Spring Data JPA

码匠君 ... 2021-10-28 大约 20 分钟

# Spring Data JPA

# JPA

JPA(Java Persistence API)意即Java持久化API,是Sun官方在JDK5.0后提出的Java持久化规范(JSR 338,这些接口所在包为javax.persistence,详细内容可参考https://github.com/javaee/jpa-spec (opens new window)

JPA的出现主要是为了简化持久层开发以及整合ORM技术,结束 Hibernate,TopLink,JDOORM框架各自为营的局面。JPA是在吸收现有ORM框架的基础上发展而来,易于使用,伸缩性强。总的来说,JPA包括以下3方面的技术:

  • ORM映射元数据: 支持XML和注解两种元数据的形式,元数据描述对象和表之间的映射关系
  • API: 操作实体对象来执行CRUD操作
  • 查询语言: 通过面向对象而非面向数据库的查询语言(JPQL)查询数据,避免程序的SQL语句紧密耦合

JPA架构

# Spring Data Jpa

Spring官方的解释https://spring.io/projects/spring-data-jpa#overview (opens new window)

Spring Data Jpa官方解释

Spring Data JPASpring Data 家族的一部分,可以轻松实现基于 JPA 的存储库。 此模块处理对基于 JPA 的数据访问层的增强支持。 它使构建使用数据访问技术的Spring驱动应用程序变得更加容易。

在相当长的一段时间内,实现应用程序的数据访问层一直很麻烦。 必须编写太多样板代码来执行简单查询以及执行分页和审计。 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,您编写repository接口,包括自定义查找器方法,Spring将自动提供实现。

Spring Data生态

# Jpa,Hibernate,Spring Data Jpa三者之间的关系

这个问题可参考https://stackoverflow.com/questions/16148188/spring-data-jpa-versus-jpa-whats-the-difference及https://blog.csdn.net/u014421556/article/details/52635000 (opens new window)

总的来说JPAORM规范,Hibernate,TopLink 等是 JPA 规范的具体实现,这样的好处是开发者可以面向 JPA 规范进行持久层的开发,而底层的实现则是可以切换的。Spring Data Jpa 则是在 JPA 之上添加另一层抽象(Repository层的实现),极大地简化持久层开发及 ORM 框架切换的成本。

Jpa,Hibernate,Spring Data Jpa三者之间的关系

# Spring Data Jpa的java配置方案

Spring Boot 没出来之前如果要采用 Java Configuration 来配置 Spring Data Jpa 你需要配置如下的Bean 参考自Spring In Action及Spring Data Jpa官方文档5.1.2. Annotation-based Configuration (opens new window)

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.data.jpa.repository.config.EnableJpaRepositories;
import org.springframework.orm.jpa.JpaTransactionManager;
import org.springframework.orm.jpa.JpaVendorAdapter;
import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;
import org.springframework.orm.jpa.vendor.Database;
import org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter;
import org.springframework.transaction.PlatformTransactionManager;
import org.springframework.transaction.annotation.EnableTransactionManagement;

import javax.persistence.EntityManagerFactory;
import javax.sql.DataSource;

/**
 * 注意:spring-data-jpa2.x版本需要spring版本为5.x
 * 否则会报Initialization of bean failed; nested exception is java.lang.AbstractMethodError错误
 * 参考:https://stackoverflow.com/questions/47558017/error-starting-a-spring-application-initialization-of-bean-failed-nested-excep
 * 搭配方案:spring4+spring-data-jpa1.x或spring5+spring-data-jpa2.x
 */
@Configuration
// 借助spring data实现自动化的jpa repository,只需编写接口无需编写实现类
// 相当于xml配置的<jpa:repositories base-package="com.example.repository" />
// repositoryImplementationPostfix默认就是Impl
// entityManagerFactoryRef默认就是entityManagerFactory
// transactionManagerRef默认就是transactionManager
@EnableJpaRepositories(basePackages = {"com.example.repository"},
        repositoryImplementationPostfix = "Impl",
        entityManagerFactoryRef = "entityManagerFactory",
        transactionManagerRef = "transactionManager")
@EnableTransactionManagement    // 启用事务管理器
public class SpringDataJpaConfig {

    // 配置jpa厂商适配器(参见spring实战p320)
    @Bean
    public JpaVendorAdapter jpaVendorAdapter() {
        HibernateJpaVendorAdapter jpaVendorAdapter = new HibernateJpaVendorAdapter();
        // 设置数据库类型(可使用org.springframework.orm.jpa.vendor包下的Database枚举类)
        jpaVendorAdapter.setDatabase(Database.MYSQL);
        // 设置打印sql语句
        jpaVendorAdapter.setShowSql(true);
        // 设置不生成ddl语句
        jpaVendorAdapter.setGenerateDdl(false);
        // 设置hibernate方言
        jpaVendorAdapter.setDatabasePlatform("org.hibernate.dialect.MySQL5Dialect");
        return jpaVendorAdapter;
    }

