session.invalidate（）拋出空指針異常

Question

當我使http會話無效時，它引發了一個空指針異常，如下所示：

2012年12月14日晚上9:49:52 org.apache.catalina.session.StandardSession過期嚴重：會話事件偵聽器引發異常java.lang.NullPointerException

建議。

Answer 1

在代碼中放置一些斷點，並在調試模式下使用喜歡的IDE來確認此“會話”引用變量是否指向空對象。

然后，只需檢查文檔以弄清圍繞getSession（）方法的所有細節（它可能會給您一些想法）： http : //tomcat.apache.org/tomcat-5.5-doc/servletapi/javax/servlet/http/ HttpServletRequest.html＃getSession％28boolean％29

Answer 2

我認為使用此方法可以解決您的問題，請嘗試一下。 由Gourav Gavate公共void doGet（HttpServletRequest請求，HttpServletResponse響應）引發ServletException，IOException {

    response.setContentType("text/html");
    PrintWriter out = response.getWriter();

    String op = request.getParameter("operation");
    System.out.println("operation valuse is ->"+op);
    LoginService service = null;
    ResultSet rs= null;
    HttpSession session=null;
    if (op.equals("Login")){
    //System.out.println("1");
    String un=request.getParameter("username");
    String psd=request.getParameter("password");
    try {
        rs = service.searchByUsernameandPassword(un, psd);
        //System.out.println("2");

         while (rs.next()) {
                //System.out.print(rp.getInt(1));
                //System.out.print("\t"+rs.getString(2));
            // System.out.println("in while");
             if(un.equals(rs.getString(3))&& psd.equals(rs.getString(4)))
             {
            //   System.out.println("in if");
             session=request.getSession(true);
             String name = rs.getString(1)+""+rs.getString(2);
             session.setAttribute("name",name);  
          //   System.out.println("set into session");
              response.sendRedirect("http://localhost:8080/Registration/Registration.jsp");
             }else{

                    response.sendRedirect("http://localhost:8080/Registration/Login.jsp");

             }
         }

    } catch (Exception e4) {
        System.out.println(e4);
    }

}   else    if (op.equals("Logout")){
    /*System.out.println("Into logout");
     if(session!=null){  

         System.out.println("Into logout if");    
            session.removeAttribute("name");
            session.invalidate();  
            response.sendRedirect("http://localhost:8080/Registration/Login.jsp");
     }else{
         System.out.println("Into logout else");
         //   session.removeAttribute("name");
              session.invalidate();  
              response.sendRedirect("http://localhost:8080/Registration/Login.jsp");
     }
    */
    HttpSession session1 = request.getSession(); 
    //session.removeAttribute("name");
    session1.invalidate();
    response.setHeader("Cache-Control","no-cache"); 
    response.setHeader("Cache-Control","no-store"); 
    response.setDateHeader("Expires", 0); 
    response.sendRedirect("http://localhost:8080/Registration/Login.jsp");

}

}