提示;本文的操作是在入门一和二上面的
hibernate对映射类型
在开始是学习的时候,我以为一对多和多对一是同样的,后来发现不是这样,比如说班主任和班级里的学生,一个班级可能有多个学生,但是只有一个班主任,学生和班主任的关系是多对一,班主任和学生的关系是一对多,两者之间的区别在于它们的指向性
- 一对一(one-to-one )
- 一对多(ont-to-many)
- 多对一(many-to-one)
- 多对多(many-to-many)
一对多(ont-to-many)和多对一(many-to-one)最常见和做常用的映射类型
一对多(ont-to-many)
1、创建班级表和学生表,通过外键进行关联(
alter table Students add constraint fk_students_gid foreign key (gid) references grade(gid);
)
2、创建班级表和学生表的实体类(属性/默认构造器、带参构造器、get和set方法,toString方法)
private int gid;
private String gname;
private String gdesc;
// 在一方定义一个多方的集合
private Set<Students> students = new HashSet<Students>();
//其他。。。略 }
public class Students {
private int sid;
private String sname;
private String gender;
private Date birthday;
//private String address;
private Blob pictur;
private Address address;
//其他。。。略
}
3、创建Grade.hbm.xml(我的Stuendt.hbm.xml已经配置好了)
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="com.hibernate.entity.Grade" table="grade">
<id name="gid" column="gid" type="int">
<generator class="increment"></generator>
</id>
<property name="gname" type="java.lang.String">
<column name="gname"/>
</property>
<property name="gdesc" type="java.lang.String">
<column name="gdesc"/>
</property>
<!--单项的一对多的关系-->
<set name="students" table="Students">
<!--指定的关联外键列-->
<key column="gid"></key>
<one-to-many class="com.hibernate.entity.Students"/>
</set>
</class>
</hibernate-mapping>
4、在cfg.xml中配置映射文件路径
<!--指定映射文件的路径-->
<mapping resource="com/hibernate/entity/Grade.hbm.xml"/>
<mapping resource="com/hibernate/entity/Students.hbm.xml"/>
5、创建HibernateUtil类
public class HibernadUtil {
private static SessionFactory sessionFactory;
private static Session session;
static {
StandardServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().configure().build();
sessionFactory = new MetadataSources(serviceRegistry).buildMetadata().buildSessionFactory();
}
//获取SessionFacory
public static SessionFactory getSessionFacory() {
return sessionFactory;
}
//获取session
public static Session getSession() {
return sessionFactory.openSession();
}
//关闭session
public static void closeSession(Session session) {
if (null != session) {
session.close();
}
}
6、进行测试
public class TestStudentsAndGrade {
public static void main(String[] args) {
//add();
//findStudensByGrade();
//update();
deleteStudentsBySid();
}
//给表中插入数据
public static void add() {
Grade grade = new Grade("道班", "道一");
Students s = new Students("道一", "男", new Date());
Students s2 = new Students("阴阳", "男", new Date());
Students s3 = new Students("三生", "女", new Date());
System.out.println(grade.getStudents().add(s));
//如果希望在学生表中添加对应道班级编号,需要在班级中添加学生,建立关联关系
grade.getStudents().add(s);
grade.getStudents().add(s2);
grade.getStudents().add(s3);
Session session = HibernadUtil.getSession();
//开启事务
Transaction transaction = session.beginTransaction();
session.save(grade);
session.save(s);
session.save(s2);
session.save(s3);
transaction.commit();
HibernadUtil.closeSession(session);
}
//查询班级中包含的学生
public static void findStudensByGrade() {
//获取session
Session session = HibernadUtil.getSession();
//获取班级信息
Grade grade = session.get(Grade.class, 1);
//输出班级信息
System.out.println(grade.getGname() + "," + grade.getGdesc());
//找出班级信息,然后找出班级学生信息
Set<Students> students = grade.getStudents();
for (Students stu:students
) {
System.out.println(stu);
}
}
//修改学生信息
public static void update(){
Grade grade =new Grade("道","道二");
Session session = HibernadUtil.getSession();
Transaction transaction = session.beginTransaction();
//修改学生所在班级信息(grade班级信息为自增长,别一不小心玩过头了)
Students students = session.get(Students.class, 2);
grade.getStudents().add(students);
session.save(grade);
transaction.commit();
HibernadUtil.closeSession(session);
}
//从班级中删除学生信息
public static void deleteStudentsBySid(){
Session session = HibernadUtil.getSession();
Transaction transaction = session.beginTransaction();
Students students = session.get(Students.class,9);
session.delete(students);
transaction.commit();
HibernadUtil.closeSession(session);
