对多一关系是最普遍也是最重要的一种对象关系，其中又包括了单向的多对一，单向的一对多以及双向的多对一关系
单向多对一
多的一方：学生（Student）
一的一方：班级（Grade）
班级类的定义以及hbm文件配置如下
1 public class Grade {
2 private int id;
3 private String name;
4 }
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC 3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 5 <hibernate-mapping > 6 <class name="com.zlt.hibernatedemo.Grade" table="grade"> 7 <id name="id" column="id"> 8 <generator class="increment"></generator> 9 </id> 10 11 <property name="name" column="name"></property> 12 </class> 13 14 </hibernate-mapping>
学生类的定义以及hbm文件配置如下
1 public class Student {
2 private int id;
3 private String name;
4 private Grade grade;
5 }
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC 3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 5 <hibernate-mapping > 6 <class name="com.zlt.hibernatedemo.Student" table="student"> 7 <id name="id" column="id"> 8 <generator class="increment"></generator> 9 </id> 10 11 <property name="name" type="java.lang.String"> 12 <column name="name" length="50" /> 13 </property> 14 15 <many-to-one name="grade" column="gradeid"></many-to-one> 16 </class> 17 18 </hibernate-mapping>
测试程序
1 public class HibernateTest {
2 public static void main(String[] args) {
3 Session session = HibernateFactory.currentSession();
4 Transaction tx = session.beginTransaction();
5
6 Grade grade = new Grade();
7 grade.setName("grade1");
8
9 Student student1 = new Student();
10 student1.setName("student1");
11 student1.setGrade(grade);
12
13 Student student2 = new Student();
14 student2.setName("student2");
15 student2.setGrade(grade);
16
17 session.save(grade);
18 session.save(student1);
19 session.save(student2);
20
21 tx.commit();
22 session.close();
23 }
24 }
结果
单向一对多
多的一方：学生（Student）
一的一方：班级（Grade）
学生类的定义以及hbm文件配置如下
1 public class Student {
2 private int id;
3 private String name;
4 }
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC 3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 5 <hibernate-mapping > 6 <class name="com.zlt.hibernatedemo.Student" table="student"> 7 <id name="id" column="id"> 8 <generator class="increment"></generator> 9 </id> 10 11 <property name="name"></property> 12 </class> 13 14 </hibernate-mapping>
班级类的定义以及hbm文件配置如下
1 public class Grade {
2 private int id;
3 private String name;
4 private Set<Student> students = new HashSet<Student>();
5 }
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC 3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 5 <hibernate-mapping > 6 <class name="com.zlt.hibernatedemo.Grade" table="grade"> 7 <id name="id" column="id"> 8 <generator class="increment"></generator> 9 </id> 10 11 <property name="name" column="name"></property> 12 <set name="students"> 13 <key column="grade_id"/> 14 <one-to-many class="com.zlt.hibernatedemo.Student"/> 15 </set> 16 </class> 17 18 </hibernate-mapping>
测试程序
1 public class HibernateTest {
2 public static void main(String[] args) {
3 Session session = HibernateFactory.currentSession();
4 Transaction tx = session.beginTransaction();
5
6 Grade grade = new Grade();
7 grade.setName("grade1");
8
9 Student student1 = new Student();
10 student1.setName("student1");
11
12 Student student2 = new Student();
13 student2.setName("student2");
14
15 grade.getStudents().add(student1);
16 grade.getStudents().add(student2);
17
18 session.save(grade);
19 session.save(student1);
20 session.save(student2);
21
22 tx.commit();
23 session.close();
24 }
25 }
结果
<set>标签中可以设置cascade属性（all,save-update,delete,none）
当不设置cascade时，保存时需要执行三条语句
1 session.save(grade); 2 session.save(student1); 3 session.save(student2);
当设置cascade="all"后对某个grade的操作会级联到它student，所以只需执行session.save(grade)就可以同时插入三条数据到数据库
不设置cascade时，执行session.delete(grade);会把grade从数据库中删去，同时把student关联到这个grade的外键gradeid的值置为空;而设置了cascade后，删除了grade同时会把该grade对应的student对象从数据库中删去
<set>标签中也可以设置inverse属性（true,false）
inverse="false"（默认）生成的sql语句
Hibernate: insert into grade (name, id) values (?, ?)
