初接触hibernate基础问题

初学者刚接触hibernate，按书本上的例子，用hibernate创建个表并插入条记录，问题是每次运行不插入新记录而是更新了原来的记录，表里永远只有第一条记录，这是哪里的原因？看书上描述和网上查询好像是能执行插入的啊
配置文件<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="hibernate.dialect">org.hibernate.dialect.SQLServerDialect</property>
    <property name="hibernate.connection.driver_class">net.sourceforge.jtds.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:jtds:sqlserver://localhost:1433/Northwind</property>
    <property name="hibernate.connection.username">sa</property>
    <property name="hibernate.connection.password">sa</property>
    <property name="hibernate.hbm2ddl.auto">create</property>
    <property name="hibernate.show_sql">true</property>
    <mapping resource="Test.hbm.xml"/>
  </session-factory>
</hibernate-configuration>Test.hbm.xml映射文件<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="db">
  <class name="Test" table="t_test">
      <id name="id" unsaved-value="null">
          <generator class="identity"/>
      </id>
      <property name="title"/>
  </class>
</hibernate-mapping>Test类package db;
public class Test {
    private int id;
    private String title;
    public void setId(int id){
        this.id=id;
    }
    public int getId(){
        return (this.id);
    }
    public void setTitle(String title){
        this.title=title;
    }
    public String getTitle(){
        return (this.title);
    }
    public void test(){
    }
}        Configuration conf=new Configuration().configure();
        SessionFactory sf=conf.buildSessionFactory();
        Session sess=sf.openSession();
        Transaction tx=sess.beginTransaction();
        Test t=new Test();
        t.setTitle("hello");
        sess.save(t);
        tx.commit();
        sess.close();