映射组件属性之⑤：组件作为复合主键

如果数据库采用简单的逻辑主键，则不会出现组件类型的主键。但在一些特殊的情况下，总会出现组件类型主键，Hibernate也为这种组件类型的主键提供了支持。

使用组件作为复合主键，也就是使用组件作为持久化类的标示符，则该组件类必须满足以下要求：

① 必须实现java.io.Serializable接口。

② 建议正确地重写equals和hashCode方法，也就是根据组件类的关键属性来区分组件对象。

hibernate.cfg.xml :

<?xml version='1.0' encoding='UTF-8'?><!DOCTYPE hibernate-configuration PUBLIC          "-//Hibernate/Hibernate Configuration DTD 3.0//EN"          "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"><!-- Generated by MyEclipse Hibernate Tools.                   --><hibernate-configuration>    <session-factory>        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>        <property name="connection.url">jdbc:mysql://localhost:3306/temp</property>        <property name="connection.username">root</property>        <property name="connection.password">mysqladmin</property>        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>        <property name="hbm2ddl.auto">update</property>        <property name="show_sql">true</property>        <property name="format_sql">true</property>        <mapping resource="db/mapping/Person.hbm.xml"/>    </session-factory></hibernate-configuration>

Person.java :

public class Person {private Name name;private int age;public Name getName() {return name;}public void setName(Name name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}}

Name.java :

public class Name implements Serializable{private String firstname;private String lastname;public Name() {}public Name(String firstname, String lastname) {this.firstname = firstname;this.lastname = lastname;}public String getFirstname() {return firstname;}public void setFirstname(String firstname) {this.firstname = firstname;}public String getLastname() {return lastname;}public void setLastname(String lastname) {this.lastname = lastname;}@Overridepublic boolean equals(Object obj) {if(this==obj){return true;}if(obj!=null && obj.getClass()==Name.class){Name target=(Name)obj;if(target.getFirstname()==this.getFirstname() && target.getLastname()==this.getLastname()){return true;}}return false;}@Overridepublic int hashCode() {return getFirstname().hashCode()*13+getLastname().hashCode();}}

Person.hbm.xml :

<?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 package="db.domain">    <class name="Person" table="persons">                <composite-id name="name" class="Name">            <key-property name="firstname" type="string"/>            <key-property name="lastname" type="string"/>        </composite-id>                <property name="age" type="integer"/>    </class></hibernate-mapping>

Test.java :

public class Test {public static void main(String[] args) {Session session=HibernateSessionFactory.getSession();Transaction txt=session.beginTransaction();Person p1=new Person();p1.setAge(22);p1.setName(new Name("张","三"));Person p2=new Person();p2.setAge(20);p2.setName(new Name("李","四"));session.save(p1);session.save(p2);txt.commit();HibernateSessionFactory.closeSession();}}