Hibernate 一对多注解 mappedby 作用

 

  1 package oneToMany; 

  2 import java.util.Set; 

  3 import javax.persistence.*; 

  4 /* 

  5 注意导入时，是导入：import javax.persistence.*;    

  6 非导入org.hibernate的相关类：import org.hibernate.annotations.Entity; 

  7 */ 

  8 @Entity 

  9 @Table(name="classes") 

 10 public class Classes implements Serializable { 

 11   @Id 

 12   @GeneratedValue(strategy=GenerationType.AUTO) 

 13   private int id; 

 14   private String name; 

 15     

 16   @OneToMany(cascade=CascadeType.ALL,mappedBy="classes")    

 17   private Set<Student> students; 

 18 //getter,setter省略 

 19 } 

 20 

 21 

 22 package oneToMany; 

 23 import javax.persistence.*; 

 24 @Entity 

 25 @Table(name="student") 

 26 public class Student implements Serializable  { 

 27   @Id 

 28   @GeneratedValue(strategy=GenerationType.AUTO) 

 29   private int sid; 

 30     

 31   private String sname; 

 32     

 33   //若有多个cascade，可以是：{CascadeType.PERSIST,CascadeType.MERGE} 

 34   @ManyToOne(cascade={CascadeType.ALL})         

 35   @JoinColumn(name="classid")     //student类中对应外键的属性：classid 

 36   private Classes classes; 

 37 //getter,setter省略 

 38 } 

 39 

 40 

 41 public class TestOneToMany { 

 42 /* 

 43 CREATE TABLE    student (    --要定义外键!!!!!!! 

 44     `sid` double NOT NULL auto_increment, 

 45     `classid` double NULL, 

 46     `sname` varchar(255) NOT NULL, 

 47     PRIMARY KEY    (sid), 

 48     INDEX par_ind (classid), 

 49     FOREIGN KEY (classid) REFERENCES classes(id) ON DELETE CASCADE ON UPDATE CASCADE 

 50 ) ENGINE=InnoDB 

 51 */    

 52   public static void main(String[] args) throws SQLException    

 53   { 

 54     try 

 55     { 

 56       SessionFactory sf = new AnnotationConfiguration().configure().buildSessionFactory(); 

 57       Session session=sf.openSession(); 

 58       Transaction tx=session.beginTransaction();         

 59 /*

 60 因为mappedBy是定义在classes中,即classes类不负责维护级联关系.即维护者是student.所以,

 61 1.要将clsses的数据,赋给student,即用student的setClasses()方法去捆定class数据;

 62 2.在进行数据插入/更新session.save()/session.update()时,最后操作的是student.

 63 */

 64       Classes classes=new Classes(); 

 65       classes.setName("access"); 

 66         

 67       Student st1=new Student(); 

 68       st1.setSname("jason"); 

 69       st1.setClasses(classes); 

 70       session.save(st1); 

 71         

 72       Student st2=new Student(); 

 73       st2.setSname("hwj"); 

 74       st2.setClasses(classes); 

 75       session.save(st2); 

 76       tx.commit();

 77 /* 

 78 输出如下:

 79 Hibernate: insert into classes (name) values (?)

 80 Hibernate: insert into student (classid, sname) values (?, ?)

 81 Hibernate: insert into student (classid, sname) values (?, ?)

 82 */

 83 /*

 84 因为一端维护关系另一端不维护关系的原因，我们必须注意避免在应用中用不维护关系的类(class)建立关系，因为这样建立的关系是不会在数据库中存储的。

 85 如上的代码倒过来,则插入时,student的外键值为空.如下:

 86 */

 87 //      Student st1=new Student(); 

 88 //      st1.setSname("jason"); 

 89 //      session.save(st1); 

 90 //        

 91 //      Student st2=new Student(); 

 92 //      st2.setSname("hwj"); 

 93 //      session.save(st2); 

 94 //        

 95 //      Set<Student> students=new HashSet<Student>(); 

 96 //      students.add(st1); 

 97 //      students.add(st2); 

 98 //        

 99 //      Classes classes=new Classes(); 

100 //      classes.setName("access"); 

101 //      classes.setStudents(students); 

102 //      session.save(classes); 

103 /*

104 输出如下:

105 Hibernate: insert into student (classid, sname) values (?, ?)

106 Hibernate: insert into student (classid, sname) values (?, ?)

107 Hibernate: insert into classes (name) values (?)

108 */

109     } 

110     catch(HibernateException e) 

111     { 

112       e.printStackTrace();        

113     } 

114   } 

115 }