`
cynhafa
  • 浏览: 153047 次
  • 性别: Icon_minigender_1
  • 来自: 北京
社区版块
存档分类
最新评论

Hibernate的fetch

 
阅读更多

http://hi.baidu.com/32ddd/blog/item/52631eb2a16724aad9335ae0.html

hibernate抓取策略fetch详解
一、hibernate抓取策略(单端代理的批量抓取fetch=select(默认)/join)
测试用例:
Student student = (Student)session.get(Student.class, 1);
System.out.println(student.getName());
System.out.println(student.getClasses().getName());

1)保持默认,同fetch="select",如:
<many-to-one name="classes" column="classesid" fetch="select"/>

fetch="select",另外发送一条select语句抓取当前对象关联实体或集合

hibernate抓取策略fetch详解
执行结果:2条语句

Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_, student0_.class_id as class3_1_0_ from student_join student0_ where student0_.id=?
学生1
Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes_join classes0_ where classes0_.id=?
高一(1)班
2)设置fetch="join",如:
<many-to-one name="classes" column="classesid" fetch="join"/>

fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合

hibernate抓取策略fetch详解
此时lazy会失效

执行结果:一条join语句

Hibernate: select student0_.id as id1_1_, student0_.name as name1_1_, student0_.class_id as class3_1_1_, classes1_.id as id0_0_, classes1_.name as name0_0_ from student_join student0_ left outer join classes_join classes1_ on student0_.class_id=classes1_.id where student0_.id=?
学生1
高一(1)班
二、hibernate抓取策略(集合代理的批量抓取,fetch=select(默认)/join/subselect)

测试用例:

Classes c = (Classes) session.load(Classes.class, new Integer(1));
System.out.println("Class.name=" + c.getName());
Set stuSet = c.getStudents();
System.out.println(stuSet.size());
if(stuSet != null && !stuSet.isEmpty()){
for(Iterator it = stuSet.iterator(); it.hasNext();){
Student s = (Student) it.next();
System.out.println("student.name=" + s.getName());
}
}

1)hibernate抓取策略fetch详解
保持默认,同fetch="select",如:
<set name="students" inverse="true" fetch="select">

fetch="select",另外发送一条select语句抓取当前对象关联实体或集合

测试结果:2条独立的查询语句

Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes_join classes0_ where classes0_.id=?
Class.name=高一(1)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
9
student.name=学生7
student.name=学生3
student.name=学生1
student.name=学生8
student.name=学生2
student.name=学生4
student.name=学生5
student.name=学生9
student.name=学生6

(2)设置fetch="join",如:
<set name="students" inverse="true" fetch="join">

fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合

此时lazy会失效

测试结果:1条独立的join查询语句

Hibernate: select classes0_.id as id0_1_, classes0_.name as name0_1_, students1_.class_id as class3_3_, students1_.id as id3_, students1_.id as id1_0_, students1_.name as name1_0_, students1_.class_id as class3_1_0_ from classes_join classes0_ left outer join student_join students1_ on classes0_.id=students1_.class_id where classes0_.id=?
Class.name=高一(1)班
9
student.name=学生6
student.name=学生4
student.name=学生9
student.name=学生7
student.name=学生2
student.name=学生3
student.name=学生8
student.name=学生1
student.name=学生5
(3)设置fetch="subselect",如:用在查询语句中
<set name="students" inverse="true" fetch="subselect">

fetch="subselect",另外发送一条select语句抓取在前面查询到的所有实体对象的关联集合

测试用例:

List classList = session.createQuery("from Classes where id in (1,2,3)").list();
for(Iterator iter = classList.iterator(); iter.hasNext();){
Classes c = (Classes)iter.next();
System.out.println("Class.name=" + c.getName());
Set stuSet = c.getStudents();
System.out.println(stuSet.size());
if(stuSet != null && !stuSet.isEmpty()){
for(Iterator it = stuSet.iterator(); it.hasNext();){
Student s = (Student) it.next();
System.out.println("student.name=" + s.getName());
}
}
}

当不设fetch="subselect" ,即:<set name="students" inverse="true">,结果如下:

执行了3条查询语句

Hibernate: select classes0_.id as id0_, classes0_.name as name0_ from classes_join classes0_ where classes0_.id in (1 , 2 , 3)
Class.name=高一(1)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
9
student.name=学生8
student.name=学生5
student.name=学生3
student.name=学生9
student.name=学生7
student.name=学生1
student.name=学生4
student.name=学生6
student.name=学生2
Class.name=高一(2)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
4
student.name=学生3
student.name=学生4
student.name=学生1
student.name=学生2
Class.name=高一(3)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id=?
0


当不设fetch="subselect" ,即:<set name="students" inverse="true" fetch="subselect">,结果如下:

执行了1条查询语句(嵌套子查询)

Hibernate: select classes0_.id as id0_, classes0_.name as name0_ from classes_join classes0_ where classes0_.id in (1 , 2 , 3)
Class.name=高一(1)班
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student_join students0_ where students0_.class_id in (select classes0_.id from classes_join classes0_ where classes0_.id in (1 , 2 , 3))

