hibernate_04_hbm.xml介绍

先贴上类文件Students.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">
 <!-- Generated 2017-5-23 0:24:09 by Hibernate Tools 3.5.0.Final -->
 <hibernate-mapping>
     <class name="com.imooc.hibernate.Students" table="STUDENTS">
         <id name="sid" type="int">
             <column name="SID" />
             <generator class="assigned" />
         </id>
         <property name="sname" type="java.lang.String">
             <column name="SNAME" />
         </property>
         <property name="gender" type="java.lang.String">
             <column name="GENDER" />
         </property>
         <property name="birthday" type="java.util.Date">
             <column name="BIRTHDAY" />
         </property>
         <property name="address" type="java.lang.String">
             <column name="ADDRESS" />
         </property>
     </class>
 </hibernate-mapping>

其中<generator class="assigned" />：适用于自然主键，由java代码设置标识符。

　　<generator class="assigned" />：适用于代理主键，hibernate自动生成标识符。

自然主键和代理主键请参考：http://blog.csdn.net/haiross/article/details/21388997

举个栗子：

students类：

 package com.imooc.hibernate;

 import java.util.Date;

 public class Students {

     private int sid;
     private String sname;
     private String gender;
     private Date birthday;
     private String address;

     public Students() {}

     public Students(int sid, String sname, String gender, Date birthday, String address) {
         super();
         this.sid = sid;
         this.sname = sname;
         this.gender = gender;
         this.birthday = birthday;
         this.address = address;
     }

     @Override
     public String toString() {
         return "Students [sid=" + sid + ", sname=" + sname + ", gender=" + gender + ", birthday=" + birthday
                 + ", address=" + address + "]";
     }

     public int getSid() {
         return sid;
     }

     public void setSid(int sid) {
         this.sid = sid;
     }

     public String getSname() {
         return sname;
     }

     public void setSname(String sname) {
         this.sname = sname;
     }

     public String getGender() {
         return gender;
     }

     public void setGender(String gender) {
         this.gender = gender;
     }

     public Date getBirthday() {
         return birthday;
     }

     public void setBirthday(Date birthday) {
         this.birthday = birthday;
     }

     public String getAddress() {
         return address;
     }

     public void setAddress(String address) {
         this.address = address;
     }
 }

hibernate.cfg.xml文件：

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE hibernate-configuration PUBLIC
         "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
         "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
 <hibernate-configuration>
     <session-factory>
         <property name="connection.username">root</property>
         <property name="connection.password">root</property>
         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
         <property name="connection.url">jdbc:mysql://localhost:3306/hibernate?useUnicode=true&amp;characterEncoding=UTF-8</property>
         <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
         <property name="show_sql">true</property>
         <property name="format_sql">true</property>
         <property name="hbm2ddl.auto">create</property>

         <mapping resource="com/imooc/hibernate/Students.hbm.xml"/>
     </session-factory>
 </hibernate-configuration>

测试类Students.java

 package com.icoom.test;
 import static org.junit.Assert.fail;

 import java.util.Date;

 import org.hibernate.Session;
 import org.hibernate.SessionFactory;
 import org.hibernate.Transaction;
 import org.hibernate.cfg.Configuration;
 import org.hibernate.service.ServiceRegistry;
 import org.hibernate.service.ServiceRegistryBuilder;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;

 import com.imooc.hibernate.Students;

 public class StudentsTest {

     private SessionFactory sessionFactory;
     private Session session;
     private Transaction transaction;

     @Before
     public void init() {
         // 1.创建配置对象
         Configuration config = new Configuration().configure();
         // 2.创建服务注册对象
         ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry();
         // 3.创建会话工厂对象
         sessionFactory = config.buildSessionFactory(serviceRegistry);
         // 会话对象
         session = sessionFactory.openSession();
         // 开启事务
         transaction = session.beginTransaction();
     }
     @After
     public void destory() {
         transaction.commit(); //提交事务
         session.close(); //关闭session
         sessionFactory.close();//关闭会话工厂
     }
     @Test
     public void testSaveStudents() {
         // 生成学生对象
 //        Students s = new Students(1, "老张", "男", new Date(), "山东");
         Students s = new Students();
         s.setSid(1);
         s.setSname("老张");
         s.setGender("男");
         s.setBirthday(new Date());
         s.setAddress("山东");
         session.save(s);//保存对象进入数据库
     }
 }

数据库产生一条数据：

将hibernate.cfg.xml文件中的<property name="hbm2ddl.auto">create</property>改为<property name="hbm2ddl.auto">update</property>，然后再执行一次，就会报错。因为主键重复。

----------------------------------

将数据表删除，将Students.hbm.xml文件中的<generator class="assigned" />改成<generator class="native" />，然后再执行一次testSaveStudents()方法，会生成一条数据；再执行一次，Junit不会报错，而是插入了一条新数据：

以上就是assigned和native的区别。