest

@RunWith(SpringRunner.class)
@SpringBootTest
@Slf4j
public class BookRepositoryTest {

    @Autowired
    private BookRepository bookRepository;

    @Test
    public void saveTest() throws Exception {

        List books = new ArrayList<>();
        books.add(new Book("Book one", new BookDetail(1)));
        books.add(new Book("Book two", new BookDetail(2)));
        books.add(new Book("Book three", new BookDetail(3)));
        List bookList = bookRepository.save(books);
        Assert.assertNotNull(bookList);
        Assert.assertEquals(3, bookList.size());
    }

    @Test
    public void findBooksTest() throws Exception{
        List books = bookRepository.findAll();
        for (Book book: books) {
            log.info(book.toString());
        }
    }

}

其它

剩下的一对一主键、一对多，多对一，多对多、多对多额外的列参考如上。

代码下载

从我的 github 中下载，https://github.com/longfeizheng/jpa-example