Hibernate查询优化：类级别查询（集合策略）

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='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'}]
3

（8）lazy="extra" 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