hibernate一对一关联OneToOne
什么用场景需要hibernate一对一关联呢？举个例子：例如用户表与账户表，我们查询用户基本信息，不希望账户信息也拥挤在基本信息中，分开表管理又与之关联，单查用户信息的时候不用查账户，如登录，那么就用一对一。
一对一外键关联，使用@OneToOne
用户表：User
@OneToOne(cascade = CascadeType.ALL,fetch = FetchType.LAZY)
一对一外键关联，使用@OneToOne，并设置了级联操作
@JoinColum设置了外键的名称为account_id如果不设置，则默认为account_id;
外键的值是唯一的(unique)，不可重复，与Account的主键一至
package com.weizhixi.entity;
import javax.persistence.*;
/**
* Created by cxq on 2017-12-26.
*/
@Entity
public class User {
private int id;
private String name;
private Account account;
@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;
}
@OneToOne(cascade = CascadeType.ALL, fetch=FetchType.LAZY)
@JoinColumn(name="account_id",unique=true)
public Account getAccount() {
return account;
}
public void setAccount(Account account) {
this.account = account;
}
}账户表：Account
@OneToOne(mappedBy="account",cascade = CascadeType.ALL)
一对一双向关联关系，使用@OneToOne
注意：需要加上mappedBy="account"，如果不加上的话，Account也会生成一个外键（user_id）。
mappedBy="account"需要指向与他关联的对象的一个属性，说明双向关联关系中，有且有一端是作为主体端存在的。
主体端负责维护关联接列，对于不需要维护这种关系的从表则通过mappedBy属性进行声明mappedBy的值指向主体的关联属性。
规律：只有是双向关联关系，不维护的那一端要加上mappedBy，mappedBy谁用谁不维护。
package com.weizhixi.entity;
import javax.persistence.*;
/**
* Created by cxq on 2017-12-26.
*/
@Entity
public class Account {
private int id;
private double money;
private User user;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public double getMoney() {
return money;
}
public void setMoney(double money) {
this.money = money;
}
@OneToOne(mappedBy="account", cascade=CascadeType.ALL)
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
}谁主谁从你自己根据业务决定。以上配置User为主表，Account为从表，由User维护双方关系。
来试下：Test
使用junit测试
//创建用户
User u = new User();
u.setName("weizhixi");
//创建用户的账户
Account a = new Account();
a.setMoney(100.00);
//把账户分配给该用户
u.setAccount(a);
//提交后就关闭session，不要自己再关闭session
session = sessionFactory.getCurrentSession();
session.beginTransaction();
//保存用户
session.save(u);
session.getTransaction().commit();从日志看到发送了2句SQL插入语句
Hibernate: insert into Account (money) values (?)
Hibernate: insert into User (account_id, name) values (?, ?)
查询：主表
session = sessionFactory.getCurrentSession(); session.beginTransaction(); //查询主表 - 懒加载 User user = (User)session.get(User.class , 1); System.out.println(user.getName()); //再获取从表信息 Account account = user.getAccount(); System.out.println(account.getMoney()); session.getTransaction().commit();
从日志看到，在查询User表的时候，并没有真正去查询Account，只发送了1条查询User表的SQL
在获取从表信息的时候才真正发送查询Account表的SQL。
Hibernate: select user0_.id as id2_0_, user0_.account_id as account3_2_0_, user0_.name as name2_0_ from User user0_ where user0_.id=? weizhixi Hibernate: select account0_.id as id3_1_, account0_.money as money3_1_, user1_.id as id2_0_, user1_.account_id as account3_2_0_, user1_.name as name2_0_ from Account account0_ left outer join User user1_ on account0_.id=user1_.account_id where account0_.id=? Hibernate: select user0_.id as id2_0_, user0_.account_id as account3_2_0_, user0_.name as name2_0_ from User user0_ where user0_.account_id=? 100.0
查询：从表
session = sessionFactory.getCurrentSession(); session.beginTransaction(); Account user = (Account)session.get(Account.class , 1); System.out.println(user.getMoney()); User user1 = user.getUser(); System.out.println(user1.getName()); session.getTransaction().commit();
Hibernate: select account0_.id as id3_1_, account0_.money as money3_1_, user1_.id as id2_0_, user1_.account_id as account3_2_0_, user1_.name as name2_0_ from Account account0_ left outer join User user1_ on account0_.id=user1_.account_id where account0_.id=? 100.0 weizhixi
用户表
mysql> select * from user; +----+----------+------------+ | id | name | account_id | +----+----------+------------+ | 1 | weizhixi | 1 | +----+----------+------------+ 1 row in set (0.00 sec)
账户表
mysql> select * from account; +----+-------+ | id | money | +----+-------+ | 1 | 100 | +----+-------+ 1 row in set (0.00 sec)
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