spring data JPA是spring团队打造的sping生态全家桶的一部分，本身内核使用的是hibernate核心源码，用来作为了解java持久层框架基本构成的样本是再好不过的选择。最近闲来无事，构建了一个demo工程，用来阅读spring data JPA源码，这对于宏观了解持久层框架的基本工作、微观分析spring data JPA的原理和优缺点、避免使用过程中采坑，将会有一定的帮助。

spring data JPA的使用需要依托于web框架，最简单快速的方式就是使用https://start.spring.io/构建一个包含spring data JPA的spring boot项目，只需要在引导的对应SQL的菜单中选中Spring Data Jpa和响应数据库的Driver即可，目前无论是idae还是eclipse都支持采用这种方式。上述方法的实质还是在最终生成的spring boot项目的pom.xml中加入了相关的依赖，所以也可以直接生成由maven管理的spring boot项目，然后pom.xml中：
<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.1.8.RELEASE</version><relativePath/> <!-- lookup parent from repository --></parent><groupId>com.example</groupId><artifactId>jpademo</artifactId><version>0.0.1-SNAPSHOT</version><name>jpademo</name><description>Demo project for Spring Boot</description><properties><java.version>1.8</java.version></properties><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-data-jpa</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId></dependency><dependency><groupId>com.querydsl</groupId><artifactId>querydsl-jpa</artifactId></dependency><dependency><groupId>com.querydsl</groupId><artifactId>querydsl-apt</artifactId></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId></dependency></dependencies><build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin><plugin><groupId>com.mysema.maven</groupId><artifactId>apt-maven-plugin</artifactId><version>1.1.3</version><executions><execution><goals><goal>process</goal></goals><configuration><outputDirectory>target/generated-sources/apt</outputDirectory><processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor></configuration></execution></executions></plugin></plugins></build></project>

CREATE TABLE `song` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` char(10) DEFAULT NULL,`year` int(11) DEFAULT NULL,`length` int(11) DEFAULT NULL,`type` varchar(64) DEFAULT NULL,PRIMARY KEY (`id`)
)
接着项目中application.properties中设置数据库配置：
#driver配置
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
#数据库配置
spring.datasource.url=jdbc:mysql://localhost:3306/test?useSSL=false&allowPublicKeyRetrieval=true
spring.datasource.username=root
spring.datasource.password=123456
#jpa配置
spring.jpa.show-sql=true
spring.jpa.hibernate.ddl-auto=update

@Entity  //定义为实体类
@Table(name = "song")  //映射表配置
@DynamicInsert         //支持动态插入
@DynamicUpdate         //支持动态更新
//以下为lombok懒癌注解
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Song {@Id              //主键id@GeneratedValue  //主键自动生成private Long id;private String name;private Integer year;private Integer length;private String type;
}

public interface SongRepository extends JpaRepository<Song,Integer>{Song findById(Long id);Song getById(Long id);Song queryByIdAndYear(Long id, Long year);void deleteById(Long id);
}

了解一个事物基本原理，首先需要全面的了解事物的外在功能。对于spirng data jpa，主要的数据操作方式有使用Repository和使用EntityManager这两种形式。
@Slf4j
@RestController
public class TestController {@Resourceprivate EntityManager entityManager;@Resourceprivate SongRepository songRepository;@GetMapping("/")public List<Song> test() {return repositoryList();}public List<Song> repositoryList() {//Repository方式List<Song> songs = songRepository.findAll();return songs;}@Transactionalpublic void repositorySave() {Song song = new Song();song.setName("MyHeart");song.setLength(230);song.setYear(2020);songRepository.save(song);}@Transactionalpublic void entityManagerMerge() {//EntityManager方式Song song = new Song();song = new Song();song.setName("Journey");song.setLength(230);song.setYear(2020);entityManager.merge(song);}public List<Song> hqlList() {//hql方式Query query = entityManager.createQuery("select s as ss from Song s");List<Song> songs = query.getResultList();return  songs;}public List<Song> sqlList() {//sql方式Query nativeQuery = entityManager.createNativeQuery("select * from song s where name LIKE 'MyHeart'");List<Song> songs = nativeQuery.getResultList();return songs;}
}

@JoinTable(name = "head", joinColumns = { @JoinColumn(nullable = false, name = "item_id", referencedColumnName = "id") })