注意事项：一对多，多对一双向关联，在一的一方的多的getSet集合上的oneToMany上加上mappedBy。告诉hibernate由多的方一来维护关系。这也符合逻辑
，本来外键就是在加在多的一方。
User类：
package com.oracle.hibernate; import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne; @Entity
public class User { private int id;
private String name;
private Group group; @ManyToOne
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类：
package com.oracle.hibernate; import java.util.HashSet;
import java.util.Set; import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Table; @Entity
@Table(name="t_group")//group是mysql的关键字，换个名
public class Group { private int id;
private String name;
//用set集合，不重复。因为表里的记录也不会重复
private Set<User> users = new HashSet<User>(); //铁律：双向关系，必设mappedBy,
//在oneTomany一方设mappedBy，告诉hibernate，在多的一方来维护关系，在Group类里就不用管我了，
@OneToMany(mappedBy="group")
public Set<User> getUsers() {
return users;
}
public void setUsers(Set<User> users) {
this.users = users;
}
@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;
} }
生成的表：
hibernate建表语句：
create table t_group (
id integer not null auto_increment,
name varchar(255),
primary key (id)
)
create table User (
id integer not null auto_increment,
name varchar(255),
group_id integer,
primary key (id)
)
alter table User
add index FK285FEB80FC033B (group_id),
add constraint FK285FEB80FC033B
foreign key (group_id)
references t_group (id)