Hibernate: insert into student (name, id) values (?, ?)
Hibernate: insert into student (name, id) values (?, ?)
Hibernate: update student set grade_id=? where id=?
Hibernate: update student set grade_id=? where id=?
inverse="true"生成的sql语句
Hibernate: insert into grade (name, id) values (?, ?)
Hibernate: insert into student (name, id) values (?, ?)
Hibernate: insert into student (name, id) values (?, ?)
同时数据库并没有关联三条数据
它意味着 grade不再作为主控方，而将关联关系的维护工作交给关联对象student来完成。在保存grade时，grade不在关心student的gradeid属性，必须由student自己去维护，即设置student.setGrade(grade); 如果需要通过student来维护关联关系，那么这个关联关系转换成双向关联。
双向关联
学生
1 public class Student {
2 private int id;
3 private String name;
4 private Grade grade;
5 }
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC 3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 5 <hibernate-mapping > 6 <class name="com.zlt.hibernatedemo.Student" table="student"> 7 <id name="id" column="id"> 8 <generator class="increment"></generator> 9 </id> 10 11 <property name="name" type="java.lang.String"> 12 <column name="name" length="50" /> 13 </property> 14 15 <many-to-one name="grade" column="gradeid"></many-to-one> 16 </class> 17 18 </hibernate-mapping>
班级
1 public class Grade {
2 private int id;
3 private String name;
4 private Set<Student> students = new HashSet<Student>();
5 }
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC 3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 5 <hibernate-mapping > 6 <class name="com.zlt.hibernatedemo.Grade" table="grade"> 7 <id name="id" column="id"> 8 <generator class="increment"></generator> 9 </id> 10 11 <property name="name" column="name"></property> 12 13 <set name="students" inverse="true" cascade="all"> 14 <key column="gradeid"/> 15 <one-to-many class="com.zlt.hibernatedemo.Student"/> 16 </set> 17 </class> 18 19 </hibernate-mapping>
测试程序
public class HibernateTest {
public static void main(String[] args) {
Session session = HibernateFactory.currentSession();
Transaction tx = session.beginTransaction();
Grade grade = new Grade();
grade.setName("grade1");
Student student1 = new Student();
student1.setName("student1");
Student student2 = new Student();
student2.setName("student2");
//只需要session.save(grade);就能插入三条数据，需要先设置cascade
grade.getStudents().add(student1);
grade.getStudents().add(student2);
//因为grade中inverse设置为ture，所以需要由student维护关联关系，设置graidid
student1.setGrade(grade);
student2.setGrade(grade);
session.save(grade);
tx.commit();
session.close();
}
}
结果：
产生3条sql语句（如果用一的那端维护关联关系，会产生5条sql，所以在多的一方设置inverse="true",有助于性能的改善）
Hibernate: insert into grade (name, id) values (?, ?)
Hibernate: insert into student (name, gradeid, id) values (?, ?, ?)
Hibernate: insert into student (name, gradeid, id) values (?, ?, ?)
自身一对多配置
1 <?xml version="1.0" encoding="utf-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 3 4 <hibernate-mapping> 5 <class name="Category" table="category"> 6 <id name="id" type="string"> 7 <column name="id"></column> 8 <generator class="uuid"></generator> 9 </id> 10 11 <property name="name" column="name" type="string"></property> 12 13 <set name="chidrenCategories" cascade="all" inverse="true"> 14 <key column="category_id"></key> 15 <one-to-many class=" Category"/> 16 </set> 17 18 <many-to-one name="parentCategory" column="category_id"></many-to-one> 19 20 </class> 21 </hibernate-mapping>
hibernate关系映射（多对一）
时间： 2024-08-09 02:08:03