hibernate--联合主键(了解+，掌握-)

如果一个表有多个主键（= =一般比较少）

8.4. 组件作为联合标识符(Components as composite identifiers)

先定义一个类OrderLineId(实现接口，implems java.io.Serializable（把当前对象序列化的接口）)，

重写

为什么要序列化？内存满和集群化的时候用，具体百度(= =！其实我就是不懂)

里面有多个成员变量(字段(这些都是主键))

package com.zjx.hibernate;public class StudentPK implements java.io.Serializable{private int id;private String name;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic boolean equals(Object o) {         //判断是否相等，相等就找它if(o instanceof StudentPK) {  //如果o是一个StudentPK对象StudentPK pk = (StudentPK)o;if(this.id == pk.getId() && this.name.equals(pk.getName())) {return true;}}return false;}@Overridepublic int hashCode() {   //计算主键的hash码  if有多个对象 就直接找hash码   return this.name.hashCode();}}

然后在 类Student中 创建OrderLineId成员变量，set,get一下，

package com.zjx.hibernate;public class Student {private StudentPK pk;private int age;private String sex;private boolean good;public boolean isGood() {return good;}public void setGood(boolean good) {this.good = good;}/*public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}*/public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public StudentPK getPk() {return pk;}public void setPk(StudentPK pk) {this.pk = pk;}}

 其他都一样 就是hbm.xml配置文件改成如下

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping><class name="com.bjsxt.hibernate.Student"><composite-id name="pk" class="com.zjx.hibernate.StudentPK"><key-property name="id"></key-property><key-property name="name"></key-property></composite-id><property name="age" /><property name="sex" /><property name="good" type="yes_no"></property>    </class></hibernate-mapping>

如果是使用注释？

2.2.. 映射主键属性

下面是定义组合主键的几种语法:

• 将组件类注解为@Embeddable,并将组件的属性注解为@Id(比较少用)
• 将组件的属性注解为@EmbeddedId       
• 将类注解为@IdClass,并将该实体中所有属于主键的属性都注解为@Id

对于EJB2的开发人员来说 @IdClass是很常见的, 但是对于Hibernate的用户来说就是一个崭新的用法. 

组合主键类对应了一个实体类中的多个字段或属性, 

而且主键类中用于定义主键的字段或属性和 实体类中对应的字段或属性在类型上必须一致.下面我们看一个例子:

@Entity@IdClass(FootballerPk.class)public class Footballer {    //part of the id key    @Id public String getFirstname() {        return firstname;    }    public void setFirstname(String firstname) {        this.firstname = firstname;    }    //part of the id key    @Id public String getLastname() {        return lastname;    }    public void setLastname(String lastname) {        this.lastname = lastname;    }    public String getClub() {        return club;    }    public void setClub(String club) {        this.club = club;    }    //appropriate equals() and hashCode() implementation}@Embeddablepublic class FootballerPk implements Serializable {    //same name and type as in Footballer    public String getFirstname() {        return firstname;    }    public void setFirstname(String firstname) {        this.firstname = firstname;    }    //same name and type as in Footballer    public String getLastname() {        return lastname;    }    public void setLastname(String lastname) {        this.lastname = lastname;    }    //appropriate equals() and hashCode() implementation}

如上, @IdClass指向对应的主键类.

Hibernate支持在组合标识符中定义关联(就像使用普通的注解一样),而EJB3规范并不支持此类用法.

@Entity@AssociationOverride( name="id.channel", joinColumns = @JoinColumn(name="chan_id") )public class TvMagazin {    @EmbeddedId public TvMagazinPk id;    @Temporal(TemporalType.TIME) Date time;}@Embeddablepublic class TvMagazinPk implements Serializable {    @ManyToOne    public Channel channel;    public String name;    @ManyToOne    public Presenter presenter;}

@IdClass(TeacherPK.class)；

那么就能使用TeacherPK的东西；

t.setId(1);
t.setName("t1");