一、簡介
MyBatis是支援自定義SQL，儲存過程和高階對映的第一類永續性框架。MyBatis消除了幾乎所有的JDBC程式碼以及手動設定引數和檢索結果。MyBatis可以使用簡單的XML或Annotations來配置和對映原語，Map介面和Java POJO（普通舊Java物件）到資料庫記錄。（官網簡介）
二、引入依賴

<dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.boot</groupId
 
>
                <artifactId>spring-boot-dependencies</artifactId>
                <version>2.0.0.RELEASE</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement
 
>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>


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


        <dependency>
            <groupId>com.mchange</groupId>
            <artifactId>c3p0</artifactId>
            <version>0.9.5.2</version>
        </dependency>


        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.9-rc</version>
        </dependency>

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.16.10</version>
        </dependency>

    </dependencies>

三、配置mybatis
首先需要配置資料來源，本例使用c3p0

@Data
@EnableAutoConfiguration
public class MybatisConfig {
    @Autowired
    private ComboPooledDataSource dataSource;

    @Bean
    public SqlSessionFactory getSqlSessionFactory(){
        try {
            SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
            sqlSessionFactoryBean.setDataSource(dataSource);
            return sqlSessionFactoryBean.getObject();
        }catch (Exception e){
            e.printStackTrace();
            return null;
        }
    }

    @Bean
    public SqlSessionTemplate getSqlSessionTemplate() {
        return new SqlSessionTemplate(getSqlSessionFactory());
    }
}

四、使用註解
建立三個表：學生表（student）、課程表(course)和中間表選課表(score)



建立StudentDao

//Mapper和Component註解同時使用的作用是使得Maybatis的sqlSessionTemplate自動掃面Mapper
@Mapper
@Component
public interface StudentDao {
    //注意#和$的區別，mybatis 在對sql語句進行預編譯之前，會對 sql 進行動態解析。
    // #{}預編譯之後是一個佔位符？
    // ${}預編譯以後直接是替換作用

    @Select({"SELECT * FROM student WHERE id = #{id}"})
    @Results(id ="id",//id是對映結果的名稱
            value = {@Result(property = "courseList", column = "id", many = @Many(select = "com.yyh.dao.CourseDao.selectCoursesByStudentId")),//這是一個一對多
                    @Result(property = "id",column = "id")//表的主鍵，不配置此項，查詢出來id會被設定為0
            }
    )
    Student findStudentById(int id);

    @Select("SELECT * FROM student")
    List<Student> findAllStudent();
}

建立CourseDao

@Mapper
@Component
public interface CourseDao {
    @Insert("INSERT INTO course (`name`,`teacher`) VALUES (#{name},#{teacher})")
    int insertCourse(Course course);


    @Select("SELECT * FROM course WHERE id IN (SELECT course_id FROM score WHERE student_id = #{studentId})")
    List<Course> selectCoursesByStudentId(@Param("studentId") int studentId);
}

實體類如下

@Data
public class Student {
    private int id;
    private String code;
    private String name;
    private String age;
    private String phoneNo;
    private List<Course> courseList;
}
@Data
public class Course {
    private int id;
    private String name;
    private String teacher;
    private List<Student> studentList;
}

五、測試
查詢一個學生資訊和他所選課程

{
    "id": 1,
    "code": "Q150",
    "name": "張三",
    "age": "12",
    "phoneNo": "18857121034",
    "courseList": [
        {
            "id": 2,
            "name": "數學",
            "teacher": "Brain"
        },
        {
            "id": 3,
            "name": "java",
            "teacher": "Snow"
        },
        {
            "id": 1,
            "name": "英語",
            "teacher": "Michael"
        },
        {
            "id": 4,
            "name": "python",
            "teacher": "Smith"
        }
    ]
}

六、常用註解說明

七、其他配置
applicant.properties檔案

#為空的屬性在轉換時忽略掉
spring.jackson.default-property-inclusion=non_empty

#將資料庫中下劃線命名風格自動轉換為駝峰式命名風格
mybatis.configuration.map-underscore-to-camel-case=true
mybatis.configuration.default-fetch-size=100
mybatis.configuration.default-statement-timeout=30
mybatis.configuration.auto-mapping-unknown-column-behavior=WARNING