目标

学习JPA框架在关系型数据库中如何存储和检索数据。

环境

JDK 1.8+
Maven 3.2+
IDE(eclipse/idea)

内容

新建gs-accessing-data-jpa工程

使用idea新建工程步骤参考《springboot学习之构建 RESTful Web服务》（）
添加依赖包：Spring Data Jpa和H2 Database

    <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-data-jpa</artifactId></dependency><dependency><groupId>com.h2database</groupId><artifactId>h2</artifactId><scope>runtime</scope></dependency>

定义Entity

@Entity
public class Customer {@Id@GeneratedValue(strategy= GenerationType.AUTO)private Long id;private String firstName;private String lastName;protected Customer() {}public Customer(String firstName, String lastName) {this.firstName = firstName;this.lastName = lastName;}public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getFirstName() {return firstName;}public void setFirstName(String firstName) {this.firstName = firstName;}public String getLastName() {return lastName;}public void setLastName(String lastName) {this.lastName = lastName;}@Overridepublic String toString() {return "Customer{" +"id=" + id +", firstName='" + firstName + '\'' +", lastName='" + lastName + '\'' +'}';}
}

• Customer有两个构造器，默认的构造器定义为protected只用于JPA，另一个构造器用于创建保存到数据库的Customer实例。
• @Entity注解用于标注是一个JPA实体，表示该实体映射表customer。
• Customer类中使用@Id注解标识主键，使用@GeneratedValue标识自动生成id，配置strategy为GenerationType.AUTO用于自动选择一个最适合底层数据库的主键生成策略。
• 另外两个字段firstName和lastName，表中映射字段分别为first_name和last_name。

定义CustomerRepository接口

public interface CustomerRepository extends CrudRepository<Customer, Long> {List<Customer> findByLastName(String lastName);Customer findById(long id);
}

CustomerRepository继承CrudRepository接口，通过继承CrudRepository实现了保存、删除和查询等方法。Spring Data JPA允许定义其它的查询方法，例如：findByLastName。

创建Application

@SpringBootApplication
public class GsAccessingDataJpaApplication {private static final Logger log = LoggerFactory.getLogger(GsAccessingDataJpaApplication.class);public static void main(String[] args) {SpringApplication.run(GsAccessingDataJpaApplication.class, args);}
}

@SpringBootApplication包含下面三个注解：

• @Configuration：装配bean的配置文件
• @EnableAutoConfiguration：启用Spring应用程序上下文配置
• @ComponentScan：扫描GsAccessingDataJpaApplication所在包下的其他组件
  修改Application类，加入如下方法：

    @Beanpublic CommandLineRunner demo(CustomerRepository repository) {return (args) -> {// save a few customersrepository.save(new Customer("Jack", "Bauer"));repository.save(new Customer("Chloe", "O'Brian"));repository.save(new Customer("Kim", "Bauer"));repository.save(new Customer("David", "Palmer"));repository.save(new Customer("Michelle", "Dessler"));// fetch all customerslog.info("Customers found with findAll():");log.info("-------------------------------");for (Customer customer : repository.findAll()) {log.info(customer.toString());}log.info("");// fetch an individual customer by IDCustomer customer = repository.findById(1L);log.info("Customer found with findById(1L):");log.info("--------------------------------");log.info(customer.toString());log.info("");// fetch customers by last namelog.info("Customer found with findByLastName('Bauer'):");log.info("--------------------------------------------");repository.findByLastName("Bauer").forEach(bauer -> {log.info(bauer.toString());});// for (Customer bauer : repository.findByLastName("Bauer")) {//  log.info(bauer.toString());// }log.info("");};}

运行日志

2022-05-17 17:38:07.448  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customers found with findAll():
2022-05-17 17:38:07.448  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : -------------------------------
2022-05-17 17:38:07.559  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=1, firstName='Jack', lastName='Bauer'}
2022-05-17 17:38:07.559  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=2, firstName='Chloe', lastName='O'Brian'}
2022-05-17 17:38:07.559  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=3, firstName='Kim', lastName='Bauer'}
2022-05-17 17:38:07.559  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=4, firstName='David', lastName='Palmer'}
2022-05-17 17:38:07.559  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=5, firstName='Michelle', lastName='Dessler'}
2022-05-17 17:38:07.559  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : 
2022-05-17 17:38:07.571  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer found with findById(1L):
2022-05-17 17:38:07.571  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : --------------------------------
2022-05-17 17:38:07.571  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=1, firstName='Jack', lastName='Bauer'}
2022-05-17 17:38:07.571  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : 
2022-05-17 17:38:07.571  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer found with findByLastName('Bauer'):
2022-05-17 17:38:07.571  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : --------------------------------------------
2022-05-17 17:38:07.616  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=1, firstName='Jack', lastName='Bauer'}
2022-05-17 17:38:07.616  INFO 15660 --- [           main] c.example.GsAccessingDataJpaApplication  : Customer{id=3, firstName='Kim', lastName='Bauer'}

H2改为Mysql

之前是使用H2数据库做的，Customer怎么映射的数据表问题还不明白，所以再使用mysql数据库试验一下。

• 屏蔽h2依赖包，添加mysql依赖：

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

• yml文件配置：

spring:jpa:hibernate:ddl-auto: updatedatasource:driver-class-name: com.mysql.cj.jdbc.Driverurl: jdbc:mysql://127.0.0.1:3306/test?useUnicode=true&characterEncoding=utf8&zeroDateTimeBehavior=convertToNull&useSSL=true&serverTimezone=GMT%2B8username: rootpassword: root

Entity、Repository和Application都不改变，运行后跟H2数据库一切正常。查看mysql库，发现数据库如下：

customer之前没有这个表，运行之后库中新建了customer表。这是因为使用如下配置项，spring.jpa.hibernate.ddl-auto: update，使用内嵌数据库时或者没有schema manager被检测到时为create-drop模式，否则为none模式。这里分别介绍下几种模式区别：
create: 先删除现有表的数据和结构，新建表然后再新建表上操作
validate: 验证表结构，不存在会引发异常
update: 检查表和列，不存在会创建
create-drop:检查表并操作，操作完后删除
none: 不做表的操作

总结

• 实体类相关注解的使用
• Repository接口的使用及扩展
• spring.jpa.hibernate.ddl-auto的配置