hibernate之主键生成策略

1. hibernate的主键生成器:

generator元素：表示了一个主键生成器，它用来为持久化类实例生成唯一的标识 。

1）数据库控制:

identity(标识列/自动增长)
sequence（不建议用：1、出现跳号2、使用这种配置的情况下可以指定序列3、跨平台）.

2）程序员自己控制：

assigned标识符生成器指定由应用程序生成并维护持久化对象的标识符。

3） hibernate控制:

increment uuid/uuid.hex
适用于代理主键，Hibernate采用128位的UUID算法生成标识符。

2. 主键生成器要求

1 assigned
数据类型不限、保存前必须赋值

2 identity(重点掌握)
数字，无需赋值

3 sequence(重点掌握)
数字，无需赋值， 默认使hibernate_sequence这个序列，
也可以通过sequence/sequence_name参数赋值

4 increment
数字，无需赋值

5 uuid/uuid.hex (是由容器自动生成的一个32位的字符串，.hex代表的是十六进制)
32位的字符串，无需赋值，

6 native(重点掌握)
等于identity+sequence

案例(基于上次的hibernate入门)

1、创建一个实体类、以及相对应的配置文件

Student

package com.wyy.two.entity;

import java.io.Serializable;

public class Student implements Serializable{
	private Integer sid;
	private String sname;
	public Integer getSid() {
		return sid;
	}
	public void setSid(Integer sid) {
		this.sid = sid;
	}
	public String getSname() {
		return sname;
	}
	public void setSname(String sname) {
		this.sname = sname;
	}
	@Override
	public String toString() {
		return "Student [sid=" + sid + ", sname=" + sname + "]";
	}
	
	
}

Worker

package com.wyy.two.entity;

public class Worker {
	private String wid;
	private String wname;
	public String getWid() {
		return wid;
	}
	public void setWid(String wid) {
		this.wid = wid;
	}
	public String getWname() {
		return wname;
	}
	public void setWname(String wname) {
		this.wname = wname;
	}
	@Override
	public String toString() {
		return "Worker [wid=" + wid + ", wname=" + wname + "]";
	}
	
}

Student.xbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.wyy.two.entity.Student" table="t_hibernate_student">
		<id name="sid" type="java.lang.Integer" column="sid">
			<!-- <generator class="assigned" /> -->
<!-- 			<generator class="increment" /> -->
			<!-- <generator class="native" /> -->
			<generator class="increment" />
			<!-- <generator class="sequence" > <param name="sequence_name">aaa</param> 
				</generator> -->
			<!-- <generator class="com.javaxl.two.id.Myts" /> -->
		</id>
		<property name="sname" type="java.lang.String" column="sname">
		</property>
	</class>
</hibernate-mapping>

Worker.xbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.wyy.two.entity.Worker" table="t_hibernate_worker">
		<id name="wid" type="java.lang.String" column="wid">
<!-- 			<generator class="uuid" /> -->
			<!-- <generator class="sequence" /> -->
			<!-- <generator class="sequence" > <param name="sequence_name">aaa</param> 
				</generator> -->
			<generator class="com.wyy.two.id.Myts" />
		</id>

		<property name="wname" type="java.lang.String" column="wname">
		</property>
	</class>
</hibernate-mapping>

2、创建一个辅助类

SessionFactoryUtils

package com.wyy.two.util;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
/**
 * 这个类是在学习hibernate的过程所用（整合SSH框架之前用）s:spring s:struts h:hibernate
 * 
 * 作用：可以用来检测所写的映射文件是否正确
 * @author 20190614
 *
 */
public class SessionFactoryUtils {
	private static SessionFactory sessionFactory;
//	存放当前会话
	private static ThreadLocal<Session> threadLocal = new ThreadLocal<Session>();
	static {
		Configuration cfg = new Configuration();
		Configuration configure = cfg.configure("/hibernate.cfg.xml");
		sessionFactory = configure.buildSessionFactory();
	}
	
	public static Session openSession() {
		Session session = threadLocal.get();
		if (null == session) {
			session = sessionFactory.openSession();
			threadLocal.set(session);
		}
		return session;
	}

	public static void closeSession() {
		Session session = threadLocal.get();
		if (null != session) {
			if (session.isOpen()) {
				session.close();
			}
			threadLocal.set(null);
		}
	}

	public static void main(String[] args) {
		Session session = openSession();
		System.out.println(session.isConnected());
		closeSession();
	}
	
}

3、创建一个dao层

DemoDao

package com.wyy.two.dao;

import java.io.Serializable;

import org.hibernate.Session;
import org.hibernate.Transaction;

import com.wyy.two.entity.Student;
import com.wyy.two.entity.Worker;
import com.wyy.two.util.SessionFactoryUtils;

public class DemoDao {
	/**
	 * 添加学生
	 * @param stu
	 * @return
	 */
	public Serializable addStudent(Student stu) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Serializable saveId = session.save(stu);
		transaction.commit();
		session.close();
		return saveId;
		
	}
	
	/**
	 * 添加工人
	 * @param worker
	 * @return
	 */
	public Serializable addWork(Worker worker) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Serializable saveId = session.save(worker);
		transaction.commit();
		session.close();
		return saveId;
		
	}
	
	
	public static void testStudent(String[] args) {
		DemoDao dao = new DemoDao();
		Student stu = new Student();
//		stu.setSid(60);
		stu.setSname("大哈");
		System.out.println(dao.addStudent(stu));
		
	}
	
	
	public static void main(String[] args) {
		DemoDao dao = new DemoDao();
		Worker worker = new Worker();
		worker.setWname("大哈");
		System.out.println(dao.addWork(worker));
		
	}
}

4、创建一个自定义主键ID生成器

创建主键生成器类
实现org.hibernate.id.IdentifierGenerator接口即可,并还可以实现org.hibernate.id.Configurable接口来读取一些配置信息

Myts

package com.wyy.two.id;

import java.io.Serializable;
import java.text.SimpleDateFormat;
import java.util.Date;

import org.hibernate.HibernateException;
import org.hibernate.engine.spi.SharedSessionContractImplementor;
import org.hibernate.id.IdentifierGenerator;

public class Myts implements IdentifierGenerator{

	@Override
	public Serializable generate(SharedSessionContractImplementor arg0, Object arg1) throws HibernateException {
		// TODO Auto-generated method stub
		SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
		return "wyy_shop_book_"+sdf.format(new Date());
	}

}

效果

id带时间效果的（native）