深入理解Hibernate中Iterate方法-CFANZ编程社区

深入理解Hibernate中Iterate方法

一、数据准备（参考前面一篇博文）

数据准备参考博文（这是链接）
在实际需求中有时需要对一些业务数据进行频繁的查询处理，可能会这样处理：
二、list查询剖析

public void testQuerySingleTable_HQL_List() {
        Session session = HbnUtils.getSession();
        try {
            session.beginTransaction();
            String hql = "from Student where age > 17";
            // 执行第一次查操作
            List<Student> list = session.createQuery(hql).list();
            for (Student stu : list) {
            System.out.println(stu);
            }
            System.out.println("\t-------第一次查询与第二次查询的分割线--------");
            // 执行第二次查操作
            List<Student> list_1 = session.createQuery(hql).list();
            for (Student stu : list) {
                System.out.println(stu);
            }
            session.getTransaction().commit();
        } catch (Exception e) {
            e.printStackTrace();
            session.getTransaction().rollback();
        }
    }

下面是控制台打印的SQL语句和提示语句

Hibernate: 
    select
        student0_.t_id as t_id1_0_,
        student0_.t_name as t_name2_0_,
        student0_.t_age as t_age3_0_,
        student0_.t_score as t_score4_0_ 
    from
        t_student student0_ 
    where
        student0_.t_age>17
Student [id=8, name=张8三, age=18, score=88.0]
Student [id=9, name=张9三, age=19, score=89.0]
Student [id=10, name=张10三, age=20, score=90.0]
    -------第一次查询与第二次查询的分割线--------
Hibernate: 
    select
        student0_.t_id as t_id1_0_,
        student0_.t_name as t_name2_0_,
        student0_.t_age as t_age3_0_,
        student0_.t_score as t_score4_0_ 
    from
        t_student student0_ 
    where
        student0_.t_age>17
Student [id=8, name=张8三, age=18, score=88.0]
Student [id=9, name=张9三, age=19, score=89.0]
Student [id=10, name=张10三, age=20, score=90.0]

从上面的输出信息可以看出，用List方法Hibernate查询了几次就访问了几次数据库，为了提高效率，本应该在第二次查询的时候可以从缓存中获取，因为第一次的结果已经保存在Session缓存中了！

三、iterate查询剖析

代码

/**
* iterate
 *  */
    @Test
    public void testQuerySingleTable_HQL_Iterate() {
        Session session = HbnUtils.getSession();
        try {
            session.beginTransaction();
            String hql = "from Student where age>17";
            // 执行(第一次查询)
            Iterator<Student> iterator = session.createQuery(hql).iterate();
            while(iterator.hasNext()){
                System.out.println(iterator.next());
            }
            System.out.println("\t-------第一次查询与第二次查询的分割线--------");
            //执行第二次查询操作
            List<Student> list_1 = (List<Student>) session.createQuery(hql).iterate();
            while(iterator.hasNext()){
                System.out.println(iterator.next());
            }
            session.getTransaction().commit();
        } catch (Exception e) {
            e.printStackTrace();
            session.getTransaction().rollback();
        }
    }

执行后控制台输出信息

Hibernate: 
    select
        student0_.t_id as col_0_0_ 
    from
        t_student student0_ 
    where
        student0_.t_age>17
Hibernate: 
    select
        student0_.t_id as t_id1_0_0_,
        student0_.t_name as t_name2_0_0_,
        student0_.t_age as t_age3_0_0_,
        student0_.t_score as t_score4_0_0_ 
    from
        t_student student0_ 
    where
        student0_.t_id=?
Student [id=8, name=张8三, age=18, score=88.0]
Hibernate: 
    select
        student0_.t_id as t_id1_0_0_,
        student0_.t_name as t_name2_0_0_,
        student0_.t_age as t_age3_0_0_,
        student0_.t_score as t_score4_0_0_ 
    from
        t_student student0_ 
    where
        student0_.t_id=?
Student [id=9, name=张9三, age=19, score=89.0]
Hibernate: 
    select
        student0_.t_id as t_id1_0_0_,
        student0_.t_name as t_name2_0_0_,
        student0_.t_age as t_age3_0_0_,
        student0_.t_score as t_score4_0_0_ 
    from
        t_student student0_ 
    where
        student0_.t_id=?
Student [id=10, name=张10三, age=20, score=90.0]
    -------第一次查询与第二次查询的分割线--------
Hibernate: 
    select
        student0_.t_id as col_0_0_ 
    from
        t_student student0_ 
    where
        student0_.t_age>17

分析输出信息可以看出：Hibernate的iterate执行过程
1）：
Hibernate: select student0_.t_id as col_0_0_ from t_student student0_ where student0_.t_age>17 Student [id=8, name=张8三, age=18, score=88.0] Student [id=9, name=张9三, age=19, score=89.0] Student [id=10, name=张10三, age=20, score=90.0]
1）：先把所有满足条件的记录的id从数据库查询出来
2）：然后根据查询出来的id逐个去查询每一条记录的详细信息
3）：先根据id在Session缓存中查找
4）：如果没有，就需要去数据库查找，有几个满足条件的id，就需要访问几次数据库
3）：当进行第二次查询时，先到数据库进行了一次主键查询，查查满足条件的id，然后直接根据id从Session缓存中读出满足条件的记录的详细信息

四、Query接口的List方法与Iterate方法比较

List方法不会从Session缓存中读取数据

Iterate方法会从Session缓存中读取数据

List方法第一次查询效率高

Iterate方法第一次查询效率很低（2n+1次）

所以为了避免n+1问题，我们取长补短，第一次查询用List,第二次查询用Iterate

五、Query接口的List方法与Iterate方法结合

public void testQuerySingleTable_HQL_IterateAndList() {
        Session session = HbnUtils.getSession();
        try {
            session.beginTransaction();
            String hql = "from Student where age>17";
            // 执行(第一次查询)
            List<Student> list = session.createQuery(hql).list();
            for (Student stu : list) {
            System.out.println(stu);
            }
            System.out.println("\t-------第一次查询与第二次查询的分割线--------");
            //执行第二次查询操作
            Iterator<Student> iterator_1 = session.createQuery(hql).iterate();
            while(iterator_1.hasNext()){
                System.out.println(iterator_1.next());
            }
            session.getTransaction().commit();
        } catch (Exception e) {
            e.printStackTrace();
            session.getTransaction().rollback();
        }
    }

看结果：———

Hibernate: 
    select
        student0_.t_id as t_id1_0_,
        student0_.t_name as t_name2_0_,
        student0_.t_age as t_age3_0_,
        student0_.t_score as t_score4_0_ 
    from
        t_student student0_ 
    where
        student0_.t_age>17
Student [id=8, name=张8三, age=18, score=88.0]
Student [id=9, name=张9三, age=19, score=89.0]
Student [id=10, name=张10三, age=20, score=90.0]
    -------第一次查询与第二次查询的分割线--------
Hibernate: 
    select
        student0_.t_id as col_0_0_ 
    from
        t_student student0_ 
    where
        student0_.t_age>17
Student [id=8, name=张8三, age=18, score=88.0]
Student [id=9, name=张9三, age=19, score=89.0]
Student [id=10, name=张10三, age=20, score=90.0]

从结果分析效率确实提高了不少！！！