javaweb工程的一个功能无法实现请求帮忙 报错

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在写一个类似于校园图书馆的网页工程，在还书功能那里遇到麻烦了
用的是mysql5.6+myeclipse10 其他功能都运行良好
在DAO层
/还书/

public void backBook(int bkid, int stid) throws Exception{ // TODO Auto-generated method stub /删除记录/ String sql = "delete record where bookid = " + bkid + " and studentid = " + stid; this.openconnection(); this.conn.setAutoCommit(false); Statement st = conn.createStatement();

st.executeUpdate(sql);
/*把书的数量+1*/
int oldnumber = 0;
String sql2 = "select booknumber from book where bookid = " + bkid;
PreparedStatement st2 = conn.prepareStatement(sql2);
ResultSet rs = st2.executeQuery();
if (rs.next())
    oldnumber = rs.getInt("booknumber");
int newnumber = oldnumber + 1;
String sql3= "update book set booknumber = " + newnumber + " where bookid = " + bkid;
Statement st3 = conn.createStatement();
st3.executeUpdate(sql3);
this.closeResource();

}

在BIZ层

public void backBook(int bkid, int stid) throws Exception{

    // TODO Auto-generated method stub
    BookDao bkd = new BookDao();
    bkd.backBook(bkid, stid);
}

SEVLET里

public void service(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    String bookid = request.getParameter("bookid");
    String studentid = request.getParameter("studentid");
    
    int bkid = Integer.parseInt(bookid);
    int stid = Integer.parseInt(studentid);
    BookBiz bkb = new BookBiz();
    try{
        bkb.backBook(bkid, stid);
        request.setAttribute("errMsg", "还书成功！");
        request.getRequestDispatcher("error.jsp").forward(request, response);
    }catch(Exception e){
        Log.logger.error(e.getMessage());
        request.setAttribute("errMsg", "还书失败，请稍后再试！");
        request.getRequestDispatcher("error.jsp").forward(request, response);
    }
    
}

运行的结果是，还书失败请稍后再试

但是没有报错，已经不知道如何改，求帮助