利用hibernate实现数据库对象关联（多对一、一对多、一对一、多对多）

核心思想：

数据库表对应java类（表-类），表的字段对应类的属性（字段-属性）

关联：把数据库中的外键映射，变成对象的关联关系。

多对一，一对多，一对一，多对多

单向，双向

一、单向，多对一（Info，Nation多个人有同一个民族）
准备两个基本类和映射文件
1.给类加关联对象

import java.util.Date;

public class Info {
    private String code;
    private String name;
    private Boolean sex;
    private Date birthday;
    private Nation nationObj;//给类加关联对象。添加一个成员nationObj，并生成getter和setter

    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Boolean getSex() {
        return sex;
    }
    public void setSex(Boolean sex) {
        this.sex = sex;
    }
    public Date getBirthday() {
        return birthday;
    }
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
    public Nation getNationObj() {
        return nationObj;
    }
    public void setNationObj(Nation nationObj) {
        this.nationObj = nationObj;
    }    
}

public class Nation {
    private String code;
    private String name;
    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }    
}

2.给hbm.xml文件加关联映射

给Info.hbm.xml加上<many-to-one>
<many-to-one name="nationObj" column="nation"></many-to-one>

<?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="maya.model">
    <class name="Info" table="Info">
        <id name="code" type="java.lang.String">
            <column name="code" />
            <generator class="assigned"></generator>
        </id>
        <property name="name" type="java.lang.String">
            <column name="name" />
        </property>
        <property name="sex" type="java.lang.Boolean">
            <column name="sex" />
        </property>
        <property name="birthday" type="java.util.Date">
            <column name="birthday" />
        </property>
        
        <!--加上<many-to-one> -->
        <!--column：列，class：类名， cascade="save-update"，加这个配置属性目的是，many表和one表一起更新 -->
        <many-to-one name="nationObj" column="nation" class="Nation" cascade="save-update">
        </many-to-one>          
    </class>
</hibernate-mapping>    

注意：

1.主要针对的是哪个对象？ --- Info
2.主表是谁？从表是谁？主表是约束别人的，从表是被约束的。 外键在从表上。
主表是nation,从表是info
3.在实体类中，外键属性与关联对象，留一个就可以。在hbm.xml文件中,外键属性与<many-to-one>也留一个就可以。
如果两个都保留，记得在<many-to-one>上加上两个属性insert=false,update=false。
4.一般需要加上cascade="save-update"，加这个配置业属性目的是，many表和one表一起更新

二、单向，一对一（Login和Info）
准备两个一对一的表和类
1.给类加关联对象
给Login加上infoObj属性

public class Login {
    private int ids;
    private String userName;
    private String password;
    private Info infoObj;
    public int getIds() {
        return ids;
    }
    public void setIds(int ids) {
        this.ids = ids;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    public Info getInfoObj() {
        return infoObj;
    }
    public void setInfoObj(Info infoObj) {
        this.infoObj = infoObj;
    }
}

2.给Login.hbm.xml加上<many-to-one>配置

<many-to-one name="infoObj" column="infoCode" unique="true"/>

<?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">
<!-- Generated 2017-3-4 12:27:40 by Hibernate Tools 5.2.1.Final -->
<hibernate-mapping>
    <class name="maya.model.Login" table="Login"><!-- 上面的hibernate-mapping不带包的路径，class就得带类的路径，下同 -->
        <id name="ids" type="int">
            <column name="ids" />
            <generator class="native"></generator>
        </id>
        <property name="userName" type="java.lang.String">
            <column name="userName" />
        </property>
        <property name="password" type="java.lang.String">
            <column name="password" />
        </property>
        
        <!-- unique的意思是唯一性 -->
        <many-to-one name="infoObj" column="InfoCode" class="maya.model.Info"  unique="true">
            
        </many-to-one>
    </class>
</hibernate-mapping>

三、单向，一对多(Info,Work)
1.给类加关联对象（集合对象）
private Set<Work> works;

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

public class Info {
    private String code;
    private String name;
    private Boolean sex;
    private Date birthday;
    private Nation nationObj;
    private Set<Work> works = new HashSet<Work>(0);//给类加关联对象（集合对象）
    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Boolean getSex() {
        return sex;
    }
    public void setSex(Boolean sex) {
        this.sex = sex;
    }
    public Date getBirthday() {
        return birthday;
    }
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
    public Nation getNationObj() {
        return nationObj;
    }
    public void setNationObj(Nation nationObj) {
        this.nationObj = nationObj;
    }
    public Set<Work> getWorks() {
        return works;
    }
    public void setWorks(Set<Work> works) {
        this.works = works;
    }
    
