Hibernate第六篇【多对多映射、一对一映射】

小鱼儿 2021-09-17 06:16 326阅读 0赞

# 前言 #

前面已经讲解了一对多和多对一的映射是怎么配置了，也讲解了inverse和cascade属性对关联关系的影响，**本博文讲解多对多的映射和一对一的映射！**

# 多对多映射 #

需求：**一个项目由多个员工开发，一个员工开发多个项目**

## 设计数据库表 ##

一般地，**如果是多对多的映射，我们都会使用一张中间表来保存它们的关联关系**….

![这里写图片描述][SouthEast]

## 设计实体 ##

我们在设计实体的时候，一般是**核心数据表对应一个JavaBean实体【中间表并不是核心数据表】**，那么我们将会设计两个JavaBean对象

![这里写图片描述][SouthEast 1]

**project.java**

package zhongfucheng.many2many;
    
    import java.util.HashSet;
    import java.util.Set;
    
    /** * Created by ozc on 2017/5/7. */
    
    
    public class Project { 
    
        private int projectId;
        private String projectName;
    
        //使用Set集合与developer实体维护关系
        private Set<Developer> developers = new HashSet<>();
    
        public int getProjectId() {
            return projectId;
        }
    
        public void setProjectId(int projectId) {
            this.projectId = projectId;
        }
    
        public String getProjectName() {
            return projectName;
        }
    
        public void setProjectName(String projectName) {
            this.projectName = projectName;
        }
    
        public Set<Developer> getDevelopers() {
            return developers;
        }
    
        public void setDevelopers(Set<Developer> developers) {
            this.developers = developers;
        }
    }

**developer.java**

package zhongfucheng.many2many; import java.util.HashSet; import java.util.Set; /** * Created by ozc on 2017/5/7. */ public class Developer { private int developerId; private String developerName; //使用Set集合来维护与Project关系 private Set<Project> projects = new HashSet<>(); public int getDeveloperId() { return developerId; } public void setDeveloperId(int developerId) { this.developerId = developerId; } public String getDeveloperName() { return developerName; } public void setDeveloperName(String developerName) { this.developerName = developerName; } public Set<Project> getProjects() { return projects; } public void setProjects(Set<Project> projects) { this.projects = projects; } }

## 映射配置文件 ##

**以项目映射文件为例：**我们不急着写，首先来**分析一下关键点**……想要在多对多映射中产生正确的关联关系，下面几步必不可少：

*  **配置映射集合的属性(developers)**
 *  **映射集合对应的中间表(developer\_project)**
 *  **中间表的外键字段(project\_id)**
 *  **集合元素的类型(Developer)**
 *  **中间表另外的外键字段(developer\_id)**

**Project和Developer的映射文件都需要这几个关键步骤**

### **Project映射文件** ###

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    
    
    <hibernate-mapping package="zhongfucheng.many2many">
    
        <class name="Project" table="Project">
    
            
            <id name="projectId" column="projectId">
                <generator class="native"></generator>
            </id>
    
            
            <property name="projectName" column="projectName"></property>
    
    
            
            
            <set name="developers" table="developer_project">
                
                <key column="project_id"></key>
    
                
                <many-to-many column="developer_id" class="Developer"></many-to-many>
            </set>
    
        </class>
    
    </hibernate-mapping>

--------------------

### **Developer映射文件** ###

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    
    
    <hibernate-mapping package="zhongfucheng.many2many">
    
        <class name="Developer" table="Developer">
    
            
            <id name="developerId" column="developerId">
                <generator class="native"></generator>
            </id>
    
            
            <property name="developerName" column="developerName"></property>
    
    
            
            
            <set name="projects" table="developer_project">
                
                <key column="developer_id"></key>
    
                
                <many-to-many column="project_id" class="Project"></many-to-many>
            </set>
    
        </class>
    
    </hibernate-mapping>

--------------------

## 测试 ##

package zhongfucheng.many2many;
    
    import org.hibernate.SessionFactory;
    import org.hibernate.Transaction;
    import org.hibernate.cfg.Configuration;
    import org.hibernate.classic.Session;
    
