MyBatis延迟加载、缓存机制与注解开发

一、延迟加载策略1.1 延迟加载的概念立即加载查询用户时默认也把该用户的所有账户信息查询出来延迟加载查询用户时不查询账户信息等需要使用账户数据时才去查询1.2 应用场景查询账户多对一时可以直接查询用户信息 → 立即加载查询用户一对多时可以先不查账户信息 → 延迟加载1.3 多对一延迟加载配置在 SqlMapConfig.xml 中开启延迟加载settings !-- 开启延迟加载 -- setting namelazyLoadingEnabled valuetrue/ !-- 将积极加载改为按需加载 -- setting nameaggressiveLazyLoading valuefalse/ /settingsAccountMapper.xml 配置resultMap idaccountMap typeAccount result propertyid columnid/ result propertyuid columnuid/ result propertymoney columnmoney/ association propertyuser javaTypeUser selectcom.qcbyjy.mapper.UserMapper.findById columnuid/ /resultMap select idfindAll resultMapaccountMap SELECT * from account /select二、MyBatis缓存机制2.1 一级缓存SqlSession缓存一级缓存是 SqlSession 级别的缓存底层使用Map集合存储key执行的SQL语句value查询的结果对象一级缓存的生命周期与SqlSession相同。以下操作会清空一级缓存session.clearCache()session.update()、insert()、delete()、commit()、close()Test public void testFindById() { User user mapper.findById(41); System.out.println(user); // 第二次查询会从缓存中获取不会查询数据库 User user2 mapper.findById(41); System.out.println(user2); }三、注解开发3.1 单表CRUD注解在 SqlMapConfig.xml 中配置Mapper接口所在的包mappers package namecom.qcbyjy.mapper/ /mappersUserMapper接口public interface UserMapper { Select(select * from user) Results(iduserMap, value{ Result(idtrue, columnid, propertyid), Result(columnusername, propertyusername), Result(columnbirthday, propertybirthday), Result(columnsex, propertysex), Result(columnaddress, propertyaddress) }) public ListUser findAll(); Select(select * from user where id #{uid}) ResultMap(userMap) public User findById(Integer uid); Insert(insert into user (username,birthday,sex,address) values (#{username},#{birthday},#{sex},#{address})) public void insert(User user); Update(update user set username #{username},birthday#{birthday},sex#{sex},address#{address} where id #{id}) public void update(User user); Delete(delete from user where id #{id}) public void delete(Integer userId); Select(select count(*) from user) public Integer findByCount(); Select(select * from user where username like #{username}) public ListUser findByUsername(String username); }3.2 多对一注解查询延迟加载public interface AccountMapper { Select(select * from account) Results(value{ Result(idtrue, columnaid, propertyid), Result(columnuid, propertyuid), Result(columnmoney, propertymoney), Result(propertyuser, javaTypeUser.class, columnuid, oneOne(selectcom.qcbyjy.mapper.UserMapper.findById, fetchTypeFetchType.LAZY)) }) public ListAccount findAll(); }3.3 一对多注解查询延迟加载public interface UserMapper { Select(select * from user) Results(iduserMap, value{ Result(idtrue, columnid, propertyid), Result(columnusername, propertyusername), Result(columnbirthday, propertybirthday), Result(columnsex, propertysex), Result(columnaddress, propertyaddress), Result(propertyaccounts, columnid, manyMany(selectcom.qcbyjy.mapper.AccountMapper.findByUid, fetchTypeFetchType.LAZY)) }) public ListUser findAll(); } public interface AccountMapper { Select(select * from account where uid #{uid}) public ListAccount findByUid(Integer uid); }四、总结特性XML方式注解方式简单语句支持支持更简洁复杂语句支持较复杂动态SQL支持良好支持但较繁琐可读性较高高简单语句推荐场景复杂SQL、动态SQL简单SQL、Spring Boot项目实际开发建议简单SQL可以使用注解方式复杂SQL和动态SQL建议使用XML方式-。