hibernate自动删除以前的表

[lg_asus]

<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
	<session-factory name="foo">
		<property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
		<property name="hibernate.connection.url">jdbc:oracle:thin:@127.0.0.1:1521:test</property>
		<property name="hibernate.connection.username">test</property>
		<property name="hibernate.connection.password">test</property>
		<property name="hibernate.dialect">org.hibernate.dialect.Oracle10gDialect</property>
		<property name="hiernate.hbm2ddl.auto">validate</property>
		<property name="hibernate.show_sql">true</property>
		<mapping resource="com/asm/hibernate/domain/Student.hbm.xml"/>
		<mapping resource="com/asm/hibernate/domain/Teacher.hbm.xml"/> 
	</session-factory>
</hibernate-configuration>

这是我的hibernate.cfg.xml文件，不管我把hbm2ddl.auto配置成什么值，结果都会把以前建的表删除掉。我的java代表中并没有创建表的语句，请问这是为什么啊？？？？
小弟刚学hibernate，请大侠们帮帮忙，只用到hibernate，没有struts spring.

[soo520]

<!-- 启动时删数据库中的表，然后创建，退出时不删除数据表
<property name="hbm2ddl.auto">create</property>-->
<!-- 启动时删数据库中的表，然后创建，退出时自动删除所有表
<property name="hbm2ddl.auto">create-drop</property>-->
<!-- 自动修改，如果表结构与实体类不一致，那么就更新表结构，数据会保留
（如果原表不存在，就创建新表;如果缺少相应的字段，就加入;对于原来存在的多余字段，不作处理）
<property name="hbm2ddl.auto">update</property>-->
<!-- 自动校验，如果表结构与实体类不一致，那么不做任何操作，报错
<property name="hbm2ddl.auto">validate</property>-->

[lg_asus]

to 1L:这些值含义我都知道啊，问题就是我无论设置成何值都会删除表。。。

附上其它的代码吧：
Student.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="com.asm.hibernate.domain">
	<class name="Student">
		<id name="id">
			<generator class="native"></generator>
		</id>
		<property name="name"></property>
		<set name="teachers" table="teacher_student">
			<key column="student_id"/>
			<many-to-many class="Teacher" column="teacher_id"></many-to-many>
		</set>
	</class>
</hibernate-mapping>

Teacher.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="com.asm.hibernate.domain">
	<class name="Teacher">
		<id name="id">
			<generator class="native"></generator>
		</id>
		<property name="name"></property>
		<set name="students" table="teacher_student">
			<key column="teacher_id"/>
			<many-to-many class="Student" column="student_id"></many-to-many>
		</set>
	</class>
</hibernate-mapping>

每次执行都会：
drop table Student cascade constraints
drop table Teacher cascade constraints
drop table teacher_student cascade constraints
drop sequence hibernate_sequence、
连hibernate_sequence都删除并重建了。真的不可思议。。
开发环境是eclipse，只用到了hibernate，如果要源码的话可以提供

[lg_asus]

com.asm.hibernate.domain就是放java文件的包了，Teacher.hbm.xml和Student.hbm.xml是在这个包下

hibernate.cfg.xml是在src根目录下

public class Student {

	private int id;
	private String name ;
	private Set<Teacher> teachers;
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Set<Teacher> getTeachers() {
		return teachers;
	}
	public void setTeachers(Set<Teacher> teachers) {
		this.teachers = teachers;
	}
	
}

上面的是Student.java,Teacher.java和这一样。
测试代码：

s = HibernateUtil.getSession();
			tr = s.beginTransaction();
			Teacher t1 = new Teacher();
			t1.setName("t1Name");
			Teacher t2 = new Teacher();
			t2.setName("t2Name");
			Student s1 = new Student();
			s1.setName("s1Name");
			Student s2 = new Student();
			s2.setName("s2Name");
			Set<Teacher> ts = new HashSet<Teacher>();
			ts.add(t1);
			ts.add(t2);
			Set<Student> ss = new HashSet<Student>();
			ss.add(s1);
			ss.add(s2);
			t1.setStudents(ss);
			t2.setStudents(ss);
			//s1.setTeachers(ts);
			//s2.setTeachers(ts);
			s.save(s1);
			s.save(s2);
			s.save(t1);
			s.save(t2);
			tr.commit();

控制台上显示的sql信息：
drop table Student cascade constraints
drop table Teacher cascade constraints
drop table teacher_student cascade constraints
drop sequence hibernate_sequence
create table Student (id number(10,0) not null, name varchar2(255 char), primary key (id))
create table Teacher (id number(10,0) not null, name varchar2(255 char), primary key (id))
create table teacher_student (student_id number(10,0) not null, teacher_id number(10,0) not null, primary key (teacher_id, student_id))
alter table teacher_student add constraint FK2E2EF2DEA1548DAC foreign key (teacher_id) references Teacher
alter table teacher_student add constraint FK2E2EF2DE90B9430C foreign key (student_id) references Student
create sequence hibernate_sequence
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: insert into Student (name, id) values (?, ?)
Hibernate: insert into Student (name, id) values (?, ?)
Hibernate: insert into Teacher (name, id) values (?, ?)
Hibernate: insert into Teacher (name, id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
每次都是这样的sql信息

[lg_asus]

时隔2周，无奈只能删除重建项目，这次竟然成功了

我都不知道为什么。。。

[王莹月]

你是成功了，哥杯具了，搞死了