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:

 1 package com.qcf.pox;
 2
 3 import java.util.HashSet;
 4 import java.util.Set;
 5
 6 import javax.persistence.CascadeType;
 7 import javax.persistence.Column;
 8 import javax.persistence.Entity;
 9 import javax.persistence.GeneratedValue;
10 import javax.persistence.GenerationType;
11 import javax.persistence.Id;
12 import javax.persistence.OneToMany;
13 import javax.persistence.Table;
14
15 import org.hibernate.annotations.Cascade;
16 import org.hibernate.annotations.Formula;
17 //实体类
18 @Entity
19 @Table(name="_class")
20 public class ClassName {
21     @Id
22     @GeneratedValue(strategy=GenerationType.AUTO)//设置id生成方式
23     private int classid;
24     @Column(name="classname")
25     private String name;
26     /*
27      * 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。
28      */
29     //@Formula("(select count(*) from student s where s.className_classid=classid)")
30     private int num;
31     /*
32      * 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
33      */
34     @OneToMany(mappedBy="classname",cascade=CascadeType.ALL)
35     private Set<Student> students=new HashSet<Student>();
36
37     public ClassName(int classid, String name, int num, Set<Student> students) {
38         super();
39         this.classid = classid;
40         this.name = name;
41         this.num = num;
42         this.students = students;
43     }
44     public Set<Student> getStudents() {
45         return students;
46     }
47     public void setStudents(Set<Student> students) {
48         this.students = students;
49     }
50     public int getClassid() {
51         return classid;
52     }
53     public void setClassid(int classid) {
54         this.classid = classid;
55     }
56     public String getName() {
57         return name;
58     }
59     public void setName(String name) {
60         this.name = name;
61     }
62     public int getNum() {
63         return num;
64     }
65     public void setNum(int num) {
66         this.num = num;
67     }
68     public ClassName(int classid, String name, int num) {
69         super();
70         this.classid = classid;
71         this.name = name;
72         this.num = num;
73     }
74     public ClassName() {
75         super();
76     }
77
78
79 }

学生表：

 1 package com.qcf.pox;
 2
 3 import java.util.Date;
 4
 5 import javax.persistence.Entity;
 6 import javax.persistence.FetchType;
 7 import javax.persistence.GeneratedValue;
 8 import javax.persistence.GenerationType;
 9 import javax.persistence.Id;
10 import javax.persistence.ManyToOne;
11
12 @Entity
13 public class Student {
14
15     @Id
16     @GeneratedValue(strategy=GenerationType.AUTO)//代表主键的生成策略
17     private int stuno;
18     private String stuname;
19     private Date birthday;
20
21     @ManyToOne
22     private ClassName classname;
23     public int getStuno() {
24         return stuno;
25     }
26     public ClassName getClassName() {
27         return classname;
28     }
29     public void setClassName(ClassName className) {
30         this.classname = className;
31     }
32     public void setStuno(int stuno) {
33         this.stuno = stuno;
34     }
35     public String getStuname() {
36         return stuname;
37     }
38     public void setStuname(String stuname) {
39         this.stuname = stuname;
40     }
41     public Date getBirthday() {
42         return birthday;
43     }
44     public void setBirthday(Date birthday) {
45         this.birthday = birthday;
46     }
47     public Student() {
48         super();
49     }
50     public Student(int stuno, String stuname, Date birthday) {
51         super();
52         this.stuno = stuno;
53         this.stuname = stuname;
54         this.birthday = birthday;
55     }
56
57 }

第二步：在hibernate.cfg.xml文件中引入这两个po类

1         <!-- 引入映射文件 -->
2
3         <mapping class="com.qcf.pox.Student"/>
4         <mapping class="com.qcf.pox.ClassName"/>

第三步：编写测试代码

 1 package com.qcf.test;
 2
 3 import java.util.Date;
 4
 5 import org.hibernate.Session;
 6 import org.hibernate.SessionFactory;
 7 import org.hibernate.Transaction;
 8 import org.hibernate.cfg.AnnotationConfiguration;
 9 import org.hibernate.cfg.Configuration;
10
11 import com.qcf.pox.ClassName;
12 import com.qcf.pox.Student;
13
14 public class TestManyToOne {
15     public static void main(String[] args) {
16         //获取hibernate配置文件并取得session对象
17         Configuration configuration=new AnnotationConfiguration().configure();
18         SessionFactory factory=configuration.buildSessionFactory();
19         Session session=factory.openSession();
20         //开启事务
21         Transaction transaction=session.beginTransaction();
22         //创建classname实例
23         ClassName className=new ClassName();
24         className.setName("java");
25
26         //创建student实例
27         Student student=new Student();
28
29         student.setStuname("zhangsan");
30         student.setBirthday(new Date());
31         student.setClassName(className);
32         session.save(className);
33         session.save(student);
34         //提交事务
35         transaction.commit();
36         //关闭session
37         session.close();
38     }
39
40 }

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？