    /** * Created by ozc on 2017/5/6. */
    public class App2 { 
        public static void main(String[] args) {
    
    
            /* * * * Project Developer 电商系统 曹吉 王春 OA系统 王春 老张 */
            //创建对象
            Developer cj = new Developer();
            Developer wc = new Developer();
            Developer lz = new Developer();
            Project ds = new Project();
            Project oa = new Project();
    
            //设置对象的数据
            cj.setDeveloperName("曹吉");
            wc.setDeveloperName("王春");
            lz.setDeveloperName("老张");
    
            oa.setProjectName("OA系统");
            ds.setProjectName("电商系统");
    
            //使用Project来关联数据【在多对多中，一样的】
            oa.getDevelopers().add(wc);
            oa.getDevelopers().add(lz);
    
            ds.getDevelopers().add(cj);
            ds.getDevelopers().add(wc);
    
    
            //获取加载配置管理类
            Configuration configuration = new Configuration();
    
            //加载User的映射文件！
            configuration.configure().addClass(Developer.class).addClass(Project.class);
    
            //创建Session工厂对象
            SessionFactory factory = configuration.buildSessionFactory();
    
            //得到Session对象
            Session session = factory.openSession();
    
            //使用Hibernate操作数据库，都要开启事务,得到事务对象
            Transaction transaction = session.getTransaction();
    
            //开启事务
            transaction.begin();
    
    
            //在Project映射文件中设置级联保存了
            session.save(oa);
            session.save(ds);
    
    
            //提交事务
            transaction.commit();
    
            //关闭Session
            session.close();
        }
    }

执行了9条SQL语句，数据库中的记录也是正确的。

![这里写图片描述][SouthEast 2]

--------------------

# 一对一的映射 #

需求：**用户与身份证信息..一个用户对应一个身份证**

## 数据库表设计 ##

对于数据库表设计我们有两种方式

*  第一种：**在身份证的数据表中设置一个外键来维护用户的关系，这个外键也应该是唯一的【一个用户对应一张身份证】**
 *  第二种：**在身份证的数据表中使用主键+外键的方式来维护用户的关系。**

![这里写图片描述][SouthEast 3]

## 设计实体 ##

**idCard.java**

package zhongfucheng.one2one;
    
    /** * Created by ozc on 2017/5/7. */
    public class IdCard { 
    
        private int idCardId;
        private String idCardName;
    
        //维护与用户之间的关系
        private User user ;
    
        public int getIdCardId() {
            return idCardId;
        }
    
        public void setIdCardId(int idCardId) {
            this.idCardId = idCardId;
        }
    
        public String getIdCardName() {
            return idCardName;
        }
    
        public void setIdCardName(String idCardName) {
            this.idCardName = idCardName;
        }
    
        public User getUser() {
            return user;
        }
    
        public void setUser(User user) {
            this.user = user;
        }
    }

**User.java**

package zhongfucheng.one2one;
    
    /** * Created by ozc on 2017/5/7. */
    public class User { 
    
        private int userId;
        private String userName;
    
        //维护与身份证一对一的关系
        private IdCard idCard ;
    
        public int getUserId() {
            return userId;
        }
    
        public void setUserId(int userId) {
            this.userId = userId;
        }
    
        public String getUserName() {
            return userName;
        }
    
        public void setUserName(String userName) {
            this.userName = userName;
        }
    
        public IdCard getIdCard() {
            return idCard;
        }
    
        public void setIdCard(IdCard idCard) {
            this.idCard = idCard;
        }
    }

--------------------

## 第一种方式映射文件 ##

我们有两种方式来设计数据库中的表实现一对一的关系，首先我们来挑比较熟悉的外键方式来写映射文件

### **user映射文件** ###

### **idCard映射文件** ###

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    
    <hibernate-mapping package="zhongfucheng.one2one">
    
        <class name="IdCard" table="IdCard">
    
            
            <id name="idCardId" column="idCardId">
                <generator class="native"></generator>
            </id>
    
            
            <property name="idCardName" column="idCardName"></property>
    
            
    