}
}
7、一对多中的set元素属性
多对一(many-to-one)
描述:多对一多关系和关系数据库中外键的参照关系最匹配,即在已方的表中的一个外键参考另一个表中的主键
实现:通过多方持有一方的引用实现,需要在“多”的一端使用<many-to-one>配置
1、在多方定义一个一方的引用,如(private Grade grade;)(属性/默认构造器、带参构造器、get和set方法,toString方法)
public class Students {
private int sid;
private String sname;
private String gender;
private Date birthday;
//private String address;
private Blob pictur;
private Address address;
private int gid;
//在多方定义一个一方的引用
private Grade grade;
//其他。。。略
}
2、增加配置文件属性(如果有上一步的话,则需要修改相应的属性,删掉grade.hbm.xml中的set元素,在Stuent.hbm.xml中添加<many-to-one>属性)
友情提示:相比于one-to-many,many-to-one是直接写的,不用添加set属性
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="com.hibernate.entity.Students" table="Students">
<!--id表示表的主键-->
<id name="sid" type="int">
<column name="SID"/>
<!--表的生成策略-->
<generator class="native"/>
</id>
<property name="sname" type="java.lang.String">
<column name="SNAME"/>
</property>
<property name="gender" type="java.lang.String">
<column name="GENDER"/>
</property>
<property name="birthday" type="time">
<column name="BIRTHDAY"/>
</property>
<property name="pictur" type="java.sql.Blob">
<column name="PICTUR"/>
</property>
<component name="address" class="com.hibernate.entity.Address" >
<property name="postcode" column="POSTCODE" type="java.lang.String"/>
<property name="phone" column="PHONE" type="java.lang.String"/>
<property name="address" column="ADDRESS" type="java.lang.String"/>
</component>
<many-to-one name="grade" class="com.hibernate.entity.Grade" column="gid"></many-to-one>
</class>
3、进行测试
public class TestStudent {
public static void main(String[] args) {
save();
}
//添加学生和班级
public static void save() {
Grade grade = new Grade("道班", "道一");
Students s = new Students("道一", "男", new Date());
Students s2 = new Students("阴阳", "男", new Date());
Students s3 = new Students("三生", "女", new Date());
//设置关系
s.setGrade(grade);
s2.setGrade(grade);
s3.setGrade(grade);
Session session = HibernadUtil.getSession();
Transaction transaction = session.beginTransaction();
session.save(grade);
session.save(s);
session.save(s2);
session.save(s3);
//提交事务
transaction.commit();
//关闭事务
HibernadUtil.closeSession(session);
}
}
同时配置 一对多(ont-to-many)和多对一(many-to-one)的关系(双向关联关系,可以双向查询,非常方便)
通过配置两个对应的hbm.xml实现(实体类也需要相应的配置)
一对多
<set name="students" table="Students">
<!--指定的关联外键列-->
<key column="gid"></key>
<one-to-many class="com.hibernate.entity.Students"/>
</set>
多对一
<many-to-one name="grade" class="com.hibernate.entity.Grade" column="gid"></many-to-one>
代码关系建立(关系建立之后,执行之后会多出update语句,对性能会有影响,所以需要通过inverse属性去掉这两句话)
//设置关系(双向关系)
grade.getStudents().add(s);
grade.getStudents().add(s2);
grade.getStudents().add(s3);
s.setGrade(grade);
s2.setGrade(grade);
s3.setGrade(grade);
inverse属性的用法
- <set>节点的inverse属性指定关联关系的控制方向,默认由one方来维护
- 关联关系中,inverse=“false”,则为主动方,由主动方负责维护关联关系
- 在一对多关联中,只能设置one方的inverse为“true”,这将有助于性能改善
<!--单项的一对多的关系,inverse设置为true,由多方维护关联关系,-->
<set name="students" table="Students" inverse="true">
<!--指定的关联外间列-->
<key column="gid"></key>
<one-to-many class="com.hibernate.entity.Students"/>
</set>
cascade属性(级联操作)
<!--单项的一对多的关系,inverse设置为true,由多方维护关联关系
当进行保存和更新时级联操作所关联的对象-->
<set name="students" table="Students" inverse="true">
<!--指定的关联外间列-->
<key column="gid"></key>
<one-to-many class="com.hibernate.entity.Students"/>
</set>
相比于之前,只需要写(session.save(grade)就可以了
session.save(grade);//级联操作,班级对学生
学生对班级的级联操作配置方案
<many-to-one name="grade" class="com.hibernate.entity.Grade" column="gid" cascade="all"></many-to-one>
代码端则为
session.save(s);
session.save(s2);
session.save(s3);
//级联操作,学生对班级
测试(查询学生所在班级信息)
public static void findGradeByStudent() {
Session session = HibernadUtil.getSession();
//一定要注意你数据里的数据,是否有这个学生id,否则会报错
Students students = session.get(Students.class, 31);
System.out.println(students.getSname()+","+students.getGender()+","+students.getBirthday());
Grade grade=students.getGrade();
System.out.println(grade.getGname()+","+grade.getGdesc());
HibernadUtil.closeSession(session);
}