源自 imooc 中的学习
Hibernate中的单向一对多关联，这里举例 班级对学生 的单向一对多关联，即一个班级可以有多个学生。
那么在Hibernate中实体对象间如何体现出单向一对多的关联关系呢？如 班级对学生 这个例子而言，单向一对多关联，可以在“一方”添加 一个集合，如 Set集合，其中集合中保存“多”方的信息，如 班级类 中添加 一个集合，这里采用Set集合（保存的学生对象将不重复），Set集合中保存多个 学生对象。（意味着 一个班级有多个学生）。当然添加该属性后还要 在对象关系映射配置文件中添加相应的配置。具体查看下文的程序。
hibernate-one2many 项目结构如下：
首先创建一个工具类 HibernateUtil.java
package com.imooc.util;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
/**
* Created by DreamBoy on 2016/5/18.
*/
public class HibernateUtil {
private static SessionFactory sessionFactory;
private static Session session;
static {
//创建Configuration对象，读取hibernate.cfg.xml文件，完成初始化
Configuration config = new Configuration().configure();
/*StandardServiceRegistryBuilder ssrb = new StandardServiceRegistryBuilder()
.applySettings(config.getProperties());
StandardServiceRegistry ssr = ssrb.build();
sessionFactory = config.buildSessionFactory(ssr);
*/
//创建服务注册对象
ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry();
//创建会话工厂对象
sessionFactory = config.buildSessionFactory(serviceRegistry);
}
//获取SessionFactory
public static SessionFactory getSessionFactory() {
return sessionFactory;
}
//获得Session
public static Session getSession() {
session = sessionFactory.openSession();
return session;
}
//关闭Session
public static void closeSession(Session session) {
if(session != null) {
session.close();
}
}
}
创建 SQL 语句在数据库 hibernate2 中建表结构和关系（student.sql）：
CREATE TABLE grade (
gid INT PRIMARY KEY,
gname VARCHAR(20) NOT NULL,
gdesc VARCHAR(50)
);
CREATE TABLE student (
sid INT PRIMARY KEY,
sname VARCHAR(20) NOT NULL,
sex CHAR(2),
gid INT
);
ALTER TABLE student ADD CONSTRAINT fk_student_gid FOREIGN KEY (gid) REFERENCES grade(gid);
先在MySQL中创建好名为hibernate2的数据库，执行上述的SQL语句。
在 com.imooc.entity 包中创建两个实体：
Student.java
package com.imooc.entity;
import java.io.Serializable;
/**
* Created by DreamBoy on 2016/5/18.
*/
public class Student implements Serializable {
private int sid;
private String sname;
private String sex;
public Student() {
}
public Student(String sname, String sex) {
this.sname = sname;
this.sex = sex;
}
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;
}
}
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.imooc.entity.Student" table="student">
<id name="sid" column="sid" type="java.lang.Integer">
<generator class="increment"/>
</id>
<property name="sname" type="java.lang.String">
<column name="sname" length="20" not-null="true"/>
</property>
<property name="sex">
<column name="sex"/>
</property>
</class>
</hibernate-mapping>
Grade.java
package com.imooc.entity;
import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;
/**
* Created by DreamBoy on 2016/5/18.
*/
public class Grade implements Serializable {
private int gid;
private String gname;
private String gdesc;
//在一方定义一个多方的集合
private Set<Student> students = new HashSet<>();
public Grade() {
}
public Grade(int gid, String gname, String gdesc, Set<Student> students) {
this.gid = gid;
this.gname = gname;
this.gdesc = gdesc;
this.students = students;
}
public Grade(String gname, String gdesc) {
this.gname = gname;
this.gdesc = gdesc;
}
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;
}
}
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.imooc.entity.Grade" table="grade">
<id name="gid" column="gid" type="java.lang.Integer">
<generator class="increment"/>
</id>
<property name="gname" type="java.lang.String">
<column name="gname" length="20" not-null="true"/>
</property>
<property name="gdesc">
<column name="gdesc"/>
</property>
<!-- 配置单向的一对多关联关系 -->
<set name="students" table="student">
<!-- 指定关联的外键列 -->
<key column="gid"></key>
<one-to-many class="com.imooc.entity.Student"/>
</set>
</class>
</hibernate-mapping>
项目中Hibernate的配置文件 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.username">root</property>
<property name="connection.password"></property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<!-- 下面的 hibernate 指的是 数据库名，即所用的数据库 -->
<property name="connection.url">jdbc:mysql:///hibernate2?useUnicode=true&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">create</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"/>
<!-- DB schema will be updated if needed -->
<!-- <property name="hbm2ddl.auto">update</property> -->
</session-factory>
</hibernate-configuration>
在 com.imooc.test 包中添加测试类 test.java（测试 增删查改操作）
package com.imooc.test;
/**
* Created by DreamBoy on 2016/5/18.
*/
import com.imooc.entity.Grade;
import com.imooc.entity.Student;
import com.imooc.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.Transaction;
import java.util.Set;
/**
* 单向 一对多关系（班级--》学生）
* 建立关联关系后，可以方便地从一个对象导航到另一个对象
* 注意关联的方向：这里只是 1到多；只能查询到一个班级所具有的学生，但不能查询一个学生属于哪个班级
*/
public class Test {
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 stu1 = new Student("张三", "男");
Student stu2 = new Student("木女神", "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(stu1);
g.getStudents().add(stu2);
Session session = HibernateUtil.getSession();
Transaction tx = session.beginTransaction();
session.save(g);
session.save(stu1);
session.save(stu2);
tx.commit();
HibernateUtil.closeSession(session);
}
//查询班级中包含的学生信息
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 stu : students) {
System.out.println(stu.getSname() + ", " + stu.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 tx = session.beginTransaction();
Student stu = (Student) session.get(Student.class, 1);
g.getStudents().add(stu);
session.save(g);
tx.commit();
HibernateUtil.closeSession(session);
}
//删除学生信息
public static void delete() {
Session session = HibernateUtil.getSession();
Transaction tx = session.beginTransaction();
Student stu = (Student) session.get(Student.class, 2);
session.delete(stu);
tx.commit();
HibernateUtil.closeSession(session);
}
}<span style="font-size:18px;">
</span>
总结：
本文内容由网友自发贡献，版权归原作者所有，本站不承担相应法律责任。如您发现有涉嫌抄袭侵权的内容，请联系:hwhale#tublm.com(使用前将#替换为@)
