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Hibernate关系映射之一对多


关系映射
一对一(one-to-one)
一对多(one-to-many)
多对一(many-to-one)
多对多(many-to-many)


一对多:通过"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"的一端的对象

1 、插入数据 (班级->学生)
**Grade.java:**

package com.lxj.entity;

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

public class Grade {
private int gid;
private String gname;
private String gdesc;

public Grade(){}

public Grade(String gname, String gdesc) {
this.gname = gname;
this.gdesc = gdesc;
}

private Set<Student> students = new HashSet<Student>();

public void setStudents(Set<Student> students){
this.students = students;
}

public Set<Student> getStudents(){
return students;
}

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 void setGdesc(String gdesc) {
this.gdesc = gdesc;
}
public String getGdesc() {
return gdesc;
}



@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;

Grade grade = (Grade) o;

if (gid != grade.gid) return false;
if (gname != null ? !gname.equals(grade.gname) : grade.gname != null) return false;
if (gdesc != null ? !gdesc.equals(grade.gdesc) : grade.gdesc != null) return false;

return true;
}

@Override
public int hashCode() {
int result = gid;
result = 31 * result + (gname != null ? gname.hashCode() : 0);
result = 31 * result + (gdesc != null ? gdesc.hashCode() : 0);
return result;
}
}


**Grade.hbm.xml:**
<?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>

<class name="com.lxj.entity.Grade" table="grade" schema="hibernate_study">
<id name="gid" column="gid" type="java.lang.Integer">
<generator class="increment"></generator>
</id>
<property name="gname" column="gname" type="java.lang.String" />
<property name="gdesc" column="gdesc" type="java.lang.String"/>
<!--配置单向的一对多关联关系-->
<set name="students" table="student">
<!--指定关联的外键-->
<key column="gid"></key>
<one-to-many class="com.lxj.entity.Student"></one-to-many>
</set>

</class>
</hibernate-mapping>


**Student.java**

package com.lxj.entity;

public class Student {
private int sid;

public Student(String sname, int sex) {
this.sname = sname;
this.sex = sex;
}

private String sname;
private int sex;
private int gid;

public Student(){

}



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 int getSex() {
return sex;
}

public void setSex(int sex) {
this.sex = sex;
}

public int getGid() {
return gid;
}

public void setGid(int gid) {
this.gid = gid;
}

@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;

Student student = (Student) o;

if (sid != student.sid) return false;
if (sex != student.sex) return false;
if (gid != student.gid) return false;
if (sname != null ? !sname.equals(student.sname) : student.sname != null) return false;

return true;
}

@Override
public int hashCode() {
int result = sid;
result = 31 * result + (sname != null ? sname.hashCode() : 0);
result = 31 * result + sex;
result = 31 * result + gid;
return result;
}
}


**Student.hbm.xml:**
<?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>

<class name="com.lxj.entity.Student" table="student" schema="hibernate_study">
<id name="sid" column="sid" type="java.lang.Integer">
<generator class="increment"></generator>
</id>
<property name="sname" column="sname"/>
<property name="sex" column="sex"/>
<property name="gid" column="gid"/>
</class>
</hibernate-mapping>

**Test.java**

import com.lxj.entity.Grade;
import com.lxj.entity.Student;
import com.lxj.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class Test {

@org.junit.Test
public void testAddGrade(){

Grade g = new Grade("java-1班","最牛逼的班级");
Student student1 = new Student("张三",0);
Student student2 = new Student("女神",1);

//设置班级、学生关联关系
g.getStudents().add(student1);
g.getStudents().add(student2);

Session session = HibernateUtil.getSession();
Transaction transaction = session.beginTransaction();
session.save(g);
session.save(student1);
session.save(student2);

transaction.commit();
HibernateUtil.closeSession();
}
}


**HibernateUtil.java:**
package com.lxj.util;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {
private static SessionFactory sessionFactory;
private static Session session;
private static Transaction transaction;

static {
Configuration configuration = new Configuration().configure();
sessionFactory = configuration.buildSessionFactory();
}

/**
* 获取session工厂
* @return
*/
public static SessionFactory getSessionFactory(){
return sessionFactory;
}

/**
* 获取session
* @return
*/
public static Session getSession(){
session = sessionFactory.openSession();
return session;
}

/**
* 关闭session
*/
public static void closeSession(){
if(session != null){
session.close();
}
}
}


**hibernate.cfg.xml:**
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>

<session-factory>
<property name="connection.url">jdbc:mysql://localhost:3306/hibernate_study</property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">root</property>

<!-- 配置hibernate的基本信息 -->

<!-- 配置数据库方言dialect -->
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="show_sql">true</property>
<property name="format_sql">true</property>
<property name="hibernate.current_session_context_class">thread</property>
<!-- 指定自动生成数据表的策略 -->
<property name="hbm2ddl.auto">update</property>


<mapping resource="com/lxj/entity/Grade.hbm.xml"/>
<mapping resource="com/lxj/entity/Student.hbm.xml"/>
</session-factory>
</hibernate-configuration>


2、查询数据(单向从班级查询学生)

/**
* 通过班级查询当前班级下的学生
*/
@org.junit.Test
public void getStudentsByGrade(){

Session session = HibernateUtil.getSession();
int gid = 1;
Grade grade = (Grade) session.get(Grade.class,gid);
System.out.println(grade.getGname());

//查询该班级下的学生
Set<Student> students = grade.getStudents();
Iterator<Student> iter = students.iterator();
while (iter.hasNext()){
Student student = (Student)iter.next();
System.out.println(student.getSname());
}
}


3、修改一个学生信息
@org.junit.Test
public void updateStudentGrade(){
Grade g = new Grade("java-3班","一般般");
Session session = HibernateUtil.getSession();
//这个人之前属于java-1班,现在修改到3班
Student stu = (Student)session.get(Student.class,1);
Transaction transaction = session.beginTransaction();
g.getStudents().add(stu);
session.save(g);
transaction.commit();
HibernateUtil.closeSession();
}


4、删除学生信息
@org.junit.Test
public void deleteStudent(){

Session session = HibernateUtil.getSession();
Transaction transaction = session.beginTransaction();
Student student = session.get(Student.class,1);
session.delete(student);
transaction.commit();
HibernateUtil.closeSession();
}


5、Entity.hbm.xml中的set标签常用属性:
<set name="students" table="student">
<!--指定关联的外键-->
<key column="gid"></key>
<one-to-many class="com.lxj.entity.Student"></one-to-many>
</set>

name:映射类属性的名称(必须)
table:表名(非必须)
lazy:指定关联对象是否使用延迟加载(非必须,proxy)
inverse:标识双象关联中被动的一方(非必须,false)


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