Hibernate学习之双向一对多映射（双向多对一映射）
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1.双向映射与单向映射
- 一对多单向映射：由一方（教室）维护映射关系，可以通过教室查询该教室下的学生信息，但是不能通过学生查询该学生所在教室信息；
删除教室时可以级联删除该教室下的所有学生信息。
- 多对一单向映射：由多方（学生）维护映射关系，可以通过学生查询该学生所在的教室信息，但是不能通过教室查询该教室下的所有学生信息；
并且删除教室的时候不能级联删除该教室下的所有学生信息。
- 一对多（多对一）双向映射：即在一方配置一对多的单向映射，在多方配置多对一的单向映射，因此默认双方都会对映射关系进行维护，所以可通过mappedBy（注解方式）
或inverse（xml方式）来指定多方不参与映射关系维护。双向映射既可以通过学生查询学生所在教室的信息，也可以通过教室查询该教室下的
所有学生信息；并且删除教室的时候可以级联删除该教室下的所有学生信息。故实际开发中多实用双向映射。
2.mappedBy和inverse
mappedBy：注解方式下使用，是在@OneToOne、@OneToMany、@ManyToMany上才有的属性，ManyToOne不存在该属性；在一的一方配置mappedBy将维护权
交由多的一方（因为外键在多的一方，所以多的一方维护起来更为方便）；当使用mappedBy时不能再使用@JoinColumn
inverse：xml方式下使用，inverse="false"表示由自身维护映射关系，因此在一方将其设置为true，将维护权交给多的一方
3.@JoinColumn：此注解的作用是指定外键，避免生成第三种关系维护表
4.fetch=FetchType.*：数据加载方式
fetch=FetchType.EAGER：积极加载，加载数据时，将与该数据关联的数据也加载到内存中；一般在多方使用，因为关联的一方数据较少
fetch=FetchType.LAZY：延迟加载，加载数据时，不加载关联数据，当使用到关联数据时再从数据库中查询；一般在一方使用，因为关联的多方数据较多
实例：
1.项目结构
2.pom.xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>org.hibernate</groupId> <artifactId>Hibernate-TwoWayOneToMany</artifactId> <version>0.0.1-SNAPSHOT</version> <packaging>jar</packaging> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> </properties> <dependencies> <!-- Junit --> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.12</version> <scope>test</scope> </dependency> <!-- hibernate --> <dependency> <groupId>org.hibernate</groupId> <artifactId>hibernate-core</artifactId> <version>5.1.7.Final</version> </dependency> <!-- MySQL --> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.42</version> </dependency> </dependencies> </project>
3.Student.java
package org.hibernate.entity;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import org.hibernate.annotations.Cascade;
import org.hibernate.annotations.CascadeType;
import org.hibernate.annotations.GenericGenerator;
@Entity
public class Student {
@Id
@GeneratedValue(generator="sid")
@GenericGenerator(name="sid", strategy="native")
private int sid;// 学号
@Column(length=100)
private String sname;// 姓名
@Column(length=2)
private String sex;// 性别
@ManyToOne(fetch=FetchType.EAGER)
@Cascade(value=CascadeType.SAVE_UPDATE)
@JoinColumn(name="gid")
private Grade grade;
public Student() {
}
public Student(String sname, String sex) {
this.sname = sname;
this.sex = sex;
}
public Student(String sname, String sex, Grade grade) {
this.sname = sname;
this.sex = sex;
this.grade = grade;
}
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;
}
public Grade getGrade() {
return grade;
}
public void setGrade(Grade grade) {
this.grade = grade;
}
}
4.Grade.java
package org.hibernate.entity;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import org.hibernate.annotations.GenericGenerator;
@Entity
public class Grade {
@Id
@GeneratedValue(generator="gid")
@GenericGenerator(name="gid", strategy="native")
private int gid;// 班号
@Column(length=100)
private String gname;// 班名
private String gdesc;// 班级描述
//mappedBy表示grade表不再参与关系维护，将主控方交给学生类;配置了mappedBy就不能再@JoinColumn
@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY, mappedBy="grade")
private Set<Student> students = new HashSet<Student>();
public Grade() {
}
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;
}
}
5.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="org.hibernate.entity.Student" table="student">
<id name="sid" type="java.lang.Integer">
<column name="sid"/>
<generator class="native"/>
</id>
<property name="sname" type="java.lang.String">
<column name="sname"/>
</property>
<property name="sex" type="java.lang.String">
