hibernate之主键生成策略

最新推荐文章于 2022-10-12 15:23:26 发布

原创最新推荐文章于 2022-10-12 15:23:26 发布 · 188 阅读

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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。

主键生产方式

程序员自己控制：assigned
数据库控制: identity(标识列/自动增长)sequence
sequence 1.标识列/自动增长，在mysql编程时一般不用，在默认的情况下会hibernate会自动调用sequence，多个表用，存在跳号现象
2.在使用sequence的情况下可以指定序列号
3.跨平台（跨数据库使用）
hibernate控制:increment（数据库没有设置自增长，序号也可以自增长） uuid/uuid.hex
其它:native 公司名_模块名_时间戳

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

案列

先导入辅助工具库

package com.tt.two.util;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
/**
 * 这个类是在学习hibernate 的过程中所用（整合SSH框架之前用）
 * 作用：
 *    可以用来检测所写的映射文件是否正确
 * @author tt
 */
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();
	}
}

实体类student

package com.tt.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 + "]";
	}
}

student.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.tt.two.entity.Student" table="t_hibernate_student">
		<id name="sid" type="java.lang.Integer" column="sid">
			<!-- <generator class="assigned" /> -->
            <!-- <generator class="increment" /> -->
			<!-- <generator class="identity" /> -->
			<!-- <generator class="sequence" > <param name="sequence_name">aaa</param> 
			</generator> -->
			<generator class="com.tt.two.id.Myts" />
		</id>
		<property name="sname" type="java.lang.String" column="sname">
		</property>
	</class>
</hibernate-mapping>

实体类worker

package com.tt.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 + "]";
	}
}

Worker.hbm.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.tt.two.entity.Worker" table="t_hibernate_worker">
		<id name="wid" type="java.lang.String" column="wid">
			<!-- <generator class="assigned" /> -->
			<!-- <generator class="uuid" /> -->
			<!-- <generator class="sequence" > <param name="sequence_name">aaa</param> 
				</generator> -->
			<generator class="com.tt.two.id.Myts" />
		</id>

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

DemoDao.java

package com.tt.two.dao;
import java.io.Serializable;
import org.hibernate.Session;
import org.hibernate.Transaction;
import com.tt.two.entity.Student;
import com.tt.two.entity.Worker;
import com.tt.two.util.SessionFactoryUtils;

public class DemoDao {
	/**
	 * 添加学生
	 * @param stu
	 * @return
	 */
	public Serializable addStudent(Student stu) {
		//获取session对象
		Session session = SessionFactoryUtils.openSession();
		//session开始
		Transaction transaction = session.beginTransaction();
		//操作数据库
		Serializable saveId = session.save(stu);
		//提交
		transaction.commit();
		//关闭
		session.close();
		return saveId;
	}

	/**
	 * 添加工人
	 * @param worker
	 * @return
	 */
	public Serializable addWorker(Worker worker) {
		//获取session对象
		Session session = SessionFactoryUtils.openSession();
		//session开始
		Transaction transaction = session.beginTransaction();
		//操作数据库
		Serializable saveId = session.save(worker);
		//提交
		transaction.commit();
		//关闭
		session.close();
		return saveId;
	}
	
	
	public static void testStudent(String[] args) {
		DemoDao dao = new DemoDao();
		Student stu = new Student();
//		stu.setSid(3);
		stu.setSname("增大包");
		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.addWorker(worker));
	}
}

写一个类来控制id的生产。格式为tt_shop_book_2019-07-14 20:26:02

Myts.java

package com.tt.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 {
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
		return "tt_shop_book_"+sdf.format(new Date());
	}

}

自定义主键生成器

1 *.hbm.xml指定主键生成器类

2 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
PersistentIdentifierGenerator.TABLE
PersistentIdentifierGenerator.PK

assigned、native、自定义主键