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
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&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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