JPA关系映射系列四：many-to-many 关联映射

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

实现

many-to-many

关联映射。

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

<?xml version="1.0" encoding="UTF-8"?>
<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>4.0.0</modelVersion>

<groupId>cn.merryyou</groupId>
<artifactId>jpa-example</artifactId>
<version>1.0-SNAPSHOT</version>
<modules>
<module>one-to-one-foreignkey</module>
<module>one-to-one-primarykey</module>
<module>one-to-many</module>
<module>many-to-many</module>
<module>many-to-many-extra-columns</module>
</modules>
<packaging>pom</packaging>

<dependencyManagement>
<dependencies>
<dependency>
<groupId>io.spring.platform</groupId>
<artifactId>platform-bom</artifactId>
<version>Brussels-SR6</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
</project>

多对多关联映射

目录结构





pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<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-example</artifactId>
<groupId>cn.merryyou</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
<modelVersion>4.0.0</modelVersion>

<artifactId>many-to-many</artifactId>

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

<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<scope>runtime</scope>
</dependency>

<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
</dependency>

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
</dependencies>

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

</project>

多对多关联

book.id

 和 

publisher.id

 多对多关联表

book_publisher

 



db.sql

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

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

DROP TABLE IF EXISTS `book`;
CREATE TABLE `book` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;

--
-- Table structure for table `publisher`
--

DROP TABLE IF EXISTS `publisher`;
CREATE TABLE `publisher` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=19 DEFAULT CHARSET=utf8;
--
-- Table structure for table `book_publisher`
--

DROP TABLE IF EXISTS `book_publisher`;
CREATE TABLE `book_publisher` (
`book_id` int(10) unsigned NOT NULL,
`publisher_id` int(10) unsigned NOT NULL,
PRIMARY KEY (`book_id`,`publisher_id`),
KEY `fk_bookpublisher_publisher_idx` (`publisher_id`),
CONSTRAINT `fk_bookpublisher_book` FOREIGN KEY (`book_id`) REFERENCES `book` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `fk_bookpublisher_publisher` FOREIGN KEY (`publisher_id`) REFERENCES `publisher` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

实体类

Book

@Entity
public class Book {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int id;
private String name;
@ManyToMany(cascade = CascadeType.MERGE)
@JoinTable(name = "book_publisher", joinColumns = @JoinColumn(name = "book_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "publisher_id", referencedColumnName = "id"))
private Set<Publisher> publishers = new HashSet<>();

public Book() {

}

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

public Book(String name, Set<Publisher> publishers) {
this.name = name;
this.publishers = publishers;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Set<Publisher> getPublishers() {
return publishers;
}

public void setPublishers(Set<Publisher> publishers) {
this.publishers = publishers;
}

@Override
public String toString() {
String result = String.format(
"Book [id=%d, name='%s']%n",
id, name);
if (publishers != null) {
for (Publisher publisher : publishers) {
result += String.format(
"Publisher[id=%d, name='%s']%n",
publisher.getId(), publisher.getName());
}
}

return result;
}
}

Publisher

@Entity
public class Publisher {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int id;
private String name;
@ManyToMany(mappedBy = "publishers")
private Set<Book> books = new HashSet<>();

public Publisher(){

}

public Publisher(String name){
this.name = name;
}

public Publisher(String name, Set<Book> books){
this.name = name;
this.books = books;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Set<Book> getBooks() {
return books;
}

public void setBooks(Set<Book> books) {
this.books = books;
}

@Override
public String toString() {
String result = String.format(
"Publisher [id=%d, name='%s']%n",
id, name);
if (books != null) {
for(Book book : books) {
result += String.format(
"Book[id=%d, name='%s']%n",
book.getId(), book.getName());
}
}

return result;
}
}

@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。
TABLE：使用表保存id值
IDENTITY：identitycolumn
SEQUENCR ：sequence
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。

@ManyToMany

 多对多关联关系

@JoinColumn

 指定关联的字段

@JoinTable

 参考

Spring Data JPA Repository

BookRepository

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

PublisherRepository

public interface PublisherRepository extends JpaRepository<Publisher, 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_manytomany
username: root
password: admin
driver-class-name: com.mysql.jdbc.Driver
jpa:
show-sql: true
properties:
hibernate:
enable_lazy_load_no_trans: true

BookRepositoryTest

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

@Autowired
private BookRepository bookRepository;

@Autowired
private PublisherRepository publisherRepository;

@Test
public void saveTest() throws Exception {

Publisher publisherA = new Publisher("Publisher One");
Publisher publisherB = new Publisher("Publisher Two");

Book bookA = new Book("Book One");
bookA.setPublishers(new HashSet<Publisher>() );

bookRepository.save(bookA);

}

@Test
public void saveTest1() throws Exception{
Publisher publisher = publisherRepository.findOne(24);
Book bookA = new Book("Book Two");
bookA.getPublishers().add(publisher);
bookRepository.save(bookA);
}

@Test
public void saveTest2() throws Exception{
Book two = bookRepository.findOne(18);
Publisher publisher = publisherRepository.findOne(25);
two.getPublishers().add(publisher);
bookRepository.save(two);
}

@Test
public void findPublisherTest() throws Exception{
Publisher publisher = publisherRepository.findOne(24);
Set<Book> books = publisher.getBooks();
for(Book book: books){
log.info(book.getName()+"..."+book.getId());
}
Assert.assertNotNull(publisher);
Assert.assertNotNull(publisher.getName());
}

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

@Test
public void findBookTest() throws Exception{
Book book = bookRepository.findOne(16);
Set<Publisher> publishers = book.getPublishers();
for(Publisher publisher: publishers){
log.info(publisher.getName()+"..."+publisher.getId());
}
Assert.assertNotNull(book);
Assert.assertNotNull(book.getName());
}
}

代码下载

从我的 github 中下载，https://github.com/longfeizheng/jpa-example/tree/master/many-to-many





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