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 }