Javaweb项目获取信息是如果先登录管理员查看用户信息就报空指针异常，如果先登其他用户再登管理员查看信息则只显示之前登录的用户信息
public String userlist(){
HttpServletRequest request = ServletActionContext.getRequest();
HttpSession session = request.getSession();
Students students = (Students)session.getAttribute("students");
String username = request.getParameter("username");
StringBuffer sb = new StringBuffer();
sb.append(" where ");
if(username !=null &&!"".equals(username)){
sb.append(" username like '%"+username+"%' ");
request.setAttribute("username", username);
}
if(students.getRole()==2){
sb.append(" stuid="+students.getStuid()+" and deletestatus=0 order by id desc ");//降序查询，ASC
}
if(students.getRole()==1){
sb.append(" role=2 and deletestatus=0 order by id desc ");//降序查询，ASC
}
if(students.getRole()==3){
sb.append(" role=2 and deletestatus=0 order by id desc ");
}
String where = sb.toString();
int currentpage = 1;
int pagesize = 10;
if(request.getParameter("pagenum") != null){
currentpage = Integer.parseInt(request.getParameter("pagenum"));
}
long total = studentsDao.selectBeanCount(where.replaceAll("order by id desc", ""));
List<Students> list = studentsDao.selectBeanList((currentpage-1)*pagesize, pagesize, where);
request.setAttribute("list", list);
String pagerinfo = Pager.getPagerNormal((int)total, pagesize, currentpage, "method!userlist", "共有"+total+"条记录");
request.setAttribute("pagerinfo", pagerinfo);
