java - servlet jsp 和 servlet 中的异常 java.lang.NullPointerException

Question

我有一个 Servlet 源代码。

SinhVien sv= new SinhVien();
    String url="../lienhepage.jsp";
    HttpSession session = request.getSession(true);
    //RequestDispatcher rD;
    //HttpSession session;
    if(request.getParameter("searchword")!= ""){
        String result= "";
        result= ZendCodeHTML.ZendCodeTag(sv.getDataTag(request.getParameter("searchword")));
        response.setContentType("text/plain");
        response.setCharacterEncoding("UTF-8"); 
        response.getWriter().write(result);
    }
    if(request.getParameter("NienKhoa") != ""){
        List<String> result= getData(request.getParameter("NienKhoa"));
        session.setAttribute("tieude", result.get(0)); 
        session.setAttribute("danhsachsinhvien", result.get(1)); 
        session.setAttribute("cacmongiangday", result.get(2)); 
        session.setAttribute("noidunghuongdanmonhoc", result.get(3));     
    }
    RequestDispatcher rD= request.getRequestDispatcher(url);
    rD.forward(request, response);

当我单击页面jsp中的链接时，url =“../lienheservlet？NienKhoa = 2010-2015”，然后代码servlet运行正常。但是当代码运行函数 rD.forward(request, response); 时会出现这样的异常:

java.lang.NullPointerException
javacore.controller.lienheservlet.doGet(lienheservlet.java:115)
javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)

不知道为什么？请帮助我!

Answer 1

我认为这是因为网址不正确，

尽量不要使用相对路径，而使用绝对路径。 (例如“/lienhepage.jsp”)