    // 配置实体管理器工厂
    @Bean
    public LocalContainerEntityManagerFactoryBean entityManagerFactory(
            DataSource dataSource, JpaVendorAdapter jpaVendorAdapter) {
        LocalContainerEntityManagerFactoryBean emfb = new LocalContainerEntityManagerFactoryBean();
        // 注入数据源
        emfb.setDataSource(dataSource);
        // 注入jpa厂商适配器
        emfb.setJpaVendorAdapter(jpaVendorAdapter);
        // 设置扫描基本包
        emfb.setPackagesToScan("com.example.entity");
        return emfb;
    }

    // 配置jpa事务管理器
    @Bean
    public PlatformTransactionManager transactionManager(EntityManagerFactory emf) {
        JpaTransactionManager transactionManager = new JpaTransactionManager();
        // 配置实体管理器工厂
        transactionManager.setEntityManagerFactory(emf);
        return transactionManager;
    }

}
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启用web支持还需要在 Spring MVC 配置类上添加 @EnableSpringDataWebSupport 注解

@Configuration
@ComponentScan(basePackages = {"cn.fulgens.controller"})
@EnableWebMvc   // 启用spring mvc
@EnableSpringDataWebSupport     // 启用springmvc对spring data的支持
public class WebMvcConfig extends WebMvcConfigurerAdapter {

}
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# Spring Boot整合Spring Data Jpa

# 导入依赖

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<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>
    <optional>true</optional>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <optional>true</optional>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
</dependency>
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# 相关配置

server:
  port: 8080
  servlet:
    context-path: /
spring:
  datasource:
    url: jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=UTF-8&zeroDateTimeBehavior=convertToNull&allowMultiQueries=true&useSSL=false
    username: root
    password: mysql123
  jpa:
    database: MySQL
    database-platform: org.hibernate.dialect.MySQL5InnoDBDialect
    show-sql: true
    hibernate:
      ddl-auto: update
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ddl-auto

  • 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

# Spring Data Jpa的使用

# Spring Data Jpa UML类图

Spring Data JPA UML

# 简单的REST CRUD示例

实体类

/src/main/java/com/example/springbootjpa/entity/User

package com.example.springbootjpa.entity;

import lombok.Data;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;

@Entity
@Table(name = "tb_user")
@Data
public class User {

    @Id
    @GenericGenerator(name = "idGenerator", strategy = "uuid")
    @GeneratedValue(generator = "idGenerator")
    private String id;

    @Column(name = "username", unique = true, nullable = false, length = 64)
    private String username;

    @Column(name = "password", nullable = false, length = 64)
    private String password;

    @Column(name = "email", length = 64)
    private String email;

}
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主键采用UUID策略

@GenericGeneratorHibernate 提供的主键生成策略注解,注意下面的 @GeneratedValue(JPA注解)使用generator = "idGenerator"引用了上面的name = "idGenerator"主键生成策略

一般简单的Demo示例中只会使用@GeneratedValue(strategy = GenerationType.IDENTITY)这种主键自增的策略,而实际数据库中表字段主键类型很少是int型的

JPA自带的几种主键生成策略

  • TABLE: 使用一个特定的数据库表格来保存主键
  • SEQUENCE: 根据底层数据库的序列来生成主键,条件是数据库支持序列。这个值要与generator一起使用,generator 指定生成主键使用的生成器(可能是orcale中自己编写的序列)
  • IDENTITY: 主键由数据库自动生成(主要是支持自动增长的数据库,如mysql)
  • AUTO: 主键由程序控制,也是GenerationType的默认值

Dao层

/src/main/java/com/example/springbootjpa/repository/UserRepository

package com.example.springbootjpa.repository;

import com.example.springbootjpa.entity.User;
import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<User, String> {
}
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Controller层

这里简单起见省略Service层

/src/main/java/com/example/springbootjpa/controller/UserController

package com.example.springbootjpa.controller;

import com.example.springbootjpa.entity.User;
import com.example.springbootjpa.repository.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;
import org.springframework.web.bind.annotation.*;

import java.util.HashMap;
import java.util.Optional;

@RestController
@RequestMapping("/users")
public class UserController {

    @Autowired
    private UserRepository userRepository;

    @PostMapping()
    public User saveUser(@RequestBody User user) {
        return userRepository.save(user);
    }

    @DeleteMapping("/{id}")
    public void deleteUser(@PathVariable("id") String userId) {
        userRepository.deleteById(userId);
    }

    @PutMapping("/{id}")
    public User updateUser(@PathVariable("id") String userId, @RequestBody User user) {
        user.setId(userId);
        return userRepository.saveAndFlush(user);
    }

    @GetMapping("/{id}")
    public User getUserInfo(@PathVariable("id") String userId) {
        Optional<User> optional = userRepository.findById(userId);
        return optional.orElseGet(User::new);
    }

