【hibernate框架】一对多(多对一)双向关联(Annotation实现)

用户和组的关联，一个用户只能属于一个组，一个组可以拥有多个用户

User.java:

package cn.edu.hpu.one2many;


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;
	
        @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.java:

package cn.edu.hpu.one2many;


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.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;
	}
	
	//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
	@OneToMany(mappedBy="group")
	public Set<User> getUsers() {
		return users;
	}
	public void setUsers(Set<User> users) {
		this.users = users;
	}
	
}

在hibernate.cfg.xml中添加：
<mapping class="cn.edu.hpu.many2many.Group"/>
<mapping class="cn.edu.hpu.many2many.User"/>


生成的sql语句：
alter table m_user 
        drop 
        foreign key FKBF71E05DAB4F2BAC


    drop table if exists m_group


    drop table if exists m_user


    create table m_group (
        id integer not null auto_increment,
        name varchar(255),
        primary key (id)
    )


    create table m_user (
        id integer not null auto_increment,
        name varchar(255),
        groupId integer,
        primary key (id)
    )


    alter table m_user 
        add index FKBF71E05DAB4F2BAC (groupId), 
        add constraint FKBF71E05DAB4F2BAC 
        foreign key (groupId) 

        references m_user (id)

转载请注明出处:http://blog.csdn.net/acmman