java.lang.IllegalStateException:在servlet中提交响应后无法转发

在我的项目中，我只在用户已经登录的情况下才禁止他访问每个页面。这就是我编写以下代码的原因。例如，当我输入browser时，我会进入

login.jsp

页面。但在我输入loginid和密码后，只会出现空白页面，GlassFish说在这个页面中有一个异常

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

doGet和doPost的完整代码：

 @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>" );
            }
      }

}

@覆盖
受保护的void doGet（HttpServletRequest请求、HttpServletResponse响应）
抛出ServletException、IOException{
HttpSession ses=request.getSession（）；
String login=（String）ses.getAttribute（“login”）；
字符串密码=（字符串）ses.getAttribute（“密码”）；
如果（（登录==null）|（密码==null））{
RequestDispatcher=request.getRequestDispatcher（“/login.jsp”）；
转发（请求、响应）；
} 
//现在我们认为我们已成功登录
字符串userPath=request.getServletPath（）；
//System.out.println（用户路径）；
if（userPath.equals（“/Login”））{
RequestDispatcher=request.getRequestDispatcher（“/login.jsp”）；
转发（请求、响应）；
}
if（userPath.equals（“/Students”））{
RequestDispatcher=request.getRequestDispatcher（“/Students.jsp”）；
转发（请求、响应）；
}
if（userPath.equals（“/Student”））{
RequestDispatcher=request.getRequestDispatcher（“/Student.jspx”）；
转发（请求、响应）；
}
if（userPath.equals（“/StudentEdit”））{
RequestDispatcher=request.getRequestDispatcher（“/StudentEdit.jsp”）；
转发（请求、响应）；
}
}
@凌驾
受保护的void doPost（HttpServletRequest请求、HttpServletResponse响应）
抛出ServletException、IOException{
//processRequest（请求、响应）；
PrintWriter out=response.getWriter（）；
字符串userPath=request.getServletPath（）；
System.out.println（用户路径）；
if（request.getRequestURI（）.equals（“/Logged”））{
字符串Login=request.getParameter（“Login”）；
字符串密码=request.getParameter（“密码”）；
request.getSession（）.setAttribute（“login”，login）；
request.getSession（）.setAttribute（“密码”，password）；
RequestDispatcher=request.getRequestDispatcher（“/Students.jsp”）；
转发（请求、响应）；
}
if（userPath.equals（“/addStudent”））{
//System.out.println（request.getContextPath（））；
字符串名称=request.getParameter（“名称”）；
字符串姓氏=request.getParameter（“姓氏”）；
字符串Login=request.getParameter（“Login”）；
字符串密码=request.getParameter（“密码”）；
学生=新学生（姓名、姓氏、登录名、密码）；
如果（学生！=null）{
插入学生（学生）；
}否则{
System.out.println（“无效参数！！！”；
}
RequestDispatcher=request.getRequestDispatcher（“/Students.jsp”）；
转发（请求、响应）；
}
if（request.getRequestURI（）.equals（“/Edit”））{
System.out.println（“我们与学生一起工作！！！”；
String delete=request.getParameter（“Add_new_student”）；
如果（删除！=null）{
RequestDispatcher=request.getRequestDispatcher（“/Student.jspx”）；
转发（请求、响应）；
}
枚举参数=request.getParameterNames（）；
while（parameters.hasMoreElements（））{
String parameterName=（String）parameters.nextElement（）；
字符串parameterValue=request.getParameter（parameterName）；
字符串norder=parameterName.substring（parameterName.indexOf（“”）+1）；
ArrayList curStudents=dao.getAllStudents（）；
int norderint=Integer.parseInt（norder）；
Student studentToWork=curStudents.get（norderint）；
String actionToDo=parameterName.substring（0，parameterName.indexOf（“”）；
if（actionToDo.equals（“编辑”））{
RequestDispatcher=request.getRequestDispatcher（“/StudentEdit.jsp”）；
ServletContext cont=request.getServletContext（）；
cont.setAttribute（“studentToEdit”，studentToWork）；
转发（请求、响应）；
}否则{
布尔attemp=dao.deleteStustudent（studentToWork）；
如果（尝试）{
RequestDispatcher=request.getRequestDispatcher（“/Students.jsp”）；
转发（请求、响应）；
}否则{
out.println（“尝试删除学生失败”）；
}
}
}
}
if（userPath.equals（“/EditStudent”））{
System.out.println（“我们与StudentEdit一起工作！”）；
Student studentToEdit=（Student）request.getSession（）.getAttribute（“studentToEdit”）；
字符串newName=request.getParameter（“名称”）；
字符串newSurname=request.getParameter（“姓氏”）；
字符串newLogin=request.getParameter（“login”）；
字符串newPassword=request.getParameter（“密码”）；
学生newStudent=新学生（新姓名、新姓名、新登录名、新密码）；
布尔更新=dao.updateStudent（studentToEdit，newStudent）；
如果（更新）{
out.println（“您已成功地将Student=“+studentToEdit.toString（）+”编辑为St
<%@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>

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

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.
}