关于把数据从里面拿出来是什么样子。
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：
将属于id=2的Group的User全部取出来了
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注:显然User里面的ManyToOne里面的Fetch默认的是EAGER
