一对多 、多对多 的检索策略

在映射文件中, 用 <set> 元素来配置一对多关联及多对多关联关系. <set> 元素有 lazy 和 fetch 属性
lazy: 主要决定 orders 集合被初始化的时机. 即到底是在加载 Customer 对象时就被初始化, 还是在程序访问                   orders 集合时被初始化
fetch: 取值为 “select” 或 “subselect” 时, 决定初始化 orders 的查询语句的形式;  若取值为”join”, 则决定 orders            集合被初始化的时机若把 fetch 设置为 “join”, lazy 属性将被忽略

<set> 元素的 batch-size属性：用来为延迟检索策略或立即检索策略设定批量检索的数量. 批量检索能减少                                                                SELECT 语句的数目, 提高延迟检索或立即检索的运行性能. 

Clazz 类 的对象关系映射，之一中的set 属性中的配置

<hibernate-mapping package="com.baidu.setStrategy"><!-- lazy 类级别的懒加载 -->    <class name="Clazz" table="CLAZZS" >            <id name="clazzId" type="java.lang.Integer">            <column name="CLAZZ_ID" />            <generator class="native" />        </id>                <property name="clazzName" type="java.lang.String">            <column name="CLAZZ_NAME" />        </property>                <set name="stus" table="STUDENTS" lazy="true">            <key>                <column name="CLAZZ_ID" />            </key>            <one-to-many class="Student" />        </set>            </class>    </hibernate-mapping>

/** * Set 的 lazy 属性 * 1.  1-n 或n-n 的集合属性默认使用懒加载检索策略 *  2.  可以通过设置set 的lazy 属性来修改默认的检索策略，默认为true *  并不建议设置为false *  3.  lazy 还可以设置为extra，增强的延迟检索。该取值会尽可能的延迟集合初始化的时机 * 4.Hibernate.initialize() 静态方法显式初始化 */

Lazy 属性：默认值是true

① 当取true 时， 

@Testpublic void testOne2ManyLevelStrategy() {Clazz clazz =(Clazz) session.get(Clazz.class, 1); System.out.println(clazz.getClass());System.out.println(clazz.getStus().size());}

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_0_     from        CLAZZS clazz0_     where        clazz0_.CLAZZ_ID=?class com.baidu.setStrategy.ClazzHibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?2

② 当取fasle 时，
运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_0_     from        CLAZZS clazz0_     where        clazz0_.CLAZZ_ID=?Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?class com.baidu.setStrategy.Clazz2

③ 当取extra 时，

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_0_     from        CLAZZS clazz0_     where        clazz0_.CLAZZ_ID=?class com.baidu.setStrategy.ClazzHibernate:     select        count(STU_ID)     from        STUDENTS     where        CLAZZ_ID =?2

在添加测试方法中添加代码

@Testpublic void testOne2ManyLevelStrategy() {Clazz clazz =(Clazz) session.get(Clazz.class, 1); System.out.println(clazz.getClass());System.out.println(clazz.getStus().size());Student student = new Student();student.setStuId(1);System.out.println(clazz.getStus().contains(student));}

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_0_     from        CLAZZS clazz0_     where        clazz0_.CLAZZ_ID=?class com.baidu.setStrategy.ClazzHibernate:     select        count(STU_ID)     from        STUDENTS     where        CLAZZ_ID =?2Hibernate:     select        1     from        STUDENTS     where        CLAZZ_ID =?         and STU_ID =?true

④ 当使用Hibernate.initialize() 静态方法 时

@Testpublic void testOne2ManyLevelStrategy() {Clazz clazz =(Clazz) session.get(Clazz.class, 1); System.out.println(clazz.getClass());System.out.println(clazz.getStus().size());Student student = new Student();student.setStuId(1);System.out.println(clazz.getStus().contains(student));Hibernate.initialize(clazz.getStus());}

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_0_     from        CLAZZS clazz0_     where        clazz0_.CLAZZ_ID=?class com.baidu.setStrategy.ClazzHibernate:     select        count(STU_ID)     from        STUDENTS     where        CLAZZ_ID =?2Hibernate:     select        1     from        STUDENTS     where        CLAZZ_ID =?         and STU_ID =?trueHibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?

