MyBatis分步查询的延迟加载

• 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<Address> 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

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC  "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="canger.study.chapter04.mapper.CityPlusPlusMapper">

    <resultMap id="cityPlusPlusResultMap" type="canger.study.chapter04.bean.CityPlusPlus">
        <id column="city_id" property="id"/>
        <result column="city" property="name"/>
        <result column="country_id" property="countryId"/>
        <result column="last_update" property="lastUpdate"/>
        <association property="country"
                     select="canger.study.chapter04.mapper.CountryMapper.selectCountryById"
                     column="country_id"/>
        <association property="addressList"
                     select="canger.study.chapter04.mapper.AddressMapper.selectAddressesByCityId"
                     column="city_id"/>
    </resultMap>

    <select id="selectCityPlusPlusById" resultMap="cityPlusPlusResultMap">
        select *
        from city
        where city_id=#{id}
    </select>
</mapper>

    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="true"/>
    </settings>

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): <==      Total: 1
*************主动查询结束*************
[DEBUG] 2018-11-21 23:37:12,632(625) --> [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): <==      Total: 1
[DEBUG] 2018-11-21 23:37:12,641(634) --> [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): <==      Total: 2
Aurora
*************Step2*************
Country{id=103, name='United States', lastUpdate=Wed Feb 15 04:44:00 CST 2006}
**************Step3************
[Address{addressId=335, address='587 Benguela Manor', address2='', district='Illinois', postalCode='91590', phone='165450987037', cityId=42, location=POINT(-88.3200762 41.760583), lastUpdate=Thu Sep 25 22:33:44 CST 2014}, Address{addressId=543, address='43 Vilnius Manor', address2='', district='Colorado', postalCode='79814', phone='484500282381', cityId=42, location=POINT(-104.8319273 39.729439), lastUpdate=Thu Sep 25 22:33:44 CST 2014}]

lazyLoadingEnabled为true和aggressiveLazyLoading为false，fetchType不设置

    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

输出日志，可以看到：

• 当调用cityPlusPlus.getCountry() 时，第一个关联查询执行
• 当调用cityPlusPlus.getAddressList()时，第二个关联查询执行

************开始主动查询**************
[DEBUG] 2018-11-21 23:56:04,599(286) --> [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): <==      Total: 1
*************主动查询结束*************
Aurora
*************Step2*************
[DEBUG] 2018-11-21 23:56:04,942(629) --> [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): <==      Total: 1
Country{id=103, name='United States', lastUpdate=Wed Feb 15 04:44:00 CST 2006}
**************Step3************
[DEBUG] 2018-11-21 23:56:04,961(648) --> [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): <==      Total: 2
[Address{addressId=335, address='587 Benguela Manor', address2='', district='Illinois', postalCode='91590', phone='165450987037', cityId=42, location=POINT(-88.3200762 41.760583), lastUpdate=Thu Sep 25 22:33:44 CST 2014}, Address{addressId=543, address='43 Vilnius Manor', address2='', district='Colorado', postalCode='79814', phone='484500282381', cityId=42, location=POINT(-104.8319273 39.729439), lastUpdate=Thu Sep 25 22:33:44 CST 2014}]