hibernate关系映射（多对一）

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: 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 (?, ?)

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 (?, ?, ?)

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>