RowMapper

RowMapper 在 ResultSet 读取每一行时进行映射操作。使用 RowMapper 无需关心 ResultSet 的遍历过程，只需将编程工作专注在每行数据的转换上。

接口定义

@FunctionalInterface
public interface RowMapper<T> {
    T mapRow(ResultSet rs, int rowNum) throws SQLException;
}

自定义实现示例

public class UserRowMapper implements RowMapper<User> {
    public User mapRow(ResultSet rs, int rowNum) throws SQLException {
        User user = new User();
        user.setId(rs.getInt("id"));
        user.setName(rs.getString("name"));
        return user;
    }
}

如何使用

例：编程式 API

UserRowMapper userRowMapper = new UserRowMapper();
List<User> result = jdbc.queryForList("select * from users", userRowMapper);

例：声明式 API

@SimpleMapper
public interface UserMapper {
    @Query(value = "select * from users where id > #{id}",
           resultRowMapper = UserRowMapper.class)
    List<User> listUsers(@Param("id") long searchId);
}

例：构造器 API

List<User> result = lambda.query(User.class)
                          .le(User::getId, 100)
                          .queryForList(userRowMapper);

例：在 Mapper File 中使用

<select id="queryListByAge" resultRowMapper="com.example.dto.UserRowMapper">
    select * from users where age = #{age}
</select>

内置实现

dbVisitor 内置了 5 种 RowMapper，用户也可自行扩展。

ColumnMapRowMapper

// 将每行转换为 Map<String, Object>，适用于任意查询
String querySql = "select * from users where age > 40";
RowMapper<Map<String, Object>> rowMapper = new ColumnMapRowMapper();
List<Map<String, Object>> result = jdbc.queryForList(querySql, rowMapper);

SingleColumnRowMapper

// 查询结果只有一列时，直接转换为目标类型
String querySql = "select name from users where age > 40";
RowMapper<String> rowMapper = new SingleColumnRowMapper<>(String.class);
List<String> result = jdbc.queryForList(querySql, rowMapper);

BeanMappingRowMapper

// 基于 ORM 映射将每行转换为 Bean
String querySql = "select * from users where age > 40";
RowMapper<TestUser> rowMapper = new BeanMappingRowMapper<>(TestUser.class);
List<TestUser> result = jdbc.queryForList(querySql, rowMapper);

MapMappingRowMapper

// 基于 ORM 映射将每行转换为 Map（仅包含映射声明的列）
String querySql = "select * from users where age > 40";
RowMapper<Map<String, Object>> rowMapper = new MapMappingRowMapper<>(TestUser.class);
List<Map<String, Object>> result = jdbc.queryForList(querySql, rowMapper);

TypeHandlerColumnRowMapper

// 查询结果只有一列时，使用指定的 TypeHandler 进行类型转换
String querySql = "select create_time from users where age > 40";
RowMapper<LocalDateTime> rowMapper = new TypeHandlerColumnRowMapper<>(new LocalDateTimeOfTimestampTypeHandler());
List<LocalDateTime> result = jdbc.queryForList(querySql, rowMapper);

ColumnMapRowMapper 与 MapMappingRowMapper 的差异

两者都返回 Map，但行为不同：

• ColumnMapRowMapper — 将查询返回的所有列都放入 Map，适合 select * 等任意查询。
• MapMappingRowMapper — 根据构造方法中的实体类型，只保留 ORM 映射中声明的列，即使查询了更多列也会被忽略。