分享到:
评论

相关推荐

    Hibernate Fetch 的作用

    正确理解hibernate fetch 的作用

    hibernate

    Hibernate fetch lazy cascade inverse 关键字

    优化Hibernate性能的几点建议

    中取出的记录条数,一般设置为30、50、100.Oracle数据库的JDBC驱动默认的Fetch Size=15,设置Fetch Size设置为:30、50,性能会有明显提升,如果继续增大,超出100,性能提升不明显,反而会消耗内存。 即在...

    hibernate 中 fetch=FetchType.LAZY 懒加载失败处理方法

    主要介绍了hibernate 中 fetch=FetchType.LAZY 懒加载失败处理方法,需要的朋友可以参考下

    Hibernate配置各种数据源详解

    Hibernate配置各种数据源 &lt;hibernate-configuration&gt; &lt;!– 各属性的配置–&gt; &lt;!—为true表示将Hibernate发送给...”jdbc.fetch_size”&gt;50 &lt;!– 设定对数据库进行批量删除 –&gt; ”jdbc.batch_size”&gt;30&lt;/property&gt;

    Hibernate3.2学习笔记

    Hibernate环境搭建 Hibernate主要接口 Hibernate主要映射 Hibernate的lazy、fetch、cascade等策略 Hibernate性能优化

    Hibernate+中文文档

    19.1.2. 调整抓取策略(Tuning fetch strategies) 19.1.3. 单端关联代理(Single-ended association proxies) 19.1.4. 实例化集合和代理(Initializing collections and proxies) 19.1.5. 使用批量抓取(Using...

    精通 Hibernate:Java 对象持久化技术详解(第2版).part2

     16.3.5 用带子查询的select语句整批量初始化orders集合(fetch属性为“subselect”)  16.3.6 迫切左外连接检索(fetch属性为“join”)  16.4 多对一和一对一关联的检索策略  16.4.1 迫切左外连接检索(fetch...

    hibernate.properties

    hibernate.max_fetch_depth 1 ## set the default batch size for batch fetching #hibernate.default_batch_fetch_size 8 ## rollback generated identifier values of deleted entities to default values #...

    hibernate3.2中文文档(chm格式)

    19.1.2. 调整抓取策略(Tuning fetch strategies) 19.1.3. 单端关联代理(Single-ended association proxies) 19.1.4. 实例化集合和代理(Initializing collections and proxies) 19.1.5. 使用批量抓取(Using...

    HibernateAPI中文版.chm

    19.1.2. 调整抓取策略(Tuning fetch strategies) 19.1.3. 单端关联代理(Single-ended association proxies) 19.1.4. 实例化集合和代理(Initializing collections and proxies) 19.1.5. 使用批量抓取(Using...

    Hibernate教程15_关联关系中的CRUD Cascade_Fetch

    http://blog.csdn.net/e421083458/article/details/8794127 该源码为Hibernate教程配套源码

    Hibernate中文详细学习文档

    19.1.2. 调整抓取策略(Tuning fetch strategies) 19.1.3. 单端关联代理(Single-ended association proxies) 19.1.4. 实例化集合和代理(Initializing collections and proxies) 19.1.5. 使用批量抓取(Using...

    Hibernate 中文 html 帮助文档

    19.1.2. 调整抓取策略(Tuning fetch strategies) 19.1.3. 单端关联代理(Single-ended association proxies) 19.1.4. 实例化集合和代理(Initializing collections and proxies) 19.1.5. 使用批量抓取(Using...

    最全Hibernate 参考文档

    19.1.1. 调整抓取策略(Tuning fetch strategies) 19.1.2. 单端关联代理(Single-ended association proxies) 19.1.3. 实例化集合和代理(Initializing collections and proxies) 19.1.4. 使用批量抓取(Using...

    SSH笔记-检索策略(lazy、fetch、batch-size)

    hibernate的类级别检索策略和检索策略的lazy、fetch、batch-size属性使用

    hibernate总结

    i. 如果不加fetch关键字,则hibernate不会抓取关系属性,但会遍历关系属性所对应的表 ii. 不加fetch关键字时,select 要指定返回的对象,否则它要返回数组 iii. 条件:持久化类之间有关系属性映射 Hibernate级联...

    Hibernate注释大全收藏

    @Basic(fetch = FetchType.LAZY) String getDetailedComment() { ... } // persistent property @Temporal(TemporalType.TIME) java.util.Date getDepartureTime() { ... } // persistent property @Enumerated(Enum...

    hibernate 体系结构与配置 参考文档(html)

    1. Hibernate入门 1.1. 前言 1.2. 第一部分 - 第一个Hibernate应用程序 1.2.1. 第一个class 1.2.2. 映射文件 1.2.3. Hibernate配置 1.2.4. 用Ant构建 1.2.5. 启动和辅助类 1.2.6. 加载并存储对象 1.3. 第...

    Hibernate注解

    fetch指定是否延迟加载,值为FetchType.LAZY表示延迟,为FetchType.EAGER表示立即加载 * 方法一 使用这种配置,在为“一端”添加“多端”时,不会修改“多端”的外键。在“一端”加载时,不会得到“多端”。如果使用...

Global site tag (gtag.js) - Google Analytics