添加mybatis依赖：

<dependency>
    <groupId>org.mybatis.spring.boot</groupId>
    <artifactId>mybatis-spring-boot-starter</artifactId>
    <version>1.3.2</version>
</dependency>

mybatis-spring-boot-starter依赖.png

步骤：

​ 1）、添加数据源依赖及连接池

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>druid</artifactId>
    <version>1.1.9</version>
</dependency>
<dependency>
    <groupId>org.mybatis.spring.boot</groupId>
    <artifactId>mybatis-spring-boot-starter</artifactId>
    <version>1.3.2</version>
</dependency>

​ 2）、全局配置文件中配置数据源

spring:
  datasource:
    url: jdbc:mysql:///springboot
    username: root
    password: admin
    type: com.alibaba.druid.pool.DruidDataSource
    initialSize: 5
    minIdle: 5
    maxActive: 20
    schema:
      - classpath*:sql/department.sql
      - classpath*:sql/employee.sql

​ 3）、准备sql文件，并存放在resources目录下的sql文件夹下

​ 4）、注解版

@Mapper
public interface DepartmentMapper {

    @Select("select * from department where id = #{id}")
    public Department getById(Integer id);

    @Delete("delete from department where id = #{id}")
    public int deleteById(Integer id);

    @Insert("insert into department(departmentName) values(#{departmentName})")
    @Options(useGeneratedKeys = true, keyProperty = "id")
    public int insert(Department department);

    @Update("update department set departmentName = #{departmentName} where id = #{id}")
    public int update(Department department);
}

问题：

​ 问题1：如果数据库字段与实体属性的不能匹配，但是数据库字段使用下划线来区分隔开，而实体类属性使用驼峰法命名，这样的字段如何使用注解版的mybatis来解决呢？

​ 解决方法1：定义一个mybatis的配置类，并配置一个ConfigurationCustomizer类型的Bean，并将驼峰命名规则设为true即可。

@Configuration
public class MybatisConfig {

    @Bean
    public ConfigurationCustomizer configurationCustomizer(){
        return configuration -> {
            configuration.setMapUnderscoreToCamelCase(true);
        };
    }
}

原理：在MybatisAutoConfiguration自动配置类中，创建了一个SqlSessionFactory的Bean：

@Bean
@ConditionalOnMissingBean
public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
    SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
    factory.setDataSource(dataSource);
    factory.setVfs(SpringBootVFS.class);
    if (StringUtils.hasText(this.properties.getConfigLocation())) {
        factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
    }
    Configuration configuration = this.properties.getConfiguration();
    if (configuration == null && !StringUtils.hasText(this.properties.getConfigLocation())) {
        configuration = new Configuration();
    }
    
    //获取所有的Mybatis中的ConfigurationCustomizer定制器，执行各自的定制方法
    if (configuration != null && !CollectionUtils.isEmpty(this.configurationCustomizers)) {
        for (ConfigurationCustomizer customizer : this.configurationCustomizers) {
            customizer.customize(configuration);
        }
    }
    factory.setConfiguration(configuration);
    if (this.properties.getConfigurationProperties() != null) {
        factory.setConfigurationProperties(this.properties.getConfigurationProperties());
    }
    if (!ObjectUtils.isEmpty(this.interceptors)) {
        factory.setPlugins(this.interceptors);
    }
    if (this.databaseIdProvider != null) {
        factory.setDatabaseIdProvider(this.databaseIdProvider);
    }
    if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {
        factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());
    }
    if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {
        factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());
    }
    if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
        factory.setMapperLocations(this.properties.resolveMapperLocations());
    }

    return factory.getObject();
}

正如上面源码中注释提到的，在配置SqlSessionFactory的Bean的时候，会依次调用每个Mybatis定制器的customize定制方法，从而修改mybatis的默认配置。

所以可以自定义一个Mybatis的ConfigurationCustomizer定制器即可。

​ 解决方法2：在全局配置文件中添加配置：

# 开启驼峰命名法规则
mybatis.configuration.map-underscore-to-camel-case=true

​ 问题2：如果使用驼峰命名法都无法映射的话，那如何解决？

​ 解决方法：使用@Results注解，用于定义之前mybatis的xml映射文件中的<resultMap />标签：

@Mapper
public interface DepartmentMapper {

    @Results({
                @Result(column = "id", property = "id", id = true),
                @Result(column = "department_name", property = "departmentName")
    })
    @Select("select * from department where id = #{id}")
    public Department getById(Integer id);

    @Delete("delete from department where id = #{id}")
    public int deleteById(Integer id);

    @Insert("insert into department(department_name) values(#{departmentName})")
    @Options(useGeneratedKeys = true, keyProperty = "id")
    public int insert(Department department);

    @Update("update department set department_name = #{departmentName} where id = #{id}")
    public int update(Department department);
}

​ 问题3：如果每个Mapper接口都使用@Mapper注解会比较麻烦，是否有更简便的方法来配置接口呢？

​ 解决方法：（通常）在主程序入口上添加@MapperScan注解，用于指定扫描mapper接口的包路径：

@MapperScan(basePackages = "mapper接口所在的包路径")

​ 如果mapper分在不同的包中，可以同时该注解配置多个mapper接口所在的包路径：

@MapperScan(basePackages = {"包1", "包2", "包3"})

​ 5）、配置版

​ 首先编写一个mybatis全局配置文件（如果不需要任何配置，则不需要该文件配置）：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
</configuration>

​ 接着编写一个mapper接口：

public interface EmployeeMapper {

    public Employee getById(Integer id);
    public Employee insert(Employee employee);
}

​ 接着编写sql映射文件：

<?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="org.com.cay.spring.boot.dao.EmployeeMapper">
    <select id="getById" resultType="employee">
        SELECT * FROM employee
        <where>
            id = #{id}
        </where>
    </select>
    
    <insert id="insert" useGeneratedKeys="true" keyProperty="id">
        INSERT INTO employee(lastName, email, gender, d_id)
        VALUES
        (
            #{lastName},
            #{email},
            #{gender},
            #{dId}
        )
    </insert>
</mapper>

​ 最后全局配置文件中设置mybatis属性：

mybatis:
# 如果无需设置，则可以忽略mybatis.config-location
#  config-location: classpath:mybatis/mybatis-config.xml
  mapper-locations: classpath:mybatis/mapper/*.xml
  type-aliases-package: org.com.cay.spring.boot.entity

4、整合SpringData JPA

4.1、JPA简介

JPA规范.png

4.2、使用JPA

​ 1）、编写一个实体类和数据库表进行映射，并且配置实体类:

@Entity
@Table
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String lastName;
    private String email;

    //getter、setter、toString
}

​ 2）、编写一个Dao接口来操作实体类对应的数据库，继承JpaRepository：

public interface UserRepository extends JpaRepository<User, Integer> {
}

​ 3）、在全局配置文件中配置jpa的基本配置，详细的配置属性在JpaProperties配置类中：

spring:
  jpa:
    hibernate:
      ddl-auto: update
    show-sql: true  #显示sql