关于hibernate配置文件中，hibernate.hbm2ddl.auto值的设置？

[fishfanfan]

配置文件中属性hibernate.hbm2ddl.auto设置为create，在调用add()方法的时候，能正确插入数据，为什么调用query方法的时候会出现空指针异常，而且查询完毕后，数据库中表的内容被清空

请大家帮兄弟看看，感激不尽！
Hibernate.cfg.xml:

<property name="hibernate.hbm2ddl.auto">create</property>


执行代码：

public static void main(String[] args) {
add();
query(2);
}

static void add() {
Session s = null;
Transaction tx = null;

Department depart = new Department();
depart.setName("depart");
Set<Employee> emps = new HashSet<Employee>();

Employee emp1 = new Employee();
emp1.setDepart(depart);
emp1.setName("emp1");
emp1.setBirthday(new Date());

Skill skiller1 = new Skill();
skiller1.setDepart(depart);
skiller1.setSkill("skill");
skiller1.setName("skiller1");

Saler saler = new Saler();
saler.setDepart(depart);
saler.setSale(100);
saler.setName("saler");

emps.add(emp1);
emps.add(skiller1);
emps.add(saler);

depart.setEmps(emps);

try {
s = HibernateUtil.getSession();
tx = s.beginTransaction();

s.save(depart);
s.save(emp1);
s.save(skiller1);
s.save(saler);

tx.commit();
} catch (Exception e) {
e.printStackTrace();
} finally {
if (s != null) {
s.close();
}
}
}

static Employee query(int id) {
Session s = null;
Transaction tx = null;

try {
s = HibernateUtil.getSession();
tx = s.beginTransaction();

Employee emp = (Employee)s.get(Employee.class, id);
System.out.println(emp.getClass());

tx.commit();
return emp;
} finally {
if (s != null) {
s.close();
}
}
}

[fishfanfan]

补充： 别人同样的设置和代码，却可以执行成功而不报异常

[xuexijava]

值设为Update