映射对象标识符(OID)

1. Hibernate用对象标识符(OID)来区分对象

      session缓存            student表
s1    student对象 OID=1      1    stu1
s2    student对象 OID=2      2    stu2
s3    student对象 OID=1

Student.java

package com.andrew.model;
public class Student {
    private long id;
    private String name;
    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + "]";
    }
}

Student.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.andrew.model">
    <class name="Student" table="t_student">
        <id column="stuId" name="id">
            <generator class="native"/>
        </id>
        <property generated="never" lazy="false" name="name"/>
    </class>
</hibernate-mapping>

hibernate.cfg.xml

<?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>
        <!-- 数据库连接设置 -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/hibernate4</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>
        <!-- 方言 -->
        <property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>
        <!-- 控制台显示SQL -->
        <property name="show_sql">true</property>
        <!-- 自动更新表结构 -->
        <property name="hbm2ddl.auto">update</property>
        <!-- Mapper -->
        <mapping resource="com/andrew/model/Student.hbm.xml"/>
    </session-factory>
</hibernate-configuration>

HibernateUtil.java

package com.andrew.util;
import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
public class HibernateUtil {
    private static final SessionFactory sessionFactory = buildSessionFactory();
    private static SessionFactory buildSessionFactory() {
        // 1. 实例化配置文件
        Configuration configuration = new Configuration().configure();
        // 2. 实例化服务登记
        ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder()
                .applySettings(configuration.getProperties()).build();
        // 3. 获取Session工厂
        return configuration.buildSessionFactory(serviceRegistry);
    }
    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
}

StudentTest.java

package com.andrew.service;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import com.andrew.model.Student;
import com.andrew.util.HibernateUtil;
public class StudentTest {
    public static void main(String[] args) {
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
        Session session = sessionFactory.openSession(); // 1. 生成一个session
        session.beginTransaction(); // 2. 开启事务
        Student s1 = (Student)session.get(Student.class, Long.valueOf(1));
        Student s2 = (Student)session.get(Student.class, Long.valueOf(2));
        Student s3 = (Student)session.get(Student.class, Long.valueOf(1));
        System.out.println(s1==s2);
        System.out.println(s1==s3);
        session.getTransaction().commit(); // 3. 提交事务
        session.close(); // 4. 关闭session
    }
}

运行结果：
false
true

2. Hibernate对象标识符生成策略

主键的分类 业务主键 VS 代理主键 代理主键是不具有业务性的；

StudentIdTest.java

package com.andrew.service;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import com.andrew.model.Student;
import com.andrew.util.HibernateUtil;
public class StudentIdTest {
    public static void main(String[] args) {
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
        Session session = sessionFactory.openSession(); // 1. 生成一个session
        session.beginTransaction(); // 2. 开启事务
        Student s = new Student();
        s.setName("张三");
        session.save(s);
        session.getTransaction().commit(); // 3. 提交事务
        session.close(); // 4. 关闭session
    }
}

1) increment由Hibernate自动以递增的方式生成标识符，适用代理主键

Student.hbm.xml

<generator class="increment"/>

StudentIdTest.java运行结果：
Hibernate: select max(stuId) from t_student
Hibernate: insert into t_student (name, stuId) values (?, ?)

2) identity由底层数据库生成标识符；适用代理主键

Student.hbm.xml

<generator class="identity"/>

StudentIdTest.java运行结果：
Hibernate: insert into t_student (name) values (?)

3) sequcence由Hibernate根据底层数据库的序列来生成标识符；适用代理主键

4) hilo由Hibernate根据high/low算法来生成标识符。适用代理主键

Student.hbm.xml

<generator class="hilo"/>

StudentIdTest.java运行结果：
Hibernate: select next_hi from hibernate_unique_key for update
Hibernate: update hibernate_unique_key set next_hi = ? where next_hi = ?
Hibernate: insert into t_student (name, stuId) values (?, ?)

5) native根据底层数据库对自动生成标识符的支持能力，来选择identity,sequence或hilo；适用代理主键

Student.hbm.xml

<generator class="native"/>

StudentIdTest.java运行结果：
Hibernate: insert into t_student (name) values (?)