学习应用 Spring Data JPA
JPA 简介
JPA(Java Persistence API)是 Java 长久化 API,是 Sun 公司提出的基于 ORM 的 Java 长久化标准。
ORM(Object Relational Mapping)的全称是对象关系映射,支流 Java ORM 框架有 Mybatis,Hibernate 等。
Spring Data JPA
Spring Data JPA 是 Spring Data 框架的一个模块,可简化 JPA 的实现。此外,Spring Data JPA 还能够帮忙咱们简化长久层的开发。对于简略查问,Spring Data JPA 能够依据办法名称进行解析,并主动生成查问语句进行查问;对于简单查问,Spring Data JPA 同样反对原生的 SQL。
Spring Data JPA 实际
创立 SpringBoot 我的项目,利用 JPA 实现简略 CRUD。
1. 引入依赖
POM 文件如下:
<?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>2.2.5.RELEASE</version> <relativePath/> <!-- lookup parent from repository --> </parent> <groupId>com.example</groupId> <artifactId>demo</artifactId> <version>0.0.1-SNAPSHOT</version> <name>demo</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>org.projectlombok</groupId> <artifactId>lombok</artifactId> <optional>true</optional> </dependency> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> <configuration> <excludes> <exclude> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> </exclude> </excludes> </configuration> </plugin> </plugins> </build></project>
2. 配置 JPA 和 MySQL
application.yml 配置文件如下:
spring: datasource: url: jdbc:mysql://localhost:3306/jpa_test?serverTimezone=UTC username: root password: 123456 driver-class-name: com.mysql.cj.jdbc.Driver jpa: hibernate: ddl-auto: update # 表内有数据时不会清空, 只会更新 # 控制台显示SQL show-sql: true
3. 编写实体类
本文以 User 类为例:
package com.example.entity;import lombok.Data;import javax.persistence.*;/** * @Author join * @Date 2021/10/13 */@Data@Entity@Table(name = "user")public class User extends BaseEntity{ @Id @Column(name = "id") @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id; @Column(name = "name", columnDefinition = "varchar(20) not null") private String name;}
User 类中蕴含 id 和 name 两个属性,并且 User 类还继承了 BaseEntity,当我的项目中存在多个实体类时,咱们无妨将数据表中独特的字段封装在 BaseEntity 中,比方 create_time、update_time 等。
package com.example.entity;import lombok.Data;import javax.persistence.*;import java.util.Date;/** * @Author join * @Date 2021/10/13 */@Data@MappedSuperclasspublic class BaseEntity { @Column(name = "create_time") @Temporal(TemporalType.TIMESTAMP) private Date createTime; @Column(name = "update_time") @Temporal(TemporalType.TIMESTAMP) private Date updateTime; @PrePersist public void prePersist() { Date now = new Date(); if (createTime == null) { createTime = now; } if (updateTime == null) { updateTime = now; } } @PreUpdate public void preUpdate() { updateTime = new Date(); } @PreRemove public void preRemove() { updateTime = new Date(); }}
注解解释:
- @Data:lombok 注解,可主动生成 get()、set()、toString() 等办法;
- @Entity:JPA 注解,申明该类为一个实体类,必须与 @Id 搭配应用;
- @Table:JPA 注解,示意该实体类的对象会映射到数据库的 user 表(name 指定表名);
- @Id:JPA 注解,申明主键;
- @GeneratedValue:JPA 注解,示意主键的生成策略,IDENTITY 示意应用自增 id;
- @Column:JPA 注解,申明实体对象的属性映射到数据表中的哪一个字段,name 指定字段名,columnDefinition 指定字段的定义。
- @MappedSuperclass:JPA 注解,利用于实体类的父类,该注解作用的类不会映射到数据表,但其属性都将映射到子类的数据表。
- @PrePersist:被 @PrePersist 润饰的办法在将实体长久化到数据库之前被调用;
- @PreUpdate:被 @PreUpdate 润饰的办法在实体的某个属性产生变动时被调用,如更新实体的 update_time;
- @PreRemove:被 @PreRemove 润饰的办法在实体从数据库删除前被调用。
4. 定义 repository
编写 UserRepository:
package com.example.repository;import com.example.entity.User;import lombok.NonNull;import org.springframework.data.jpa.repository.JpaRepository;import java.util.Optional;/** * @Author join * @Date 2021/10/13 */public interface UserRepository extends JpaRepository<User, Integer> { @NonNull Optional<User> findByName(@NonNull String name);}
UserRepository 只需继承 JpaRepository,便可主动生成根本的 CRUD 办法,如 findById(),save() 等。上述代码中,咱们还自定义了一个办法 findByName(),JpaRepository 可依据办法名称主动实现对应的逻辑。
留神理解 JpaRepository 的命名标准。
5. 定义 controller
编写 UserController:
package com.example.controller;import com.example.entity.User;import com.example.repository.UserRepository;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Controller;import org.springframework.web.bind.annotation.*;import java.util.Optional;/** * @Author join * @Date 2021/10/13 */@Controller@RequestMapping("/user")public class UserController { @Autowired private UserRepository userRepository; @RequestMapping(path = "addUser", method = RequestMethod.POST) @ResponseBody public void addUser(@RequestBody User user) { userRepository.save(user); } @RequestMapping(path = "deleteUser", method = RequestMethod.POST) @ResponseBody public void deleteUser(@RequestBody User user) { userRepository.delete(user); } @RequestMapping(path = "/getById/{id}", method = RequestMethod.GET) @ResponseBody public User queryUserById(@PathVariable("id") int id) { return userRepository.findById(id).orElse(null); } @RequestMapping(path = "/getByName/{name}", method = RequestMethod.GET) @ResponseBody public User queryUserByName(@PathVariable("name") String name) { Optional<User> optional = userRepository.findByName(name); return optional.orElse(null); }}
上述代码中,咱们实现了四个办法,别离用来创立、删除和查问用户。
6. 测试
启动我的项目,利用 Navicat 查看 'jpa_test' 库下的数据表,发现 JPA 主动为咱们创立了 user 表:
表中蕴含 User 类中定义的 id、name 以及 BaseEntity 类中定义的 create_time、update_time 等字段。
增加 user
利用 Postman 发送 post 申请,增加 user:
查问 user
利用 id 查问:
利用 name 查问:
7. JPA 解析原理
JPA 遵循约定大概配置(Convention over configuration)的准则,且遵循 Spring 以及 JPQL 定义的命名规定。Spring 提供了依据命名规定进行查问构建的机制,该机制会从办法名中过滤出一些关键字,如 find…By, read…By, query…By, count…By 和 get…By 等。零碎会依据关键字将办法名解析成两个子句,第一个 By 是辨别这两个子语句的关键字。By 之前的子句是查问子句,指明返回的对象。残余局部为条件子句,如果办法名为 findBy…,那么返回的就是定义 Respository 时指定的对象。下表中是一些关键字的应用阐明:
Keyword | Sample | JPQL snippet |
---|---|---|
And | findByLastnameAndFirstname | … where x.lastname = ?1 and |
Or | findByLastnameOrFirstname | … where x.lastname = ?1 or x.firstname = ?2 |
Is,Equals | 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 | … 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 ages) | … where x.age in ?1 |
NotIn | findByAgeNotIn(Collection age) | … 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) |