一、一对多单向关良

一对多单向关联与多对一相似

假设一个组有多个用户，即一(Group)对多(User)

需要在Group类中添加一个User类的Set集合(数据库中的用户不可能是重复的，所以要用Set)属性:private Set<User> users;然后在getUser()方法上添加注解：

@OneToMany和@JoinColumn(name="groupId"),如果不加@JoinColumn则会自动生成一个中间表

Group类：

package cn.orlion.hibernate.model;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="t_group")public class Group {    private int id;        private String name;        private Set
     
       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;    }    @OneToMany    @JoinColumn(name="groupId")    public Set
      
        getUsers() {        return users;    }    public void setUsers(Set
       
         users) {        this.users = users;    }}
       
      
     

User类：

package cn.orlion.hibernate.model;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Table;@Entity@Table(name="t_user")public class User {    private int id;        private String name;    @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sql：

create table t_group (        id integer not null auto_increment,        name varchar(255),        primary key (id)    )create table t_user (        id integer not null auto_increment,        name varchar(255),        groupId integer,        primary key (id)    )alter table t_user         add constraint FK_7ktm6l2qkykpqrf6oq01ys8wy         foreign key (groupId)         references t_group (id)

 

二、一对多双向关联

配置方式：在Group类中的getUsers()方法上添加注解@OneToMany(mappedBy="group"),如果不写mappedBy则会在user表中生成两个groupid字段

；然后在User类中添加group属性，在getGroup()方法上添加注解@ManyToOne

Group类：

 

package cn.orlion.hibernate.model;import java.util.Set;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.OneToMany;import javax.persistence.Table;@Entity@Table(name="t_group")public class Group {    private int id;        private String name;        private Set
     
       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;    }    @OneToMany(mappedBy="group")    public Set
      
        getUsers() {        return users;    }    public void setUsers(Set
       
         users) {        this.users = users;    }}
       
      
     

 

User类：

package cn.orlion.hibernate.model;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToOne;import javax.persistence.Table;@Entity@Table(name="t_user")public class User {    private int id;        private String name;        private 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;    }    @ManyToOne    public Group getGroup() {        return group;    }    public void setGroup(Group group) {        this.group = group;    }}

生成的sql

create table t_group (        id integer not null auto_increment,        name varchar(255),        primary key (id)    )create table t_user (        id integer not null auto_increment,        name varchar(255),        group_id integer,        primary key (id)    )alter table t_user         add constraint FK_e5f24mh6aryt9hsy99oydps6g         foreign key (group_id)         references t_group (id)