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
    
      users=new HashSet
     
      ();	//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
      
        getUsers() {		return users;	}	public void setUsers(Set
       
         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)

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