hibernate插入数据错误！
第一次写hibernate 一个简单的插入数据代码  
数据库为mysql 表格名stu 列 username varchar primary key,password varchar
持久化类 reg 
对应的xml reg.hbm.xml
<?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="HPO">
  <class name="reg" table="stu">
<id name="username" column="username">
<generator class="native"/>
</id>
<property name="password" type="string" column="password"/>
</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://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<!-- Generated by MyEclipse Hibernate Tools. -->
<hibernate-configuration>
  <session-factory>
  <property name="connection.username">root</property>
  <property name="connection.url">jdbc:mysql://localhost:3306/student</property>
  <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
  <property name="myeclipse.connection.profile">com.mysql.jdbc.Driver</property>
  <property name="connection.password">root</property>
  <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
  <mapping resource="HPO/reg.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

代码运行后

javax.servlet.ServletException: could not insert: [HPO.reg]
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:523)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


root cause 

org.hibernate.exception.GenericJDBCException: could not insert: [HPO.reg]
org.hibernate.exception.SQLStateConverter.handledNonSpecificException(SQLStateConverter.java:103)
org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:91)
org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:1986)
org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2405)
org.hibernate.action.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:37)
org.hibernate.engine.ActionQueue.execute(ActionQueue.java:248)
org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:269)
org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:167)
org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:101)
org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:186)
org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)