Hibernate中的一对多单向关联

一时失言乱红尘 2023-10-17 11:35 60阅读 0赞

Hibernate中的一对多单向关联  
  
**意思是：一方可以看到多方，多方看不到一方**  
**实例：国家与城市——国家为1，城市为n**

### **一、基本概念** ###

**域属性：对象自身的属性**  
  
**关联属性：关联关系的属性**  
  
**有Country（国家）和City（城市）两个实体**  
  
**在国家实体中，countryName、countryID为域属性，citys为关联属性**  
  
![这里写图片描述][SouthEast]  
  
**关联方向——分为单向和双向**  
  
**单向关联：只在一方把另一方作为关联属性，即只有一个实体对象可以加载和访问对方**  
  
**Country中有citys，而City中没有country;**  
  
**双向关联：彼此把对方设为关联属性，彼此都可以加载和访问**  
  
**Country中有citys，City中也有country;**  
  
**关联关系的维护：**  
**关联关系在Java代码上的维护：**  
  
![这里写图片描述][SouthEast 1]  
  
**关联关系在数据库上的维护：**  
**本质上是对外键值的维护，当少的一方维护关系时**  
  
![这里写图片描述][SouthEast 2]  
  
**当多的一方维护关系时：**  
![这里写图片描述][SouthEast 3]  
**总结一下就是session.save(1)维护外键关系的是update语句，session.save(n)维护外键关系的是insert语句**  
  
  
**牢记一点，Java代码谁在维护关联关系，session就save谁**

### 1）实体类（1、Country ） ###

package com.hibernate.beans;
    
    import java.util.HashSet;
    import java.util.Set;
    
    public class Country {
        
        //域属性
        private Integer countryId;
        private String countryName;
        //关联属性
        private Set<City> citys;
        public Country() {
            citys = new HashSet<City>();
        }
        public Country(String countryName) {
            this();
            this.countryName = countryName;
        }
    
        public Integer getCountryId() {
            return countryId;
        }
        public void setCountryId(Integer countryId) {
            this.countryId = countryId;
        }
        public String getCountryName() {
            return countryName;
        }
        public void setCountryName(String countryName) {
            this.countryName = countryName;
        }
        @Override
        public String toString() {
            return "Country [countryId=" + countryId + ", countryName="
                    + countryName +  "]";
        }
        public Set<City> getCitys() {
            return citys;
        }
        public void setCitys(Set<City> citys) {
            this.citys = citys;
        }
    
    
    }

### 2）实体类（2、City） ###

package com.hibernate.beans;
    
    public class City {
        
    
        private Integer cityId;
        private String cityName;
        public City() {
        }
    
        public City(String cityName) {
            this.cityName = cityName;
        }
    
        public Integer getCityId() {
            return cityId;
        }
    
        public void setCityId(Integer cityId) {
            this.cityId = cityId;
        }
    
        public String getCityName() {
            return cityName;
        }
    
        public void setCityName(String cityName) {
            this.cityName = cityName;
        }
    
    
        @Override
        public String toString() {
            return "City [cityId=" + cityId + ", cityName=" + cityName
                    +  "]";
        }
    
    }

### 3）映射文件（Country.hbm.xml） ###

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
     <hibernate-mapping package="com.hibernate.beans">
         <class name="Country" table="t_country">
            
            <id name="countryId" column="t_countryId">
                
                <generator class="native"></generator>
            </id>
            <property name="countryName" column="t_countryName"></property>
            
            <set name="citys" cascade="save-update">
                
                <key column="t_countryId"></key>
                
                <one-to-many class="City"/>
            </set>
         </class>
     </hibernate-mapping>

### 4）映射文件（City.hbm.xml） ###

### 5）主映射文件（Hibernate.hbm.xml） ###

**略**

### 测试方法： ###

public void test() {
            // 获取连接
            Session session = HbnUtils.getSession();
            try {
                // 开启事务
                session.beginTransaction();
                Country country = new Country("CN");
                City city0 = new City("南昌");
                City city1 = new City("武汉");
                City city2 = new City("长沙");
    
                country.getCitys().add(city0);
                country.getCitys().add(city1);
                country.getCitys().add(city2);
                // 执行业务
                session.save(country);
                // 提交事务
                session.getTransaction().commit();
            } catch (Exception e) {
                e.printStackTrace();
                // 回滚
                session.getTransaction().rollback();
            }
        }

### 分析运行过程 ###

**从控制台日志可以看出Hibernate运行过程**

Hibernate: 
        insert 
        into
            t_country
            (t_countryName) 
        values
            (?)
    Hibernate: 
        insert 
        into
            t_city
            (t_cityName) 
        values
            (?)
    Hibernate: 
        insert 
        into
            t_city
            (t_cityName) 
        values
            (?)
    Hibernate: 
        insert 
        into
            t_city
            (t_cityName) 
        values
            (?)
    Hibernate: 
        update
            t_city 
        set
            t_countryId=? 
        where
            t_cityId=?
    Hibernate: 
        update
            t_city 
        set
            t_countryId=? 
        where
            t_cityId=?
    Hibernate: 
        update
            t_city 
        set
            t_countryId=? 
        where
            t_cityId=?

**首先保存了City对象除了外键以外的所有的字段，然后保存Country对象的所有字段，最后根据Country的id去更新City的外键字段**  
  
**控制台打印顺序并不是Hibernate的执行顺序，执行顺序参考官方文档**  
  
  
**如有疑问，欢迎入群讨论：511906138**

[SouthEast]: https://img-blog.csdn.net/20170314163908835?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGVpc3VyZV9saWZl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 1]: https://img-blog.csdn.net/20170314152425784?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGVpc3VyZV9saWZl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 2]: https://img-blog.csdn.net/20170314154601528?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGVpc3VyZV9saWZl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 3]: https://img-blog.csdn.net/20170314154909249?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvbGVpc3VyZV9saWZl/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast