标签：
一对多与多对一区别:
映射原理是一致的,都是在多的一端加入一个外键,指向一的一端
区别在于维护的关系不同:
多对一维护的关系是:多指向一的,有了此关系,在加载多的时候可以将一加载上来
一对多维护的关系是:一指向多的关系,有了此关系,加载一
的时候可以将多加载上来
适用情况:一个班级有多个学生(班级依赖学生,必须先有学生,班级维护关系)
关系图:
注:都是在多的一端加上外键,指向一的一端
<span style="font-size:18px;">实体:
Student:
public class Student {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Class:
public class Classes {
private int id;
private String name;
private Set students;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set getStudents() {
return students;
}
public void setStudents(Set students) {
this.students = students;
}
}
映射文件:
Student:
<hibernate-mapping>
<class name="com.bjpowernode.hibernate.Student" table="t_student">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
Class:
<hibernate-mapping>
<class name="com.bjpowernode.hibernate.Classes" table="t_classes">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students">
<key column="classesid"/>
<one-to-many class="com.bjpowernode.hibernate.Student"/>
</set>
</class>
</hibernate-mapping>
客户端:
public void testSave2() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
Student student1 = new Student();
student1.setName("张三");
session.save(student1);
Student student2 = new Student();
student2.setName("李四");
session.save(student2);
Classes classes = new Classes();
classes.setName("动力节点");
Set students = new HashSet();
students.add(student1);
students.add(student2);
classes.setStudents(students);
// 可以成功保存数据
// 但是会发出多余的update语句来维持关系
session.save(classes);
session.getTransaction().commit();
} catch (Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
} finally {
HibernateUtils.closeSession(session);
}
}</span>
结果:
适用情况:
两端都知道关系，解决一对多单向关联的缺陷
关系图:
<span style="font-size:18px;">实体:
student：
public class Student {
private int id;
private String name;
private Classes classes;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Classes getClasses() {
return classes;
}
public void setClasses(Classes classes) {
this.classes = classes;
}
}
class：
public class Classes {
private int id;
private String name;
private Set students;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set getStudents() {
return students;
}
public void setStudents(Set students) {
this.students = students;
}
}
映射文件:
student：
<hibernate-mapping>
<class name="com.bjpowernode.hibernate.Student" table="t_student">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<many-to-one name="classes" column="classesid"/>
</class>
</hibernate-mapping></span><span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-size:18px;">class：</span></span>
<span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-size:18px;"> <hibernate-mapping></span></span>
<span style="font-size:18px;"> <class name="com.bjpowernode.hibernate.Classes" table="t_classes"> <id name="id"> <generator class="native"/> </id> <property name="name"/> <set name="students" inverse="true"> <key column="classesid"/> <one-to-many class="com.bjpowernode.hibernate.Student"/> </set> </class> </hibernate-mapping></span>
客户端:
<span style="font-size:18px;"> 情况1：
public void testSave2() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
Student student1 = new Student();
student1.setName("张三");
session.save(student1);
Student student2 = new Student();
student2.setName("李四");
session.save(student2);
Classes classes = new Classes();
classes.setName("动力节点");
Set students = new HashSet();
students.add(student1);
students.add(student2);
classes.setStudents(students);
//可以成功保存数据
//但是会发出多余的update语句来维持关系
session.save(classes);
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
finally {
HibernateUtils.closeSession(session);
}
}</span>
注：可以保存数据，但会发出多余的update语句来维护关系
情况2：
<span style="font-size:18px;">public void testSave3() {
Session session = null;
try {
<span style="white-space:pre"> </span>session = HibernateUtils.getSession();
session.beginTransaction();
Classes classes = new Classes();
classes.setName("动力节点");
session.save(classes);
Student student1 = new Student();
student1.setName("张三");
student1.setClasses(classes);
session.save(student1);
Student student2 = new Student();
student2.setName("李四");
student2.setClasses(classes);
session.save(student2);
session.getTransaction().commit();
catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}</span>
结果：
标签：
原文地址：http://blog.csdn.net/gxq741718618/article/details/46285029
