java.lang.IllegalStateException: Cannot forward after response has been committed in servlet [duplicate]

Question

In my project, I prohibit a user every page only if he is already logged on. That is why I wrote the below code. When I type in browser, for example, http://localhost:8080/JSP1/Students, I come to the login.jsp page. But after I input the loginid and password, only blank page http://localhost:8080/JSP1/Logged appears and GlassFish says there is an exception in

if (userPath.equals("/Students")){
        RequestDispatcher requestDispatcher = request.getRequestDispatcher("/Students.jsp");
        requestDispatcher.forward(request, response);
    }

java.lang.IllegalStateException: PWC1227: Cannot forward after response has been committed

Complete code for doGet and doPost:

 @Override
 protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

     HttpSession ses = request.getSession();
     String login = (String)ses.getAttribute("login");
     String password = (String)ses.getAttribute("password");
     if ((login==null)|(password==null)){
         RequestDispatcher requestDispatcher = request.getRequestDispatcher("/login.jsp");
         requestDispatcher.forward(request, response);
     } 

     //Now we think that we are successfully logged in

    String userPath = request.getServletPath();
   // System.out.println(userPath);

    if (userPath.equals("/Login")){
        RequestDispatcher requestDispatcher = request.getRequestDispatcher("/login.jsp");
        requestDispatcher.forward(request, response);
    }

    if (userPath.equals("/Students")){
        RequestDispatcher requestDispatcher = request.getRequestDispatcher("/Students.jsp");
        requestDispatcher.forward(request, response);
    }

     if (userPath.equals("/Student")){
        RequestDispatcher requestDispatcher = request.getRequestDispatcher("/Student.jspx");
        requestDispatcher.forward(request, response);
    }

     if (userPath.equals("/StudentEdit")){
        RequestDispatcher requestDispatcher = request.getRequestDispatcher("/StudentEdit.jsp");
        requestDispatcher.forward(request, response);
    }

}


@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
  //  processRequest(request, response);
    PrintWriter out = response.getWriter();

    String userPath = request.getServletPath();
    System.out.println(userPath);

     if (request.getRequestURI().equals("/Logged")){
            String Login = request.getParameter("login");
            String Password = request.getParameter("password");
            request.getSession().setAttribute("login", Login);
            request.getSession().setAttribute("password", Password);
            RequestDispatcher requestDispatcher = request.getRequestDispatcher("/Students.jsp");
            requestDispatcher.forward(request, response);

        }

    if (userPath.equals("/addStudent")) {

       //     System.out.println(request.getContextPath());
            String Name = request.getParameter("name");
            String Surname = request.getParameter("surname");
            String Login = request.getParameter("login");
            String Password = request.getParameter("password");
            Student student = new Student(Name,Surname,Login,Password);
            if (student != null) {
                dao.insertStudent(student);
            } else {
                System.out.println("Not valid parameter!!!");
            }
             RequestDispatcher requestDispatcher = request.getRequestDispatcher("/Students.jsp");
             requestDispatcher.forward(request, response);
        }

    if (request.getRequestURI().equals("/Edit")) {
            System.out.println("We work with students!!!");
            String delete = request.getParameter("Add_new_student");
            if (delete != null){
                RequestDispatcher requestDispatcher = request.getRequestDispatcher("/Student.jspx");
                requestDispatcher.forward(request, response);
            }

            Enumeration parameters = request.getParameterNames();
            while (parameters.hasMoreElements()) {
                String parameterName = (String) parameters.nextElement();
                String parameterValue = request.getParameter(parameterName);
                String norder = parameterName.substring(parameterName.indexOf("_")+1);

                ArrayList<Student> curStudents = dao.getAllStudents();
                int norderint = Integer.parseInt(norder);
                Student studentToWork = curStudents.get(norderint);

                String actionToDo = parameterName.substring(0, parameterName.indexOf("_"));

                if (actionToDo.equals("Edit")){

                RequestDispatcher requestDispatcher = request.getRequestDispatcher("/StudentEdit.jsp");
                ServletContext cont = request.getServletContext();
                cont.setAttribute("studentToEdit", studentToWork);
                requestDispatcher.forward(request, response);
                } else {
                    boolean attemp = dao.deleteStudent(studentToWork);
                    if (attemp){
                         RequestDispatcher requestDispatcher = request.getRequestDispatcher("/Students.jsp");
                         requestDispatcher.forward(request, response);
                    } else {
                        out.println("Unsuccessfull attemp to delete a Student");
                    }
                }
            }
        }

      if (userPath.equals("/EditStudent")){
            System.out.println("We work with StudentEdit!");
            Student studentToEdit = (Student)request.getSession().getAttribute("studentToEdit");
            String newName = request.getParameter("name");
            String newSurname = request.getParameter("surname");
            String newLogin = request.getParameter("login");
            String newPassword = request.getParameter("password");
            Student newStudent = new Student(newName, newSurname,newLogin,newPassword);
            boolean update = dao.updateStudent(studentToEdit, newStudent);
            if (update){
            out.println("<p>You have successfully edited a Student=" + studentToEdit.toString() + " to Student="+ newStudent.toString());
            } else {
                 out.println("<p>Unsuccessful attempt to edit!</p>" );
            }
      }

}

The login.jsp is simple:

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
</head>
<body>
    <form action="/JSP1/Logged" method="POST">
    <table>           
    <tr>
        <td>Login:</td>
        <td><input type="text" name="login" value=""/>  </td>   
    </tr>
    <tr>
         <td>Password </td>
         <td><input type="password" name="password"/> ></td>

    </tr>
     <tr>
         <td><input type="submit" name="OK" value="OK" /> </td>
    </tr>
    </table>
</form>
</body>

I can't figure out what's happening.

Answer 1

You aren't returning after the forward when the login and/or password is not been supplied. It's a common misconception among starters that the forward() method magically terminates the code execution and jumps out of the method somehow. This is thus not true. You have to return from the method and stop the execution of the remnant of the code yourself.

You need to either add a return;

if ((login==null)|(password==null)){
    RequestDispatcher requestDispatcher = request.getRequestDispatcher("/login.jsp");
    requestDispatcher.forward(request, response);
    return;
} 

Or to add an else

if ((login==null)|(password==null)){
    RequestDispatcher requestDispatcher = request.getRequestDispatcher("/login.jsp");
    requestDispatcher.forward(request, response);
} else {
    // Now we think that we are successfully logged in.

    // Yes, that above comment is now finally true.
    // Put your bunch of non-DRY if-else code here.
}

See also:

• java.lang.IllegalStateException: Cannot (forward | sendRedirect | create session) after response has been committed