Hibernate查询优化——类级别查询（集合策略）
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1、类级别查询：
get方法和load方法：
（1）get方法：
public static void testSel() { Session session = HibernateUtils.openSession(); Transaction transaction = session.beginTransaction(); Student student=session.get(Student.class,201811); System.out.println(student); transaction.commit(); session.close(); }
特点：执行get方法时，立即发送查询语句查询结果。
（2）load方法（延迟加载，即：只有在使用的时候才会进行查询）：
public static void testSel() { Session session = HibernateUtils.openSession(); Transaction transaction = session.beginTransaction(); Student student=session.load(Student.class,201811); System.out.println(student); transaction.commit(); session.close(); }
特点：在执行load方法时，不会发送任何sql语句，不会产生任何查询结果，但是会产生一个对象，在使用该对象时才会产生查询结果。
延迟加载与配置文件的关系：
延迟加载（默认）：
<class name="Student" table="student" lazy="true">
不延迟加载：
<class name="Student" table="student" lazy="false">
延迟加载可以提高效率（对象取出来可以不用，到真正需要的时候才进行查询）。
2、关联级别延迟加载和抓取策略：
集合级别的关联：
lazy属性:决定是否延迟加载 true（默认值）：延迟加载，懒惰加载 false：立即加载 extra：极其懒惰 fetch属性：决定加载策略，使用什么类型的sql语句加载集合数据 select（默认值）：单表查询加载 join：多表查询加载集合 subselect：使用子查询加载集合
（1）lazy=true，fetch=select
public static void testSel() { Session session = HibernateUtils.openSession(); Transaction transaction = session.beginTransaction(); Student student=session.load(Student.class,201811);//不执行查询 Clas clas= student.getaClas(); System.out.println(clas);//执行查询 transaction.commit(); session.close(); }
<hibernate-mapping package="pers.zhb.domain"> <class name="Clas" table="class"> <id name="classno" column="classno"> <generator class="native"></generator> </id><!--主键--> <property name="department" column="department"></property> <property name="monitor" column="monitor"></property> <property name="classname" column="classname"></property> <set name="students" table="student" lazy="true" fetch="select"><!--一对多关系配置--> <key column="classno" update="false" ></key><!--指定了集合表的外键--> <one-to-many class="Student"></one-to-many> </set> </class> </hibernate-mapping>
表中的数据：
class表：
student表：
（2）lazy=false,fetch=select
public static void testSel() { Session session = HibernateUtils.openSession(); Transaction transaction = session.beginTransaction(); Student student=session.load(Student.class,201811);//没有使用即开始执行查询 Clas clas= student.getaClas(); System.out.println(clas); transaction.commit(); session.close(); }
（3）lazy=extra,fetch=select
极其懒惰与懒加载几乎一致。
获取查询结果的size：
public class Test{ public static void testSel() { Session session = HibernateUtils.openSession(); Transaction transaction = session.beginTransaction(); Student student=session.load(Student.class,201811); Set<Clas> clas= Collections.singleton(student.getaClas()); System.out.println(clas); System.out.println(clas.size()); transaction.commit(); session.close(); }
Hibernate: select student0_.studentno as studentn1_1_0_, student0_.birthday as birthday2_1_0_, student0_.classno as classno3_1_0_, student0_.phone as phone4_1_0_, student0_.sex as sex5_1_0_, student0_.sname as sname6_1_0_, student0_.point as point7_1_0_ from student student0_ where student0_.studentno=? Hibernate: select clas0_.classno as classno1_0_0_, clas0_.department as departme2_0_0_, clas0_.monitor as monitor3_0_0_, clas0_.classname as classnam4_0_0_ from class clas0_ where clas0_.classno=? Hibernate: select students0_.classno as classno3_1_0_, students0_.studentno as studentn1_1_0_, students0_.studentno as studentn1_1_1_, students0_.birthday as birthday2_1_1_, students0_.classno as classno3_1_1_, students0_.phone as phone4_1_1_, students0_.sex as sex5_1_1_, students0_.sname as sname6_1_1_, students0_.point as point7_1_1_ from student students0_ where students0_.classno=? [Clas{classno=tx171, department=\'信工学院\', monitor=\'张伟\', classname=\'通信171\',
students=[Student{studentno=\'201811\', sname=\'zhai\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=890.0, phone=\'19837372532\'},
