Hibernate的fetch

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, 43);
			
System.out.println("student name  "+student.getName());
			
System.out.println("class name    "+student.getClasses().getName());

 

1）保持默认，同fetch="select",如：

<many-to-one name="classes" column="classesid" fetch="select"/>

 

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

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 student0_ where student0_.id=?

student name  ccc 

Hibernate: select myclass0_.id as id0_0_, myclass0_.name as name0_0_ from class myclass0_ where myclass0_.id=?

class name    09002

 
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，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_, myclass1_.id as id0_0_, myclass1_.name as name0_0_ from student student0_ inner join class myclass1_ on student0_.class_id=myclass1_.id where student0_.id=?

student name  ccc
class name    09002

 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：

MyClass class1 = (MyClass) session.load(MyClass.class, 28);
			
         System.out.println("班级名：" + class1.getName());

         Set<Student> students = class1.getStudents();
         System.out.println("班级人数：" + students.size());
			
                   if (students != null && !students.isEmpty()) {
				
                         Iterator<Student> iterator = students.iterator();
	       System.out.print("学生姓名：");
				
                          while (iterator.hasNext()) {
					
                                               Student student = iterator.next();
					
                                                System.out.println(student.getName());
				}
			}

 

1）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 myclass0_.id as id0_0_, myclass0_.name as name0_0_ from class myclass0_ where myclass0_.id=?
班级名：09002

Hibernate: select students0_.class_id as class3_0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id=?
班级人数：3
学生姓名：ccc
bbb
aaa

 

(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 myclass0_.id as id0_1_, myclass0_.name as name0_1_, students1_.class_id as class3_0_3_, students1_.id as id3_, students1_.id as id1_0_, students1_.name as name1_0_, students1_.class_id as class3_1_0_ from class myclass0_ left outer join student students1_ on myclass0_.id=students1_.class_id where myclass0_.id=?
班级名：09002
班级人数：3
学生姓名：aaa
ccc
bbb

 (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<MyClass> classes = session.createQuery("from MyClass where id in (27,28,29)").list();
			
            Iterator<MyClass> iterator1 = classes.iterator();
			
              while(iterator1.hasNext()){
				
                 MyClass class1 = iterator1.next();
				
                 System.out.println("班级名：" + class1.getName());

				
                  Set<Student> students = class1.getStudents();
				
                  System.out.println("班级人数：" + students.size());
				
                   if (students != null && !students.isEmpty()) {
					
                         Iterator<Student> iterator = students.iterator();
					
                         System.out.print("学生姓名：");
					
                         while (iterator.hasNext()) {
						
                              Student student = iterator.next();
                              System.out.println(student.getName());
					}
				}
				
			}

 

 

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

结果如下：执行了4条查询语句,其中有3条关联查询语句。

Hibernate: select myclass0_.id as id0_, myclass0_.name as name0_ from class myclass0_ where myclass0_.id in (27 , 28 , 29)
班级名：09002
Hibernate: select students0_.class_id as class3_0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id=?
班级人数：0

班级名：09003
Hibernate: select students0_.class_id as class3_0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id=?
班级人数：3
学生姓名：ccc
aaa
bbb

班级名：09004
Hibernate: select students0_.class_id as class3_0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id=?
班级人数：3
学生姓名：ccc
bbb
aaa

 

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

执行了2条查询语句，其中有1条子查询语句（嵌套子查询）

Hibernate: select myclass0_.id as id0_, myclass0_.name as name0_ from class myclass0_ where myclass0_.id in (27 , 28 , 29)
班级名：09002
Hibernate: select students0_.class_id as class3_0_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id in (select myclass0_.id from class myclass0_ where myclass0_.id in (27 , 28 , 29))
班级人数：0
班级名：09003
班级人数：3
学生姓名：ccc
aaa
bbb
班级名：09004
班级人数：3
学生姓名：bbb
ccc
aaa

 

总结：

hibernate抓取策略（单端代理的批量抓取）

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

 

2.hibernate抓取策略（单端代理的批量抓取）

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

此时lazy会失效

 

3.hibernate抓取策略（集合代理的批量抓取）

保持默认，同fetch="select",如：
<set name="students" inverse="true" cascade="all" 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

 

4.hibernate抓取策略（集合代理的批量抓取）

设置fetch="join",如：
<set name="students" inverse="true" cascade="all" 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

 

5.hibernate抓取策略（集合代理的批量抓取）

设置fetch="subselect",如：
<set name="students" inverse="true" cascade="all" 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

 

6.hibernate抓取策略，,batch-szie在<class>上的应用

batch-size属性，可以批量加载实体类，参见：Classes.hbm.xml
<class name="Classes" table="t_classes" batch-size="3">

当查classes对象时发出9条hql语句配置过后batch-size=3后会之发9/3=3条hql语句，提高性能

 

7.hibernate抓取策略,batch-szie在集合上的应用

batch-size属性，可以批量加载实体类，参见：Classes.hbm.xml
<set name="students" inverse="true" cascade="all" batch-size="5">

当查students对象时发出10条hql语句配置过后batch-size=5后会之发10/5=2条hql语句，提高性能

 

 

总体上分析：默认是fetch="select" 当配置fetch="join"时直接查询包含的对象或者集合lazy失效。