创建java模块，导入依赖

<!--springboot父工程-->
<parent>
    <artifactId>spring-boot-starter-parent</artifactId>
    <groupId>org.springframework.boot</groupId>
    <version>2.3.12.RELEASE</version>
</parent>

<dependencies>
    <!--数据库驱动-->
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.45</version>
    </dependency>

    <!--连接池-->
    <dependency>
        <groupId>com.alibaba</groupId>
        <artifactId>druid-spring-boot-starter</artifactId>
        <version>1.2.3</version>
    </dependency>

    <!--持久层框架-mybatisPlus-->
    <dependency>
        <groupId>com.baomidou</groupId>
        <artifactId>mybatis-plus-boot-starter</artifactId>
        <version>3.4.1</version>
    </dependency>

    <!--lombok-->
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
    </dependency>

    <!--web环境依赖-->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <!--配置提示-->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-configuration-processor</artifactId>
    </dependency>

    <!--测试依赖-->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
    </dependency>

</dependencies>
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添加配置

spring:
  datasource:
    driver-class-name: com.mysql.jdbc.Driver
    url: jdbc:mysql://localhost:3306/springboot?useUnicode=true&characterEncoding=utf8&useSSL=false&serverTimezone=Asia/Shanghai
    username: root
    password: root
    type: com.alibaba.druid.pool.DruidDataSource
    druid:
      #初始化连接池的的连接数据量
      initial-size: 5
      #连接池最小连接数
      min-idle: 5
      #获取连接等待超时时间 (毫秒)
      max-wait: 6000
      #最大连接数
      max-active: 20
      # 检测连接是否有效。建议配置为true，不影响性能，并且保证安全性,默认false
      test-while-idle: true
      # 申请连接时执行validationQuery检测连接是否有效，做了这个配置会降低性能 默认true
      test-on-borrow: false
      # 归还连接时执行validationQuery检测连接是否有效，做了这个配置会降低性能。默认false
      test-on-return: false
mybatis-plus:
  #别名包路径
  type-aliases-package: com.eponine.domain
  #mapper文件路径,默认位置classpath:/mapper/**/*.xml
  #xml用于扩展自定义sql
  mapper-locations: classpath:/mapper/**/*.xml
  configuration:
    #开启驼峰命名规则,默认开启
    map-underscore-to-camel-case: true

#日志
logging:
  level:
    com.eponine.mapper: debug
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启动类

@MapperScan("com.eponine.mapper") //mapper接口扫描
@SpringBootApplication //标注为springboot启动类
public class MybatisPlusApplication {

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

实体类

@Data
@AllArgsConstructor
@NoArgsConstructor
//把实体类和表绑定关系,同名可以不写
@TableName("tb_user")
public class User {

    //指定id属性和主键生成方式(AUTO-自增)
    @TableId(type = IdType.AUTO)
    private Long id;
    //属性和表字段绑定关系
    @TableField("username")
    private String username;//同名或符合驼峰命名规则可以省略
    private String password;
    private String nickName;
    private Integer age;
    private String email;
}
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UserMapper接口

//继承BaseMapper类,BaseMapper里封装了通用的CRUD方法
public interface UserMapper extends BaseMapper<User> {
}
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UserService

//继承Mybatis-plus提供的基础接口,IService接口,封装了service需要的基础的CRUD方法
public interface UserService extends IService<User> {
}
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//继承service的基础实现类,实现UserService接口
//ServiceImpl: myabtis-plus提供,封装了通用CURD,包含mapper接口注入
@Service
@Transactional
public class UserServiceImpl extends ServiceImpl<UserMapper, User> implements UserService {
}
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UserController

@RestController
@RequestMapping("/user")
public class UserController {
    @Autowired
    UserService userService;

    @GetMapping("/{id}")
    public void SelectById(@PathVariable Long id){
        User user = userService.getById(id);
        System.out.println(user);
    }

    @GetMapping("/list")
    public void SelectUser(){
        List<User> list = userService.list();
        for (User user : list) {
            System.out.println(user);

        }
    }
}
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Service CRUD接口

mybatis-plus 在IService接口中封装了大量的基础CRUD方法

@RunWith(SpringRunner.class)
@SpringBootTest
public class MyTest {
    @Autowired
    UserService userService;

    @Test
    public void TestMP(){
        //User user = new User(10l, "dover", null, null, null, null);
        //userService.save(user);
        //保存或修改(修改时空值不会覆盖旧值)
        //userService.saveOrUpdate(user);
        //删除
        //userService.removeById(11l);

        List<Long> ids = new ArrayList<>();
        ids.add(1l);
        ids.add(3l);
        ids.add(4l);
        //批量查询
        List<User> list = userService.listByIds(ids);
        for (User user : list) {
            System.out.println(user);
        }
        //批量删除
        //userService.removeByIds(ids);
    }
}
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Mapper CRUD接口

