JPA关系映射系列三：one-to-many和many-to-one

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

one-to-many

和

many-to-one

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>

一对多和多对一

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>one-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>

一对多和多对一关联

book.book-category_id

和

book_category.id

db.sql

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

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

DROP TABLE IF EXISTS `book_category`;
CREATE TABLE `book_category` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
PRIMARY KEY (`id`,`name`)
) ENGINE=InnoDB AUTO_INCREMENT=9 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_category_id` int(11) unsigned DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fk_book_bookcategoryid_idx` (`book_category_id`),
CONSTRAINT `fk_book_bookcategoryid` FOREIGN KEY (`book_category_id`) REFERENCES `book_category` (`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;

@ManyToOne
@JoinColumn(name = "book_category_id")
private BookCategory bookCategory;

public Book() {

}

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

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

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 BookCategory getBookCategory() {
return bookCategory;
}

public void setBookCategory(BookCategory bookCategory) {
this.bookCategory = bookCategory;
}
}

BookCategory

@Entity
@Table(name = "book_category")
public class BookCategory {

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

@OneToMany(mappedBy = "bookCategory", cascade = CascadeType.ALL)
private Set<Book> books;

public BookCategory(){

}

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

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(
"Category[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。

@OneToMany

一对多关联关系

@ManyToOne

多对一关联关系

@JoinColumn

指定关联的字段

Spring Data JPA Repository

public interface BookCategoryRepository extends JpaRepository<BookCategory, 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_onetomany
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 BookCategoryRepositoryTest {

@Autowired
private BookCategoryRepository repository;

@Test
public void saveCategoryTest(){
BookCategory categoryOne = new BookCategory("Category One");
Set books = new HashSet<Book>(){{
add(new Book("Book One", categoryOne));
add(new Book("Book Two", categoryOne));
add(new Book("Book Three", categoryOne));
}};
categoryOne.setBooks(books);

BookCategory categoryTwo = new BookCategory("Category Two");
Set bookBs = new HashSet<Book>(){{
add(new Book("Book Four", categoryTwo));
add(new Book("Book Five", categoryTwo));
add(new Book("Book Six", categoryTwo));
}};
categoryTwo.setBooks(bookBs);

Set allBooks = new HashSet();

allBooks.add(categoryOne);
allBooks.add(categoryTwo);

List list = repository.save(allBooks);

Assert.assertNotNull(list);
}

@Test
public void findAll() throws Exception{

for (BookCategory bookCategory : repository.findAll()) {
log.info(bookCategory.toString());
}
}
}

代码下载

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





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