无法克服exam.jsp中的空指针异常

我正在使用oracle数据库和html制作一个java在线考试系统。。。页面位于.jsp中。我正在eclipse中运行代码。除了空指针异常之外，其他一切似乎都正常工作。当我尝试运行

exam.jsp

时，它是一个错误。我正在上传代码和错误截图：

---

当您尝试访问空对象时，会引发

NullPointerException

。在图片中的stacktrace中，您显然可以看到它将您指向第21行：

int currentQuestion = ((Exam)request.getSession().getAttribute("currentExam")).getCurrentQuestion();

这意味着此行中的某些内容为空，而您仍在尝试访问它。您仅尝试访问

request

、

request.getSession（）

和

request.getSession（）.getAttribute（“currentExam”）

最有可能导致问题的原因是

request.getSession（）.getAttribute（“currentExam”）

返回null。在运行代码之前，请尝试检查它是否不为null

你可能想看一看，然后

int currentQuestion = ((Exam)request.getSession().getAttribute("currentExam")).getCurrentQuestion();

 can u please tell me about it in a bit of detail...
 i am uploading another code ExamController.java which contains the        
 request.getSession().getAttribute("currentExam) method

package co.etest.quiz.controller;

import java.io.IOException;


import java.text.SimpleDateFormat;
import java.util.Date;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import co.etest.quiz.Exam;
import co.etest.quiz.QuizQuestion;

/**
* Servlet implementation class ExamController
*/
 @WebServlet("/exam")
public class ExamController extends HttpServlet {
private static final long serialVersionUID = 1L;

protected void doGet(HttpServletRequest request, HttpServletResponse     
response) throws ServletException, IOException {
// TODO Auto-generated method stub
doPost(request,response);
}

protected void doPost(HttpServletRequest request, HttpServletResponse       
response) throws ServletException, IOException {
    // TODO Auto-generated method stub

    boolean finish=false;

    HttpSession session=request.getSession();       
    try
    {
    if(session.getAttribute("currentExam")==null)
    {  session=request.getSession();    
    String selectedExam= 
    (String)request.getSession().getAttribute("exam"); 
    System.out.println("Setting Exam "+selectedExam);
    Exam newExam=new Exam(selectedExam);          
    session.setAttribute("currentExam",newExam);
    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd    
    HH:mm:ss a");
    Date date = new Date();
    String started=dateFormat.format(date);
    session.setAttribute("started",started);
    }

    }catch(Exception e){e.printStackTrace();}

    Exam exam=(Exam)request.getSession().getAttribute("currentExam");       

    if(exam.currentQuestion==0){    
    exam.setQuestion(exam.currentQuestion);
    QuizQuestion q=exam.questionList.get(exam.currentQuestion); 
    session.setAttribute("quest",q);
    }
    String action=request.getParameter("action");

    String radio=request.getParameter("answer");
    int selectedRadio=-1;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    if("1".equals(radio))
    {
    selectedRadio=1;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }
    else if("2".equals(radio))
    {
    selectedRadio=2;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }
    else if("3".equals(radio))
    {
    selectedRadio=3;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }
    else if("4".equals(radio))
    {
    selectedRadio=4;
    exam.selections.put(exam.currentQuestion, selectedRadio);
    System.out.println("You selected "+selectedRadio);
    }


    if("Next".equals(action)){
    exam.currentQuestion++;
    exam.setQuestion(exam.currentQuestion);
    QuizQuestion q=exam.questionList.get(exam.currentQuestion); 
    session.setAttribute("quest",q);
    }
    else if("Previous".equals(action))
    {   System.out.println("You clicked Previous Button");
    exam.currentQuestion--;
    exam.setQuestion(exam.currentQuestion);
    QuizQuestion q=exam.questionList.get(exam.currentQuestion); 
    session.setAttribute("quest",q);
    }
    else if("Finish Exam".equals(action))
    {   finish=true;
    int result=exam.calculateResult(exam);              
    request.setAttribute("result",result);
    request.getSession().setAttribute("currentExam",null);
    request.getRequestDispatcher("result.jsp").forward(request,response);       

    }

    if(finish!=true){
    request.getRequestDispatcher("exam.jsp").forward(request,response);
    }

    }

    }