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？