<column name="sex"/>
</property>
<!-- 配置多对一映射关系 -->
<many-to-one name="grade" column="gid" cascade="save-update" class="org.hibernate.entity.Grade"/>
</class>
</hibernate-mapping>
6.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="org.hibernate.entity.Grade" table="grade">
<id name="gid" type="java.lang.Integer">
<column name="gid"/>
<generator class="native"/>
</id>
<property name="gname" type="java.lang.String">
<column name="gname"/>
</property>
<property name="gdesc" type="java.lang.String">
<column name="gdesc"/>
</property>
<!-- 配置一对多映射关系 -->
<set name="students" cascade="all" lazy="true" inverse="true">
<key column="gid"/>
<one-to-many class="org.hibernate.entity.Student"/>
</set>
</class>
</hibernate-mapping>
7.hibernate.cfg.xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <!-- 配置SessionFactory --> <session-factory> <!-- 配置数据库连接信息 --> <property name="connection.drvier_class">com.mysql.jdbc.Driver</property> <property name="connection.url"> jdbc:mysql:///hibernate?useSSL=true&characterEncoding=UTF-8 </property> <property name="connection.user">root</property> <property name="connection.password">***</property> <!-- 配置hibernate常用配置 --> <property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property><!-- 方言 --> <property name="hbm2ddl.auto">update</property><!-- 是否自动创建表结构 --> <property name="show_sql">true</property><!-- 是否展示SQL --> <!-- 引入映射类 --> <mapping class="org.hibernate.entity.Student"/> <mapping class="org.hibernate.entity.Grade"/> <!-- 引入映射文件 --> <!-- <mapping resource="hbm/Student.hbm.xml"/> <mapping resource="hbm/Grade.hbm.xml"/> --> </session-factory> </hibernate-configuration>
8.TestTwoWayOneToMany.java
package org.hibernate.test;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.entity.Grade;
import org.hibernate.entity.Student;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
public class TestTwoWayOneToMany {
private SessionFactory sessionFactory;
private Session session;
private Transaction transaction;
@Before
public void before() {
// 创建SessionFactory对象
sessionFactory = new Configuration().configure().buildSessionFactory();
// 获取Session对象
session = sessionFactory.openSession();
// 开启事务
transaction = session.beginTransaction();
}
@After
public void after() {
// 提交事务
transaction.commit();
// 关闭Session
session.close();
// 关闭SessionFactory
sessionFactory.close();
}
@Test
public void testAdd() {
Grade grade = new Grade("Java一班", "Java应用开发一班");
Student student1 = new Student("张三", "男", grade);
Student student2 = new Student("李四", "女", grade);
session.save(student1);
session.save(student2);
session.save(grade);
}
@Test
public void testQuery() {
Grade grade = session.get(Grade.class, 1);
System.out.println(grade.getGname() + " - " + grade.getGdesc());
for (Student student: grade.getStudents()) {
System.out.println(student.getSname() + " - " + student.getSex());
}
Student student = session.get(Student.class, 1);
System.out.println(student.getSname() + " - " + student.getSex() + " - "
+ student.getGrade().getGname() + student.getGrade().getGdesc());
}
@Test
public void testUpdate() {
Grade grade = new Grade("Java二班", "Java应用开发二班");
Student student = session.get(Student.class, 2);
student.setGrade(grade);
session.update(student);
}
@Test
public void testDelete() {
Grade grade = session.get(Grade.class, 2);
session.delete(grade);
Student student = session.get(Student.class, 1);
session.delete(student);
}
}
9.效果预览
9.1 执行testAdd方法
9.2 执行testQuery方法
9.3 执行testUpdate方法
9.4 执行testDelete方法
© 版权声明：本文为博主原创文章，转载请注明出处
