Servlets Servlet view.forward（请求、响应）方法中出现空指针异常

main.jsp文件中的Javascript

    <script>
        var form = null;
        var item = null;
        window.onload = init;
        function init(){
            form = document.getElementsByTagName('form')[0];                    
            item = document.getElementById("selectedItem"); 

        }
        function handleCellClick(e)
        {
            item.value = e.innerHTML;
            alert(item.value);
            form.submit();
        }
    </script>

登录并提交后，它将转到main.jsp 当我点击出纳信息时，它会提醒提交执行servlet 但servlet在最后一行给出了NullPointerException 该行包含

            view.forward(request, response);

HTTP状态500-java.lang.NullPointerException .... .... com.program.web.MainProgram.doGetMainProgram.java:31

我现在停下来思考 我不明白为什么会这样

请对此进行指导

您已发布到…/Main.do，并尝试在Main.jsp获取servlet的dispacher。服务器遵从并在…/Main.do/Main.jsp处查找servlet：-

我假设它找不到任何内容，并且视图为空

相对URL是一个持续的危险，可以使用它，但要小心

但是+1用于直接提供所有相关信息

public class MainProgram extends HttpServlet {

    public void doGet (HttpServletRequest request, HttpServletResponse response) 
        throws IOException, ServletException {

    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String jsp =null;
    String selectedItem = request.getParameter("selectedItem");



    HttpSession session = request.getSession(false);
    if(session==null){
        jsp = "login.jsp";
        RequestDispatcher view = request.getRequestDispatcher(jsp);
        view.forward(request, response);
    }else{
        jsp = "user_info.jsp";
        RequestDispatcher view = request.getRequestDispatcher(jsp);
        view.forward(request, response);     // this line gives NullPointerException
    }
}

public void doPost (HttpServletRequest request, HttpServletResponse response) 
        throws IOException, ServletException {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        String userName = request.getParameter("txtUserName");  
        String jsp = null;                      //clickme page

        if(userName.equals("Admin") || userName.equals("Operator") || userName.equals("Guest")){
            HttpSession session = request.getSession();
            session.setAttribute("regUser", userName);
            jsp = "main.jsp";
        }else{
            jsp = "login.jsp";      
        }
        RequestDispatcher view = request.getRequestDispatcher(jsp);
        view.forward(request, response);
    }
}

            view.forward(request, response);