MyBatis核心机制与实战踩坑全指南 —— 从 JDBC 底层到企业级开发规范(最新)

// 1. 创建数据源 DataSource，配置 JDBC 连接信息
HikariDataSource dataSource = new HikariDataSource();
dataSource.setJdbcUrl("jdbc:mysql://127.0.0.1:3306/mybatis_test");
dataSource.setUsername("root");
dataSource.setPassword("123456");
// 2. 从数据源获取数据库连接 Connection
Connection connection = dataSource.getConnection();
// 3. 编写带 ? 占位符的预编译 SQL 语句
String sql = "SELECT id, username, delete_flag, create_time FROM user_info WHERE age = ? AND gender = ?";
// 4. 通过连接 + SQL 创建 PreparedStatement 操作命令对象
PreparedStatement statement = connection.prepareStatement(sql);
// 5. 替换占位符：按索引位置赋值（索引从 1 开始）
statement.setInt(1, 18);
statement.setInt(2, 1);
// 6. 执行 SQL 语句
ResultSet resultSet = statement.executeQuery();
// 7. 遍历结果集 ResultSet
List<UserInfo> list = new ArrayList<>();
while (resultSet.next()) {
    UserInfo user = new UserInfo();
    // 8. 核心步骤：手动完成 数据库字段 → Java 实体类属性 的映射
    user.setId(resultSet.getInt("id"));
    user.setUsername(resultSet.getString("username"));
    user.setDeleteFlag(resultSet.getInt("delete_flag"));   // 下划线字段 → 驼峰属性，手动映射
    user.setCreateTime(resultSet.getDate("create_time"));
    list.add(user);
}
// 9. 关闭释放数据库资源（这里省略了 try-catch-finally 的冗长写法）
resultSet.close();
statement.close();
connection.close();

mybatis:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

==>  Preparing: SELECT * FROM `user_info` WHERE age = ? AND gender = ?
==> Parameters: 18(Integer), 1(Integer)
<==    Columns: id, username, password, age, gender, phone, delete_flag, create_time, update_time
<==        Row: 1, 张三, 123456, 18, 1, 13800138000, 0, 2025-01-15 10:30:00, 2025-06-01 14:20:00
<==        Row: 2, 李四, 654321, 18, 1, 13900139000, 0, 2025-02-20 08:15:00, 2025-05-10 09:45:00
<==      Total: 2

CREATE TABLE `user_info` (
    `id`          INT PRIMARY KEY AUTO_INCREMENT,
    `username`    VARCHAR(50),
    `password`    VARCHAR(50),
    `age`         INT,
    `gender`      INT,
    `phone`       VARCHAR(20),
    `delete_flag` INT DEFAULT 0,      -- 下划线命名
    `create_time` DATETIME,           -- 下划线命名
    `update_time` DATETIME            -- 下划线命名
);

@Data
public class UserInfo {
    private Integer id;
    private String username;
    private String password;
    private Integer age;
    private Integer gender;
    private String phone;
    private Integer deleteFlag;    // 驼峰命名 → 对应数据库 delete_flag
    private Date createTime;       // 驼峰命名 → 对应数据库 create_time
    private Date updateTime;       // 驼峰命名 → 对应数据库 update_time
}

@Select("SELECT id, username, password, age, gender, phone, " +
        "delete_flag AS deleteFlag, " +   // 手动指定别名 → 属性名
        "create_time AS createTime, " +
        "update_time AS updateTime " +
        "FROM `user_info`")
List<UserInfo> selectList2();

@Results({
    @Result(column = "delete_flag", property = "deleteFlag"),
    @Result(column = "create_time", property = "createTime"),
    @Result(column = "update_time", property = "updateTime")
})
@Select("SELECT * FROM `user_info`")
List<UserInfo> selectList3();

// 定义可复用的映射规则，id = "BaseMap"
@Results(id = "BaseMap", value = {
    @Result(column = "delete_flag", property = "deleteFlag"),
    @Result(column = "create_time", property = "createTime"),
    @Result(column = "update_time", property = "updateTime"),
})
@Select("SELECT * FROM `user_info`")
List<UserInfo> selectList4();   // 这里定义了 BaseMap
// 其他方法直接引用 BaseMap，无需重复声明映射规则
@ResultMap(value = "BaseMap")
@Select("SELECT * FROM `user_info` WHERE age = #{age}")
List<UserInfo> selectByAge(Integer age);

mybatis:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl   # 开启 SQL 日志
    map-underscore-to-camel-case: true                       # 核心配置

@Select("SELECT * FROM `user_info`")
List<UserInfo> selectList5();   // 无需任何映射注解，驼峰自动转换

// ❌ 错误示例：多参数直接写形参名
@Select("SELECT * FROM `user_info` WHERE age = #{age} AND gender = #{gender}")
List<UserInfo> selectByAgeAndGender(Integer age, Integer gender);

org.apache.ibatis.binding.BindingException: Parameter 'age' not found.
Available parameters are [arg1, arg0, param1, param2]

@Select("SELECT * FROM `user_info` WHERE age = #{param1} AND gender = #{param2}")
List<UserInfo> selectByAgeAndGender(Integer age, Integer gender);

// ✅ 规范写法：占位符名称 = 方法形参名称，清晰直观
@Select("SELECT * FROM `user_info` WHERE age = #{age} AND gender = #{gender}")
List<UserInfo> selectByAgeAndGender(Integer age, Integer gender);

// 🏆 最佳实践：用 @Param 显式指定参数别名，兼容性最强
@Select("SELECT * FROM `user_info` WHERE age = #{age} AND gender = #{gender}")
List<UserInfo> selectByAgeAndGender2(
    @Param("age") Integer age,
    @Param("gender") Integer gender
);

// 直接传入实体对象，SQL 中直接写 #{实体属性名}
@Options(useGeneratedKeys = true, keyProperty = "id")  // 回填自增主键
@Insert("INSERT INTO user_info (username, password, age) " +
        "VALUES (#{username}, #{password}, #{age})")
Integer insertUser(UserInfo userInfo);

UserInfo userInfo = new UserInfo("java", "123456", 18);
Integer rows = userInfoMapper.insertUser(userInfo);
System.out.println("插入 " + rows + " 条，自增 id：" + userInfo.getId());
// 输出：插入 1 条，自增 id：6
// userInfo.getId() 能拿到数据库自动生成的主键值，因为 @Options 配置了 keyProperty = "id"

// ⚠️ 加了 @Param("userInfo")，SQL 中必须用 #{userInfo.属性名}
@Insert("INSERT INTO user_info (username, password, age) " +
        "VALUES (#{userInfo.username}, #{userInfo.password}, #{userInfo.age})")
Integer insertUser2(@Param("userInfo") UserInfo userInfo);