    @GetMapping("/list")
    public Page<User> pageQuery(@RequestParam(value = "pageNum", defaultValue = "1") Integer pageNum,
                                @RequestParam(value = "pageSize", defaultValue = "10") Integer pageSize) {
        return userRepository.findAll(PageRequest.of(pageNum - 1, pageSize));
    }

}
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# Spring Data Jpa使用详解

# Spring Data查询方法

使用Spring Data创建查询只需四步:

  1. 声明一个接口继承自Repository或Repositoy的一个子接口,对于Spring Data Jpa通常是JpaRepository,如:
interface PersonRepository extends Repository<Person, Long> {}
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  1. 在接口中声明查询方法,如:
interface PersonRepository extends Repository<Person, Long> {
  List<Person> findByLastname(String lastname);
}
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  1. 使用 JavaConfig (opens new window)XML configuration (opens new window)配置Spring,让 Spring 为声明的接口创建代理对象 3.1 JavaConfig参见上文 3.2 使用Xml配置,可以像下面这样使用jpa命名空间进行配置:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:jpa="http://www.springframework.org/schema/data/jpa"
   xsi:schemaLocation="http://www.springframework.org/schema/beans
     http://www.springframework.org/schema/beans/spring-beans.xsd
     http://www.springframework.org/schema/data/jpa
     http://www.springframework.org/schema/data/jpa/spring-jpa.xsd">

   <jpa:repositories base-package="com.acme.repositories"/>

</beans>
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对于不同的 Spring Data 子项目Spring提供了不同的xml命名空间,如对于 Spring Data MongoDB 可以将上面的jpa改为mongodb。当然,使用 Spring Boot 这一步基本可以省略,我们需要做的就是在application.propertiesapplication.yml文件中配置几个属性即可

  1. 注入Repository实例并使用,如:
class SomeClient {

  private final PersonRepository repository;

  SomeClient(PersonRepository repository) {
    this.repository = repository;
  }

  void doSomething() {
    List<Person> persons = repository.findByLastname("Matthews");
  }
}
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# 定义Repository接口

选择性暴露CRUD方法

一种方法是定义一个BaseRepository接口继承Repository接口,并从CrudRepository中copy你想暴露的CRUD方法

src/main/java/com/example/springbootjpa/repository/MyBaseRepository

package com.example.springbootjpa.repository;

import org.springframework.data.repository.NoRepositoryBean;
import org.springframework.data.repository.Repository;

import java.util.Optional;

/**
 * 自定义Repository,选择性暴露CRUD方法
 * @param <T>
 * @param <ID>
 */
@NoRepositoryBean
public interface MyBaseRepository<T, ID> extends Repository<T, ID> {

    Optional<T> findById(ID id);

    <S extends T> S save(S entity);

}
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注意:MyBaseRepository上面加了 @NoRepositoryBean 注解

src/main/java/com/example/springbootjpa/repository/UserRepository2

package com.example.springbootjpa.repository;

import com.example.springbootjpa.entity.User;
import org.springframework.stereotype.Repository;

public interface UserRepository2 extends MyBaseRepository<User, String> {
}
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# Repository方法的Null值处理

Spring Data2.0 开始对于返回单个聚合实例的CRUD方法可以使用java8 Optional接口作为方法返回值来表明可能存在的缺省值,典型示例为 CrudRepositoryfindById 方法

另外Spring也提供了几个注解来处理Null值

  • @NonNullApi: 在包级别使用来声明参数和返回值不能为Null
  • @NonNull: 在参数或返回值上使用,当它们不能为Null时(如果在包级别上使用了@NonNullApi注解则没有必要再使用@NonNull注解了)
  • @Nullable: 在参数或返回值上使用,当它们可以为Null时

# 查询方法

# 查询创建Query Creation

Spring Data Jpa 通过解析方法名创建查询,框架在进行方法名解析时,会先把方法名多余的前缀find…By, read…By, query…By, count…By以及 get…By 截取掉,然后对剩下部分进行解析,第一个By会被用作分隔符来指示实际查询条件的开始。 我们可以在实体属性上定义条件,并将它们与And和Or连接起来,从而创建大量查询:

User findByUsername(String username);

List<User> findByUsernameIgnoreCase(String username);

List<User> findByUsernameLike(String username);

User findByUsernameAndPassword(String username, String password);

User findByEmail(String email);

List<User> findByEmailLike(String email);

List<User> findByIdIn(List<String> ids);

List<User> findByIdInOrderByUsername(List<String> ids);

void deleteByIdIn(List<String> ids);

