11. Spring Data JPA

Spring Data JPA旨在通过减少实际需要的工作量来显著改善数据访问层的实现。 作为开发人员，仅需编写repository接口，包括自定义查找器方法，Spring将自动提供实现。

11.1. 依赖

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

11.2. 创建和删除JPA数据库

默认情况下，仅当您使用嵌入式数据库（H2，HSQL或Derby）时，才会自动创建JPA数据库。您可以使用spring.jpa.*属性显式配置JPA设置。例如，要创建和删除表，可以将以下行添加到application.properties:

spring.jpa.hibernate.ddl-auto=create-drop

值可以是这些：

  • create：每次运行程序时，都会重新创建表，故而数据会丢失
  • create-drop：每次运行程序时会先创建表结构，然后待程序结束时清空表
  • upadte：每次运行程序，没有表时会创建表，如果对象发生改变会更新表结构，原有数据不会清空，只会更新（推荐使用）
  • validate：运行程序会校验数据与数据库的字段类型是否相同，字段不同会报错
  • none: 禁用DDL处理

11.3. Query Creation

public interface UserRepository extends Repository<User, Long> {
    List<User> findByEmailAddressAndLastname(String emailAddress, String lastname);
}

https://docs.spring.io/spring-data/jpa/docs/2.2.0.RELEASE/reference/html/#jpa.query-methods.query-creation

11.4. Using @Query

public interface UserRepository extends JpaRepository<User, Long> {
@Query("select u from User u where u.emailAddress = ?1")
User findByEmailAddress(String emailAddress);
}

https://docs.spring.io/spring-data/jpa/docs/2.2.0.RELEASE/reference/html/#jpa.query-methods.at-query

11.5. Using Named Parameters

public interface UserRepository extends JpaRepository<User, Long> {

@Query("select u from User u where u.firstname = :firstname or u.lastname = :lastname")
User findByLastnameOrFirstname(@Param("lastname") String lastname,
                                @Param("firstname") String firstname);
}

11.6. Using SpEL Expressions

@Entity
public class User {

@Id
@GeneratedValue
Long id;

String lastname;
}

public interface UserRepository extends JpaRepository<User,Long> {

@Query("select u from #{#entityName} u where u.lastname = ?1")
List<User> findByLastname(String lastname);
}

https://docs.spring.io/spring-data/jpa/docs/2.2.0.RELEASE/reference/html/#jpa.query.spel-expressions

11.7. Modifying Queries

@Modifying
@Query("update User u set u.firstname = ?1 where u.lastname = ?2")
int setFixedFirstnameFor(String firstname, String lastname);

https://docs.spring.io/spring-data/jpa/docs/2.2.0.RELEASE/reference/html/#jpa.modifying-queries