如果CURD需要添加条件，可以使用 Wrapper` 的4个实现类

• UpdateWrapper

  封装修改条件

• QueryWrapper

  封装查询、删除条件

• LambdaQueryWrapper

  用法基本和 QueryWrapper 一致，区别在于获取 column 的方式不一样

• LambdaUpdaeWrapper

  用法基本和 UpdateWrapper 一致，区别在于获取 column 的方式不一样

  条件查询

官网提供的QueryWrapper对象，用于生成 sql 的 where 条件

查询方法	说明
eq （equals）	等于=
ne （not equals）	不等与<>
gt (greater than)	大于>
ge (greater equals)	大于等于>=
lt (less than)	小于<
le (less equals)	小于等于<=
like	模糊查询 LIKE
notLike	模糊查询 NOT LIKE
in	IN 查询
notIn	NOT IN 查询
isNull	NULL 值查询
isNotNull	IS NOT NULL

@Test
public void TestWrapper() {
    User user = new User();
    user.setId(3l);
    user.setAge(22);
    user.setNickName("张");
    /*
    -- 查询 id > 3 并且 年龄 <= 22 或者 昵称以 张开头的用户信息，以ID倒序排序
    SELECT *
            FROM tb_user
            WHERE
    (id > 3 AND age <= 22)
    OR nick_name LIKE "张%"
    ORDER BY id DESC
    */
    QueryWrapper<User> wrapper = new QueryWrapper<>();
    if (user.getId()!=null){
        wrapper.gt("id",user.getId());
    }
    if (user.getAge()!=null){
        wrapper.le("age",user.getAge());
    }
    wrapper.or();
    if (user.getNickName()!=null){
        wrapper.likeRight("nick_name",user.getNickName());
    }
    wrapper.orderByDesc("id");
    List<User> list = userMapper.selectList(wrapper);
    for (User thisUser : list) {
        System.out.println(thisUser);

    }
}
    //优化1: 使用重载方法添加查询条件
    @Test
    public void TestBetter1() {
        User user = new User();
        user.setId(3l);
        user.setAge(22);
        user.setNickName("张");
        QueryWrapper<User> wrapper = new QueryWrapper<>();

        wrapper.gt(user.getId() != null, "id", user.getId());
        wrapper.le(user.getAge() != null, "age", user.getAge());
        wrapper.or();
        wrapper.likeRight(user.getNickName() != null, "nick_name", user.getNickName());
        wrapper.orderByDesc("id");
        List<User> list = userMapper.selectList(wrapper);
        for (User thisUser : list) {
            System.out.println(thisUser);

        }
    }
    //优化2: 使用链式调用
    @Test
    public void TestBetter2() {
        User user = new User();
        user.setId(3l);
        user.setAge(22);
        user.setNickName("张");
        QueryWrapper<User> wrapper = new QueryWrapper<>();

        wrapper.gt(user.getId() != null, "id", user.getId())
                .le(user.getAge() != null, "age", user.getAge())
                .or()
                .likeRight(user.getNickName() != null, "nick_name", user.getNickName())
                .orderByDesc("id");
        List<User> list = userMapper.selectList(wrapper);
        for (User thisUser : list) {
            System.out.println(thisUser);

        }
    }
    //优化3: 使用lambda表达式动态获取字段名
    @Test
    public void TestBetter3() {
        User user = new User();
        user.setId(3l);
        user.setAge(22);
        user.setNickName("张");
        //lambdaQueryWrapper可以从实体类的 `@TableField` 上统一获取列名称
        LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();

        wrapper.gt(user.getId() != null, User::getId, user.getId())
                .le(user.getAge() != null, User::getAge, user.getAge())
                .or()
                .likeRight(user.getNickName() != null, User::getNickName, user.getNickName())
                .orderByDesc(User::getId);
        List<User> list = userMapper.selectList(wrapper);
        for (User thisUser : list) {
            System.out.println(thisUser);

        }
    }
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自定义SQL

注解方式

// 根据年龄分组，并统计每组人数
@Select("select age,count(*)num from tb_user group by age")
List<Map> orderByAge();
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xml配置方式

//继承BaseMapper接口,BaseMapper里封装了通用的CRUD方法
public interface UserMapper extends BaseMapper<User> {

    // 根据年龄分组，并统计每组人数
    @Select("select age,count(*)num from tb_user group by age")
    List<Map> orderByAge();

    User selectById(Long id);
}
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<?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.eponine.mapper.UserMapper">
    <select id="selectbyId" resultType="user">
        select * from tb_user where id=#{value}
    </select>
</mapper>
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分页查询

配置分页拦截器

@Configuration
public class MybatisPlusConfig {
    
    @Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
        return interceptor;
    }
}
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编写分页代码

@Test
public void TestPage(){
    //开启分页查询
    Page<User> page = new Page<>(1, 3);

    //条件
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.eq(User::getPassword,"123456");
    Page<User> userPage = userMapper.selectPage(page, wrapper);
    //分页查询结果
    System.out.println("查询总记录数："+ page.getTotal());
    System.out.println("查询数据列表："+ page.getRecords());
    System.out.println("总页数："+ page.getPages());
    System.out.println("当前数："+ page.getCurrent());
    System.out.println("是否有下一页："+ page.hasNext());
    System.out.println("是否有上一页："+ page.hasPrevious());
}

链接：https://juejin.cn/post/7032661933337608228