一、通过JDBC访问数据库

1、思路

• 可以把jdbc理解为访问数据库的接口，访问数据库的细节由驱动实现。驱动由数据库提供商遵守jdbc规范来实现。

2、示例

• maven引入mysql驱动

<dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.33</version>
</dependency>

• 代码

public class MainClass {public static void main(String[] args) {try (Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/learn_mybatis","root", "xxx")) {PreparedStatement selectStatement = conn.prepareStatement("select * from user where id =  ?");selectStatement.setInt(1, 1);ResultSet rs = selectStatement.executeQuery();while (rs.next()) {Integer id = rs.getInt("id");String lastName = rs.getString("name");Integer age = rs.getInt("age");UserDO userDO = new UserDO().setId(id).setName(lastName).setAge(age);System.out.println(userDO);}} catch (Exception e) {e.printStackTrace();}}
}

3、思考

• 从示例代码可知，实际开发中，直接通过JDBC进行增删改查太不方便了，例如，我们需要手动完成数据库的记录和Java实体类的映射。
• 因此，我们迫切需要ORM（对象关系映射）框架来简化开发。

二、通过ORM框架访问数据库（主要是Mybatis）

官方文档

1、示例

框架 = 配置 + API

1.1 配置

• src/main/resources/mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""https://mybatis.org/dtd/mybatis-3-config.dtd"><configuration><!-- 配置日志   --><settings><setting name="logImpl" value="LOG4J2"/></settings><environments default="development"><!-- 配置连接数据库的基本信息       --><environment id="development"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="com.mysql.cj.jdbc.Driver"/><property name="url" value="jdbc:mysql://localhost:3306/learn_mybatis"/><property name="username" value="root"/><property name="password" value="xxx"/></dataSource></environment></environments><!-- 配置映射文件（写SQL的地方）--><mappers><mapper resource="mapper/UserMapper.xml"/></mappers>
</configuration>

• src/main/resources/log4j2.xml

<?xml version="1.0" encoding="UTF-8"?>
<Configuration xmlns="http://logging.apache.org/log4j/2.0/config"><Appenders><Console name="stdout" target="SYSTEM_OUT"><PatternLayout pattern="%5level [%t] - %msg%n"/></Console></Appenders><Loggers><Logger name="com.forrest.learn.mapper.UserMapper" level="trace"/><Root level="error" ><AppenderRef ref="stdout"/></Root></Loggers></Configuration>

1.2 SQL写在xxxMapper.xml中：mapper/UserMapper.xml

• src/main/resources/mapper/UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""https://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.forrest.learn.mapper.UserMapper"><select id="selectById" resultType="com.forrest.learn.pojo.UserDO">select id, name, age from user where id = #{id}</select>
</mapper>

• src/main/java/com/forrest/learn/pojo/UserDO.java

@Data
@Accessors(chain = true)
public class UserDO {private Integer id;private String name;private Integer age;
}

1.3 xxxMapper.xml对应的xxxMapper接口（Application通过该接口访问数据库）

• src/main/java/com/forrest/learn/mapper/UserMapper.java

public interface UserMapper {UserDO selectById(Integer id);
}

1.4 测试

• src/test/java/com/forrest/learn/mapper/UserMapperTest.java

public class UserMapperTest {@Testpublic void selectById() {try (InputStream is = Resources.getResourceAsStream("mybatis-config.xml")) {SqlSessionFactoryBuilder ssfBuilder = new SqlSessionFactoryBuilder();SqlSessionFactory ssFactory = ssfBuilder.build(is);SqlSession sqlSession = ssFactory.openSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);UserDO userDO = userMapper.selectById(1);System.out.println(userDO);} catch (IOException e) {System.out.println("读取配置文件失败, caused by " + e.getMessage());}}
}

• 结果：

DEBUG [main] - ==>  Preparing: select id, name, age from user where id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 张三, 18
DEBUG [main] - <==      Total: 1
UserDO(id=1, name=张三, age=18)

