hql实例 jpa_SpringBoot学习笔记九：Spring Data Jpa的使用

Spring Data Jpa 简介

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)

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、JDO等ORM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 Data Jpa官方解释

Spring Data JPA是Spring 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

总的来说JPA是ORM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

在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

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

// 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;

}

}

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 {

}

Spring Boot整合Spring Data Jpa

导入依赖

org.springframework.boot

spring-boot-starter-web

org.springframework.boot

spring-boot-starter-data-jpa

mysql

mysql-connector-java

runtime

org.projectlombok

lombok

true

org.springframework.boot

spring-boot-devtools

true

org.springframework.boot

spring-boot-starter-test

test

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

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

注意：

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;

}

主键采用UUID策略

@GenericGenerator是Hibernate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 {

}

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 optional = userRepository.findById(userId);

return optional.orElseGet(User::new);

}

@GetMapping("/list")

public Page pageQuery(@RequestParam(value = "pageNum", defaultValue = "1") Integer pageNum,

@RequestParam(value = "pageSize", defaultValue = "10") Integer pageSize) {

return userRepository.findAll(PageRequest.of(pageNum - 1, pageSize));

}

}

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：

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 { … }

在接口中声明查询方法，如：

interface PersonRepository extends Repository {

List findByLastname(String lastname);

}

使用 JavaConfig 或 XML configuration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：

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">

顺带一提，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.properties或application.yml文件中配置几个属性即可

注入Repository实例并使用，如：

class SomeClient {

private final PersonRepository repository;

SomeClient(PersonRepository repository) {

this.repository = repository;

}

void doSomething() {

List persons = repository.findByLastname("Matthews");

}

}

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

* @param

*/

@NoRepositoryBean

public interface MyBaseRepository extends Repository {

Optional findById(ID id);

S save(S entity);

}

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 {

}

Junit测试

package com.example.springbootjpa.repository;

import org.junit.Assert;

import org.junit.Test;

import org.junit.runner.RunWith;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.boot.test.context.SpringBootTest;

import org.springframework.test.context.junit4.SpringRunner;

import java.util.Optional;

@RunWith(SpringRunner.class)

@SpringBootTest

public class UserRepository2Test {

@Autowired

private UserRepository2 userRepositoy;

@Test

public void findByIdTest() {

Optional optional = userRepositoy.findById("40289f0c65674a930165674d54940000");

Assert.assertNotNull(optional.get());

}

}

这里启动Junit测试时报了一个错，记录一下

java.lang.IllegalStateException: Failed to load ApplicationContext

...

Caused by: java.lang.ClassNotFoundException: javax.xml.bind.JAXBException

at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582)

at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:190)

at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:499)

... 50 more

错误很明显Spring应用上下文加载失败，原因是找不到javax.xml.bind.JAXBException

手贱从java8升级到java10，JAXB API是java EE 的API，在java SE 9.0 中已经不再包含这个 Jar 包。java9 中引入了模块的概念，默认情况下，Java SE中将不再包含java EE 的Jar包，而在 java 6/7 / 8 时关于这个API 都是捆绑在一起的，解决方法添加如下jar包

javax.xml.bind

jaxb-api

2.3.0

com.sun.xml.bind

jaxb-impl

2.3.0

com.sun.xml.bind

jaxb-core

2.3.0

javax.activation

activation

1.1.1

另一种方法是使用@RepositoryDefinition注解，并从CrudRepository中copy你想暴露的CRUD方法

src/main/java/com/example/springbootjpa/repository/UserRepository3

package com.example.springbootjpa.repository;

import com.example.springbootjpa.entity.User;

import org.springframework.data.repository.RepositoryDefinition;

import java.util.Optional;

@RepositoryDefinition(domainClass = User.class, idClass = String.class)

public interface UserRepository3 {

Optional findById(String id);

User save(User user);

}

Repository方法的Null值处理

从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CrudRepository的findById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 findByUsernameIgnoreCase(String username);

List findByUsernameLike(String username);

User findByUsernameAndPassword(String username, String password);

User findByEmail(String email);

List findByEmailLike(String email);

List findByIdIn(List ids);

List findByIdInOrderByUsername(List ids);

void deleteByIdIn(List ids);

Long countByUsernameLike(String username);

支持的关键字、示例及JPQL片段如下表所示：

Keyword

Sample

JPQL snippet

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

findByAgeIsNull

… where x.age is null

IsNotNull,NotNull

findByAge(Is)NotNull

… where x.age not null

Like

findByFirstnameLike

… where x.firstname like ?1

NotLike

findByFirstnameNotLike

... findByFirstnameNotLike

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 ages)

… where x.age in ?1

NotIn

findByAgeNotIn(Collection ages)

… where x.age not in ?1

True

findByActiveTrue()

… where x.active = true

False

findByActiveFalse()

… where x.active = false

IgnoreCase

findByFirstnameIgnoreCase

… where UPPER(x.firstame) = UPPER(?1)

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 findTop10ByUsername(String username, Sort sort);

List findTop10ByUsername(String username, Pageable pageable);

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 getByUsernameLike(String username);

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);

Using SpEL Expressions

从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);

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 queryByUsernameLike(String username, Pageable pageable);

分页查询及排序

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

@Query("select u from User u where u.username like %?1%")

Page findByUsernameLike(String username, Pageable pageable);

@Query("select u from User u where u.username like %?1%")

List findByUsernameAndSort(String username, Sort sort);

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PageRequest的of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"));

注意：Spring Data PageRequest的page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);

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 roles;

}

/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;

}

测试

@Test

public void findByIdTest() {

Optional optional = userRepository.findById("40289f0c65674a930165674d54940000");

Set roles = optional.get().getRoles();

System.out.println(optional.get());

}

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

原因：Spring Boot整合JPA后Hibernate的Session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

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 employees;

}

/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;

}

测试

@Test

public void findByIdTest() {

Optional optional = employeeRepository.findById("93fce66c1ef340fa866d5bd389de3d79");

System.out.println(optional.get());

}

结果报错了...

java.lang.StackOverflowError

at java.base/java.lang.Exception.(Exception.java:102)

at java.base/java.lang.ReflectiveOperationException.(ReflectiveOperationException.java:89)

at java.base/java.lang.reflect.InvocationTargetException.(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)

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=?

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的@Data注解的锅。Lombok的@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() + ")";

}

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 +

'}';

}

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'}}

再来测试查询Department

@Test

public void findByIdTest() {

Optional optional = departmentRepository.findById("0a4fe7234fff42afad34f6a06a8e1821");

Set employees = optional.get().getEmployees();

Assert.assertNotEquals(0, employees.size());

}

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)

依旧是Lombok的锅，@EqualsAndHashCode为我们生成的equals()和hashCode()方法会使用所有属性，注意，Department中employees是Set集合，当我们调用department.getEmployees()时，Employee的hashCode()方法会被调用，Employee中的hashCode()又依赖于Department的HashCode()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;

}

自己动手重写equals()和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);

}

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

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，@LastModifiedDate4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 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;

}

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 {

@Override

public Optional getCurrentAuditor() {

return Optional.of("admin");

}

}

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

在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);

}

}