Long countByUsernameLike(String username);
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支持的关键字,示例及 JPQL 片段如下表所示:

| Keyword | Sample | JPQL snippet | | ------- | ------ | ------------ || | Distinct | findDistinctByLastnameAndFirstname | select distinct …​ where x.lastname = ?1 and x.firstname = ?2 | | And | findByLastnameAndFirstname | … where x.lastname = ?1 and x.firstname = ?2 | | Or | findByLastnameOrFirstname | … where x.lastname = ?1 or x.firstname = ?2 | | Is, Equals | findByFirstname,findByFirstnameIs,findByFirstnameEquals | … where x.firstname = ?1 | | Between | findByStartDateBetween | … where x.startDate between ?1 and ?2 | | LessThan | findByAgeLessThan | … where x.age < ?1 | | LessThanEqual | findByAgeLessThanEqual | … where x.age <= ?1 | | GreaterThan | findByAgeGreaterThan | … where x.age > ?1 | | GreaterThanEqual | findByAgeGreaterThanEqual | … where x.age >= ?1 | | After | findByStartDateAfter | … where x.startDate > ?1 | | Before | findByStartDateBefore | … where x.startDate < ?1 | | IsNull, Null | findByAge(Is)Null | … where x.age is null | | IsNotNull, NotNull | findByAge(Is)NotNull | … where x.age not null | | Like | findByFirstnameLike | … where x.firstname like ?1 | | NotLike | findByFirstnameNotLike | … where x.firstname not like ?1 | | StartingWith | findByFirstnameStartingWith |… where x.firstname like ?1 (parameter bound with appended %) | | EndingWith | findByFirstnameEndingWith | … where x.firstname like ?1 (parameter bound with prepended %) | | Containing | findByFirstnameContaining | … where x.firstname like ?1 (parameter bound wrapped in %) | | OrderBy | findByAgeOrderByLastnameDesc | … where x.age = ?1 order by x.lastname desc | | Not | findByLastnameNot | … where x.lastname <> ?1 | | In | findByAgeIn(Collection<Age> ages) | … where x.age in ?1 | | NotIn | findByAgeNotIn(Collection<Age> ages) | … where x.age not in ?1 | | True | findByActiveTrue() | … where x.active = true | | False | findByActiveFalse() | … where x.active = false | | IgnoreCase | findByFirstnameIgnoreCase | … where UPPER(x.firstname) = UPPER(?1) |

具体 Spring Data Jpa 对方法名的解析规则可参看官方文档 (opens new window)

# 限制查询结果

Spring Data Jpa 支持使用first,top 以及 Distinct 关键字来限制查询结果,如:

User findFirstByUsernameOrderByUsernameAsc(String username);

List<User> findTop10ByUsername(String username, Sort sort);
    
List<User> findTop10ByUsername(String username, Pageable pageable);
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# 自定义查询Using @Query

@Query 注解的使用非常简单,只需在声明的方法上面标注该注解,同时提供一个 JPQL 查询语句即可

@Query("select u from User u where u.email = ?1")
User getByEmail(String eamil);

@Query("select u from User u where u.username = ?1 and u.password = ?2")
User getByUsernameAndPassword(String username, String password);

@Query("select u from User u where u.username like %?1%")
List<User> getByUsernameLike(String username);
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# 使用命名参数Using Named Parameters

默认情况下,Spring Data JPA 使用基于位置的参数绑定,如前面所有示例中所述。 这使得查询方法在重构参数位置时容易出错。 要解决此问题,可以使用 @Param 注解为方法参数指定具体名称并在查询中绑定名称,如以下示例所示:

@Query("select u from User u where u.id = :id")
User getById(@Param("id") String userId);

@Query("select u from User u where u.username = :username or u.email = :email")
User getByUsernameOrEmail(@Param("username") String username, @Param("email") String email);
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# Using SpEL Expressions

Spring Data JPA release 1.4 开始,Spring Data JPA 支持名为 entityName 的变量。 它的用法是select x from #{#entityName} x

entityName的解析方式如下:如果实体类在 @Entity 注解上设置了name属性,则使用它。否则,使用实体类的简单类名。为避免在 @Query 注解使用实际的实体类名,就可以使用#{#entityName}进行代替。如以上示例中,@Query注解的查询字符串里的User都可替换为#{#entityName}

@Query("select u from #{#entityName} u where u.email = ?1")
User getByEmail(String eamil);
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# 原生查询Native Queries

@Query 注解还支持通过将 nativeQuery 标志设置为true来执行原生查询,同样支持基于位置的参数绑定及命名参数,如:

@Query(value = "select * from tb_user u where u.email = ?1", nativeQuery = true)
User queryByEmail(String email);

@Query(value = "select * from tb_user u where u.email = :email", nativeQuery = true)
User queryByEmail(@Param("email") String email);
注意:Spring Data Jpa目前不支持对原生查询进行动态排序,但可以通过自己指定计数查询countQuery来使用原生查询进行分页,排序,如:

@Query(value = "select * from tb_user u where u.username like %?1%",
            countQuery = "select count(1) from tb_user u where u.username = %?1%",
            nativeQuery = true)
Page<User> queryByUsernameLike(String username, Pageable pageable);
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# 分页查询及排序

Spring Data Jpa 可以在方法参数中直接传入PageableSort 来完成动态分页或排序,通常 PageableSort 会是方法的最后一个参数,如:

@Query("select u from User u where u.username like %?1%")
Page<User> findByUsernameLike(String username, Pageable pageable);

@Query("select u from User u where u.username like %?1%")
List<User> findByUsernameAndSort(String username, Sort sort);
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那调用repository方法时传入什么参数呢?

