hibernate_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

 1 package com.imooc.entity; 2  3 import java.io.Serializable; 4  5 public class Student implements Serializable { 6  7     private int sid; 8     private String sname; 9     private String sex;10     // 在多方定义一个一方的引用11     private Grade grade;12     13     public int getSid() {14         return sid;15     }16     public void setSid(int sid) {17         this.sid = sid;18     }19     public String getSname() {20         return sname;21     }22     public void setSname(String sname) {23         this.sname = sname;24     }25     public String getSex() {26         return sex;27     }28     public void setSex(String sex) {29         this.sex = sex;30     }31     public Grade getGrade() {32         return grade;33     }34     public void setGrade(Grade grade) {35         this.grade = grade;36     }37     38     public Student() {39         super();40     }41     42     public Student(String sname, String sex) {43         super();44         this.sname = sname;45         this.sex = sex;46     }47     48 }

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创建班级类Grade

 1 package com.imooc.entity; 2  3 import java.io.Serializable; 4 import java.util.HashSet; 5 import java.util.Set; 6  7  8 public class Grade implements Serializable { 9 10     private int gid;11     private String gname;12     private String gdesc;13     private Set<Student> students = new HashSet<Student>();14     15     public int getGid() {16         return gid;17     }18     public void setGid(int gid) {19         this.gid = gid;20     }21     public String getGname() {22         return gname;23     }24     public void setGname(String gname) {25         this.gname = gname;26     }27     public String getGdesc() {28         return gdesc;29     }30     public void setGdesc(String gdesc) {31         this.gdesc = gdesc;32     }33     public Set<Student> getStudents() {34         return students;35     }36     public void setStudents(Set<Student> students) {37         this.students = students;38     }39     40     public Grade() {41         super();42     }43     44     public Grade(int gid, String gname, String gdesc) {45         super();46         this.gid = gid;47         this.gname = gname;48         this.gdesc = gdesc;49     }50     51     public Grade(String gname, String gdesc) {52         super();53         this.gname = gname;54         this.gdesc = gdesc;55     }56 }

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创建Student类的映射文件Student.hbm.xml

 1 <?xml version="1.0"?> 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 4 <!-- Generated 2017-6-1 14:49:09 by Hibernate Tools 3.5.0.Final --> 5 <hibernate-mapping> 6     <class name="com.imooc.entity.Student" table="STUDENT"> 7         <id name="sid" type="int"> 8             <column name="SID" /> 9             <generator class="increment" />10         </id>11         <property name="sname" type="java.lang.String">12             <column name="SNAME" />13         </property>14         <property name="sex" type="java.lang.String">15             <column name="SEX" />16         </property>17     </class>18 </hibernate-mapping>

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创建Grade类的映射文件Grade.hbm.xml

 1 <?xml version="1.0"?> 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 4 <!-- Generated 2017-6-1 14:49:09 by Hibernate Tools 3.5.0.Final --> 5 <hibernate-mapping> 6     <class name="com.imooc.entity.Grade" table="GRADE"> 7         <id name="gid" type="int"> 8             <column name="GID" /> 9             <generator class="increment" />10         </id>11         <property name="gname" type="java.lang.String">12             <column name="GNAME" length="20" not-null="true" />13         </property>14         <property name="gdesc" type="java.lang.String">15             <column name="GDESC" />16         </property>17         <!-- 指定关联的外键列 -->18         <set name="students" table="STUDENT">19             <key>20                 <column name="GID" />21             </key>22             <one-to-many class="com.imooc.entity.Student" />23         </set>24     </class>25 </hibernate-mapping>

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创建hibernate的配置文件

 1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-configuration PUBLIC 3         "-//Hibernate/Hibernate Configuration DTD 3.0//EN" 4         "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"> 5 <hibernate-configuration> 6     <session-factory> 7         <property name="connection.username">root</property> 8         <property name="connection.password">root</property> 9         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>10         <property name="connection.url">11             <![CDATA[12                 jdbc:mysql://localhost:3306/hibernate?useUnicode=true&amp;characterEncoding=UTF-813             ]]>14         </property>15         <property name="dialect">org.hibernate.dialect.MySQLDialect</property>16         <property name="show_sql">true</property>17         <property name="format_sql">true</property>18         <property name="hbm2ddl.auto">update</property>19         20         <!-- 指定映射文件的路径 -->21         <mapping resource="com/imooc/entity/Grade.hbm.xml" />22         <mapping resource="com/imooc/entity/Student.hbm.xml" />23     </session-factory>24 </hibernate-configuration>

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写一个增删改查的测试文件

 1 package com.imooc.test; 2  3 import java.util.Set; 4  5 import org.hibernate.Session; 6 import org.hibernate.Transaction; 7  8 import com.imooc.entity.Grade; 9 import com.imooc.entity.Student;10 import com.imooc.util.HibernateUtil;11 12 /*13  * 单向一对多关系关系（班级--->学生）14  * 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15  * 注意关联的方向16  */17 public class Test01 {18 19     public static void main(String[] args) {20         //add();21         //findStudentsByGrade();22         //update();23         delete();24     }25     26     //将学生添加到班级27     public static void add() {28         Grade g = new Grade("Java一班", "Java软件开发一班");29         Student s1 = new Student("杨康", "男");30         Student s2 = new Student("穆念慈", "女");31         32         //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33         g.getStudents().add(s1);34         g.getStudents().add(s2);35         36         Session session = HibernateUtil.getSession();37         Transaction tr = session.beginTransaction();38         session.save(g);39         session.save(s1);40         session.save(s2);41         tr.commit();42         HibernateUtil.closeSession(session);43     }44     45     //查询班级中包含的学生46     public static void findStudentsByGrade() {47         Session session = HibernateUtil.getSession();48         Grade grade = (Grade) session.get(Grade.class, 1);49         System.out.println( grade.getGname() + "," + grade.getGdesc() );50         51         Set<Student> students = grade.getStudents();52         for(Student s : students) {53             System.out.println( s.getSname() + "," + s.getSex() );54         }55     }56     57     //修改学生信息58     public static void update() {59         Grade g=new Grade("Java二班", "Java软件开发二班");60         Session session = HibernateUtil.getSession();61         Transaction tr = session.beginTransaction();62         Student s = (Student) session.get(Student.class, 1);63         g.getStudents().add(s);64         session.save(g);65         tr.commit();66         HibernateUtil.closeSession(session);67     }68     69     //删除学生信息70     public static void delete() {71         Session session = HibernateUtil.getSession();72         Transaction tr = session.beginTransaction();73         Student s = (Student) session.get(Student.class, 2);74         session.delete(s);75         tr.commit();76         HibernateUtil.closeSession(session);77     }78 }

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