hibernate_08_关联映射_一对多

hibernate的映射关系

一对多、多对一、一对一、多对多。

常用的是一对多和多对一。

在数据库中可以通过添加主外键的关联，表现一对多的关系；在hibernate中通过在一方持有多方的集合实现，即在“一”的一端中使用<set>元素表示持有“多”的一端对象。

下面实现一个增删改查的“一对多”demo：一个班级对应多个学生。

首先创建学生类Student

 package com.imooc.entity;
 
 import java.io.Serializable;
 
 public class Student implements Serializable {
 
     private int sid;
     private String sname;
     private String sex;
     // 在多方定义一个一方的引用
     private Grade grade;
 
     public int getSid() {
         return sid;
     }
     public void setSid(int sid) {
         this.sid = sid;
     }
     public String getSname() {
         return sname;
     }
     public void setSname(String sname) {
         this.sname = sname;
     }
     public String getSex() {
         return sex;
     }
     public void setSex(String sex) {
         this.sex = sex;
     }
     public Grade getGrade() {
         return grade;
     }
     public void setGrade(Grade grade) {
         this.grade = grade;
     }
 
     public Student() {
         super();
     }
 
     public Student(String sname, String sex) {
         super();
         this.sname = sname;
         this.sex = sex;
     }
 
 }

创建班级类Grade

 package com.imooc.entity;
 
 import java.io.Serializable;
 import java.util.HashSet;
 import java.util.Set;
 
 public class Grade implements Serializable {
 
     private int gid;
     private String gname;
     private String gdesc;
     private Set<Student> students = new HashSet<Student>();
 
     public int getGid() {
         return gid;
     }
     public void setGid(int gid) {
         this.gid = gid;
     }
     public String getGname() {
         return gname;
     }
     public void setGname(String gname) {
         this.gname = gname;
     }
     public String getGdesc() {
         return gdesc;
     }
     public void setGdesc(String gdesc) {
         this.gdesc = gdesc;
     }
     public Set<Student> getStudents() {
         return students;
     }
     public void setStudents(Set<Student> students) {
         this.students = students;
     }
 
     public Grade() {
         super();
     }
 
     public Grade(int gid, String gname, String gdesc) {
         super();
         this.gid = gid;
         this.gname = gname;
         this.gdesc = gdesc;
     }
 
     public Grade(String gname, String gdesc) {
         super();
         this.gname = gname;
         this.gdesc = gdesc;
     }
 }

创建Student类的映射文件Student.hbm.xml

 <?xml version="1.0"?>
 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 <!-- Generated 2017-6-1 14:49:09 by Hibernate Tools 3.5.0.Final -->
 <hibernate-mapping>
     <class name="com.imooc.entity.Student" table="STUDENT">
         <id name="sid" type="int">
             <column name="SID" />
             <generator class="increment" />
         </id>
         <property name="sname" type="java.lang.String">
             <column name="SNAME" />
         </property>
         <property name="sex" type="java.lang.String">
             <column name="SEX" />
         </property>
     </class>
 </hibernate-mapping>

创建Grade类的映射文件Grade.hbm.xml

 <?xml version="1.0"?>
 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 <!-- Generated 2017-6-1 14:49:09 by Hibernate Tools 3.5.0.Final -->
 <hibernate-mapping>
     <class name="com.imooc.entity.Grade" table="GRADE">
         <id name="gid" type="int">
             <column name="GID" />
             <generator class="increment" />
         </id>
         <property name="gname" type="java.lang.String">
             <column name="GNAME" length="20" not-null="true" />
         </property>
         <property name="gdesc" type="java.lang.String">
             <column name="GDESC" />
         </property>
         <!-- 指定关联的外键列 -->
         <set name="students" table="STUDENT">
             <key>
                 <column name="GID" />
             </key>
             <one-to-many class="com.imooc.entity.Student" />
         </set>
     </class>
 </hibernate-mapping>

创建hibernate的配置文件

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE hibernate-configuration PUBLIC
         "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
         "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
 <hibernate-configuration>
     <session-factory>
         <property name="connection.username">root</property>
         <property name="connection.password">root</property>
         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
         <property name="connection.url">
             <![CDATA[
                 jdbc:mysql://localhost:3306/hibernate?useUnicode=true&amp;characterEncoding=UTF-8
             ]]>
         </property>
         <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
         <property name="show_sql">true</property>
         <property name="format_sql">true</property>
         <property name="hbm2ddl.auto">update</property>
 
         <!-- 指定映射文件的路径 -->
         <mapping resource="com/imooc/entity/Grade.hbm.xml" />
         <mapping resource="com/imooc/entity/Student.hbm.xml" />
     </session-factory>
 </hibernate-configuration>

写一个增删改查的测试文件

 package com.imooc.test;
 
 import java.util.Set;
 
 import org.hibernate.Session;
 import org.hibernate.Transaction;
 
 import com.imooc.entity.Grade;
 import com.imooc.entity.Student;
 import com.imooc.util.HibernateUtil;
 
 /*
  * 单向一对多关系关系（班级--->学生）
  * 建立关联关系后，可以方便的从一个对象导航到另一个对象
  * 注意关联的方向
  */
 public class Test01 {
 
     public static void main(String[] args) {
         //add();
         //findStudentsByGrade();
         //update();
         delete();
     }
 
     //将学生添加到班级
     public static void add() {
         Grade g = new Grade("Java一班", "Java软件开发一班");
         Student s1 = new Student("杨康", "男");
         Student s2 = new Student("穆念慈", "女");
 
         //如果希望在学生表中添加对应的班级编号，需要在班级中添加学生，建立关联关系
         g.getStudents().add(s1);
         g.getStudents().add(s2);
 
         Session session = HibernateUtil.getSession();
         Transaction tr = session.beginTransaction();
         session.save(g);
         session.save(s1);
         session.save(s2);
         tr.commit();
         HibernateUtil.closeSession(session);
     }
 
     //查询班级中包含的学生
     public static void findStudentsByGrade() {
         Session session = HibernateUtil.getSession();
         Grade grade = (Grade) session.get(Grade.class, 1);
         System.out.println( grade.getGname() + "," + grade.getGdesc() );
 
         Set<Student> students = grade.getStudents();
         for(Student s : students) {
             System.out.println( s.getSname() + "," + s.getSex() );
         }
     }
 
     //修改学生信息
     public static void update() {
         Grade g=new Grade("Java二班", "Java软件开发二班");
         Session session = HibernateUtil.getSession();
         Transaction tr = session.beginTransaction();
         Student s = (Student) session.get(Student.class, 1);
         g.getStudents().add(s);
         session.save(g);
         tr.commit();
         HibernateUtil.closeSession(session);
     }
 
     //删除学生信息
     public static void delete() {
         Session session = HibernateUtil.getSession();
         Transaction tr = session.beginTransaction();
         Student s = (Student) session.get(Student.class, 2);
         session.delete(s);
         tr.commit();
         HibernateUtil.closeSession(session);
     }
 }