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。