’Caused by: java.sql.SQLException: 违反了 PRIMARY KEY 约束'PK_book',在Hibernate的TestUn

使用Middlegen-Hibernate-r5，hbm2java.bat生成的.hbm.xml和Java文件
 
  但在Hibernate的TestUnit中插入对象时报错：
  “Caused by: java.sql.SQLException: 违反了 PRIMARY KEY 约束'PK_book'。不能在对象 'book' 中插入重复键。”
 
  下面是我的代码：
  /*
  * Created on 2005-3-20 dahe
  */
  package org.hibernate.sample;
 
  import java.util.List;
 
  import net.sf.hibernate.*;
  import junit.framework.*;
  import org.hibernate.sample.Book;
  import net.sf.hibernate.cfg.*;
 
  public class HibernateTest extends TestCase {
  Session session = null;
  protected void setUp() {
  try {
  Configuration config = new Configuration().configure();
  SessionFactory sessionFactory = config.buildSessionFactory();
  session = sessionFactory.openSession();
  } catch (HibernateException e) {
  e.printStackTrace();
  }
  }
 
  protected void tearDown() {
  try {
  session.close();
  } catch (HibernateException e) {
  e.printStackTrace();
  }
  }
 
  public void testInsert() {
  try {
  Book book = new Book();
  book.setBook("newbook"); //已经换了几次不同的名称，仍然说是重复插入
  book.setSn("aaa");
  Transaction tran=session.beginTransaction();
  session.save(book);
  tran.commit();
  } catch (HibernateException e) {
  e.printStackTrace();
  Assert.fail(e.getMessage());
  }
  }
 
  public void testSelect() {
  String hql = "FROM Book where book='Hibernate'";