一个非常奇怪的问题，你回答了一定有成就感。

小弟最近在使用hibernate做一个简单实践的时候出现一个非常奇怪的问题，先将我本机环境说一下myeclipse+mysql5
没有使用中间件，插入数据和得到数据显示，我是直接在myeclipse控制台打印出来的。

1.在mysql上建立表

-- Table "student" DDL

CREATE TABLE `student` (

  `id` int(100) NOT NULL AUTO_INCREMENT,

  `username` varchar(100) DEFAULT NULL,

  `sex` varchar(10) DEFAULT NULL,

  `age` int(100) DEFAULT NULL,

  PRIMARY KEY (`id`)

) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=gbk;

2.工程加入hibernate支持，在hibernate 的date视图上直接将表反转生成相关代码，唯一修改代码的Student.hbm.xml 内容如下:

<hibernate-mapping>

    <class name="dao.Student" table="student" catalog="db">

        <id name="id" type="java.lang.Integer">

            <column name="id" />

            <generator class="increment" />

        </id>

        <property name="userName" type="java.lang.String">

            <column name="userName" length="100" />

        </property>

        <property name="age" type="java.lang.Integer">

            <column name="age" />

        </property>

        <property name="sex" type="java.lang.String">

            <column name="sex" length="10" />

        </property>

    </class>

</hibernate-mapping>

3.我写了一个类来验证studentDao的 save()方法

package test;



import java.util.List;



import org.hibernate.Transaction;



import dao.Student;

import dao.StudentDAO;



public class testDate {



	/**

	 * @param args

	 */

	public static void main(String[] args) {

		StudentDAO dao = new StudentDAO();

		Student stu  = new Student();

		stu.setUserName("wwww");

		stu.setSex("女");

		stu.setAge(24);

		

		Transaction t = dao.getSession().beginTransaction();

		dao.save(stu);

		t.commit();

		

		//StudentDAO dao = new StudentDAO();

		List<Student> stus = dao.findAll();

		

		for(Student st: stus){

			System.out.println(st.getId());

			System.out.println(st.getUserName());

		}



	}



}

4.如果mysql数据库没有数据的话插入正常，如果有数据则提示如下错误

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
select
max(id)
from
student
Hibernate:
insert
into
db.student
(userName, age, sex, id)
values
(?, ?, ?, ?)
Exception in thread "main" org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:71)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:202)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:235)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:139)
at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:297)
at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:27)
at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:985)
at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:333)
at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:106)
at test.testDate.main(testDate.java:24)
Caused by: java.sql.BatchUpdateException: Duplicate entry '3' for key 'PRIMARY'
at com.mysql.jdbc.ServerPreparedStatement.executeBatch(ServerPreparedStatement.java:853)
at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:58)
at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:195)
... 8 more


请问何解？？？求达人赐教，在线等啊！！！！！！！