mybatis+mysql环境下 uuid存储优化

1问题

uuid采用char(32)或char(36)存储的话，需要占用32或36个字节。为节省存储空间，改为binary(16)，占用16字节。对于500W行的表，可节省7.4G的空间。

mybatis中没有默认的type handler来完成uuid类型<->binary类型的相互转换，需要自定义一个type handler。下面就详细地介绍如何实现。

2 环境

mybatis 3.5.3

mysql 5.7.21

java 8

3 自定义TypeHandler

public class UuidHandler extends BaseTypeHandler<UUID> {

    @Override
    public void setNonNullParameter(PreparedStatement ps,
                                    int columnIndex, UUID uuid,
                                    JdbcType jdbcType) throws SQLException {

        long mostBit = uuid.getMostSignificantBits();
        byte[] bytes = new byte[16];
        bytes[0] = (byte) ((mostBit >> 56) & 0xFF);
        bytes[1] = (byte) ((mostBit >> 48) & 0xFF);
        bytes[2] = (byte) ((mostBit >> 40) & 0xFF);
        bytes[3] = (byte) ((mostBit >> 32) & 0xFF);
        bytes[4] = (byte) ((mostBit >> 24) & 0xFF);
        bytes[5] = (byte) ((mostBit >> 16) & 0xFF);
        bytes[6] = (byte) ((mostBit >> 8) & 0xFF);
        bytes[7] = (byte) (mostBit & 0xFF);
        //
        long leastBit = uuid.getLeastSignificantBits();
        bytes[8] = (byte) ((leastBit >> 56) & 0xFF);
        bytes[9] = (byte) ((leastBit >> 48) & 0xFF);
        bytes[10] = (byte) ((leastBit >> 40) & 0xFF);
        bytes[11] = (byte) ((leastBit >> 32) & 0xFF);
        bytes[12] = (byte) ((leastBit >> 24) & 0xFF);
        bytes[13] = (byte) ((leastBit >> 16) & 0xFF);
        bytes[14] = (byte) ((leastBit >> 8) & 0xFF);
        bytes[15] = (byte) (leastBit & 0xFF);
        //
        ps.setBytes(columnIndex, bytes);
    }

    @Override
    public UUID getNullableResult(ResultSet rs, String columnName) throws SQLException {
        byte[] bytes = rs.getBytes(columnName);

        long mostBit = ((((long) bytes[0] & 0xFF) << 56)
                | (((long) bytes[1] & 0xFF) << 48)
                | (((long) bytes[2] & 0xFF) << 40)
                | (((long) bytes[3] & 0xFF) << 32)
                | (((long) bytes[4] & 0xFF) << 24)
                | (((long) bytes[5] & 0xFF) << 16)
                | (((long) bytes[6] & 0xFF) << 8)
                | (((long) bytes[7] & 0xFF)));

        long leastBit = ((((long) bytes[8] & 0xFF) << 56)
                | (((long) bytes[9] & 0xFF) << 48)
                | (((long) bytes[10] & 0xFF) << 40)
                | (((long) bytes[11] & 0xFF) << 32)
                | (((long) bytes[12] & 0xFF) << 24)
                | (((long) bytes[13] & 0xFF) << 16)
                | (((long) bytes[14] & 0xFF) << 8)
                | (((long) bytes[15] & 0xFF)));
        return new UUID(mostBit, leastBit);
    }


    @Override
    public UUID getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        // 略
        return null;
    }

    @Override
    public UUID getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        // 略
        return null;
    }
}

 以 上代码有2个关键点：

1、在setNonNullParameter()方法中，将UUID转换成byte[]。UUID使用128位（16字节）存储，其中，uuid.getMostSignificantBits()返回高64位，uuid.getLeastSignificantBits()返回低64位。

2、在getNullableResult()方法中 , 将byte[]转换成UUID。

4 测试

4.1 表

CREATE TABLE `user_uuid` (
  `id` binary(16) NOT NULL,
  `name` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

4.2 实体

public class User {

    private UUID id;

    private String name;

   // 略
}

4.3 mapper 

public interface UserMapper {

    void insert(User record);

    User selectByPrimaryKey(UUID id);

    List<User> selectAll();

}

    <insert id="insert" parameterType="leo.domain.User">
    insert into user_uuid (id, name)
    values (#{id}, #{name})
    </insert>

    <select id="selectByPrimaryKey" resultType="leo.domain.User"
            parameterType="java.util.UUID">
    select id,  name
    from user_uuid
    where id = #{id}
    </select>


    <select id="selectAll" resultType="leo.domain.User">
      select id,  name
      from user_uuid
    </select>

4.4 配置

<typeHandlers>
    <typeHandler handler="leo.handler.UuidHandler"/>
</typeHandlers>

在typeHandlers节点中注册UuidHandler

4.5 测试用例

@FixMethodOrder(MethodSorters.NAME_ASCENDING)
public class UserMapperTest {

    SqlSession session;
    UserMapper mapper;

    @Before
    public void init() {
        InputStream inputStream = App3_uuid.class.getResourceAsStream("/mybatis-config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        session = sqlSessionFactory.openSession();
        mapper = session.getMapper(UserMapper.class);
    }


    @Test
    public void test1Insert() {
        // 插入用户1
        UUID uuid1 = UUID.fromString("0eaa9bf6-0c70-4678-8658-8737d3a5eba1");
        User user1 = new User();
        user1.setId(uuid1);
        user1.setName("用户1");
        mapper.insert(user1);
        session.commit();

        // 插入用户2
        UUID uuid2 = UUID.fromString("0eaa9bf6-0c70-4678-8658-8737d3a5eba2");
        User user2 = new User();
        user2.setId(uuid2);
        user2.setName("用户2");
        mapper.insert(user2);
        session.commit();

    }

    @Test
    public void test2SelectByPrimaryKey() {
        UUID uuid1 = UUID.fromString("0eaa9bf6-0c70-4678-8658-8737d3a5eba1");
        User user1 = mapper.selectByPrimaryKey(uuid1);
        System.out.println("user = " + user1);
    }

    @Test
    public void test3SelectAll() {
        List<User> list = mapper.selectAll();
        System.out.println("list = " + list);

    }
}