对于 Pageable 参数,在Spring Data 2.0之前我们可以new一个org.springframework.data.domain.PageRequest对象,现在这些构造方法已经废弃,取而代之Spring推荐我们使用 PageRequestof 方法

new PageRequest(0, 5);
new PageRequest(0, 5, Sort.Direction.ASC, "username");
new PageRequest(0, 5, new Sort(Sort.Direction.ASC, "username"));
        
PageRequest.of(0, 5);
PageRequest.of(0, 5, Sort.Direction.ASC, "username");
PageRequest.of(0, 5, Sort.by(Sort.Direction.ASC, "username"));
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注意:Spring Data PageRequestpage参数是从0开始的 zero-based page index

对于Sort参数,同样可以new一个org.springframework.data.domain.Sort,但推荐使用Sort.by方法

# 自定义修改,删除 Modifying Queries

单独使用 @Query 注解只是查询,如涉及到修改,删除则需要再加上 @Modifying 注解,如:

@Transactional()
@Modifying
@Query("update User u set u.password = ?2 where u.username = ?1")
int updatePasswordByUsername(String username, String password);

@Transactional()
@Modifying
@Query("delete from User where username = ?1")
void deleteByUsername(String username);
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注意:Modifying queries can only use void or int/Integer as return type!

# 多表查询

这里使用级联查询进行多表的关联查询

# 多对多

/src/main/java/com/example/springbootjpa/entity/User

package com.example.springbootjpa.entity;

import lombok.Data;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;
import java.util.Date;
import java.util.Set;
import java.util.UUID;

@Entity
@Table(name = "tb_user")
@Data
public class User {

    @Id
    @GenericGenerator(name = "idGenerator", strategy = "uuid")
    @GeneratedValue(generator = "idGenerator")
    private String id;

    @Column(name = "username", unique = true, nullable = false, length = 64)
    private String username;

    @Column(name = "password", nullable = false, length = 64)
    private String password;

    @Column(name = "email", unique = true, length = 64)
    private String email;

    @ManyToMany(targetEntity = Role.class, cascade = {CascadeType.PERSIST, CascadeType.MERGE}, fetch = FetchType.LAZY)
    @JoinTable(name = "tb_user_role", joinColumns = {@JoinColumn(name = "user_id", referencedColumnName = "id")},
            inverseJoinColumns = {@JoinColumn(name = "role_id", referencedColumnName = "id")})
    private Set<Role> roles;

}
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/src/main/java/com/example/springbootjpa/entity/Role

package com.example.springbootjpa.entity;

import lombok.Data;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;

@Entity
@Table(name = "tb_role")
@Data
public class Role {

    @Id
    @GenericGenerator(name = "idGenerator", strategy = "uuid")
    @GeneratedValue(generator = "idGenerator")
    private String id;

    @Column(name = "role_name", unique = true, nullable = false, length = 64)
    private String roleName;

}
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测试

@Test
public void findByIdTest() {
    Optional<User> optional = userRepository.findById("40289f0c65674a930165674d54940000");
    Set<Role> roles = optional.get().getRoles();
    System.out.println(optional.get());
}
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不出意外会报 Hibernate 懒加载异常,无法初始化代理类,No Session:

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.example.springbootjpa.entity.User.roles, could not initialize proxy - no Session
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原因:Spring Boot 整合 JPAHibernateSession 就交付给Spring去管理。每次数据库操作后,会关闭Session,当我们想要用懒加载方式去获得数据的时候,原来的Session已经关闭,不能获取数据,所以会抛出这样的异常。

解决方法: 在application.yml中做如下配置:

spring:
  jpa:
    open-in-view: true
    properties:
      hibernate:
        enable_lazy_load_no_trans: true
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# 一对多(多对一)

/src/main/java/com/example/springbootjpa/entity/Department

package com.example.springbootjpa.entity;

import lombok.Data;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;
import java.util.Set;

@Entity
@Table(name = "tb_dept")
@Data
public class Department {

    @Id
    @GenericGenerator(name = "idGenerator", strategy = "uuid")
    @GeneratedValue(generator = "idGenerator")
    private String id;

    @Column(name = "dept_name", unique = true, nullable = false, length = 64)
    private String deptName;

    @OneToMany(mappedBy = "department", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private Set<Employee> employees;

}
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/src/main/java/com/example/springbootjpa/entity/Employee

package com.example.springbootjpa.entity;

import lombok.Data;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;
import java.util.UUID;

@Entity
@Table(name = "tb_emp")
@Data
public class Employee {

    @Id
    @GenericGenerator(name = "idGenerator", strategy = "uuid")
    @GeneratedValue(generator = "idGenerator")
    private String id;

