MyBatis分步查询的延迟加载 |
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MyBatis分步查询的延迟加载
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延迟加载的概念只存在于分步查询时；延迟加载的本质是为第一步查询返回的Java Bean创建了一个代理对象；延迟加载的全局设置有两个：
lazyLoadingEnabled，作用为设置select语句的默认延迟加载是否开启； aggressiveLazyLoading，对于开启了延迟加载的select作用如下： 值为true时，假设select返回的Java Bean为xxx，那么获取任何xxx属性的动作都会导致resultMap中定义的所有关联查询马上执行；值为false时，假设yyy为xxx的一个属性，由关联查询获取，那么只有在yyy的某一属性被使用时，获取yyy的关联查询才会执行延迟加载针对单个查询的具体设置为resultMap的 fetchType属性；全局设置lazyLoadingEnabled和具体设置fetchType的关系如下： 如果设置了resultMap的 fetchType属性，那么全局设置lazyLoadingEnabled便不再对该select生效，是否延迟加载要以fetchType属性为准；如果select的resultMap没有设置 fetchType属性，那么该select会采用lazyLoadingEnabled的设置；aggressiveLazyLoading只对延迟加载的select生效 演示如下需要注意的是，一下演示在DEBUG模式下不成立，均为RUN模式；DEBUG模式下（IDEA环境）MyBatis不会延迟加载 E-R图：一个City隶属于一个Country，同时一个City包含多个Address
实体类：CityPlusPlus中包含一个Country和多个Address CityPlusPlus public class CityPlusPlus { private Long id; private String name; private Long countryId; private Date lastUpdate; private Country country; private List addressList; }
Country public class Country { Long id; String name; Date lastUpdate; }
Address public class Address { Long addressId; String address; String address2; String district; String postalCode; String phone; Long cityId; Location location; Date lastUpdate; } lazyLoadingEnabled和aggressiveLazyLoading均为true，fetchType不设置 DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> select * from city where city_id=#{id}测试代码如下： @Test public void testCityPlusPlusMapper(){ SqlSessionFactory sqlSessionFactory = SqlSessionFactoryUtil.initSqlSessionFactory(); SqlSession sqlSession = null; try { sqlSession = sqlSessionFactory.openSession(); CityPlusPlusMapper mapper = sqlSession.getMapper(CityPlusPlusMapper.class); System.out.println("************开始主动查询**************"); CityPlusPlus cityPlusPlus = mapper.selectCityPlusPlusById(42L); System.out.println("*************主动查询结束*************"); System.out.println(cityPlusPlus.getName()); System.out.println("*************Step2*************"); System.out.println(cityPlusPlus.getCountry()); System.out.println("**************Step3************"); System.out.println(cityPlusPlus.getAddressList()); System.out.println(); System.out.println(); sqlSession.commit(); } catch (Exception e) { e.printStackTrace(); sqlSession.rollback(); } finally { sqlSession.close(); } }
日志输出如下，可以看到，当调用 cityPlusPlus.getName() 时，两个关联查询均马上执行 ************开始主动查询************** [DEBUG] 2018-11-21 23:37:12,307(300) --> [main] org.apache.ibatis.transaction.jdbc.JdbcTransaction.openConnection(JdbcTransaction.java:137): Opening JDBC Connection [DEBUG] 2018-11-21 23:37:12,552(545) --> [main] org.apache.ibatis.datasource.pooled.PooledDataSource.popConnection(PooledDataSource.java:406): Created connection 1514160588. [DEBUG] 2018-11-21 23:37:12,553(546) --> [main] org.apache.ibatis.transaction.jdbc.JdbcTransaction.setDesiredAutoCommit(JdbcTransaction.java:101): Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@5a4041cc] [DEBUG] 2018-11-21 23:37:12,555(548) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Preparing: select * from city where city_id=? [DEBUG] 2018-11-21 23:37:12,575(568) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Parameters: 42(Long) [DEBUG] 2018-11-21 23:37:12,631(624) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Preparing: select * from country where country_id=? [DEBUG] 2018-11-21 23:37:12,637(630) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Parameters: 103(Integer) [DEBUG] 2018-11-21 23:37:12,640(633) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Preparing: select address_id,address,address2,district,city_id,postal_code,phone,astext(location) as location,last_update from address where city_id=? [DEBUG] 2018-11-21 23:37:12,641(634) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Parameters: 42(Integer) [DEBUG] 2018-11-21 23:37:12,651(644) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): [main] org.apache.ibatis.transaction.jdbc.JdbcTransaction.openConnection(JdbcTransaction.java:137): Opening JDBC Connection [DEBUG] 2018-11-21 23:56:04,829(516) --> [main] org.apache.ibatis.datasource.pooled.PooledDataSource.popConnection(PooledDataSource.java:406): Created connection 1514160588. [DEBUG] 2018-11-21 23:56:04,830(517) --> [main] org.apache.ibatis.transaction.jdbc.JdbcTransaction.setDesiredAutoCommit(JdbcTransaction.java:101): Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@5a4041cc] [DEBUG] 2018-11-21 23:56:04,844(531) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Preparing: select * from city where city_id=? [DEBUG] 2018-11-21 23:56:04,865(552) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Parameters: 42(Long) [DEBUG] 2018-11-21 23:56:04,941(628) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Preparing: select * from country where country_id=? [DEBUG] 2018-11-21 23:56:04,943(630) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Parameters: 103(Integer) [DEBUG] 2018-11-21 23:56:04,956(643) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Preparing: select address_id,address,address2,district,city_id,postal_code,phone,astext(location) as location,last_update from address where city_id=? [DEBUG] 2018-11-21 23:56:04,961(648) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): ==> Parameters: 42(Integer) [DEBUG] 2018-11-21 23:56:04,971(658) --> [main] org.apache.ibatis.logging.jdbc.BaseJdbcLogger.debug(BaseJdbcLogger.java:159): |
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