/** * Set 的 batch-size 属性: 设定一次初始化 set 集合的数量 */

 batch-size 属性

@Testpublic void testSetBatchSize() {List<Clazz> clazzs = session.createQuery("FROM Clazz").list();System.out.println(clazzs.size());for(Clazz clazz:clazzs ){if(clazz.getStus() != null)System.out.println(clazz.getStus().size());}}

① 当没有使用batch-size 属性时，

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_     from        CLAZZS clazz0_3Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?2Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?2Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?2

当在clazz的配置文件的set属性中添加batch-size = "5" ：表示一次初始化5条记录

        <set name="stus" table="STUDENTS" lazy="true" batch-size="5">            <key>                <column name="CLAZZ_ID" />            </key>            <one-to-many class="Student" />        </set>

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_     from        CLAZZS clazz0_3Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID in (            ?, ?, ?        )222

fetch属性: 取值为 “select” 或 “subselect” 时, 决定初始化 students 的查询语句的形式;  

                   若取值为”join”, 则决定orders 集合被初始化的时机

/** * Set 的 fetch 属性:  * 1. 默认值为select：通过正常的方式来初始化set元素 *  2. 可以取值subselect：  *  ① 通过子查询的方式来初始化所有的 set 集合。子查询作为where 子句的 in的条件出现， *  子查询查询所有1 的一端的ID *      ② 此时lazy 有效，但是batch-size 属性将被忽略，不再起作用。 */

/** * Set 的 fetch 属性:  * 3. 可以取值为join:  batch-size有效 * ① 在加载 1 的一端的对象时，使用迫切左外连接的方式检索 n 的一端的集合属性 *  * ② 会忽略lazy 属性 *  * ③ 使用HQL 查询时，会 忽略fetch=join *  *  * 迫切左外连接：使用左外连接进行查询，且把集合属性进行初始化 */

fetch: 
         ①取值为 select

        <set name="stus" table="STUDENTS"         lazy="true" batch-size="2" fetch="subselect">            <key>                <column name="CLAZZ_ID" />            </key>            <one-to-many class="Student" />        </set>

@Testpublic void testSetFetchSelectOrSubselect() {List<Clazz> clazzs = session.createQuery("FROM Clazz").list();System.out.println(clazzs.size());for(Clazz clazz:clazzs ){if(clazz.getStus() != null)System.out.println(clazz.getStus().size());}}

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_     from        CLAZZS clazz0_3Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID in (            ?, ?        )22Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?2

        ② 取值为usubselect

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_     from        CLAZZS clazz0_3Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID in (            select                clazz0_.CLAZZ_ID             from                CLAZZS clazz0_        )222

         ③ 取值为 join

        <set name="stus" table="STUDENTS"         lazy="true" batch-size="2" fetch="join">            <key>                <column name="CLAZZ_ID" />            </key>            <one-to-many class="Student" />        </set>

@Testpublic void testSetFetchJoin() {Clazz clazz = (Clazz) session.get(Clazz.class, 1);System.out.println(clazz.getStus().size());}

运行的SQL

使用HQL 查询时，batch-size有效， 但会 忽略fetch=join ,

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_     from        CLAZZS clazz0_3Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID in (            ?, ?        )22Hibernate:     select        stus0_.CLAZZ_ID as CLAZZ_ID3_0_1_,        stus0_.STU_ID as STU_ID1_1_1_,        stus0_.STU_ID as STU_ID1_1_0_,        stus0_.STU_NAME as STU_NAME2_1_0_     from        STUDENTS stus0_     where        stus0_.CLAZZ_ID=?2

@Testpublic void testSetFetchJoin() {Clazz clazz = (Clazz) session.get(Clazz.class, 1);System.out.println(clazz.getStus().size());}

运行的SQL

Hibernate:     select        clazz0_.CLAZZ_ID as CLAZZ_ID1_0_1_,        clazz0_.CLAZZ_NAME as CLAZZ_NA2_0_1_,        stus1_.CLAZZ_ID as CLAZZ_ID3_0_3_,        stus1_.STU_ID as STU_ID1_1_3_,        stus1_.STU_ID as STU_ID1_1_0_,        stus1_.STU_NAME as STU_NAME2_1_0_     from        CLAZZS clazz0_     left outer join        STUDENTS stus1_             on clazz0_.CLAZZ_ID=stus1_.CLAZZ_ID     where        clazz0_.CLAZZ_ID=?2