    @Column(name = "emp_name", nullable = false, length = 64)
    private String empName;

    @Column(name = "emp_job", length = 64)
    private String empJob;

    @Column(name = "dept_id", insertable = false, updatable = false)
    private String deptId;

    @ManyToOne(targetEntity = Department.class, cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "dept_id")
    private Department department;

}
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测试

@Test
public void findByIdTest() {
    Optional<Employee> optional = employeeRepository.findById("93fce66c1ef340fa866d5bd389de3d79");
    System.out.println(optional.get());
}
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结果报错了...

java.lang.StackOverflowError
    at java.base/java.lang.Exception.<init>(Exception.java:102)
    at java.base/java.lang.ReflectiveOperationException.<init>(ReflectiveOperationException.java:89)
    at java.base/java.lang.reflect.InvocationTargetException.<init>(InvocationTargetException.java:73)
    at jdk.internal.reflect.GeneratedConstructorAccessor54.newInstance(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
    at com.mysql.jdbc.PreparedStatement.getInstance(PreparedStatement.java:761)
    at com.mysql.jdbc.ConnectionImpl.clientPrepareStatement(ConnectionImpl.java:1404)
    at com.mysql.jdbc.ConnectionImpl.prepareStatement(ConnectionImpl.java:4121)
    at com.mysql.jdbc.ConnectionImpl.prepareStatement(ConnectionImpl.java:4025)
    at com.zaxxer.hikari.pool.ProxyConnection.prepareStatement(ProxyConnection.java:318)
    at com.zaxxer.hikari.pool.HikariProxyConnection.prepareStatement(HikariProxyConnection.java)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$5.doPrepare(StatementPreparerImpl.java:145)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$StatementPreparationTemplate.prepareStatement(StatementPreparerImpl.java:171)
    at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.prepareQueryStatement(StatementPreparerImpl.java:147)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.prepareQueryStatement(AbstractLoadPlanBasedLoader.java:226)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeQueryStatement(AbstractLoadPlanBasedLoader.java:190)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:121)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:86)
    at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87)
    at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:688)
    at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:75)
    at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2223)
    at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:565)
    at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:247)
    at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:561)
    at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:132)
    at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:430)
    at com.example.springbootjpa.entity.Department.hashCode(Department.java:14)
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通过日志看sql的输出,发现了sql重复执行了好多次。以下我截取了前10条sql记录。

Hibernate: select employee0_.id as id1_1_0_, employee0_.dept_id as dept_id2_1_0_, employee0_.emp_job as emp_job3_1_0_, employee0_.emp_name as emp_name4_1_0_ from tb_emp employee0_ where employee0_.id=?
Hibernate: select department0_.id as id1_0_0_, department0_.dept_name as dept_nam2_0_0_ from tb_dept department0_ where department0_.id=?
Hibernate: select employees0_.dept_id as dept_id2_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.dept_id as dept_id2_1_1_, employees0_.emp_job as emp_job3_1_1_, employees0_.emp_name as emp_name4_1_1_ from tb_emp employees0_ where employees0_.dept_id=?
Hibernate: select department0_.id as id1_0_0_, department0_.dept_name as dept_nam2_0_0_ from tb_dept department0_ where department0_.id=?
Hibernate: select employees0_.dept_id as dept_id2_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.dept_id as dept_id2_1_1_, employees0_.emp_job as emp_job3_1_1_, employees0_.emp_name as emp_name4_1_1_ from tb_emp employees0_ where employees0_.dept_id=?
Hibernate: select employees0_.dept_id as dept_id2_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.dept_id as dept_id2_1_1_, employees0_.emp_job as emp_job3_1_1_, employees0_.emp_name as emp_name4_1_1_ from tb_emp employees0_ where employees0_.dept_id=?
Hibernate: select employees0_.dept_id as dept_id2_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.dept_id as dept_id2_1_1_, employees0_.emp_job as emp_job3_1_1_, employees0_.emp_name as emp_name4_1_1_ from tb_emp employees0_ where employees0_.dept_id=?
Hibernate: select employees0_.dept_id as dept_id2_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.dept_id as dept_id2_1_1_, employees0_.emp_job as emp_job3_1_1_, employees0_.emp_name as emp_name4_1_1_ from tb_emp employees0_ where employees0_.dept_id=?
Hibernate: select employees0_.dept_id as dept_id2_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.dept_id as dept_id2_1_1_, employees0_.emp_job as emp_job3_1_1_, employees0_.emp_name as emp_name4_1_1_ from tb_emp employees0_ where employees0_.dept_id=?
Hibernate: select employees0_.dept_id as dept_id2_1_0_, employees0_.id as id1_1_0_, employees0_.id as id1_1_1_, employees0_.dept_id as dept_id2_1_1_, employees0_.emp_job as emp_job3_1_1_, employees0_.emp_name as emp_name4_1_1_ from tb_emp employees0_ where employees0_.dept_id=?
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通过观察发现,第一条sql是执行查询Employee的sql,第二条sql是执行查询Department的sql,第三条sql是执行Department里面所有员工的sql,第四条sql是执行查询Department的sql,后面所有的sql都是执行查询Department里面所有员工的sql。

