关于把数据从里面拿出来是什么样子。
User.java:
Group.java:
测试代码:
看看输出的sql语句:
Hibernate:
select
user0_.id as id1_1_,
user0_.group_id as group3_1_1_,
user0_.name as name1_1_,
group1_.id as id0_0_,
group1_.name as name0_0_
from
m_user user0_
left outer join
m_group group1_
on user0_.group_id=group1_.id
where
user0_.id=?
取出了di为2的User,并且取出了相应的group
那么取1的时候会不会把多取出来呢?取一个组,会把组的user全取出来吗?
经过实验,不会。
实验:
输出的sql语句:
ibernate:
select
group0_.id as id0_0_,
group0_.name as name0_0_
from
m_group group0_
where
group0_.id=?
测试user是否取出:
for (User u:g.getUsers()) {
System.out.println(u.getName());
}
发现并没有取出所有在id=2的group的user。
cascade并不影响你的读取。读取使用另外一个参数设定的:Fetch。
Fetch管读,cascade管增删改。
fetch里面的属性:依然是enumeration(枚举),有两个参数:
EAGER:渴望的,着急的,立马想要得到的。
LAZY:懒的,缓慢的,不是立马要得到的。
OneToMany的默认的fetch是LAZY。
现在将fecth改成EAGER:
结果:
Hibernate:
select
group0_.id as id0_1_,
group0_.name as name0_1_,
users1_.group_id as group3_0_3_,
users1_.id as id3_,
users1_.id as id1_0_,
users1_.group_id as group3_1_0_,
users1_.name as name1_0_
from
m_group group0_
left outer join
m_user users1_
on group0_.id=users1_.group_id
where
group0_.id=?
g1
u4
u5
u6
u3
将属于id=2的Group的User全部取出来了
User.java:
package cn.edu.hpu.one2many; import javax.persistence.CascadeType; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.ManyToOne; import javax.persistence.Table; @Entity @Table(name="m_user") public class User { private int id; private String name; private Group group; //只要有双向就要指定制定一个属性(mapedby) //不指定的话会有两个相同的字段产生 @ManyToOne(cascade={CascadeType.ALL}) public Group getGroup() { return group; } public void setGroup(Group group) { this.group = group; } @Id @GeneratedValue public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } }
Group.java:
package cn.edu.hpu.one2many; import java.util.HashSet; import java.util.Set; import javax.persistence.CascadeType; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.OneToMany; import javax.persistence.Table; @Entity @Table(name="m_group") public class Group { private int id; private String name; private Set<User> users=new HashSet<User>(); //选择set的原因是因为,set互相之间不会有重复的 //跟数据库模型比较匹配 @Id @GeneratedValue public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } @OneToMany(mappedBy="group", cascade={CascadeType.ALL} ) public Set<User> getUsers() { return users; } public void setUsers(Set<User> users) { this.users = users; } }
测试代码:
@Test public void testGetUser(){ sessionFactory=new AnnotationConfiguration().configure().buildSessionFactory(); Session s=sessionFactory.openSession(); s.beginTransaction(); User u=(User)s.get(User.class,2);//当取user的时候,会把相应的group取出来 System.out.println(u.getName()); s.getTransaction().commit(); }
看看输出的sql语句:
Hibernate:
select
user0_.id as id1_1_,
user0_.group_id as group3_1_1_,
user0_.name as name1_1_,
group1_.id as id0_0_,
group1_.name as name0_0_
from
m_user user0_
left outer join
m_group group1_
on user0_.group_id=group1_.id
where
user0_.id=?
取出了di为2的User,并且取出了相应的group
那么取1的时候会不会把多取出来呢?取一个组,会把组的user全取出来吗?
经过实验,不会。
实验:
@Test public void testGetGroup(){ sessionFactory=new AnnotationConfiguration().configure().buildSessionFactory(); Session s=sessionFactory.openSession(); s.beginTransaction(); Group g=(Group)s.get(Group.class,2); System.out.println(g.getName()); s.getTransaction().commit(); }
输出的sql语句:
ibernate:
select
group0_.id as id0_0_,
group0_.name as name0_0_
from
m_group group0_
where
group0_.id=?
测试user是否取出:
for (User u:g.getUsers()) {
System.out.println(u.getName());
}
发现并没有取出所有在id=2的group的user。
cascade并不影响你的读取。读取使用另外一个参数设定的:Fetch。
Fetch管读,cascade管增删改。
fetch里面的属性:依然是enumeration(枚举),有两个参数:
EAGER:渴望的,着急的,立马想要得到的。
LAZY:懒的,缓慢的,不是立马要得到的。
OneToMany的默认的fetch是LAZY。
现在将fecth改成EAGER:
@OneToMany(mappedBy="group", cascade={CascadeType.ALL}, fetch=FetchType.EAGER ) public Set<User> getUsers() { return users; }测试例子加:
for (User u:g.getUsers()) { System.out.println(u.getName()); }
结果:
Hibernate:
select
group0_.id as id0_1_,
group0_.name as name0_1_,
users1_.group_id as group3_0_3_,
users1_.id as id3_,
users1_.id as id1_0_,
users1_.group_id as group3_1_0_,
users1_.name as name1_0_
from
m_group group0_
left outer join
m_user users1_
on group0_.id=users1_.group_id
where
group0_.id=?
g1
u4
u5
u6
u3
将属于id=2的Group的User全部取出来了
注:显然User里面的ManyToOne里面的Fetch默认的是EAGER
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