hibernate 注解配置一对多关系简单实例

本文介绍如何使用Hibernate注解配置一对多关系,通过具体示例展示了Person和Address实体类之间的关联设置,并解释了mappedBy的用法。

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hibernate 注解配置一对多关系


     从Hibernate 2.5开始就可以使用annotation实现实体关系的映射了,减少了配置hbm文件的繁琐,而且annotation也是一种趋势,现在的SSH2的整合都是完全可以用annotation来实现。在以前实现一对多关联的关联式都是使用hbm文件,今天我们来使用annotation试试,同样也能实现其功能。

1.数据库:oracle数据库,有person和address二张表,一对多的关系。

数据库:
create table person
        (
                id number(4) not null,
                uname varchar (20)
        );
        create table address
                (
                        id number(4) not null,
                address varchar(20) not null,
                personId number(4) not null
        )
        alter table person
        add constraint primary_id parmary key(id)
        deferrable initially deferred;


2.Person实体类:

package com.zengguo.po;
import java.io.Serializable;
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.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import org.hibernate.annotations.Fetch;
import org.hibernate.annotations.FetchMode;
import org.hibernate.annotations.GenericGenerator;
import org.hibernate.annotations.Parameter;
@SuppressWarnings(“serial”)
@Entity
@Table(name = “person”)
public class Person implements Serializable {
//oracle 序列
@Id
@GenericGenerator(name = “personGenericGenerator”, strategy = “sequence”,
parameters = { @Parameter(value = “hibernate_seq”, name = “sequence”) })
@GeneratedValue(generator=”personGenericGenerator”)
private long id;
@Column(name=”uname”)
private String uname;
@OneToMany(targetEntity=Address.class,cascade=CascadeType.ALL)
@Fetch(FetchMode.JOIN)
//updatable=false很关键,如果没有它,在级联删除的时候就会报错(反转的问题)
@JoinColumn(name=”personId”,updatable=false)
private Set<Address> sets = new HashSet<Address>();
public Set<Address> getSets() {
return sets;
}
public void setSets(Set<Address> sets) {
this.sets = sets;
}
public Person() {
super();
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getUname() {
return uname;
}
public void setUname(String uname) {
this.uname = uname;
}
}


3.Address实体类

package com.zengguo.po;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
import org.hibernate.annotations.GenericGenerator;
import org.hibernate.annotations.Parameter;
@SuppressWarnings(“serial”)
@Entity
@Table(name = “address”)
public class Address implements java.io.Serializable {
//使用序列生成主键
@Id
@GenericGenerator(name = “addressGenericGenerator”, strategy = “sequence”,
parameters = { @Parameter(value = “hibernate_seq”, name = “sequence”) })
@GeneratedValue(generator = “addressGenericGenerator”)
private long id;
@Column(name = “address”)
private String address;
//多对一,@JoinColumn与@column类似,指定映射的数据库字段
@ManyToOne(targetEntity = Person.class)
@JoinColumn(name=”personId”,updatable=false)
private Person person;
public Address() {
super();
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public Person getPerson() {
return person;
}
public void setPerson(Person person) {
this.person = person;
}
}




hibernate一对多mappedBy的用法:(班级classes和学生student,一对多的关系)

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();        
    } 
  } 
}
 


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