很明显发生了循环依赖的情况。这是Lombok@Data注解的锅。Lombok@Data注解相当于@Getter,@Setter,@RequiredArgsConstructor,@ToString,@EqualsAndHashCode这几个注解。

我们可以通过反编译看一下Lombok生成的toString()方法

// Employee
public String toString() {
  return "Employee(id=" + getId() + ", empName=" + getEmpName() + ", empJob=" + getEmpJob() + ", deptId=" + getDeptId() + ", department=" + getDepartment() + ")";
}
// Department
public String toString() {
  return "Department(id=" + getId() + ", deptName=" + getDeptName() + ", employees=" + getEmployees() + ")";
}
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可以发现Lombok为我们生成的toString()方法覆盖了整个类的所有属性 现在将 @Data 注解去掉,替换为 @Setter,@Getter,@EqualsAndHashCode,重写 toString() 方法

// Department
@Override
public String toString() {
    return "Department{" +
            "id='" + id + '\'' +
            ", deptName='" + deptName + '\'' +
            '}';
}
// Employee
@Override
public String toString() {
    return "Employee{" +
            "id='" + id + '\'' +
            ", empName='" + empName + '\'' +
            ", empJob='" + empJob + '\'' +
            ", deptId='" + deptId + '\'' +
            ", department=" + department +
            '}';
}
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再次运行测试用例,测试通过,以上Employee toString()方法打印的department会触发懒加载,最终日志输出的sql如下:

Hibernate: select employee0_.id as id1_1_0_, employee0_.dept_id as dept_id2_1_0_, employee0_.emp_job as emp_job3_1_0_, employee0_.emp_name as emp_name4_1_0_ from tb_emp employee0_ where employee0_.id=?
Hibernate: select department0_.id as id1_0_0_, department0_.dept_name as dept_nam2_0_0_ from tb_dept department0_ where department0_.id=?
Employee{id='93fce66c1ef340fa866d5bd389de3d79', empName='jack', empJob='hr', deptId='0a4fe7234fff42afad34f6a06a8e1821', department=Department{id='0a4fe7234fff42afad34f6a06a8e1821', deptName='人事部'}}
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再来测试查询Department

@Test
public void findByIdTest() {
    Optional<Department> optional = departmentRepository.findById("0a4fe7234fff42afad34f6a06a8e1821");
    Set<Employee> employees = optional.get().getEmployees();
    Assert.assertNotEquals(0, employees.size());
}
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同样还是报了堆栈溢出,错误定位在Department和Employee的hashCode()方法上

java.lang.StackOverflowError
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:439)
    at com.mysql.jdbc.ResultSetImpl.getInstance(ResultSetImpl.java:342)
    at com.mysql.jdbc.MysqlIO.buildResultSetWithRows(MysqlIO.java:3132)
    at com.mysql.jdbc.MysqlIO.getResultSet(MysqlIO.java:477)
    at com.mysql.jdbc.MysqlIO.readResultsForQueryOrUpdate(MysqlIO.java:3115)
    at com.mysql.jdbc.MysqlIO.readAllResults(MysqlIO.java:2344)
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2739)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2486)
    at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1858)
    at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:1966)
    at com.zaxxer.hikari.pool.ProxyPreparedStatement.executeQuery(ProxyPreparedStatement.java:52)
    at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeQuery(HikariProxyPreparedStatement.java)
    at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.extract(ResultSetReturnImpl.java:60)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.getResultSet(AbstractLoadPlanBasedLoader.java:419)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeQueryStatement(AbstractLoadPlanBasedLoader.java:191)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:121)
    at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:86)
    at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.initialize(AbstractLoadPlanBasedCollectionInitializer.java:87)
    at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:688)
    at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:75)
    at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:2223)
    at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:565)
    at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:247)
    at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:561)
    at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:132)
    at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:430)
    at com.example.springbootjpa.entity.Department.hashCode(Department.java:17)
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依旧是Lombok的锅,@EqualsAndHashCode为我们生成的equals()hashCode()方法会使用所有属性,注意,DepartmentemployeesSet集合,当我们调用department.getEmployees()时,EmployeehashCode()方法会被调用,Employee中的hashCode()又依赖于DepartmentHashCode()方法,这样又形成了循环引用...

