这个为什么不能正常查询显示呢（显示空指针异常）？

查询页面：

<form action="search" method="post" name="myform">
    <table>
        <tr>
            <td>学生学号：</td>
            <td><input type="text" class="sid"></td>
        </tr>
        <tr>
            <td>学生姓名:</td>
            <td><input type="text" class="name"></td>
        </tr>
        <tr>
            <td>学生性别：</td>
            <td><input type="text" class="sex"></td>
        </tr>
        <tr>
            <td>学生政治面貌：</td>
            <td><input type="text" name="politicsstatus"></td>
        </tr>
        <tr>
            <td align="center"><input type="submit" name="submit" value="查询"></td>
            <td><input type="reset" name="reset" value="重置"></td>
        </tr>
    </table>
</form>

执行的servlet：

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    String sid=req.getParameter("sid");
    String name=req.getParameter("name");
    String sex=req.getParameter("sex");
    String politicsstatus=req.getParameter("politicsstatus");
    dbconnection dbc=new dbconnection();
    List<Student> list=new ArrayList<Student>();
    try {
        list=dbc.searchstudent(sid,name,sex,politicsstatus);
        req.setAttribute("lists",list);
        RequestDispatcher requestDispatcher = req.getRequestDispatcher("out.jsp");
        requestDispatcher.forward(req,resp);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

//dbconnetion部分：

 //模糊查询学生信息
public List<Student> searchstudent(String sid,String name,String sex,String politicsstatus) throws Exception{
     List<Student> list=new ArrayList<Student>();
     String sql=" SELECT * FROM student WHERE (1=1  ";
     if(name != null){
         sql+=" AND Sname LIKE '%"+name+"%' ";
     }
     if (sid !=null){
         sql+=" AND Sid LIKE '%"+sid+"%' ";
     }
     if (sex !=null){
         sql+=" AND SSex LIKE '%"+sex+"%' ";
     }
     if (politicsstatus != null){
         sql+=" AND SPoliticsStatus LIKE '%"+politicsstatus+"%' ";
     }
     sql+=")";
     ResultSet rs=this.executeQuery1(sql);
     try {
         while (rs.next()){
             Student student=new Student();
             student.setSid(rs.getString(1));
             student.setName(rs.getString(2));
             student.setClasses(((rs.getInt(3))));
             student.setSex(rs.getString(4));
             student.setEmail(rs.getString(5));
             student.setPoliticsstatus(rs.getString(6));
             student.setPassword(rs.getString(7));
             student.setAge(rs.getInt(8));
             student.setRole(rs.getString(rs.getString(9)));
             list.add(student);
         }
     }
     catch (SQLException e){
         return null;
     }
    return list;
 }