Hibernat之关系的处理一对多/多对一

第一步：编写两个pojo，比如一个学生表一个班级表  这里使用注解。

需要

班级表:

 package com.qcf.pox;

 import java.util.HashSet;
 import java.util.Set;

 import javax.persistence.CascadeType;
 import javax.persistence.Column;
 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
 import javax.persistence.GenerationType;
 import javax.persistence.Id;
 import javax.persistence.OneToMany;
 import javax.persistence.Table;

 import org.hibernate.annotations.Cascade;
 import org.hibernate.annotations.Formula;
 //实体类
 @Entity
 @Table(name="_class")
 public class ClassName {
     @Id
     @GeneratedValue(strategy=GenerationType.AUTO)//设置id生成方式
     private int classid;
     @Column(name="classname")
     private String name;
     /*
      * 用一个查询语句动态的生成一个类的属性. 表示这个属性是一个虚拟的列，表中并没有这个列。需要通过查询语句计算出来。
      */
     //@Formula("(select count(*) from student s where s.className_classid=classid)")
     private int num;
     /*
      * mappedBy指定谁来维护关系，mappedBy在一方表示由多方来维护关系，在进行操作时要注意不要使用这一方来save,update等操作
      */
     @OneToMany(mappedBy="classname",cascade=CascadeType.ALL)
     private Set<Student> students=new HashSet<Student>();

     public ClassName(int classid, String name, int num, Set<Student> students) {
         super();
         this.classid = classid;
         this.name = name;
         this.num = num;
         this.students = students;
     }
     public Set<Student> getStudents() {
         return students;
     }
     public void setStudents(Set<Student> students) {
         this.students = students;
     }
     public int getClassid() {
         return classid;
     }
     public void setClassid(int classid) {
         this.classid = classid;
     }
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     public int getNum() {
         return num;
     }
     public void setNum(int num) {
         this.num = num;
     }
     public ClassName(int classid, String name, int num) {
         super();
         this.classid = classid;
         this.name = name;
         this.num = num;
     }
     public ClassName() {
         super();
     }

 }

学生表：

 package com.qcf.pox;

 import java.util.Date;

 import javax.persistence.Entity;
 import javax.persistence.FetchType;
 import javax.persistence.GeneratedValue;
 import javax.persistence.GenerationType;
 import javax.persistence.Id;
 import javax.persistence.ManyToOne;

 @Entity
 public class Student {

     @Id
     @GeneratedValue(strategy=GenerationType.AUTO)//代表主键的生成策略
     private int stuno;
     private String stuname;
     private Date birthday;

     @ManyToOne
     private ClassName classname;
     public int getStuno() {
         return stuno;
     }
     public ClassName getClassName() {
         return classname;
     }
     public void setClassName(ClassName className) {
         this.classname = className;
     }
     public void setStuno(int stuno) {
         this.stuno = stuno;
     }
     public String getStuname() {
         return stuname;
     }
     public void setStuname(String stuname) {
         this.stuname = stuname;
     }
     public Date getBirthday() {
         return birthday;
     }
     public void setBirthday(Date birthday) {
         this.birthday = birthday;
     }
     public Student() {
         super();
     }
     public Student(int stuno, String stuname, Date birthday) {
         super();
         this.stuno = stuno;
         this.stuname = stuname;
         this.birthday = birthday;
     }

 }

第二步：在hibernate.cfg.xml文件中引入这两个po类

         <!-- 引入映射文件 -->

         <mapping class="com.qcf.pox.Student"/>
         <mapping class="com.qcf.pox.ClassName"/>

第三步：编写测试代码

 package com.qcf.test;

 import java.util.Date;

 import org.hibernate.Session;
 import org.hibernate.SessionFactory;
 import org.hibernate.Transaction;
 import org.hibernate.cfg.AnnotationConfiguration;
 import org.hibernate.cfg.Configuration;

 import com.qcf.pox.ClassName;
 import com.qcf.pox.Student;

 public class TestManyToOne {
     public static void main(String[] args) {
         //获取hibernate配置文件并取得session对象
         Configuration configuration=new AnnotationConfiguration().configure();
         SessionFactory factory=configuration.buildSessionFactory();
         Session session=factory.openSession();
         //开启事务
         Transaction transaction=session.beginTransaction();
         //创建classname实例
         ClassName className=new ClassName();
         className.setName("java");

         //创建student实例
         Student student=new Student();

         student.setStuname("zhangsan");
         student.setBirthday(new Date());
         student.setClassName(className);
         session.save(className);
         session.save(student);
         //提交事务
         transaction.commit();
         //关闭session
         session.close();
     }

 }

问题：

　　在测试代码中，我让学生多方来维护关系，而且级联设置的为全部，这里为什么在测试代码中还有先save班级表再save学生呢？为什么不能只在save学生表？