Hibernate主键生成
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、assigned:
在调用session.save()之前,必须手动setId(主键值),否则出错。
没有指定时的默认生成策略。
2、increment:自增长,为long, short或者int类型生成主键
自动增长,由hibernate维护,适用于所有的数据库,每次插入数据前先查询最大id(select max(id) from xxx),即使id已经设好值,无视。
缺点:不适合多进程并发更新数据库,适合单一进程访问数据库。不能用于群集环境。
3、identity:自增长
与increment不同的是:
a)identity不是由hibernate控制,而是由数据库控制自增长,所插入数据前不会查询最大id。
b)以Mysql为例,如果主键字段没有开启auto_increment,会抛异常"Field ‘id’ doesn’t have a default value"。
c)increment的insert语句包括id,identity的insert语句没有id字段。
c)假设有五条数据,id分别为1、2、3、4,删除(3、4)后,用increment增加数据id为3,用identity增加数据id为5.
不同的数据库有不同的自增长方式。如MySql是auto_increment,SQL Server 中是Identity。支持的数据库:MySql、SQL Server、DB2、Sybase和HypersonicSQL。
优点: 不需要Hibernate和用户的干涉,使用较为方便。
缺点:但不便于在不同的数据库之间移植程序。
4、其它:native等等
写一个辅助类,用来判断映射文件是否正确
package com.wangshaoyang.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框架之前应用)
* 作用:
* 可以用来检测所写的映射文件是否正确
* @author 20190320
*
*/
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.wangshaoyang.two.entity;
public class Student {
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.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.wangshaoyang.two.entity.Student" table="t_hibernate_student">
<id name="sid" type="java.lang.Integer" column="sid">
<generator class="assigned" />
<!-- <generator class="increment" /> -->
<!-- <generator class="sequence" /> -->
<!-- <generator class="sequence" > <param name="sequence_name">aaa</param>
</generator> -->
<!-- <generator class="com.wangshaoyang.id.Myts" /> -->
</id>
<property name="sname" type="java.lang.String" column="sname">
</property>
</class>
</hibernate-mapping>
Worker
package com.wangshaoyang.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.wangshaoyang.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.wangshaoyang.id.Myts" />
</id>
<property name="wname" type="java.lang.String" column="wname">
</property>
</class>
</hibernate-mapping>
配置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>
<!-- 1. 数据库相关 -->
<property name="connection.username">root</property>
<property name="connection.password">123</property>
<property name="connection.url">jdbc:mysql://localhost:3306/mysql?useUnicode=true&characterEncoding=UTF-8
</property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<!-- 配置本地事务(No CurrentSessionContext configured!) -->
<property name="hibernate.current_session_context_class">thread</property>
<!-- 2. 调试相关 -->
<property name="show_sql">true</property>
<property name="format_sql">true</property>
<!-- 3. 添加实体映射文件 -->
<mapping resource="com/wangshaoyang/entity/User.hbm.xml" />
<!-- 主键生成策略 -->
<mapping resource="com/wangshaoyang/two/entity/Student.hbm.xml" />
<mapping resource="com/wangshaoyang/two/entity/Worker.hbm.xml" />
</session-factory>
</hibernate-configuration>
写一个dao方法实现这些功能 Demodao
package com.wangshaoyang.two.dao;
import java.io.Serializable;
import org.hibernate.Session;
import org.hibernate.Transaction;
import com.wangshaoyang.two.entity.Student;
import com.wangshaoyang.two.entity.Worker;
import com.wangshaoyang.two.util.SessionFactoryUtils;
public class DemoDao {
/**
* 添加学生
* @param stu
* @return
*/
public Serializable addStu(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 testSudent(String[] args) {
DemoDao demoDao = new DemoDao();
Student stu = new Student();
// stu.setSid(1);
stu.setSname("aa");
System.out.println(demoDao.addStu(stu));
}
public static void main(String[] args) {
DemoDao demoDao = new DemoDao();
Worker worker = new Worker();
// stu.setSid(1);
worker.setWname("aa");
System.out.println(demoDao.addWork(worker));
}
}
时间生成
package com.wangshaoyang.two.id;
import java.io.Serializable;
import java.text.SimpleDateFormat;
import java.util.Date;
import org.hibernate.HibernateException;
import org.hibernate.engine.spi.SharedSessionContractImplementor;
public class Myts implements org.hibernate.id.IdentifierGenerator{
@Override
public Serializable generate(SharedSessionContractImplementor arg0, Object arg1) throws HibernateException {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
return "shop_book_"+sdf.format(new Date());
}
}
这样我们就得到了
今天就更新到这里
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