2、思考

• 上面的方式还不够方便（在增删改查前，还有一些模板代码），而实际开发中，直接注入UserMapper，就可以增删改查了，怎么实现的？

三、SpringBoot整合Mybatis

1、引入依赖

1.1 为什么需要引入spring-boot-starter-parent？（不引入会出现什么问题？）

<dependencies><!-- web开发需要 --><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><version>2.6.3</version></dependency><!-- 整合mybatis需要 --><dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>2.2.2</version></dependency><!-- mybatis驱动 --><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.28</version></dependency></dependencies>

• 两个starter会引入一堆依赖，如果没有“spring-boot-starter-parent”进行依赖管理，那么很容易导致依赖冲突：
  

1.2 引入spring-boot-starter-parent（进行依赖管理）

<parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.6.3</version>
</parent><dependencies><!-- web开发需要 --><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><!-- 整合mybatis需要 --><dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>2.2.2</version></dependency><!-- mybatis驱动 --><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId></dependency>
</dependencies>

• spring-boot-starter-parent和mybatis-spring-boot-starter版本怎么选？
• 如果用的是java8，那么可以选支持java8的最新版本。

• spring官网，支持java8的最新版本为：Spring Boot 2.7.18
  
  
  

• 通过官网可以找到合适的mybatis-spring-boot-starter版本：2.3.x（选最大的：2.3.2）
  

• 升级版本

<dependencies><!-- web开发需要 --><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><!-- 整合mybatis需要 --><dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>2.3.2</version></dependency><!-- mybatis驱动 --><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.33</version></dependency>
</dependencies>

• 升级版本后，mysql驱动必须指定版本了，估计是starter删除了对mysql驱动的依赖管理。那选什么版本呢？（8.0.33）
  

当然了，选版本不用精细到x.y.z的z。找到符合要求的x.y，z能在maven仓库中搜到即可。

2、写配置文件

2.1 application.properties

src/main/resources/application.properties

2.1.1 使用Spring Boot的默认配置

• 直接在application.properties（或application.yml）文件中配置数据库连接信息即可，而不用写mybatis-config.xml。

spring.datasource.url=jdbc:mysql://localhost:3306/learn_mybatis
spring.datasource.username=root
spring.datasource.password=xxx
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

当使用Spring Boot时，它会自动加载与spring.datasource.*相关的配置。数据源会自动注入到sqlSessionFactory中，而sqlSessionFactory会自动注入到Mapper中。因此，直接通过xxxMapper就能进行增删改查了。

3、xxxDO、xxxMapper.xml、xxxMapper.java

3.1 目录结构

• 基于"Free Mybatis Tool"生成UserDO（放model包下）、UserMapper.java（放mapper包下）、UserMapper.xml（resources/mapper）
  
  

检查生成的代码是否满足需求。

引入lombok（选最新版本，通常都是支持java8的）

<dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.30</version>
</dependency>

• 如果Mapper接口和Mapper XML文件放置在不同的包路径下（如上图所示），那么必须在MyBatis的配置中正确指定Mapper XML文件的位置。
• 在Spring Boot应用中，可以通过在application.properties或application.yml中配置mybatis.mapper-locations属性来指定Mapper XML文件的位置。

mybatis.mapper-locations=classpath:mapper/*.xml

不配置的话，必须保证：Mapper接口的包路径和Mapper XML文件的路径是一致的。

3.2 UserDO

@Data
public class UserDO implements Serializable {private static final long serialVersionUID = 3178995131329607398L;private Integer id;private String name;private Integer age;
}

3.3 UserMapper.java

@Mapper
public interface UserMapper {int insert(UserDO record);int deleteById(Integer id);int update(UserDO record);UserDO selectById(Integer id);
}

