JPA关系映射系列四：many-to-many 关联映射-白红宇

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


    
        
     
      4.0.0
         
     
      cn.merryyou
         
     
      jpa-example
         
     
      1.0-SNAPSHOT
         
             
      
       one-to-one-foreignkey
              
      
       one-to-one-primarykey
              
      
       one-to-many
              
      
       many-to-many
              
      
       many-to-many-extra-columns
          
         
     
      pom
         
             
                  
                       
        
         io.spring.platform
                        
        
         platform-bom
                        
        
         Brussels-SR6
                        
        
         pom
                        
        
         import
                    
               
          
     
    复制代码

多对多关联映射

目录结构

pom.xml


    
        
             
      
       jpa-example
              
      
       cn.merryyou
              
      
       1.0-SNAPSHOT
          
         
     
      4.0.0
         
     
      many-to-many
         
             
      
       UTF-8
              
      
       1.8
          
         
             
                  
       
        org.springframework.boot
                   
       
        spring-boot-starter-data-jpa
               
              
                  
       
        mysql
                   
       
        mysql-connector-java
                   
       
        runtime
               
              
                  
       
        org.projectlombok
                   
       
        lombok
               
              
                  
       
        org.springframework.boot
                   
       
        spring-boot-starter-test
                   
       
        test
               
          
         
             
                  
                       
        
         org.springframework.boot
                        
        
         spring-boot-maven-plugin
                    
                   
                       
        
         org.apache.maven.plugins
                        
        
         maven-compiler-plugin
                        
        
         3.6.1
                        
                            
         1.8                    
         
          1.8
                         
                    
               
          
     
    复制代码

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;复制代码

实体类

Book

@Entitypublic 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
    
      publishers = new HashSet<>();    public Book() {    }    public Book(String name) {        this.name = name;    }    public Book(String name, Set
     
       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
      
        getPublishers() {        return publishers;    }    public void setPublishers(Set
       
         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;    }}复制代码

Publisher

@Entitypublic class Publisher {    @Id    @GeneratedValue(strategy = GenerationType.AUTO)    private int id;    private String name;    @ManyToMany(mappedBy = "publishers")    private Set
    
      books = new HashSet<>();    public Publisher(){    }    public Publisher(String name){        this.name = name;    }    public Publisher(String name, Set
     
       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
      
        getBooks() {        return books;    }    public void setBooks(Set
       
         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;    }}复制代码

@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。

@ManyToMany 多对多关联关系

@JoinColumn 指定关联的字段

@JoinTable

Spring Data JPA Repository

BookRepository

public interface BookRepository extends JpaRepository
    
      {}复制代码

PublisherRepository

public interface PublisherRepository extends JpaRepository
    
      {}复制代码

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复制代码

BookRepositoryTest

@RunWith(SpringRunner.class)@SpringBootTest@Slf4jpublic 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
    
     () {
   {            add(publisherA);            add(publisherB);        }});        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
     
       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
      
        publishers = book.getPublishers();        for(Publisher publisher: publishers){            log.info(publisher.getName()+"..."+publisher.getId());        }        Assert.assertNotNull(book);        Assert.assertNotNull(book.getName());    }}复制代码

代码下载

从我的 github 中下载，

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