Student{studentno=\'201813\', sname=\'zhai3\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=892.0, phone=\'19837372534\'},
Student{studentno=\'201812\', sname=\'zhai2\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=893.0, phone=\'19837372533\'}]}] 1
（4）fetch=select
当fetch="select" 时lazy无论是 true | false | extra都失效,都是立即加载。
Hibernate: select student0_.studentno as studentn1_1_0_, student0_.birthday as birthday2_1_0_, student0_.classno as classno3_1_0_, student0_.phone as phone4_1_0_, student0_.sex as sex5_1_0_, student0_.sname as sname6_1_0_, student0_.point as point7_1_0_ from student student0_ where student0_.studentno=? Hibernate: select clas0_.classno as classno1_0_0_, clas0_.department as departme2_0_0_, clas0_.monitor as monitor3_0_0_, clas0_.classname as classnam4_0_0_ from class clas0_ where clas0_.classno=? Hibernate: select students0_.classno as classno3_1_0_, students0_.studentno as studentn1_1_0_, students0_.studentno as studentn1_1_1_, students0_.birthday as birthday2_1_1_, students0_.classno as classno3_1_1_, students0_.phone as phone4_1_1_, students0_.sex as sex5_1_1_, students0_.sname as sname6_1_1_, students0_.point as point7_1_1_ from student students0_ where students0_.classno=? [Clas{classno=tx171, department=\'信工学院\', monitor=\'张伟\', classname=\'通信171\',
students=[Student{studentno=\'201811\', sname=\'zhai\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=890.0, phone=\'19837372532\'},
Student{studentno=\'201813\', sname=\'zhai3\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=892.0, phone=\'19837372534\'},
Student{studentno=\'201812\', sname=\'zhai2\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=893.0, phone=\'19837372533\'}]}]
（5）lazy="true" fetch="subselect"
Hibernate: select student0_.studentno as studentn1_1_0_, student0_.birthday as birthday2_1_0_, student0_.classno as classno3_1_0_, student0_.phone as phone4_1_0_, student0_.sex as sex5_1_0_, student0_.sname as sname6_1_0_, student0_.point as point7_1_0_ from student student0_ where student0_.studentno=? Hibernate: select clas0_.classno as classno1_0_0_, clas0_.department as departme2_0_0_, clas0_.monitor as monitor3_0_0_, clas0_.classname as classnam4_0_0_ from class clas0_ where clas0_.classno=? Hibernate: select students0_.classno as classno3_1_0_, students0_.studentno as studentn1_1_0_, students0_.studentno as studentn1_1_1_, students0_.birthday as birthday2_1_1_, students0_.classno as classno3_1_1_, students0_.phone as phone4_1_1_, students0_.sex as sex5_1_1_, students0_.sname as sname6_1_1_, students0_.point as point7_1_1_ from student students0_ where students0_.classno=? [Clas{classno=tx171, department=\'信工学院\', monitor=\'张伟\', classname=\'通信171\',
students=[Student{studentno=\'201811\', sname=\'zhai\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=890.0, phone=\'19837372532\'},
Student{studentno=\'201813\', sname=\'zhai3\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=892.0, phone=\'19837372534\'},
Student{studentno=\'201812\', sname=\'zhai2\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=893.0, phone=\'19837372533\'}]}]
（6）lazy="true" fetch="subselect"
Hibernate: select clas0_.classno as classno1_0_, clas0_.department as departme2_0_, clas0_.monitor as monitor3_0_, clas0_.classname as classnam4_0_ from class clas0_ Hibernate: select students0_.classno as classno3_1_1_, students0_.studentno as studentn1_1_1_, students0_.studentno as studentn1_1_0_, students0_.birthday as birthday2_1_0_, students0_.classno as classno3_1_0_, students0_.phone as phone4_1_0_, students0_.sex as sex5_1_0_, students0_.sname as sname6_1_0_, students0_.point as point7_1_0_ from student students0_ where students0_.classno in ( select clas0_.classno from class clas0_ ) Clas{classno=80501, department=\'计算机学院\', monitor=\'刘国平\', classname=\'计算机0801\', students=[]} [] 0 Clas{classno=80601, department=\'机械学院\', monitor=\'王善执\', classname=\'机械0801\', students=[]} [] 0 Clas{classno=90501, department=\'计算机学院\', monitor=\'马文斐\', classname=\'计算机0901\', students=[]} [] 0 Clas{classno=90502, department=\'计算机学院\', monitor=\'章成楠\', classname=\'计算机0902\', students=[]} [] 0 Clas{classno=90801, department=\'管理学院\', monitor=\'党海\', classname=\'管理0901\', students=[]} [] 0 Clas{classno=90802, department=\'管理学院\', monitor=\'张晓\', classname=\'管理0802\', students=[]} [] 0 Clas{classno=tx161, department=\'信息工程学院\', monitor=\'张丽\', classname=\'通信161\', students=[]} [] 0 Clas{classno=tx172, department=\'信工学院\', monitor=\'李月\', classname=\'通信172\', students=[]} [] 0 Clas{classno=tx171, department=\'信工学院\', monitor=\'张伟\', classname=\'通信171\', students=[Student{studentno=\'201811\', sname=\'zhai\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=890.0, phone=\'19837372532\'}, Student{studentno=\'201813\', sname=\'zhai3\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=892.0, phone=\'19837372534\'}, Student{studentno=\'201812\', sname=\'zhai2\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=893.0, phone=\'19837372533\'}]} [Student{studentno=\'201811\', sname=\'zhai\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=890.0, phone=\'19837372532\'}, Student{studentno=\'201813\', sname=\'zhai3\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=892.0, phone=\'19837372534\'}, Student{studentno=\'201812\', sname=\'zhai2\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=893.0, phone=\'19837372533\'}] 3
（7）lazy="false" fetch="subselect"
Hibernate: select clas0_.classno as classno1_0_, clas0_.department as departme2_0_, clas0_.monitor as monitor3_0_, clas0_.classname as classnam4_0_ from class clas0_ Hibernate: select students0_.classno as classno3_1_1_, students0_.studentno as studentn1_1_1_, students0_.studentno as studentn1_1_0_, students0_.birthday as birthday2_1_0_, students0_.classno as classno3_1_0_, students0_.phone as phone4_1_0_, students0_.sex as sex5_1_0_, students0_.sname as sname6_1_0_, students0_.point as point7_1_0_ from student students0_ where students0_.classno in ( select clas0_.classno from class clas0_ ) Clas{classno=80501, department=\'计算机学院\', monitor=\'刘国平\', classname=\'计算机0801\', students=[]} [] 0 Clas{classno=80601, department=\'机械学院\', monitor=\'王善执\', classname=\'机械0801\', students=[]} [] 0 Clas{classno=90501, department=\'计算机学院\', monitor=\'马文斐\', classname=\'计算机0901\', students=[]} [] 0 Clas{classno=90502, department=\'计算机学院\', monitor=\'章成楠\', classname=\'计算机0902\', students=[]} [] 0 Clas{classno=90801, department=\'管理学院\', monitor=\'党海\', classname=\'管理0901\', students=[]} [] 0 Clas{classno=90802, department=\'管理学院\', monitor=\'张晓\', classname=\'管理0802\', students=[]} [] 0 Clas{classno=tx161, department=\'信息工程学院\', monitor=\'张丽\', classname=\'通信161\', students=[]} [] 0 Clas{classno=tx172, department=\'信工学院\', monitor=\'李月\', classname=\'通信172\', students=[]} [] 0 Clas{classno=tx171, department=\'信工学院\', monitor=\'张伟\', classname=\'通信171\', students=[Student{studentno=\'201813\', sname=\'zhai3\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=892.0, phone=\'19837372534\'}, Student{studentno=\'201811\', sname=\'zhai\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=890.0, phone=\'19837372532\'}, Student{studentno=\'201812\', sname=\'zhai2\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=893.0, phone=\'19837372533\'}]} [Student{studentno=\'201813\', sname=\'zhai3\', sex=\'男\', birthday=\'1998-11-11\', classno=\'tx171\', point=892.0, phone=\'19837372534\'}, Student{studentno=\'201811\', sname=\'zhai\', sex=\'Hibernate框架学习——查询优化[原创]java WEB学习笔记93：Hibernate学习之路---Hibernate 缓存介绍，缓存级别，使用二级缓存的情况，二级缓存的架构集合缓存，二级缓存的并发策略，实现步骤，集合缓存，查询缓