            
            <many-to-one name="user" column="user_id" class="User" unique="true" cascade="save-update"></many-to-one>
    
        </class>
    
    </hibernate-mapping>

## 测试 ##

**要使用IdCart来维护User的关联关系。**

*  **如果使用User来维护idCart的关联关系，idCart的外键列是为NULL的，因为重头到尾我们都没有给它赋值**
 *  **而使用IdCart来维护User，是外键值是根据User的主键id来生成的**

package zhongfucheng.one2one; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration; import org.hibernate.classic.Session; /** * Created by ozc on 2017/5/6. */ public class App3 { public static void main(String[] args) { //创建对象 User user = new User(); IdCard idCard = new IdCard(); //设置对象的数据 user.setUserName("你好"); idCard.setIdCardName("身份证001"); //一对一关联数据 idCard.setUser(user); //获取加载配置管理类 Configuration configuration = new Configuration(); //加载User的映射文件！ configuration.configure().addClass(User.class).addClass(IdCard.class); //创建Session工厂对象 SessionFactory factory = configuration.buildSessionFactory(); //得到Session对象 Session session = factory.openSession(); //使用Hibernate操作数据库，都要开启事务,得到事务对象 Transaction transaction = session.getTransaction(); //开启事务 transaction.begin(); //保存对象的数据,idCard配置文件使用级联保存 session.save(idCard); //提交事务 transaction.commit(); //关闭Session session.close(); } }

![这里写图片描述][SouthEast 4]

--------------------

## 第二种方式映射文件 ##

**因为IdCart使用userId作为了主键，因此需要在JavaBean中配置多一个属性UserId…其他的都不用变**

private int userId;
        public int getUserId() {
            return userId;
        }
    
        public void setUserId(int userId) {
            this.userId = userId;
        }

在Hibernate第三篇讲解配置的时候，在generator节点下还有一个属性没有讲解，也就是foreign属性…现在来填坑了..

![这里写图片描述][SouthEast 5]

### **idCard映射文件** ###

idCart的映射文件主要在于:**将主键也映射成外键来使用，这就需要用到foreign属性值了**

**使用`<one-to-one>`标签来配置基于主键的映射**

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    
    <hibernate-mapping package="zhongfucheng.one2one2.one2one">
    
        <class name="IdCard" table="IdCard">
    
            
            <id name="userId" column="userId">
                
                <generator class="foreign">
                    <param name="property">user</param>
                </generator>
            </id>
    
            
            <property name="idCardName" column="idCardName"></property>
            <property name="idCardId" column="idCartId"></property>
    
    
    
            
            <one-to-one name="user" class="User" constrained="true"></one-to-one>
    
    
    
    
        </class>
    
    </hibernate-mapping>

### **user映射文件** ###

## 测试 ##

package zhongfucheng.one2one2.one2one;
    
    import org.hibernate.SessionFactory;
    import org.hibernate.Transaction;
    import org.hibernate.cfg.Configuration;
    import org.hibernate.classic.Session;
    
    /** * Created by ozc on 2017/5/6. */
    public class App4 { 
        public static void main(String[] args) {
    
            //创建对象
            User user = new User();
            IdCard idCard = new IdCard();
    
            //设置对象的数据
            user.setUserName("你好3");
            idCard.setIdCardName("身份证003");
            idCard.setIdCardId(4235);
    
            //一对一关联数据
            idCard.setUser(user);
    
            //获取加载配置管理类
            Configuration configuration = new Configuration();
    
            //加载User的映射文件！
            configuration.configure().addClass(User.class).addClass(IdCard.class);
    
            //创建Session工厂对象
            SessionFactory factory = configuration.buildSessionFactory();
    
            //得到Session对象
            Session session = factory.openSession();
    
            //使用Hibernate操作数据库，都要开启事务,得到事务对象
            Transaction transaction = session.getTransaction();
    
            //开启事务
            transaction.begin();
    
            //保存对象的数据,idCard配置文件使用级联保存
            session.save(idCard);
    
            //提交事务
            transaction.commit();
    
            //关闭Session
            session.close();
        }
    }

![这里写图片描述][SouthEast 6]

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