3.4 UserMapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.forrest.springboot.mybatis.dao.mapper.UserMapper"><resultMap id="UserMap" type="com.forrest.springboot.mybatis.dao.model.UserDO"><id column="id" jdbcType="INTEGER" property="id" /><result column="name" jdbcType="VARCHAR" property="name" /><result column="age" jdbcType="INTEGER" property="age" /></resultMap><sql id="tb">user</sql><sql id="cols_exclude_id">`name`, age</sql><sql id="cols_all">id, <include refid="cols_exclude_id" /></sql><insert id="insert" keyColumn="id" keyProperty="id" parameterType="com.forrest.springboot.mybatis.dao.model.UserDO" useGeneratedKeys="true">insert into <include refid="tb" />(<include refid="cols_exclude_id" />)values(#{name,jdbcType=VARCHAR}, #{age,jdbcType=INTEGER})</insert><delete id="deleteById" parameterType="java.lang.Integer">delete from <include refid="tb" />where id = #{id,jdbcType=INTEGER}</delete><update id="update" parameterType="com.forrest.springboot.mybatis.dao.model.UserDO">update <include refid="tb" /><set><if test="name != null">`name` = #{name,jdbcType=VARCHAR},</if><if test="age != null">age = #{age,jdbcType=INTEGER}</if></set>where id = #{id,jdbcType=INTEGER}</update><select id="selectById" parameterType="java.lang.Integer" resultMap="UserMap">select<include refid="cols_all" />from<include refid="tb" />whereid = #{id,jdbcType=INTEGER}</select>
</mapper>

我个人更喜欢这种xml的方式，java代码和sql代码分离。

4、测试

4.1 引入spring-boot-starter-test依赖

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope>
</dependency>

• 注意：不要同时引入junit5依赖，否则可能因为版本冲突导致报错“TestEngine with ID ‘junit-jupiter‘ failed to discover test”。
  

4.2 测试类：UserMapperTest

src/test/java/com/forrest/springboot/mybatis/dao/mapper/UserMapperTest.java

@SpringBootTest
class UserMapperTest {@Autowiredprivate UserMapper userMapper;@Testvoid insert() {UserDO user = new UserDO();user.setName("John");user.setAge(30);userMapper.insert(user);assertNotNull(user.getId(), "Insert should generate ID");UserDO retrievedUser = userMapper.selectById(user.getId());assertNotNull(retrievedUser, "User should be retrieved after insertion");assertEquals("John", retrievedUser.getName(), "Name should match");assertEquals(30, retrievedUser.getAge(), "Age should match");}@Testvoid deleteById() {UserDO user = new UserDO();user.setName("Alice");user.setAge(25);userMapper.insert(user);Integer userId = user.getId();assertNotNull(userId, "Insert should generate ID");userMapper.deleteById(userId);UserDO deletedUser = userMapper.selectById(userId);assertNull(deletedUser, "User should be deleted");}@Testvoid update() {UserDO user = new UserDO();user.setName("Bob");user.setAge(35);userMapper.insert(user);Integer userId = user.getId();assertNotNull(userId, "Insert should generate ID");user.setName("UpdatedName");user.setAge(40);userMapper.update(user);UserDO updatedUser = userMapper.selectById(userId);assertNotNull(updatedUser, "User should exist after update");assertEquals("UpdatedName", updatedUser.getName(), "Name should be updated");assertEquals(40, updatedUser.getAge(), "Age should be updated");}@Testvoid selectById() {UserDO user = new UserDO();user.setName("Eve");user.setAge(28);userMapper.insert(user);Integer userId = user.getId();assertNotNull(userId, "Insert should generate ID");UserDO retrievedUser = userMapper.selectById(userId);assertNotNull(retrievedUser, "User should be retrieved by ID");assertEquals("Eve", retrievedUser.getName(), "Name should match");assertEquals(28, retrievedUser.getAge(), "Age should match");}
}

5、自定义mybatis配置

mybatis.config-location=classpath:mybatis/mybatis-config.xml
mybatis.mapper-locations=classpath:mybatis/mapper/*.xml

• 为啥需要这两个？通过mybatis官方文档就知道了，需要一个配置文件和一堆映射文件。
  
• 为啥key是mybatis.config-location和mybatis.mapper-locations？这个要看：
  

5.1 mybatis-config.xml

习惯将mybatis的配置文件命名为mybatis-config.xml

• 具体怎么配置，看官方文档。