什么是JPA

JPA(Java Persistence API)，也称之为Java持久层API，简单来说就是基本上不需要写SQL，通过方法名就可以实现相应的数据库操作，并且也可以自动生成表结构，比如我们有个实体对象users，在通过JPA的save，就可以进行insert操作了，更加突出的地方是根据方法名就可以执行相应的操作，不需要手写SQL了，但是也仅仅是简单的SQL，复杂的SQL效果不是很理想了。

比如我们要通过Username来获取这个实体对象，就可以getUserEntitiesByUsername这样定义。

代码

maven依赖

<?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>2.7.8</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>top.zunmx</groupId>
    <artifactId>jpa_demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>jpa_demo</name>
    <description>jpa_demo</description>
    <properties>
        <java.version>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-jdbc</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>com.mysql</groupId>
            <artifactId>mysql-connector-j</artifactId>
            <version>8.0.33</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </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>

springboot配置文件

spring:
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    password: ******
    url: jdbc:mysql://localhost:3306/jpa # 需要提前创建好数据库
    username: root
  jpa:
    hibernate:
      ddl-auto: update # ddl策略
    show-sql: true # 控制台输出sql

ddl-auto 参数

create 启动的时候删除表，然后创建，退出的时候不删除表，启动时执行drop table if exists ···
create-drop 启动时删除表，然后创建，退出时删除数据表如果表不存在报错 启动和结束时执行drop table if exists ···
update 如果启动时表格式不一致则更新表，原有数据保留
validate 项目启动表结构进行校验，如果与实体对象不一致则报错

启动类

package top.zunmx.jpa_demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class JpaDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(JpaDemoApplication.class, args);
    }

}

实体类

package top.zunmx.jpa_demo.entity;

import lombok.Data;

import javax.persistence.*;
@Data
@Table(name = "users") // 表名
@Entity
public class UserEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY) //MySQL 自增字段
    private int id;

    private String username;

    private String password;

}

控制器

package top.zunmx.jpa_demo.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import top.zunmx.jpa_demo.entity.UserEntity;
import top.zunmx.jpa_demo.jpa.userRepository;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Optional;

@RestController
public class base {
    @Resource
    userRepository repository;

    @GetMapping("/insert")
    public UserEntity insert(String username, String password) {
        UserEntity UserEntity = new UserEntity();
        UserEntity.setUsername(username);
        UserEntity.setPassword(password);
        repository.save(UserEntity);
        return UserEntity;
    }
    @GetMapping("/getbyid")
    public UserEntity getbyid(int id) {

        Optional<UserEntity> byId = repository.findById(id);
        UserEntity UserEntity = byId.get();
        return UserEntity;

    }
    @GetMapping("/update")
    public UserEntity getbyid(int id, String username, String password) {
        repository.updateAllById(id,password,username);
        return repository.findById(id).get();
    }
    @GetMapping("delete")
    public int delete(int id){
        boolean b = repository.existsById(id);
        if(b) {
            repository.deleteById(id);
            return 1;
        }else{
            return 0;
        }
    }

    @GetMapping("getUsersByName")
    public ArrayList<UserEntity> getByName(String name){
        UserEntity userEntity = new UserEntity();
        userEntity.setUsername(name);
        ArrayList<UserEntity> userEntitiesByUsername = repository.getUserEntitiesByUsername(name);
        return userEntitiesByUsername;
    }
}

jpa映射

package top.zunmx.jpa_demo.jpa;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Modifying;
import org.springframework.data.jpa.repository.Query;
import org.springframework.stereotype.Repository;
import top.zunmx.jpa_demo.entity.UserEntity;

import javax.transaction.Transactional;
import java.util.ArrayList;

@Repository
public interface userRepository extends JpaRepository<UserEntity, Integer> {

    @Transactional //开启事务
    @Modifying
    @Query("update UserEntity set password=?2,username=?3 where id=?1") 
/*自定义sql，?相当于mybatis中的#，预编译，当实体和表名不一致的情况下，UserEntity要和实体对象一致。也就是HQL语句中表名应该是ORM映射的类名*/
    void updateAllById(int id,String password,String username);

    ArrayList<UserEntity> getUserEntitiesByUsername(String username);
}

测试结果

http://127.0.0.1:8080/insert?username=12345&password=1234

{"id":11,"username":"12345","password":"1234"}

http://127.0.0.1:8080/getbyid?id=1

{"id":1,"username":"123","password":"1234"}

http://127.0.0.1:8080/update?id=1&username=aaa&password=aaa

{"id":1,"username":"aaa","password":"aaa"}

http://127.0.0.1:8080/delete?id=9

http://127.0.0.1:8080/getUsersByName?name=12345

[{"id":7,"username":"12345","password":"1234"},{"id":8,"username":"12345","password":"1234"},{"id":10,"username":"12345","password":"1234"},{"id":11,"username":"12345","password":"1234"}]

http://127.0.0.1:8080/getUsersByName?name=aaa

[{"id":1,"username":"aaa","password":"aaa"}]

日志

Hibernate: insert into users (password, username) values (?, ?)
Hibernate: select userentity0_.id as id1_0_0_, userentity0_.password as password2_0_0_, userentity0_.username as username3_0_0_ from users userentity0_ where userentity0_.id=?
Hibernate: update users set password=?, username=? where id=?
Hibernate: select userentity0_.id as id1_0_0_, userentity0_.password as password2_0_0_, userentity0_.username as username3_0_0_ from users userentity0_ where userentity0_.id=?
Hibernate: select count(*) as col_0_0_ from users userentity0_ where userentity0_.id=?
Hibernate: select userentity0_.id as id1_0_, userentity0_.password as password2_0_, userentity0_.username as username3_0_ from users userentity0_ where userentity0_.username=?

springboot + jpa 入门

什么是JPA

代码

maven依赖

springboot配置文件

ddl-auto 参数

启动类

实体类

控制器

jpa映射

测试结果

日志

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什么是JPA

代码

maven依赖

springboot配置文件

ddl-auto 参数

启动类

实体类

控制器

jpa映射

测试结果

日志

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