我有一个servlet，它为用户做一些工作，然后减少用户的信用。当我实时查看数据库中的用户信用时，如果有来自同一用户的许多并发请求，则由于并发控制，信用被错误地扣除。假设我有一台服务器，并且数据库管理处于hibernate状态。我正在使用事务控制来处理整个请求，请参见代码以获取详细信息。我有几个问题：

1. 当面对来自同一用户的多个并发请求时，为什么db中的信用计数器到处跳？为什么我的交易控制不起作用？

2. 如果在检索用户帐户然后尝试更新之后修改了基础数据，为什么我没有得到任何数据HibernateException(eg.StaleObjectException)？

3. 我的交易跨整个用户请求，是否有更好的方法？请批评。如果您觉得我做错了所有事情，请随时重写示例代码结构。

Main servlet class:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

            try{
                Manager.beginTransaction();
                cmdDowork(request, response);
                Manager.commitTransaction();
            }catch(Exception exp){
                Manager.rollbackTransaction();
                exp.printStackTrace();
            }
            finally{
                Manager.closeSession();
            }
}

public void cmdDowork(){
try{
     UserAccount userAccount = lazyGetUserAccount(request.getParameter("userName"));

     doWorkForUser(userAccount);//time and resource consuming process

     if(userAccount!=null) {

    decUserAccountQuota(userAccount);

     }

}catch (HibernateException e){
    e.printStackTrace();

}
}

public static UserAccount lazyGetUserAccount(String userName) {
        UserAccount userAccount = Manager.getUserAccount(userName);
        if(userAccount == null){
            userAccount = new UserAccount(userName);
            userAccount.setReserve(DEFAULT_USER_QUOTA);
            userAccount.setBalance(DEFAULT_USER_QUOTA);
            Manager.saveUserAccount(userAccount);
        }
     return userAccount;
}
    private boolean decUserAccountQuota(UserAccount userAccount) {

        if(userAccount.getBalance() 

Edit: code I used to test optimistic locking as suggested by the answer, I am not getting a
any StaleObjectException, the update were committed successfully..

Session em1=Manager.sessionFactory.openSession();
         Session em2=Manager.sessionFactory.openSession();



    em1.getTransaction().begin();
    em2.getTransaction().begin();
    UserAccount c1 = (UserAccount)em1.get( UserAccount.class, "jonathan" );
    UserAccount c2 = (UserAccount)em2.get( UserAccount.class, "jonathan" );
    c1.setBalance( c1.getBalance() -1 );
    em1.flush();
    em1.getTransaction().commit();






    System.out.println("balance1 is "+c2.getBalance());
    c2.setBalance( c2.getBalance() -1 );
    em2.flush(); // fail
    em2.getTransaction().commit();

    System.out.println("balance2 is "+c2.getBalance());