// Department
public int hashCode() {
    int i = 43;
    String $id = getId();
    int result = ($id == null ? 43 : $id.hashCode()) + 59;
    String $deptName = getDeptName();
    result = (result * 59) + ($deptName == null ? 43 : $deptName.hashCode());
    Set $employees = getEmployees();
    int i2 = result * 59;
    if ($employees != null) {
        i = $employees.hashCode();
    }
    return i2 + i;
}
// Employee
public int hashCode() {
    int i = 43;
    String $id = getId();
    int result = ($id == null ? 43 : $id.hashCode()) + 59;
    String $empName = getEmpName();
    result = (result * 59) + ($empName == null ? 43 : $empName.hashCode());
    String $empJob = getEmpJob();
    result = (result * 59) + ($empJob == null ? 43 : $empJob.hashCode());
    String $deptId = getDeptId();
    result = (result * 59) + ($deptId == null ? 43 : $deptId.hashCode());
    Department $department = getDepartment();
    int i2 = result * 59;
    if ($department != null) {
        i = $department.hashCode();
    }
    return i2 + i;
}
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自己动手重写 equals()hashCode() 方法,去掉 @EqualsAndHashCode 注解

// Department
@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Department that = (Department) o;
    return Objects.equals(id, that.id) &&
            Objects.equals(deptName, that.deptName);
}

@Override
public int hashCode() {
    return Objects.hash(id, deptName);
}
// Employee
@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Employee employee = (Employee) o;
    return Objects.equals(id, employee.id) &&
            Objects.equals(empName, employee.empName) &&
            Objects.equals(empJob, employee.empJob) &&
            Objects.equals(deptId, employee.deptId);
}

@Override
public int hashCode() {
    return Objects.hash(id, empName, empJob, deptId);
}
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再次运行测试用例,测试通过

总结:慎用 @Data 注解,使用 @Getter,@Setter注解,需要时自己重写 toString(),equals()以及 hashCode()方法

# 审计Auditing

参考自官方文档5.9Auditing (opens new window)

一般数据库表在设计时都会添加4个审计字段,Spring Data Jpa 同样支持审计功能。Spring Data 提供了@CreatedBy@LastModifiedBy@CreatedDate@LastModifiedDate 4个注解来记录表中记录的创建及修改信息。

实体类

package com.example.springbootjpa.entity;

import lombok.Data;
import org.hibernate.annotations.GenericGenerator;
import org.springframework.data.annotation.CreatedBy;
import org.springframework.data.annotation.CreatedDate;
import org.springframework.data.annotation.LastModifiedBy;
import org.springframework.data.annotation.LastModifiedDate;
import org.springframework.data.jpa.domain.support.AuditingEntityListener;

import javax.persistence.*;
import java.util.Date;
import java.util.Set;

@Entity
@EntityListeners(AuditingEntityListener.class)
@Table(name = "tb_user")
@Data
public class User {

    @Id
    @GenericGenerator(name = "idGenerator", strategy = "uuid")
    @GeneratedValue(generator = "idGenerator")
    private String id;

    @Column(name = "username", unique = true, nullable = false, length = 64)
    private String username;

    @Column(name = "password", nullable = false, length = 64)
    private String password;

    @Column(name = "email", unique = true, length = 64)
    private String email;

    @ManyToMany(targetEntity = Role.class, cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinTable(name = "tb_user_role", joinColumns = {@JoinColumn(name = "user_id", referencedColumnName = "id")},
            inverseJoinColumns = {@JoinColumn(name = "role_id", referencedColumnName = "id")})
    private Set<Role> roles;

    @CreatedDate
    @Column(name = "created_date", updatable = false)
    private Date createdDate;

    @CreatedBy
    @Column(name = "created_by", updatable = false, length = 64)
    private String createdBy;

    @LastModifiedDate
    @Column(name = "updated_date")
    private Date updatedDate;

    @LastModifiedBy
    @Column(name = "updated_by", length = 64)
    private String updatedBy;

}
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实体类上还添加了 @EntityListeners(AuditingEntityListener.class),而 AuditingEntityListener 是由 Spring Data Jpa 提供的

# 实现AuditorAware接口

光添加了4个审计注解还不够,得告诉程序到底是谁在创建和修改表记录

/src/main/java/com/example/springbootjpa/auditing/AuditorAwareImpl

package com.example.springbootjpa.auditing;

import org.springframework.data.domain.AuditorAware;
import org.springframework.stereotype.Component;

import java.util.Optional;

@Component
public class AuditorAwareImpl implements AuditorAware<String> {

    @Override
    public Optional<String> getCurrentAuditor() {
        return Optional.of("admin");
    }

}
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这里简单的返回了一个"admin"字符串来代表当前用户名

# 启用Jpa审计功能

Spring Boot启动类上添加 @EnableJpaAuditing 注解用于启用Jpa的审计功能

package com.example.springbootjpa;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.data.jpa.repository.config.EnableJpaAuditing;

@SpringBootApplication
@EnableJpaAuditing
public class SpringBootJpaApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBootJpaApplication.class, args);
    }
}
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更多关于Jpa Specifications (opens new window),Example (opens new window)查询请查阅官方文档

上次编辑于: 2021年10月28日 23:24
贡献者: herodotus