Hibernate 一对多注解 mappedby 作用
package oneToMany; import java.util.Set; import javax.persistence.*; /* 注意导入时,是导入:import javax.persistence.*; 非导入org.hibernate的相关类:import org.hibernate.annotations.Entity; */ @Entity @Table(name="classes") public class Classes implements Serializable { @Id @GeneratedValue(strategy=GenerationType.AUTO) private int id; private String name; @OneToMany(cascade=CascadeType.ALL,mappedBy="classes") private Set<Student> students; //getter,setter省略 } package oneToMany; import javax.persistence.*; @Entity @Table(name="student") public class Student implements Serializable { @Id @GeneratedValue(strategy=GenerationType.AUTO) private int sid; private String sname; //若有多个cascade,可以是:{CascadeType.PERSIST,CascadeType.MERGE} @ManyToOne(cascade={CascadeType.ALL}) @JoinColumn(name="classid") //student类中对应外键的属性:classid private Classes classes; //getter,setter省略 } public class TestOneToMany { /* CREATE TABLE student ( --要定义外键!!!!!!! `sid` double NOT NULL auto_increment, `classid` double NULL, `sname` varchar(255) NOT NULL, PRIMARY KEY (sid), INDEX par_ind (classid), FOREIGN KEY (classid) REFERENCES classes(id) ON DELETE CASCADE ON UPDATE CASCADE ) ENGINE=InnoDB */ public static void main(String[] args) throws SQLException { try { SessionFactory sf = new AnnotationConfiguration().configure().buildSessionFactory(); Session session=sf.openSession(); Transaction tx=session.beginTransaction(); /* 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, 1.要将clsses的数据,赋给student,即用student的setClasses()方法去捆定class数据; 2.在进行数据插入/更新session.save()/session.update()时,最后操作的是student. */ Classes classes=new Classes(); classes.setName("access"); Student st1=new Student(); st1.setSname("jason"); st1.setClasses(classes); session.save(st1); Student st2=new Student(); st2.setSname("hwj"); st2.setClasses(classes); session.save(st2); tx.commit(); /* 输出如下: Hibernate: insert into classes (name) values (?) Hibernate: insert into student (classid, sname) values (?, ?) Hibernate: insert into student (classid, sname) values (?, ?) */ /* 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。 如上的代码倒过来,则插入时,student的外键值为空.如下: */ // Student st1=new Student(); // st1.setSname("jason"); // session.save(st1); // // Student st2=new Student(); // st2.setSname("hwj"); // session.save(st2); // // Set<Student> students=new HashSet<Student>(); // students.add(st1); // students.add(st2); // // Classes classes=new Classes(); // classes.setName("access"); // classes.setStudents(students); // session.save(classes); /* 输出如下: Hibernate: insert into student (classid, sname) values (?, ?) Hibernate: insert into student (classid, sname) values (?, ?) Hibernate: insert into classes (name) values (?) */ } catch(HibernateException e) { e.printStackTrace(); } } }