文章结束给大家来个程序员笑话：[M]
有时候我们在项目中可能要需结合主键来现实我们的业务，比如说 对于User类说来，我们可以把cardId和name它们两个结合起来作为User类的主键。
官方API给我们解释的如下：
You can use a component as an identifier of an entity class. Your component class must satisfy certain requirements:
1.它必须现实java.io.Serializable接口
2.It must re-implement equals() and hashCode() consistently with the database's notion of composite key equality.
通过面上的我们可以晓得，结合主键的实体必须现实Serializable接口以及写重equals()和hashCode()方法。
为什么要这样做呢？
写重这两个方法是因为hibernate要根据数据库的结合主键来断判某两行记载是不是是一样的，如果一样那么就认为是同一个对象，如果不一样，那么认为是不同的对象，这反应到程序域领中就是根据hashCode与equals方法来断判某两个对象是不是可以放到诸如Set这样的集合当中。
结合主键的实体类现实Serializable接口的原因在于用使get或load方法的时候要需先构建出来该实体的对象，并且将查询据依(结合主键)置设进去，然后作为get或load方法第二个数参传进去便可。
通过例子应当更好的助帮我们解理：
User.java 用户实体类
package com.bawei.demo.vo;
public class User {
private UserPK id;
public UserPK getId() {
return id;
}
public void setId(UserPK id) {
this.id = id;
}
private String sex;
private String address;
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
@Override
public String toString() {
return "User [id=" + id + ", sex=" + sex + ", address=" + address + "]";
}
}
UserPK.java
package com.bawei.demo.vo;
import java.io.Serializable;
/**
* @author shenyiya
* 实体主键
*
*/
public class UserPK implements Serializable {
private String cardId;
private String name;
public String getCardId() {
return cardId;
}
public void setCardId(String cardId) {
this.cardId = cardId;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((cardId == null) ? 0 : cardId.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
UserPK other = (UserPK) obj;
if (cardId == null) {
if (other.cardId != null) {
return false;
}
} else if (!cardId.equals(other.cardId)) {
return false;
}
if (name == null) {
if (other.name != null) {
return false;
} else if (!name.equals(other.name)) {
return false;
}
}
return true;
}
}
User.hbm.xml 实体映射文件
<?xml version="1.0" encoding="gb2312"?> <!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.bawei.demo.vo.User" table="user"> <composite-id name="id" class="com.bawei.demo.vo.UserPK"> <key-property name="cardId" type="string"></key-property> <key-property name="name" type="string"></key-property> </composite-id> <property name="sex"></property> <property name="address"></property> </class> </hibernate-mapping>
Hibernate.cfg.xml hibernate配置文件
<?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/demo
</property>
<property name="connection.username">root</property>
<property name="connection.password">123</property>
<property name="connection.driver_class">
com.mysql.jdbc.Driver
</property>
<property name="myeclipse.connection.profile">conn</property>
<property name="show_sql">false</property>
<property name="hbm2ddl.auto">update</property>
<mapping resource="com/bawei/demo/dao/Author.hbm.xml" />
<mapping resource="com/bawei/demo/dao/Post.hbm.xml" />
<mapping resource="com/bawei/demo/dao/User.hbm.xml" />
</session-factory>
</hibernate-configuration>
Main 函数 ：
public static void main(String[] args) {
// 1.初始化配置文件
Configuration configuration = new Configuration().configure();
// 2.立建session工厂
SessionFactory sessionFactory = configuration.buildSessionFactory();
// 3.从工厂中得取一个连接
Session session = sessionFactory.openSession();
// 4.开启务事
Transaction tx = session.beginTransaction();
// 保存
// UserPK usersPK = new UserPK();
// usersPK.setName("zhangsan");
// usersPK.setCardId("110");
// User user = new User();
// user.setId(usersPK);
// user.setAddress("beijing");
// user.setSex("M");
// session.save(user);
//查询
UserPK usersPK = new UserPK();
usersPK.setName("zhangsan");
usersPK.setCardId("110");
User user=(User) session.load(User.class, usersPK);
System.out.println(user.toString());
// 5.提交务事
tx.commit();
// 6.闭关连接
session.close();
}
效果如下图：
文章结束给大家分享下程序员的一些笑话语录：
姿势要丰富，经常上百度！