}

import java.util.Date;

public class Work {
    private int ids;
    private String infoCode;
    private Date startDate;
    private Date endDate;
    private String firm;
    private String depart;
    private int orders;
    private Info infoObj;//
    public int getIds() {
        return ids;
    }
    public void setIds(int ids) {
        this.ids = ids;
    }
    public String getInfoCode() {
        return infoCode;
    }
    public void setInfoCode(String infoCode) {
        this.infoCode = infoCode;
    }
    public Date getStartDate() {
        return startDate;
    }
    public void setStartDate(Date startDate) {
        this.startDate = startDate;
    }
    public Date getEndDate() {
        return endDate;
    }
    public void setEndDate(Date endDate) {
        this.endDate = endDate;
    }
    public String getFirm() {
        return firm;
    }
    public void setFirm(String firm) {
        this.firm = firm;
    }
    public String getDepart() {
        return depart;
    }
    public void setDepart(String depart) {
        this.depart = depart;
    }
    public int getOrders() {
        return orders;
    }
    public void setOrders(int orders) {
        this.orders = orders;
    }
    public Info getInfoObj() {
        return infoObj;
    }
    public void setInfoObj(Info infoObj) {
        this.infoObj = infoObj;
    }
    
}

2.给Info.hbm.xml配置

<set name="实体类中集合成员的变量名(works)">
<one-to-many class="Work">
<key column="infoCode">
</set>

<?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="maya.model">
    <class name="Info" table="Info">
        <id name="code" type="java.lang.String">
            <column name="code" />
            <generator class="assigned"></generator>
        </id>
        <property name="name" type="java.lang.String">
            <column name="name" />
        </property>
        <property name="sex" type="java.lang.Boolean">
            <column name="sex" />
        </property>
        <property name="birthday" type="java.util.Date">
            <column name="birthday" />
        </property>
        
                  
         <set name="works" table="Work" cascade="save-update" ><!-- name="实体类中集合成员的变量名(works)"-->
            <key>
                <column name="infoCode" />
            </key>
            <one-to-many class="Work" />
        </set>
    </class>
</hibernate-mapping>

四、多对多(Student和Course，中间表Score)
数据库中没有多对多的关系。只有程序里面有。数据库中是使用两个一对多和多对一来实现多对多的。典型的是：中间表
在程序中，如果中间表中没有其它信息，那就可以不用写中间表的实体类。
多对多，还可以用在：用户和角色的关系上。
1.给类加关联对象（集合对象）

import java.util.Date;
import java.util.Set;

public class Student {
    private String sno;
    private String sname;
    private String ssex;
    private Date sbirthday;
    private String sclass;
    private Set<Course> courses;
    public String getSno() {
        return sno;
    }
    public void setSno(String sno) {
        this.sno = sno;
    }
    public String getSname() {
        return sname;
    }
    public void setSname(String sname) {
        this.sname = sname;
    }
    public String getSsex() {
        return ssex;
    }
    public void setSsex(String ssex) {
        this.ssex = ssex;
    }
    public Date getSbirthday() {
        return sbirthday;
    }
    public void setSbirthday(Date sbirthday) {
        this.sbirthday = sbirthday;
    }
    public String getSclass() {
        return sclass;
    }
    public void setSclass(String sclass) {
        this.sclass = sclass;
    }
    public Set<Course> getCourses() {
        return courses;
    }
    public void setCourses(Set<Course> courses) {
        this.courses = courses;
    }
    
}

import java.util.Set;

public class Course {
    private String cno;
    private String cname;
    private String tno;
    private Set<Student> students;
    public String getCno() {
        return cno;
    }
    public void setCno(String cno) {
        this.cno = cno;
    }
    public String getCname() {
        return cname;
    }
    public void setCname(String cname) {
        this.cname = cname;
    }
    public String getTno() {
        return tno;
    }
    public void setTno(String tno) {
        this.tno = tno;
    }
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }
    
}

2.给hbm.xml配置

<set name="当前类的集合属性" table="中间表">
<key column="中间表中与此表对应的外键"></key>
<many-to-many class="另外多方对象的类名" column="另外多方对象在中间表中的外键"></many-to-many>
</set>

<?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">
<!-- Generated 2017-3-4 12:27:40 by Hibernate Tools 5.2.1.Final -->
<hibernate-mapping package="maya.model">
    <class name="Course" table="Course">
        <id name="cno" type="java.lang.String">
            <generator class="assigned"></generator>
        </id>
        <property name="cname"/>
        <property name="tno" />
          
          <set name="students" table="score">
              <key column="cno"></key>
              <many-to-many class="Student" column="sno"></many-to-many>
          </set>
    </class>
</hibernate-mapping>

<?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">
<!-- Generated 2017-3-4 12:27:40 by Hibernate Tools 5.2.1.Final -->
<hibernate-mapping package="com.itnba.maya.model">
    <class name="Student" table="Student">
        <id name="sno" type="java.lang.String">
            <generator class="assigned"></generator>
        </id>
        <property name="sname"/>
        <property name="ssex" />
          <property name="sbirthday" />
          <property name="sclass" column="class" />
          <set name="courses" table="score">
              <key column="sno"></key>
              <many-to-many class="Course" column="cno"></many-to-many>
          </set>
    </class>
</hibernate-mapping>